The Nitro group in organic sysnthesis - Feuer
.pdf5.5 ALKYLATION OF NITRO COMPOUNDS USING TRANSITION METAL CATALYSIS 139
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LnPd(0) |
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H X |
Pd L |
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+ L |
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Scheme 5.5. Catalytic cycle in the palladium-catalyzed telomerization of 1,3-butadiene
telomerization of 1,3-butadiene is shown in Scheme 5.5; a wide range of H-X trapping reagents (X = nucleophilic carbon, oxygen, nitrogen, or sulfur) are used.69
When nitromethane is used as a trapping reagent in the telomerization of butadiene using Pd(OAc)2 and Ph3P, mono-, diand trialkylated compounds of nitromethane are formed (Eq. 5.47).70 They are selectively formed by changing the ratio of nitromethane and butadiene. As the nitro groups are transformed into various functional groups, the reaction of Eq. 5.47 is very useful in organic synthesis.
+ CH3NO2 |
Pd(OAc)2 |
NO2 |
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Ph3P |
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+ |
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NO2 |
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+ |
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(5.47) |
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NO2 |
Butadiene telomerization using nitroethane as a trapping reagent is applied to the total synthesis of the natural product, recifeiolide, where the secondary nitro group is converted into the ketogroup by the Nef reaction, and the terminal double bond is converted into the iodide via hydro alumination (Scheme 5.6).71
Extension to carbocyclization of butadiene telomerization using nitromethane as a trapping reagent is reported (Eq. 5.48).72 Palladium-catalyzed carbo-annulation of 1,3-dienes by aryl halides is also reported (Eq. 5.49).73 The nitro group is removed by radical denitration (see Section 7.2), or the nitroalkyl group is transformed into the carbonyl group via the Nef reaction (see Section 6.1).
O |
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O |
NO2 |
EtO |
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Pd(PPh3)4, PPh3 EtO |
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+ CH3NO2 |
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EtO |
EtO |
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O |
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O |
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79% |
(5.48) |
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NO2 |
NO2 |
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Pd(OAc)2, PPh3, LiCO3 |
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+ |
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(5.49) |
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H |
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I |
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73%
140 ALKYLATION, ACYLATION, AND HALOGENATION OF NITRO COMPOUNDS
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Pd(OAc) |
NO2 |
1) TiCl3 |
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+ |
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2) (HOCH2)2, H+ |
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Ph3P |
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NO2 |
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56% |
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O |
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O O |
1) LiAlH4, TiCl3, I2 |
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SPh |
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OH |
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Cl |
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2) NaBH4 |
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I
70%
O O
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1) KN(SiMe3)2 |
O O |
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SPh 2) Raney Ni |
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I
82%
Scheme 5.6. Synthesis of recifeiolide
Nitronate anions react with (π-allyl)cobalt complexes prepared from acylation of 1,3-dienes by acetylcobalt tetracarbonyl to produce nitro enones (Eq. 5.50).74
O |
+ - |
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O2N |
O |
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+ |
+ NaCH2NO2 |
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Me |
(5.50) |
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Me |
Co(CO)3 |
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74% |
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5.5.2 Pd-Catalyzed Allylic C-Alkylation of Nitro Compounds
The palladium-catalyzed allylic alkylation of soft nucleophiles (Tsuji-Trost reaction) represents one of the most useful organic transformations. Various allylating reagents and nucleophiles can participate in this reaction, as summarized in Scheme 5.7. Although allyl acetates, allyl carbonates, or 1,3-dienemonoepoxides are generally used as alkylating reagents, allylic nitro compounds also can be used as alkylating reagents (see Section 7.1). Active methylene compounds are generally used as nucleophiles in Tsuji-Trost reaction. Nitroalkanes, α-nitro ketones, α-nitro esters, and α-nitro sulfones can be also used as nucleophiles for this transformation.
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Pd(0) |
R |
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R |
Nu– |
Nu + Pd(0) + X– |
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X |
R |
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Pd |
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X |
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X = OAc , OC(O)OR, SO2Ph, NO2, OP(O)(OR)2, NR2, SR, etc.
NuH = CH2(CO2R)2, CH2(CN)CO2R, CH2(SO2Ph)CO2R, CH2(NO2)CO2R, CH2(NO2)SO2Ph,
CH2(SO2Ph)2, etc.
Scheme 5.7. Pd(0) catalyzed allylic alkylation
5.5 ALKYLATION OF NITRO COMPOUNDS USING TRANSITION METAL CATALYSIS 141
The monoanions of primary nitroalkanes, phenylnitromethane, and α-nitro esters are all preferentially C-alkylated by cinnamyl acetate and 2-butenyl acetate in 50–89% yield in the presence of Pd catalyst (Eq. 5.51).75 The α-nitro ester gives the C-alkylate in 89% yield, but 2-nitropropane gives the C-alkylate in only 29% yield. The main product is cinnamaldehyde, which is derived from O-alkylation.75a
Ph |
OAc |
R1 |
R2 |
Yield |
A/B |
+Pd(PPh3)4
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Li+ |
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CO2Et |
Et |
89% |
97/3 |
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NO2 |
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R1 |
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Ph |
Me |
Me |
29% |
93/7 |
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R2 |
+ |
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Ph |
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R1 |
NO2 |
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NO2 |
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R2 |
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A |
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B |
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(5.51) |
Wade and coworkers have found that α-nitro sulfones are useful reagents in organic synthesis because they are converted into nitroalkanes, nitriles, or carboxylic acids (see Eq. 5.52).76
(Phenylsulfonyl)nitromethane is preferentially C-alkylated by allylic acetates in the presence of Pd(PPh3)4 (5 mol%) to give various α-nitro sulfones as shown in Eq. 5.53.76
H H O
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NH2 |
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RCH2NO2 |
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N |
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CH2Ph |
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hυ |
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R SO2Ph |
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TiCl3 |
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RCN |
(5.52) |
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NO2 |
KMnO4 |
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RCO2H |
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+ |
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NO2 |
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Pd(PPh3)4, PPh3 |
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OAc |
PhO2S |
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Li+ |
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THF, reflux, 5 h |
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NO2 |
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(5.53) |
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83% |
SO2Ph |
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Allylic carbonates are better electrophiles than allylic acetates for the palladium-catalyzed allylic alkylation.77 Reaction of Eq. 5.54 shows the selective allylic alkylation of α-nitro ester with allylic carbonates without affecting allylic acetates.78
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OCO Et + |
NO2 |
Pd(dppe) |
AcO |
NO2 |
AcO |
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2 |
EtO2C |
THF, RT |
81% |
CO Et |
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2 |
(5.54)
142 ALKYLATION, ACYLATION, AND HALOGENATION OF NITRO COMPOUNDS
As the nitro group is removed by radical denitration with Bu3SnH, allylic alkylation of α-nitro ketones with allyl carbonates in the presence of Pd(0) followed by denitration with
Bu3SnH provides a new regio-selective allylation of ketones under neutral conditions (Eq. 5.55).79
NO2 |
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Pd(PPh3)4 |
O2N |
+ Ph |
OCO Et |
THF, RT |
Ph |
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2 |
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O |
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O |
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70% |
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H |
Ph |
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Bu3SnH, AIBN |
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(5.55) |
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O |
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85% |
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2-Nitrocycloalkanones can be successfully C-allylated by Pd(0)-catalyzed reaction with various allyl carbonates and 1,3-dienemonoepoxides under neutral conditions, as shown in Eqs. 5.56 and 5.57, respectively.80a The product of Eq. 5.56 is converted into cyclic nitrone via the reduction of nitro group with H2-Pd/C followed by hydrolysis and cyclization.80b
O |
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O |
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NO2 |
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NO2 |
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CO2Me |
Pd(PPh3)4 |
CO2Me |
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+ |
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(5.56) |
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OCO2Et |
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92% |
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O |
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O |
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NO2 |
O |
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NO2 |
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Pd(PPh3)4 |
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+ |
OH (5.57) |
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Me |
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Me |
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90% |
Recent papers have disclosed that Pd(0) catalyzed allylic alkylations under neutral conditions are not limited to allylic carbonates or epoxides but also can be extended in many cases to the more popular allylic acetates (Eq. 5.58).81a
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CO2Me |
Pd(dba) |
Ph |
NO2 |
Ph |
OAc |
+ |
3 |
(5.58) |
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PPh3, DMSO |
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O2N |
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CO2Me |
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80% |
Wong and co-workers have prepared various quaternary α-nitro-α-methyl carboxylic acid esters by the palladium-catalyzed allylic alkylation of α-nitropropionate ester (Eq. 5.59). The products can be kinetically resolved by using α-chymotrypsin and are converted into optical active α-methyl α-amino acids. Such amino acids are important due to the unique biological activity of these nonproteinogenic α-amino acids.82
5.5 ALKYLATION OF NITRO COMPOUNDS USING TRANSITION METAL CATALYSIS 143
Me |
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Pd(PPh3)4 |
Me NO2 |
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OAc + |
CO2Me |
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(5.59) |
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PPh3 |
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O2N |
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CO2Me |
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94% |
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Rajappa and co-workers have reported synthesis of dipeptides with an α,α-bisallylglycine residue at the NH2-terminal, which are biologically important.83 Their strategy is based on (1) nitroacetylation of an amino acid derivative, (2) regioselective bisallylation by Pd-catalyzed reaction, and (3) generation of the free terminal NH2 from the NO2 group as shown in Scheme 5.8. Esters of L-proline, L-valine, and L-phenylalanine are converted into the corresponding N-nitroacetyl derivatives using 1,1-bis(methylthio)-2-nitroethylene84 (see Section 4.2, Michael addition). Subsequent palladium-catalyzed allylation followed by reduction with Zn in AcOH gives the desired dipeptides.
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O2N |
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SMe |
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SMe |
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O2N |
+ |
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CO2CH2Ph |
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TsOH |
N |
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N |
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CO2CH2Ph |
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SMe |
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H |
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O |
CO2CH2Ph |
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O |
CO2CH2Ph |
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HgCl2 |
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Ph |
OAc |
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N |
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Ph |
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MeCN, H2O |
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DBU, Pd(PPh3)4 |
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O2N |
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NO2 |
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50–60% |
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NO2 |
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75% |
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Ph |
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Ph |
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Ph OAc
O N
DBU, Pd(PPh3)4
PhH2CO2C
95%
Scheme 5.8.
Hydroxamic acids constitute an important class of siderophores, which play a major role in iron solubilization and transport. Some of them are important as therapeutic agents. The Michael addition of nitroacetyl proline esters to allyl acrylate followed by Pd(0)-catalyzed intramolecular allyl transfer and subsequent reduction of the nitro group yields a novel class of cyclic hydroxamic acids related to pyroglutamic acid (Scheme 5.9).85
O |
O |
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O2N H |
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KF |
O |
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OMe |
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O2N |
+ |
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O |
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O |
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O |
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OMe |
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85% |
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OMe |
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O |
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O |
Pd(dba)2, PPh3 |
O2N |
Zn/AcOH |
OMe |
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OMe |
O |
HON |
+ |
AcON |
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DBU, MeCN |
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Ac2O |
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HO |
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30 ºC |
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O |
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63% |
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55% |
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40% |
Scheme 5.9.
144 ALKYLATION, ACYLATION, AND HALOGENATION OF NITRO COMPOUNDS
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Me |
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Me |
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(CO2Me)2O |
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Pd(PPh3)4 |
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HO |
N |
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MeO |
O |
N |
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NHBoc DMAP (cat.) |
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NHBoc |
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CH3NO2 |
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O |
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H |
Me |
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S |
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1) TFA, CH2Cl2 |
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Ph |
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O2N |
N |
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O2N |
N |
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NHBoc 2) PhNCS |
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H |
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O |
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60–65% |
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89% |
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TFA |
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NH2•TFA |
Boc2O, Et3N |
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NHBoc |
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O2N |
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NHBoc |
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Nef reaction |
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HO |
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Scheme 5.10.
1,4-cis-Disubstituted cyclopentene precursors of carbocyclic nucleosides are prepared by acyl-nitroso hetero Diels-Alder reaction and subsequent Pd(0)-catalyzed allylic alkylation. Acylnitroso dienophiles derived from amino acids are used for asymmetric hetero Diels-Alder reaction. The alcohol in Scheme 5.10, prepared by this route,86 is converted into the corresponding nitromethyl group by Pd(0)-mediated alkylation.87 Removal of the L-alanine side chain followed by the Nef reaction leads to an important intermediate for the preparation of carbovir, aristermycin, and related analogs, which show potent and selective anti-HIV activity.88a A short, enantioselective synthesis of the carbocyclic nucleoside carbovir is also reported, in which the reaction of Pd catalyzed allylation of nitro compounds is used in a key step.
Miller and coworker report a total synthesis of carbocyclic polyoxin C from cis-(N-tert- butylcarbamoyl)cyclopent-2-en-1-ol, as shown in Scheme 5.11.89 This synthesis features a
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OMe |
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NO2 |
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AcO |
NHBoc |
+ |
O2N |
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Pd(PPh3)4 |
MeO |
NHBoc |
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NaOAc |
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O |
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O |
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95% |
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NHCbz |
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MeO |
O |
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1) |
TiCl3, tartaric acid, |
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MeO |
NHBoc |
H NCO |
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NaBH4 |
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2) CbzCl |
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O |
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1) TFA |
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2) DBU |
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87% |
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H |
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H |
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O N |
O |
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O |
N |
O |
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NHCbz |
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NHCbz |
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1) NaOH |
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MeO |
N |
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MeO |
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NH |
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2) NH4OH |
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O |
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OMe |
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3) BnBr |
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O |
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80% |
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84% |
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H |
O |
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NH3+ |
O N |
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1) OsO4, NMO |
–O2C |
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N |
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2) H2, Pd/C |
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O
HO OH
Scheme 5.11.
5.5 ALKYLATION OF NITRO COMPOUNDS USING TRANSITION METAL CATALYSIS |
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145 |
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Ph O |
O Ph |
Pd (dba) |
•CHCl |
3 |
Ph |
O |
N3 |
PPh |
3 |
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2 |
3 |
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+ Me3SiN3 |
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(BOC)2O |
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O |
O |
THF |
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O |
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Ph |
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Ph |
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77% (98% ee) |
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O |
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O |
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NH |
HN |
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PPh2 Ph2P |
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SO2Ph |
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NO2 |
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Ph O |
NHBoc |
O2N |
PhO2S |
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NHBoc |
DBU |
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Pd2(dba)3•CHCl3 |
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tetrabutylammonium |
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O |
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88% |
PPh3 |
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96% |
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oxone |
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O |
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NH2 |
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N |
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N |
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O |
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H |
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N |
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N |
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NHBoc |
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HO |
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N |
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N |
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MeO |
HO |
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N |
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N |
NH2 |
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47% |
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HO |
OH |
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carbovir |
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aristeromycin |
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NH2 |
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O |
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N |
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N |
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NH2 |
O |
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N |
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N |
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N |
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H |
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EtHN |
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+H N |
NH |
– |
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2 |
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2Cl |
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HO |
OH |
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amidinomycin |
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C-NECA |
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Scheme 5.12.
Pd(0)-catalyzed substitution reaction, a novel, mild reduction of α-nitro ester to an amino acid ester with TiCl3, and an improved procedure for uracil ring formation.
Trost and co-workers have explored asymmetric transition metal-catalyzed allylic alkylations. Details on this subject have been well reviewed by Trost and others.90 With the use of asymmetric palladium-catalyzed desymmetrization of meso-2-ene-1,4-diols, cis-1,4-dibenzoy- loxy-2-cyclopentene can be converted to the enantiometrically pure cis-4-tert-butoxycar- bamoyl-1-methoxycarbonyl-2-cyclopentene.91 The product is a useful and general building block for synthesis of carbocyclic analogs of nucleosides as presented in Scheme 5.12.
Another approach to asymmetric syntheses of carbonucleosides is presented in Scheme 5.13. The reaction of cis-1,4-dibenzoyloxy-2-cyclopentene with the lithium salt of (phenylsulfonyl)nitromethane in the presence of Pd catalyst and a chiral ligand gives a chiral isoxazoline N-oxide, in which C-alkylation and O-alkylation of nitronates take place simultaneously. Deoxygenation with SnCl2 2H2O in MeCN gives the isoxazoline in 94% yield, which is converted into the corresponding hydroxy ester on treatment with MeOH in the presence of K2CO3 followed by reduction with Mo(CO)6. Thus, diastereoand enantioselective hydroxyalkoxycabonylation of cyclopentene ring provides useful building blocks for the synthesis of important antiviral carbanucleosides, as shown in Scheme 5.13.92
Enantioselective allylations of α-nitro ketones and α-nitro esters with allyl acetates are carried out in the presence of 2 equiv of alkali metal fluorides (KF, RbF, CsF) and 1 mol% palladium catalysts prepared in situ from Pd2(dba)3 CHCl3 and chiral phosphine ligands. Moderate enantio-selectivity (ca 50% ee) is reported for allylation of α-nitroketones (Eq. 5.60). The highest selectivity (80% ee) is observed for allylation of the reaction of tert-butyl ester (Eq. 5.61).93
146 ALKYLATION, ACYLATION, AND HALOGENATION OF NITRO COMPOUNDS
|
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SO2Ph |
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SO2Ph |
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Ph O |
O |
Ph O N |
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1) SnCl2•2H2O |
CO2Me |
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2 |
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O |
O |
Pd2(dba)3•CHCl3 |
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N |
O |
2) MeOH/K2CO3 |
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O |
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THF |
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3) Mo(CO)6 |
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OH |
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Ph |
Ph |
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1) 94% |
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O |
O |
94% (95% e.e.) |
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2) 91% |
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PPh2 Ph2P |
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3) 94% |
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chiral ligand |
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NH2 |
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O |
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LiAlH4 |
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ClCO2Me |
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O |
OMe |
Pd(OAc)2 |
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N |
N |
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OH |
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OH |
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O |
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adenine |
N N |
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O |
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OH |
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OMe |
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95% |
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98% |
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96% |
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Scheme 5.13.
|
O |
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O |
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NO2 |
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Pd2(dba)3•CHCl3 |
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NO2 |
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+ |
OAc |
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(5.60) |
||
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RbF, chiral ligand |
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CH2Cl2, –20 ºC |
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95% (58% ee) |
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O |
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O2N |
O |
+ |
|
OAc |
O |
O |
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N |
Me |
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|||||
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X |
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Me |
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O |
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O |
nMe |
(5.61) |
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Pd2(dba)3•CHCl3 |
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* |
OR |
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H |
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RbF, chiral ligand |
|
Me NO2 |
|
|
Ph2P Fe |
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Ph2P |
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|||||
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92% (80% ee) |
chiral ligand |
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Asymmetric synthesis of tricyclic nitro ergoline synthon (up to 70% ee) is accomplished by intramolecular cyclization of nitro compound Pd(0)-catalyzed complexes with classical C2 symmetry diphosphanes.94 Palladium complexes of 4,5-dihydrooxazoles are better chiral ligands to promote asymmetric allylic alkylation than classical catalysts. For example, allylic substitution with nitromethane gives enantioselectivity exceeding 99% ee (Eq. 5.62).95 Phosphinoxazolines can induce very high enatioselectivity in other transition metal-catalyzed reactions.96 Diastereoand enantioselective allylation of substituted nitroalkanes has also been reported.95b
|
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Ph |
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Ph |
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OCO2Me CH |
NO |
2 |
NO2 |
PdL+ |
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Me N |
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3 |
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Ph |
= |
Me |
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Ph |
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PdL+ |
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O |
PPh2 |
|||||
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Me |
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THF |
|
87% (99% ee) |
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+ Pd2(dba)3•CHCl3 |
|||||
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(5.62)
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5.6 |
ARYLATION OF NITRO COMPOUNDS 147 |
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OH |
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OH |
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OAc acetylcholine esterase |
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LiCH(NO2)SO2Ph |
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Phosphate buffer |
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25–27 ºC, 23 h |
|
OAc |
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Pd(OAc)2, PPh3 |
|
|
O2N SO2Ph |
|||||||
OAc |
|
77% (92% ee) |
|
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THF, 60 ºC |
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|
||||||||||||||||
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89% |
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OH |
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O |
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Me |
|||||
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KO2CN NCO2K |
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1) O3, NaOMe, MeOH |
|
|
|
|||||||||||||
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O |
||||||||||||||||||
|
AcOH, DMSO |
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2) H+ |
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Me |
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O2N |
SO2Ph |
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65% |
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OH |
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|
OCO |
Me 1) PhSO2CH2NO2 |
|
|
PhO2S NO2 |
||||||||||||
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|
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|||||||||||||
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ClCO2Me |
|
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2 |
|
Pd(OAc)2, PPh3 |
|
|
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|
|||||
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|||||
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pyridine |
|
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|
|
2) KOH |
|
|
OH |
|||||
OAc |
|
OAc |
|
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||||||||||||
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|||
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|
91% |
|
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|
O |
95% |
|
|
|||||||
|
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||
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|
1) KO2CN |
NCO2K |
|
|
|
Me |
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|||||||
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O |
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|||||||
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|
2) O3, NaOMe, MeOH |
|
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Me |
|
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||||||
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|
3) H+ |
|
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|||
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|
58% |
|
|
|
|
|
|
Scheme 5.14.
An enantio-selective enzymatic hydrolysis of meso-(E)-2,5-diacetoxy-3-hexene gives (+)- (E)-(2S,5R)-5-acetoxy-3-hexen-2-ol in 77% yield (92% ee).97 The monoacetate with its two allylic groups offers possibilities for stereo-controlled introduction of nucleophiles via Pd(0) catalysis. Synthesis of both enantiomers of the Carpenter bee pheromone based on this strategy is presented in Scheme 5.14.98
Tamura and coworkers have reported a novel C-C-bond formation reaction using organotellurium chlorides and lithium nitronates. A combination of the bis(organo)tellurium dichlorides [(R1COCH2)2TeCl2] and LiC(NO2)R2R3 leads to the coupling products R1COCH = CR2R3 in good yields. The reaction proceeds by a polar mechanism that is initiated by coordination of the nitronate oxygen atom to the tellurium followed by intramolecular C-C bond formation and subsequent elimination of nitro and tellurium moieties.99
5.6 ARYLATION OF NITRO COMPOUNDS
Arylations of nitro compounds can be achieved by aromatic nucleophilic substitution using aromatic nitro compounds, as discussed in Chapter 9.100 Kornblum and coworkers reported displacement of the nitro group of nitrobenzenes by the anion of nitroalkanes. The reactions are usually carried out in dipolar aprotic solvents such as DMSO or HMPA, and nitroaromatic rings are substituted by a variety of electron-withdrawing groups (see Eq. 5.63).101
O2N |
X |
Me Li+ |
|
Me |
|
|
HMPA |
O2N |
X |
|
|||
|
+ |
|
|
Me |
|
(5.63) |
|
|
|
|
|||
|
Me |
NO2 |
|
|
|
|
|
|
|
|
|
||
X = NO2, CN, SO2Ph |
|
|
70–90% |
|
|
|
CO2Me, C(O)Ph |
|
|
|
|
|
There are many cine substitution reactions of aromatic nitro compounds using various nucleophiles.100 In this chapter, the cine-substitution reactions using the anion of nitroalkanes
148 ALKYLATION, ACYLATION, AND HALOGENATION OF NITRO COMPOUNDS
are summarized. 1-Nitronaphthalene reacts with the anion of nitromethane to give the nitromethylated product, as shown in Eq. 5.64.102 Suzuki and coworkers have extended this reaction to m-dinitrobenzene (Eq. 5.65). Although the reaction proceeds slowly, this is the first example of nitromethylation of monocyclic nitrobenzenes. The reaction of Eq. 5.64 requires additional oxidizing agents to complete the reaction, but that of Eq. 5.65 does not need the external oxidizing agents.103
NO2 |
|
|
|
|
|
NO2 |
|
|
|
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|
|
|
+ Na+ |
CH NO |
|
1) DMSO |
|||
2 |
|
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||
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|
|||
|
2 |
2) Br2 |
||||
|
|
|
3) Et3N |
|||
|
|
|
35% |
|||
NO2 |
|
|
|
t-BuOLi (8.0 equiv) |
||
|
|
|
|
|||
+ |
CH3NO2 |
|
|
|
O2N |
|
|
|
|
||||
O2N |
|
|
|
DMI, 24 h |
(DMI: 1,3-dimethyl-2-imidazolizinone)
NO2 (5.64)
NO2
NO2
(5.65)
44%
In general, heterocyclic nitro compounds undergo cine substitution reactions more readily than nitrobenzenes. For example, the reaction of 5-acyl- or 5-alkoxycarbonyl-2-nitrofurans with the anion of nitroalkanes gives cine substitution products in excellent yields (Eq. 5.66).104
|
|
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|
|
Me Me |
|
|
|
Me |
+ |
DMF |
|
NO2 |
EtO |
|
+ |
|
|
|||
|
|
|
(5.66) |
||||
NO2 |
|
Li |
|
EtO |
|||
O |
Me |
NO2 |
|
O |
|||
|
|
|
|
O
O
90%
The reaction of 1,2-dimethyl-5-nitroimidazole with 2-nitropropane anion gives the new highly branched imidazole derivative, which is formed via cine-substitution and SRN1 substitution (Eq. 5.67).51b
|
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Me |
Me Me |
|
|
N |
|
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|
NO2 |
|
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|
|
Me Bu |
N+ |
toluene-H |
O |
|
N |
|
||
|
|
|
|
Me |
|
|||||
Me |
N |
NO2 + |
4 |
|
2 |
|
|
(5.67) |
||
Me NO2 |
|
reflux, 24 h |
Me |
N |
NO2 |
|||||
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|||||||
|
Me |
|
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||||
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|
Me
Barton and coworkers have explored the arylation of various nucleophiles including nitroalkanes using bismuth reagents.105 Reaction of 2-nitropropane with triphenylbismuth carbonate gives 2-nitro-2-phenylpropane in 80% yield.106 Recently, this arylation has been used for the synthesis of unusual amino acids. Arylation of α-nitro esters with triphenylbismuth dichloride followed by reduction gives unique α-amino acids (Eq. 5.68).107
NO2 |
|
|
Ph |
NO2 |
|
|
DBU |
|
OMe |
||
OMe |
+ Ph3BiCl2 |
O |
|||
(5.68) |
|||||
toluene |
|||||
O |
|
Me |
O |
Me O
77%