The Nitro Group in Organic Synthesis

4.3 MICHAEL ADDITION OF NITROALKANES

111

The sequence of the Michael addition of nitroalkanes and denitration provides a general method for conjugate addition of primary and secondary alkyl groups to electron deficient alkenes (Eq. 4.122).168

		O	O2N	Me		CO2Me			CO2Me
MeNO2									Me
								O2N
			O						O
		PhO2S	CO2Me			Bu3SnH		PhO2S	CO2Me
SO2Ph
			Me			AIBN			Me
DBU						benzene
		O2N							H
						80 ºC
			O						O

(4.122)

					NO2
TBDMSO	CHO	1) MeONa, MeNO2,		TBDMSO
	Me	MeOH, RT, 2 h			Me
		2) MsCl, Et3N, CH2Cl2
Me		0 ºC, 1 h		Me
				62%
		NO2
1) NaBH4, CHCl3/i-PrOH,				amberlyst A-21, Et2O,
RT, 40 min		RO
2) CH3CN/H2O, HF,			Me	RT, 2 h
RT, 24 h				O	OTBDMS
3) (S)-2-methylbutyric		Me
anhydride, DMAP,
		59%
40 ºC, 20 min
		O
	O	OTBDMS
		n-Bu3SnH, AIBN
	O	NO2	toluene
	Me	Me
			reflux, 30 min
	Me
		70%
	O				HO	O
O	OTBDMS			O		O
O					O
Me	Me			Me		Me
Me				Me
	55%			(+)-dihydromevinolin

Scheme 4.25.

112 MICHAEL ADDITION

Hanessian and coworkers have used this strategy for a total synthesis of (+)-dihydromevi- nolin (Scheme 4.25).169

Ballini and coworkers have developed a new strategy for alkenylation of carbonyl compounds based on the Michael addition followed by elimination of HNO2 (see Section 7.3). A variety of 2-alkylidene 1,4-dioles have been conveniently prepared, in two steps, by the Michael addition of a nitroalkane to the appropriate enedione derivatives under basic conditions, followed by chemoselective reduction with LiAlH4 (Eq. 4.123).170

Me	NO2	CO2Me		Me	CO2Me		Me	OH
			DBU			LiAlH4		OH
		+
Me	H		MeCN Me		CO2Me		Me
		CO2Me						82%
					94%

(4.123)

The synthesis of 2,3,5-trialkylpyrroles can be easily achieved by conjugate addition of nitroalkanes to 2-alken-1,4-dione (prepared by oxidative cleavage of 2,5-dialkylfuran) with DBU in acetonitrile, followed by chemoselective hydrogenation (10% Pd/C as catalyst) of the C-C- double bond of the enones obtained by elimination of HNO2 from the Michael adduct. The Paal-Knorr reaction (Chapter 10) gives 2,3,5-trialkylpyrroles (Eq. 4.124).171

O

Me

Me

DBU

Me

Me

+

PhCH2CH2NO2

O

MeCN

O

O

CH2Ph

H

O

77%

Ph

Pd/C

Me

Me

PhCH2NH2 Me

Me

N

(4.124)

H2

O

CH2Ph

H CH2Ph

95%

92%

The Michael addition of cyclic α-nitro ketones to acrolein or methyl vinyl ketone followed by reduction of the carbonyl group and treatment with base results in the ring expansion (Eq. 4.125).172 Hesse and coworkers have used this strategy for the preparation of various macrolides.173 For example, Michael addition of 2-nitrocyclodecanone methyl vinyl ketone followed by reduction with (S)-alpine-hydride gives the nitrolactone in 72% yield. Radical

denitration of the nitrolactone with Bu3SnH gives (+)-(S)-tetradecan-13-olide in 44% yield (Eq. 4.126).173c

	O			(H2C)n	O
C)
2 n			CHO
	n NO2						CHO
			PPh3		NO2
n = 0, 1, 2, 6					84–93%
				O				O
			(H2C)n					O
	NaBH4				OH			(H2C)n
							NaH
				NO2			DME
				90–95%				NO2
								75–81% (4.125)

		4.3 MICHAEL ADDITION OF NITROALKANES 113
O	O	O	O
	NO2
	PPh3		NO2
	O	Me	O	Me
	(S)-alpine		Bu3SnH
	hydride
		NO2	AIBN
				(4.126)
		72%		44%

The Michael addition of α-nitro ketones to α,β-unsaturated ketones followed by radical denitration provides a useful strategy for the preparation of 1,4-diketones.134b 1-Phenylheptane-

1,5-dione, isolated from the decayed heart wood of aspens, is prepared by this strategy (Eq. 4.127).174

							O	O
	O		+		PPh3		Ph
		NO2
Ph			O					NO2
				O		O		71%
			Bu3SnH	Ph				(4.127)
			AIBN
				70%

The Michael addition of nitro compounds to α,β-unsaturated ketones or esters followed by reduction of the nitro to amino group is useful for the preparation of various heterocycles. This is presented in Chapter 10 (Synthesis of Heterocycles).

4.3.2 Intramolecular Addition

Intramolecular Michael addition of nitro compounds proceeds in a stereoselective way to give various types of cyclic nitro compounds with high stereoselectivity. The Michael addition of 1-acetylcyclohexene to nitrostyrene followed by treatment with MeONa in MeOH gives 4-nitro-3-phenyldecalone with high stereoselectivity (Eq. 4.128).175

O			O Ph
				NO2
	1) LDA
	2) Ph	NO2
			75%
			O	O
			H	H
	MeONa-MeOH		+	(4.128)
	RT		Ph	Ph
			H	H
			NO2	NO2
			45% (7:93)

114 MICHAEL ADDITION

Double Michael additions of nitro compounds bearing tethered acidic carbons to 3-butyn- 2-one under NaH catalysis give nitrocyclohexanes with high stereoselectivity. The products are transformed into trans-fused bicyclic compounds via the Dickmann reaction on treatment with base. (Eq. 4.129).176

	O	CN			Me	CNO
CN		Me	CO2Et	NaOEt
	Me
	CO2Et NaH, THF		Me
NO		O2N		O	N
Me	2		O	2		OEt
		58%				80%

(4.129)

The intramolecular Michael addition is used as a key step for synthesis of epibatidine (Scheme 4.26).177 Epibatidine is an analgesic, operating by a nonopioid mechanism, it is several hundred times more potent than morphine.

O

O2N

1) NaBH4/EtOH

KF/Al2O3

O

2) MeSO2Cl

THF, RT

CH2Cl2, py

Cl

N

Cl

NO2

N

59%

OR

OMs

SnCl2•2H2O

toluene

NO2

EtOH

NH2

Cl

N

Cl

R = H (67%)

N

R = Ms (91%)

80%

R = CO-menthyl

H

N

H

Cl

H

t-BuOK

N

N

N

t-BuOH

H

Cl

50%

60%

Scheme 4.26.

Barco and coworkers have reported a more elegant synthesis of the nitrocyclohexanone via the double Michael addition of nitromethane with enones. (Eq. 4.130).178

		O
TolSO2	MeNO2
O		(4.130)
	KF
Cl N	Cl	NO2
		N

4.4 ASYMMETRIC MICHAEL ADDITION

115

The sequential process consisting of palladium-catalyzed alkylation and the intramolecular Michael addition of nitro compound provides a nitrocyclohaxane derivative, which is a good precursor for synthesis of Erythrina alkaloids (Eq. 4.131).179

O	O- Li+		AcO	Pd(PPh3)4, PPh3
	N		+	THF
O		O-
			MeO2C
O				O
	NO2
O				D	C
			CO2Me	O	N	(4.131)
	1	H			B
					A 6
	H2C			3
				MeO
	57%

4.4 ASYMMETRIC MICHAEL ADDITION

4.4.1 Chiral Alkenes and Chiral Nitro Compounds

Diastereoselective conjugate addition of nucleophiles to enones, enals, and enoates occurs with high stereocontrol and constitutes a powerful method in stereoselective synthesis.185

The conjugate addition of nucleophiles to optically pure γ-oxygenated substrates is of special interest in the enantioselective synthesis of natural products. Highly syn-selectivity is observed in the conjugate addition of ammonia,186 alkoxides,187 and alkyllithium derivatives.188 High anti-selective addition is observed in the addition of metal enolates and cuprates.189 The Michael addition of nitromethane to enoates derived from glyceraldehyde acetonide leads to syn-adducts with high diastereoselectivity (de 76–90%) (Eq. 4.132).190 Primary and secondary nitroalkanes afford syn-adducts diastereoselectively in the Bu4NFor DBU-catalyzed Michael addition to the same enoates of Eq. 4.132.191

	O				O
			Bu4NF·3H2O	O			OEt
		+ MeNO2
O	OEt						(4.132)
			THF
							O
	O				O2N

70% (syn/anti = 95/5)

Syn-selectivity of the DBU-catalyzed Michael addition of nitromethane to chiral γ-oxygen- ated enones is quite general, as shown in Scheme 4.27.192

Feringa and coworkers have used the optical active furanone or pyranone as an acceptor for the diastereoselective Michael reactions (Eq. 4.133).193

								O
	O	O H					O	O H
O			Me	CHNO		2			(4.133)
			2				O2N
	O				N			O	O
		O	N		N		Me	Me
							76% (90% de)
					H

116 MICHAEL ADDITION
					O	N
	O	N		MeNO2	O		S
O		S		DBU		O
					O2N
	O
					92% (88% de)
						NO2
	O	O			O	O
				MeNO2	O		S
O			S
				DBU
						OBn	N
	OBn	N
					94% (83% de)
							O	O	NO2
	O	O		O					O
					MeNO2	ButMe2SiO			S
t	Me2SiO			S
Bu					DBU
							O	O	N
		O	O	N

92% (86% de)

Scheme 4.27.

Krief and coworkers have found that 2-lithio-2-sulfonylpropane and 2-lithio-2-nitropropane behave differently in the addition to dimethyl alkylidenemalonate. Thus, 2-lithio-2-sulfonyl- propane reacts with it almost exclusively on the (Si)-face and leads to the anti-adduct, whereas 2-lithio-2-nitropropane reacts under similar reaction conditions, exclusively on the (Re)-face, providing the syn-product (Eq. 4.134).194

		Me SO2Ph		Me	SO2Ph
				Me	CO2Me
		Me	Li	O
					CO2Me
	CO2Me	THF
				O
O
	CO2Me	Me NO2		72% (92% de)		(4.134)
				Me	NO2
O
		Me	Li
				Me	CO2Me
		THF		O	CO2Me

O

71% (100% de)

The Michael addition of nitromethane to vinylogous esters of N-protected amino acids proceeds with good yields and with good diastereoselectivity (Eq. 4.135).195

					NHBoc
	NHBoc		MeNO2	EtO			Me
	EtO
		Me		O	(4.135)
			DBU
	O			O2N
				95% (60% de)

Levoglucosenone, a cellulose-derived α,β-unsaturated ketone, is an interesting material because it is both chiral and contains an activated double bond. This compound has been used

4.4 ASYMMETRIC MICHAEL ADDITION

117

as a chiral starting material for synthesis of a variety of natural compounds such as alkaloids or antibiotics.196 The Michael addition of nitro compounds to levoglucosenone provides a useful tool for the synthesis of various chiral compounds. Nitromethane undergoes TMG-catalyzed addition to levoglucosenone, affording 2:1 and 1:2 adducts in high yield (95%); the products result from initial Michel addition exclusively at the exo-face of the alkene anti to the 1,6-anhydro bridge (Eq. 4.136).197 If nitromethane is used as a solvent, the 2:1 adduct is obtained. A reaction is carried out by stirring a mixture of nitromethane and levoglucosenone (2:1 ratio) in CH2Cl2 in the presence of catalytic amounts of TMG to give a 1:2 adduct.

			O	O
O	MeNO2	O	MeNO2	O
O	TMG		TMG	NO2
		O2N	O	OH
O	MeNO2			O2N
	TMG		O
		O	OH	(4.136)

O2N O

O O

A variety of important synthetic reactions can be promoted by electrogenerated bases (EGB). The Michael addition and alkylation of compounds with activated hydrogen atoms are two examples in which using EGB has been very successful. The electrochemical method is very successful for functionalization of levoglucosenone as shown in Scheme 4.28.198

	O			O
	O			O
		O		O
EtO		OEt	Me	OEt
	NO2	O	O2N	O
	O
O		EtO2CCH(NO2)CH2CH2CO2Et	MeCH(NO2)CO2Et			O
						O
O
	O	O	O2NCH2CO2Et			O
						OEt
		Me2CHNO2		O2N
Me NO		O
Me	2					O
			O
		cyclo-C5H9NO2	CH3NO2
	O					O
	O	O2NCH2CO2Et	CH2(NO2)2		O	OH
	O	CH2=CHCH2Br
O2N		O	O	O2N		O
		O		O		O
		O	O
			O
	EtO
		NO2	O2N NO2
		O

Scheme 4.28.

118 MICHAEL ADDITION

4.4.2 Chiral Catalysts

Catalytic enantioselective nucleophilic addition of nitroalkanes to electron-deficient alkenes is a challenging area in organic synthesis. The use of cinchona alkaloids as chiral catalysts has been studied for many years. Asymmetric induction in the Michael addition of nitroalkanes to enones has been carried out with various chiral bases. Wynberg and coworkers have used various alkaloids and their derivatives, but the enantiomeric excess (ee) is generally low (up to 20%).199 The Michael addition of methyl vinyl ketone to

2-nitrocycloalkanes catalyzed by the cinchona alkaloid cinchonine affords adducts in high yields in up to 60% ee (Eq. 4.137).200

			N
		HO
		H	H	O
O			O
		O
	NO2		N
		+		(4.137)
			NO2
		CCl4, –20 ºC, 60 h

84% (54.6% ee)

Matsumoto and coworkers have introduced a new strategy for asymmetric induction under high pressure. The Michael addition of nitromethane to chalcone is performed at 10 kbar in the presence of a catalytic amount of chiral alkaloids. The extent of asymmetric induction reaches up to 50% ee with quinidine in toluene.201

Chiral monoaza-crown ethers containing glucose units have been applied as phase-transfer

catalysts in the Michael addition of 2-nitropropane to a chalcone to give the corresponding adduct in up to 90% ee. (Eq. 4.138).202

O							O
Me	Me	NaOtBu, toluene
+							*
	NO2
H		MeO		H		O
					O		O2N Me
		O
						N R
							Me
				H O
	1	1	O			O	82% (90% ee)
	R	O R

(4.138)

Yamaguchi and coworkers have found that proline rubidium salts catalyze the asymmetric Michael addition of nitroalkanes to prochiral acceptors. When (2S)-L-prolines are used, acyclic (E)-enones give (S)-adducts. Cyclic (Z)-enones give (R)-adducts predominantly (Eq. 4.139).203 Recently, Hanessian has reported that L-proline (3 ~ 7% mol equiv) and 2,5-dimethylpiperazine are more effective to induce catalytic asymmetric conjugate addition of nitroalkanes to cycloalkanones.204

O	NO2	N CO2Rb	O
		H
	+	(5–10 mol%)	(4.139)
		20 h
			O2N
			84% (84% ee)

REFERENCES 119

Heterobimetallic asymmetric complexes contain both Bronsted basic and Lewis acidic functionalities. These complexes have been developed by Shibasaki and coworkers and have proved to be highly efficient catalysts for many types of asymmetric reactions, including catalytic asymmetric nitro-aldol reaction (see Section 3.3) and Michael reaction. They have reported that the multifunctional catalyst (R)-LPB [LaK3tris(R)-binaphthoxide] controls the Michael addition of nitromethane to chalcones with >95% ee (Eq. 4.140).205

O		(R)-LPB, tBuOH		O2N		O
	Ph	CH3NO2 (1.2 equiv)		(S)		Ph	(4.140)

toluene

–20 ºC, 109 h

59% (97% ee)

The catalytic asymmetric nitro Mannich-type reaction using the complex Yb, K, and binaphthol gives the best result (see Eq. 4.141).206 The reaction conditions are important to get a good ee, where nitromethane is added very slowly over 27 h.

			catalyst (20 mol%)	NO2
	O	+ CH3NO2	(Yb/K/binaphthol = 1/1/3)	O
	PPh2			Ph N PPh2 (4.141)
			toluene/THF (7/1)
		(5 equiv)
	Ph N		–40 ºC	H
				79% (91% ee)

REFERENCES

1a. Feringa, B. L., and J. F. G. A. Jansen. In Houben-Weyl Methoden der Organischen Chemie, 4th Ed Vol. E21, Chapter 1.5.2.4. Thieme, Stuttgart, 1995, p 2104.

1b. Perlmutter, P. In Conjugate Addition Reactions in Organic Synthesis, Tetrahedron Organic Chemistry Series, Vol 9, Pergamon Press, Oxford, 1992.

1c. Boyd, G. V. In The Chemistry of Amino, Niroso, Nitro and Related Groups,Part 1, ed. by S. Patai, John Wiley, New York, 1996, p. 533–626.

2a. Parham, W. E., and F. L. Lamp. J. Am. Chem. Soc., 73, 1293 (1951).

2b. Ono, N., T. Hashimoto, T. X. Jun, and A. Kaji. Tetrahedron Lett., 28, 2277 (1987).

3.Vasilevskis, J., J. A. Gualtieri, S. D. Hutchings, R. C. West, J. W. Scott, D. R. Parrish, F. Bi zzarro, T., and G. F. Field. J. Am. Chem. Soc., 100, 7423 (1978).

4.Ono, N., A. Kamimura, and A. Kaji. J. Org. Chem., 51, 2139 (1978).

5.Marx, M., F. Maeri, J. Reisdorf, R. Sandmeier, and S. Clark. J. Am. Chem. Soc., 99, 6754 (1977).

6.Confalone, P. N., G. Pizzolato, D. L. Lollar-Confalone, and M. R. Uskokvic. J. Am. Chem. Soc., 102, 1954 (1980).

7.Ahrach, M., R. Schneidert, P. Gerardin, and B. Loubinoux, Synth. Commun., 27, 1865 (1997).

8.Ono, N., A. Kamimura, and A. Kaji. Tetrahedron Lett., 25, 5319 (1984).

9.Nagao, Y., K. Kaneko, and E. Fujita. Tetrahedron Lett., 1215 (1976).

10.Kamimura, A., H. Sasatani, T. Hashimoto, and N. Ono. J. Org. Chem., 54, 4998 (1989).

11.Kamimura, A., H. Sasatani, T. Hashimoto, T. Kawai, K. Hori, and N. Ono. J. Org. Chem. 55, 2437 (1990).

12.Ono, N., A. Kamimuta, T. Kawai, and A. Kaji. J. Chem. Soc., Chem. Commun., 1550 (1987).

13.Kamimura, A., and N. Ono. Tetrahrdron Lett., 30, 731 (1989).

14.Hori, H., S. Higuchi, and A. Kamimura. J. Org. Chem., 55, 5900 (1990).

15.Ender, D., A. Haertwig, G. Raabe, and J. Runsink. Angew. Chem. Int. Ed. Engl., 35, 2388 (1996).

16.Feuer, H. The Chemistry of Nitro and Nitroso Groups, part 2, Interscience, New York, p. 131 (1970).

17.Duffy, J. L., J. A. Kurth, and M. J. Kurth. Tetrahedron Lett., 34, 1259 (1993).

18a. Rajender, S. V., K. Mirayam, and W. K. Geroge. Synthesis, 486 (1986).

120 MICHAEL ADDITION

18b. Dauzonne, D., and R. Royer. Synthesis, 348 (1984).

19. Ono, N., K. Sugi, T. Ogawa, and S. Aramaki. J. Chem. Soc., Chem. Commun., 1781 (1993). 20a. Hassner, A., O. Friedmam, and W. Dehaen. Liebigs Ann., 587 (1997).

20b. Yao, C. Y., C. S. Yang, and H. Y. Fang. Tetrahedron Lett., 38, 6419 (1997).

21.Yakura, T., T. Tsuda, Y. Matsumura, S. Yamada, and M. Ikeda. Synlett, 985 (1996).

22.Dulcere, J. P., and E. Dumez. Chem. Commun., 971 (1997).

23.Dumez, E., J. Rodriguez, and J. P. Dulcere. Chem. Commun., 1831 (1997).

24a. Dumez, E., and J. P. Delcere. Chem. Commun., 479 (1998).

24b. Dumez, E., J. Rodriguez, and J. P. Dulcere. Chem. Commun., 2009 (1999). 25. Rodriguez, J. Synlett, 505 (1999).

26a. Perekalin, V. V., E. S. Lipina, V. M. Berestoviskaya, and D. A. Efremov. Nitroalkenes, Conjugated Nitro Compounds; Wiley, Chichester, 1994, Chapter 3, p 169.

26b. Rajappa, S. Tetrahedron, 55, 7065 (1999).

27a. Faulques, M., L. Rene, and R. Royer. Synthesis, 260 (1982). 27b. Rene, L., and R. Royer. Synthesis, 878 (1981).

27c. Tokumitsu, T., and T. Hayashi. J. Org. Chem., 50, 1547 (1985). 27d. Kamimura, A., and T. Nagashima. Synthesis, 694 (1990).

27e. Kamimura, A., and N. Ono. Synthesis, 921 (1988).

28.Ono, N., K, Matsumoto, T. Ogawa, H. Tani, and H. Uno, J. Chem. Soc. Perkin Trans 1, 1905 (1996).

29.Seko, S., and I. Komoto. J. Chem. Soc. Perkin Trans 1, 2975 (1998).

30.Imagawa, K., E. Hata, T. Yamada, and T. Mukaiyama. Chem. Lett., 291 (1996).

31.Ender, D., and M. Klatt. Synthesis, 1403 (1996).

32a. Morris, M. L., and M. A. Sturgess. Tetrahedron Lett., 34, 42 (1993). 32b. Sturgess, M. A., and D. J. Yarberry. Tetrahedron Lett., 34, 4743 (1993).

33.Ender, D., and J. Wiedemann. Synthesis, 1443 (1996).

34.Lucet, D., T. Toupet, T. L. Gall, and C. Mioskowski. J. Org. Chem., 62, 2682 (1997).

35.Adams, H., J. C. Anderson, S. Peace, and A. M. K Ponnell. J. Org. Chem., 63, 9932 (1998).

36.Sabelle, S., D. Lcet, T. L. Gall, and C. Mioskowski. Tetrahedron Lett., 39, 2111 (1998).

37.Shibuya, M., M. Kuretani, and S. Kubota. Tetrahedron Lett., 22, 4453 (1981).

38a. Ranganathan, D., C. B. Rao, and S. Ranganatham. J. Chem. Soc., Chem. Commun., 975 (1979). 38b. Yamada, M., and M. Yamashita. Synthesis, 1026 (1982).

38c. Yamasita, M., M. Sugiura, Y. Tamada, T. Oshikawa, and J. Clardy. Chem. Lett., 1407 (1987). 38d. Yamamoto, H., A. Noguchi, K. Torii, K. Ohno, T. Hanaya, H. Kawamoto, and S. Inokawa. Chem.

Lett., 1575 (1988).

39.Ono, N., N. Bansho, S. Ito, T. Murashima, and T. Ogawa. J. Heterocyclic Chem., 34, 1243 (1997).

40.Somanathan, R., I. A. Rivero, G. Aguirre, M. Ramirez, L. H. Hellberg, and F. Bakir. Synthetic. Commun., 26, 1023 (1996).

41a. Barrett, A. G. M. Chem. Soc. Rev., 20, 95 (1991).

41b. Banks, B., A. G. M. Barrett, and M. A. Russell. J. Chem. Soc., Chem. Commun., 670 (1984). 41c. Barrett, A. G. M., G. G. Graboski, and M. A. Russell. J. Org. Chem., 51, 1012 (1986).

42a. Ashwell, M., and R. F. K. Jackson. J. Chem. Soc., Chem. Commun., 282 (1988). 42b. Ashwell, M., R. F. K. Jackson, and J. M. Kirk. Tetrahedron, 46, 7429 (1990).

43.Barrett, A. G. M., M. C. Cheng, C. D. Spilling, and S. J. Taylor. J. Org. Chem., 54, 992 (1989).

44.Barrett, A. G. M., G. G. Graboski, and M. A. Russell. J. Org. Chem., 50, 2603 (1985).

45.Barrett, A. G. M., and S. Sakdarat. J. Org. Chem., 55, 5110 (1990).

46.Barrett, A. G. M., and S. A. Lebold. J. Org. Chem., 55, 3853 (1990).

47.Barrett, A. G. M., P. D. Weipert, D. Dhanak, R. K. Husa, and S. A. Lebold. J. Am. Chem. Soc., 113, 9820 (1991).

48.Barrett, A. G. M., and S. A. Lebold. J. Org. Chem., 56, 4875 (1991).

49.Barrett, A. G. M., D. C. Braddock, P. W. N., Christian, D. Pilipauskas, A. J. P. White, and D. J . Williams. J. Org. Chem., 63, 5818 (1998).

50.Barrett, A. G. M., J. A. Flygare, and C. D. Spilling. J. Org. Chem., 54, 4723 (1989).

51a. Jackson, R. F. W., J. M. Kirk, N. J. Palmer, D. Waterson, and M. J. Wythers. J. Chem. Soc., Chem. Commun., 889 (1993).

51b. Jackson, R. F. W., N. J. Palmer, and M. J. Wythes. J. Chem. Soc., Chem. Commun., 95 (1994).