Solid-Phase Synthesis and Combinatorial Technologies

8.4 SOLID-PHASE ASSISTED SOLUTION-PHASE LIBRARY SYNTHESIS

393

			F
			N
			L19a
		N	O
N	N		N
	L19c		L19d

N CN	N
L19f	L19g

		N
	L19i	N
		L19j

O

N

N O

L19l

N

N

L19m

N

L19b

N

L19e

N

L19h

O

N

N

L19k

	O
N	N
H

N

L19n

Figure 8.42 Synthesis of the SP, discrete tertiary arnine library L19: structure of the library individuals L19a–n.

The synthesis of hybrid solution/solid-phase small arrays of 1,2,3-thiadiazoles, L20 and L21, has also been described (239). An SP strategy was first set up (Fig. 8.43) based on commercially available PS–sulfonyl hydrazide resin that was reacted with a small set of commercially available ketones (step a) to form intermediate resin-bound sulfonyl hydrazones 8.84, then submitted to cyclative cleavage to form the thiadiazoles L22 (step b) with good yields (84–99%) and GC purities (94–99%).

A more diverse set of noncommercially available ketones was prepared in solution (Fig. 8.44) by the reaction of a Weinreb methoxymethyl amide (the 4-bromophenyl derivative 8.85 in the example) with a Grignard reagent (six representatives), purifying the crude mixtures using supported tosic acid 8.86 (step a). The solutions containing the ketones were transferred to other vessels containing PS–sulfonyl hydrazide resin, and the same reaction protocol described in Fig. 8.43 produced the final thiadiazoles

394 SYNTHETIC ORGANIC LIBRARIES: SOLUTION-PHASE LIBRARIES

O O

S

NHNH2

+

P

b	R1
	N
R2	N
	S

L20

			O	O
	O		S		N
		a		N	R2
		R2		H	R
	R1
		P	8.84		1
	M1

a:10% AcOH/THF, 50°C, 4 hrs; b: SOCl2, DCE, 60°C, 5 hrs.

M1 : commercially available ketones (7)

7 discretes

1,2,3-thiadiazole array

Figure 8.43 Synthesis of the SP, discrete thiadiazole library L20.

L21 with high yields (59–98%) and purities as measured by GC (94–100%). A hybrid approach would have benefited from further SP transformations before cleavage, and the authors proved the concept (Fig. 8.45) starting from a noncommercial ketone that was processed as in Fig. 8.44 to give a resin-bound sulfonyl hydrazone 8.88. This compound was submitted to a Stille coupling with an aryl stannane to give 8.89, which was submitted to cyclative cleavage to give 8.90 with 71% yield and 85% purity measured by GC (Fig. 8.45).

O

OMe

	N	a
Br	8.85	8.86 Br
	O O
	S	N
	N	R1
b	H	c
P	8.87
		Br

a: R1CH2MgX, THF, 0°C, 3 hrs; b: 10% AcOH/THF, 50°C, 4 hrs; c: SOCl2, DCE, 60°C, 5 hrs.

M1 : non-commercially available ketones (6)

O	O
O	S
R	NHNH2
1

+ P

M1

Br	SO3H
	P
	8.86
	N
R1	N
	S

L21

6 discretes

1,2,3-thiadiazole array

Figure 8.44 Resin capture: synthesis and purification of the discrete thiadiazole library L21 using the supported reagent 8.86.

	8.4	SOLID-PHASE ASSISTED SOLUTION-PHASE LIBRARY SYNTHESIS 395
	O	O			O	O
	S	N	N		S	N
			a			N
		H				H
P	8.88			P	8.89

Br

	b		a: PhSnBu3, Pd(PPh3)2Cl2, DMF, 90°C;
			b: SOCl2, DCE, 60°C, 5 hrs.
	8.90
			N
		R1	N
			S

Figure 8.45 Synthesis and purification of the hybrid, discrete thiadiazole, library L21 using resin capture: SP modification of the captured intermediate 8.88 to give the biaryl 8.90.

8.4.9 A Comprehensive Example: Synthesis and Purification of Libraries of Triand Tetrasubstituted Pyrroles

Caldarelli et al. (240) have recently reported a five-step synthesis of substituted pyrrole libraries L22 and L23 using solid-supported reagents and scavengers. The synthesis involved oxidation of benzyl alcohols M1 to aldehydes (step a, Fig. 8.46), Henry reaction of aldehydes 8.91 with nitroalkanes M2 (step b), and acylation and elimination of nitroalcohols 8.93 (steps c and d) to give the nitrostyrenes 8.94, which were subjected to 1,3-dipolar cycloaddition with an isocyanoacetate (step e) to give the pyrroles 8.95. N-alkylation of these pyrroles with alkyl halides (step f) and final “library-from-a-library” hydrolysis/decarboxylation of L22 gave a library of trisubstituted pyrroles L23 (step g, Fig. 8.46).

The oxidation of the alcohol was performed with supported perruthenate (8.48, Fig. 8.46) to produce clean aldehydes 8.91 after filtration. The Henry reaction was performed in the presence of a commercially available, supported strong base 8.92 and an excess of volatile nitroalkenes, giving clean nitroalcohols 8.93 after filtration and evaporation. The reaction mixtures from the trifluoroacetylation/elimination steps were purified with commercially available amino PS resin 8.58 to scavenge the trifluoroacetates and with acidic ion-exchange resin 8.76 to remove the TEA-derived salts. Again, the nitrostyrenes 8.94 were obtained cleanly after filtration and evaporation. Cycloaddition with isocyanoacetate was promoted by the commercially available, supported guanidine base 8.95, while the subsequent N-alkylation of the pyrroles 8.96 was performed with an excess of halide in the presence of the commercially available, supported phosphazene 8.97. In this case, the excess halide was removed by treatment with supported 8.58, and filtra-

396 SYNTHETIC ORGANIC LIBRARIES: SOLUTION-PHASE LIBRARIES

								OH
	OH	8.48		CHO		8.92		NO2
R1				+ R
		a	R1		NO	b	R1	R2
	M1			2 M2	2
			8.91					8.93
				R1
c,d	8.58			NO2 8.95		R2		8.97 8.58
						+	X
	R1			e
			R2		N		R3	f
	8.76			tBuOOC
			8.94	8.96 H
			R		R1
			1
				R			R2
				2
				g
			tBuOOC	N			N
				R			R3
				3
				L22			L23
			16 discretes			16 discretes
			tetrasubstituted pyrrole library		trisubstituted pyrrole library

a:DCM, rt; b: neat, rt; c: TFAA, DCM, rt; d: TEA, DCM, rt; e: tBuOcoCH2NC, THF/iPrOH, rt;

f:DCM, rt; g: TFA, DCM, rt.

	NMe +RuO -			NH2		SO	H	NMe3+OH-
P	3	4	P		P	3		P
	8.48			8.58		8.76		8.92
			N			N		N
							P
	P		N	N	P	N		N
		8.95				8.97

Figure 8.46 Synthesis and purification of the solution-phase, discrete pyrrole libraries L22 and L23 using the supported reagents 8.48, 8.58, 8.76, 8.92, 8.95, 8.97.

tion/evaporationprovidedthelibraryL22 (Fig. 8.46). Further elaboration of L22 to L23 did not require any purification to remove volatile TFA and t-butyl alcohol. The yields of all the steps were almost quantitative (with the exception of the acetylation/elimination reactions steps c and d; Fig. 8.46) producing overall yields ranging from 30 to 80%. The purities of intermediates were checked after each step by LC-MS and were always >90%, more often being >95%.

8.5 SOLUBLE SUPPORTS IN SOLUTION-PHASE COMBINATORIAL SYNTHESIS

397

8.5 SOLUBLE SUPPORTS IN SOLUTION-PHASE COMBINATORIAL SYNTHESIS

8.5.1 General Considerations

Solid supports for SP synthesis are an important option for the synthesis of libraries of both discretes and pools. We have extensively reviewed their properties, but we have also highlighted some critical issues such as the different reaction kinetics in heterogeneous reactions due to accessibility of inner reaction sites, the difficulty of monitoring reactions on SP, and the need for suitable expertise and instrumentation.

A few years ago, Han et al. (241) introduced the so-called liquid-phase combinatorial chemistry by applying the concept of liquid-phase synthesis (242) to combinatorial synthesis through the use of a soluble, linear homopolymer as a support for library synthesis. A support of this type must be soluble in most organic solvents and possibly should impart solubility onto the molecules attached to it to allow reactions to be performed and monitored in the homogeneous phase. The support must also be prone to crystallization, or at least precipitation, under well-defined experimental conditions to allow the handling of a solid material during the work-up and purification stages. Such a support would have the advantages of both solutionand solid-phase combinatorial synthesis in the different phases of a synthesis. The original work (241) used PEG-derived polymers that possessed the properties mentioned above and reported the synthesis of both a pool library of peptides and a small array of arylsulfonamides.

Since this first report, many other contributions have appeared, some of which have focused on the synthesis of libraries, whereas others describe the characterization of new polymers. The main classes of soluble supports will now be examined through examples, and their properties and use for specific applications will be reviewed.

8.5.2 PEG-Based Polymers

The use of polyethylene glycol (PEG) supports in natural oligomer synthesis has been known for some time (242–244). Polyethylene glycol polymers with average MW in the range 2000–20,000 daltons are identified as PEGs, either as bifunctional polymers with two free hydroxyls (PEG) or as monoethers capped with a methoxy function at one end (MeO-PEG). These polymers, the general structure of which is shown in Fig. 8.47, fulfill most of the criteria for an ideal soluble support:

•They are commercially available at reasonable cost in a wide range of average MWs and loadings.

•They have good solubility in most organic solvents and in water (see Fig. 8.47), but they are insoluble in n-hexane, diethyl ether, and cold ethanol, which induce their precipitation/crystallization, a property that can be used to advantage in purification protocols.

•They have excellent solubilizing properties that allow the dissolution of loaded insoluble molecules, providing that the loading is not high enough to overpower the influence of the PEG.

398 SYNTHETIC ORGANIC LIBRARIES: SOLUTION-PHASE LIBRARIES

O	OH
HO	n O

PEG

	2000<MW<20,000
O	OH
O	n O

MeO-PEG

Solubility data of MeO-PEG 5000:

-Water, DCM, CHCl3, DMF, Pyridine >40%

-MeOH, EtOH (34°C), Benzene 10%-20%

-Cold EtOH 0.1%

-Et2O 0.01%

Figure 8.47 Polyethylene glycol as a soluble support: structure and properties.

•Their structure does not interfere with analytical methods used to monitor the reactions (TLC of PEG-derivatized compounds can be carried out) and to characterize the reaction products (NMR signals of PEG are concentrated around 3.64 ppm, and MeO-PEG provides an internal standard at 3.38 ppm).

Soluble PEG supports have been used for different applications such as the synthesis of peptides (241, 245) and peptidomimetics (246); soluble-supported catalysts (247–249), reagents (250–253), scavengers (254), and traceless linkers (255– 257); to improve purification protocols (258, 259); and high-loading PEG-derived soluble supports (260), particularly in the synthesis of arrays of small organic molecules (261–275). Several recent reviews (276–281) illustrate their usefulness but also show how additional efforts could make PEG liquid-phase combinatorial synthesis more reliable.

8.5.3 An Example: Synthesis of an Aminoimidazolinedione Array

Joon et al. (282) recently reported the synthesis of a 10-member array of 3-aminoimi- dazoline-2,4-diones L24 (Fig. 8.48) using MeO-PEG-amine soluble support, the aromatic linker 8.98, and the monomer sets M1 (six α-amino acid isocyanates) (283) and M2 (four aza α-amino acids) (247). The linker was coupled to the support via an amide bond (step a, Fig. 8.48), and deprotection of the phenolic hydroxyl (step b) gave supported 8.99. Coupling with monomer set M1 (step c) and removal of the butyl ester (step b) produced 8.100, which was coupled to monomer set M2 (step d) and deprotected (step b) to give 8.101. Each deprotection step was followed by precipita-

8.5 SOLUBLE SUPPORTS IN SOLUTION-PHASE COMBINATORIAL SYNTHESIS

399

tBuO

OH

R1

a,b

O

MeO-PEG-NH2 +

MeO-PEG-NH

+

N COOtBu

8.98

COOH

8.99

O

M1

H

COOH

c,b

O N

+

Boc

MeO-PEG-NH

O

R1

N

8.100 O

H2N R2

M2

O

H

H

R1

O

O

N

N

R

d,b

N

e

H

2

+ HN N

MeO-PEG-NH

O

R

8.99

NH

1

8.101 O

R2

O

L24

M1 : α-amino acid N-isocyanate esters (6)

ten discretes

aminoimidazoline dione array

M2 : Boc aza-amino acids (4)

a: DCC, DMAP, rt; b: TFA, DCM, rt; c: TEA, DCM, rt; d: DCC, DCM, rt; e: DIPEA, dilution, rt.

Figure 8.48 Synthesis and purification of the solution-phase discrete aminoimidazolinedione array L24 using PEG soluble supports.

tion of the PEG-supported intermediates with diethyl ether to quickly and efficiently purify them. Monitoring of the reactions was performed by TLC and NMR spectroscopy of aliquots of the mixtures. Compounds 8.101 were easily cyclized/cleaved under weakly basic conditions (step e, Fig. 8.48) to give L24 with yields varying from 62 to 80% and >90% purities (NMR, MS, IR). The soluble supported construct 8.99 could be recovered with good yields after the cleavage.

8.5.4 Other Soluble Polymeric Supports

While PEG-based supports are widely used for liquid-phase combinatorial chemistry, other non-PEG-based soluble polymers have also been reported for combinatorial applications. A recent review (276) contains an exhaustive list of homoand copolymeric soluble supports used in peptide, oligonucleotide, and oligosaccharide synthesis, including combinatorial chemistry. Two of these supports have also been used for small organic molecule synthesis. Homopolymeric polyvinyl alcohol was used in conjunction with PEG for a protection/derivatization strategy in solution (284), and the copolymer between isopropylacrylamide and acrylic acid was used in the catalytic hydrogenation of a Cbz group (285).

400 SYNTHETIC ORGANIC LIBRARIES: SOLUTION-PHASE LIBRARIES

A recent paper (286) reported the use of a commercially available soluble hybrid support prepared by copolymerization of styrene and allylic alcohol that exhibits a lipophilicity intermediate between that of PS and PEG chains. This support was grafted with a number of functional groups that were reacted with various monomer sets to produce pyrazolo[3,4-b]pyridines and coumarins with high yields and purities after precipitation by addition of ethanol or water. Most of the soluble support was recovered after the cleavage and retained the same efficiency in further reaction cycles. Another example is thoroughly described in the next subsection.

Further development of reliable, high-loading soluble supports with complementary solubilizing properties and with high stability to organic reaction conditions can be expected. An intriguing true combinatorial approach to the optimization of the physico-chemical properties of soluble supports has recently been reported by Gravert et al. (287) and is described in Section 11.3.2.

8.5.5 An Example: Synthesis of a Prostaglandin on a Soluble Support

Chen and Janda (288, 289) recently reported the use of non-cross-linked polystyrene (NCPS) (290) as a soluble support with complementary solubility properties with respect to PEG resins. The synthesis of a complex prostaglandin structure 8.106 (291) using the chloromethyl NCPS support 8.102, the acid-labile THP linker, and the three synthons 8.103 (291), 8.104 (292), and 8.105 (293) is shown in Fig. 8.48.

During the complex synthetic procedure, it was necessary to use a support with a high solubility in THF even at –78 °C, in conditions in which PEG is poorly soluble (steps c, d, and f, Fig. 8.49). The extreme solubility of PEG in water would also not allow the complete removal of large quantities of salts during the aqueous removal of organometallic/inorganic salts required in steps c and f. The lipophilic, water-insoluble NCPS resin 8.102 was compatible with these requirements. Moreover, reaction conditions included solvents such as cyclohexane and benzene at temperature below 0 °C, in which NCPS is fully soluble and PEG is not and the purification protocols involved precipitation with methanol, in which NCPS is completely insoluble, in contrast to PEG.

The two single compounds prepared can be seen as a successful validation of the use of NCPS supports, which are complementary to PEG-based soluble supports. This and other similarly lipophilic supports will undoubtedly become more popular for the preparation of combinatorial libraries of small organic molecules.

8.5.6 Dendrimers as Soluble Supports

The advent of dendrimers in organic chemistry has led to many applications of these branching oligomers generated from a central core unit. Kim et al. (294) first reported their application as soluble supports in the field of combinatorial technologies, naming the approach dendrimer-supported combinatorial chemistry (DCC), then renamed by the same group combinatorial synthesis on multivalent oligomeric supports (COSMOS) (295). The elaboration of a commercially available dendrimer based on a polyamidoamine structure produced a high-loading support useful for combinatorial chemistry. For example, 7 mg of the dendrimer was equivalent in terms of loading to 100 mg of

8.5 SOLUBLE SUPPORTS IN SOLUTION-PHASE COMBINATORIAL SYNTHESIS				401
	O		COOMe
Cl
8.102	HO	OTBS	TfO
	8.103	8.104	8.105

Cl a	O	b	O	O O
	O			O

8.102

OTBS

	c,d,e
	O		OTMS
	O	O
	OTBS
			COOMe
	f,g
	O		O
	O O
		OTBS	COOMe
	h
	O		O
		O
	O		a: linker-OH, NaH, DMA, rt, 24 hrs;
			b: 8.103, PPTS, DCM, 40°C, 16 hrs;
	OTBS	COOMe	c: 8.104, Li2CuCNMe2, THF, -78°C,
			15'; d: TMSCl, -78°C, 30'; e: TEA,
			0°C, 15'; f: MeLi, THF, -23°C, 30';
	i		g: 8.105, -78°C, 10', then -23°C, 30'; h:
			H2, 5% Pd/BaSO4, quinoline,
	8.106		C6H6, cyclohexane, rt, 48 hrs; i: 48%

O	aq. HF/THF (3/20 v/v), 45°C, 6 hrs.
HO

Figure 8.49 Synthesis of the prostaglandin 8.106 using a non-cross linked polystyrene (NCPS) soluble support.

a 0.23-mmol/g resin (typical capacity for a Tentagel PS). The supported intermediates were purified from reagents, salts, and by-products by size exclusion chromatography (SEC) or ultrafiltration, taking advantage of the increased MW of dendrimers supporting multiple copies of the library individuals. The compounds released after cleavage were purified by SEC, recovering the eluate solution of the small-MW library indi-

402 SYNTHETIC ORGANIC LIBRARIES: SOLUTION-PHASE LIBRARIES

viduals and possibly recycling the high-MW dendrimers after elution from the SEC column. SEC is applicable to automated purification of small libraries, while its use for larger arrays could become somewhat rate limiting, and technological improvements would be necessary to increase its throughput.

Other reports have followed, including a paper from the same group describing the synthesis of a 48-member guanidine library (295). Gitsov et al. (296) has described the synthesis and characterization of high-loading PEG-derived dendrimers. Langer et al. (297) have reported the assembly of dendritic glycoclusters using one-pot reaction schemes starting from aliphatic polyols as core structures. Worl and Koster (298, 299) has reported the use of dendritic structures for large-scale synthesis of oligonucleotides. A hybrid approach where dendrimers are anchored onto PS resins to increase exponentially their loading has been recently presented by Mahajan et al. (300).

Cheaper, high loading hyperbranched polymers were introduced by Kantchev and Parquette and used to prepare oligosaccharides (301); their potential as solid supports is considerable. Newkome et al. have reported the combinatorial synthesis of dendritic materials with different properties (302, see also Section 11.3.2).

		H2N	NH2
		NH	HN
		O	O
NH2			N
HN	O	O	NH
H		O	H	O
	N	N
N			N
H2N	N	N	N	N
	H			H
O			O	NH2
		HN O	O	NH
		8.107
		N		NH2
	O	O
	NH	HN
	H2N	NH2

Figure 8.50 Structure of the polyamidoamine dendrimer 8.107.