Solid-Phase Synthesis and Combinatorial Technologies

Figure 8.34 Solid-supported scavengers: purification of the β-amino alcohol 8.64 using the covalent scavenger 8.62.

and amines, and the tagged molecules were removed from the reaction mixtures with basic supports.

The use of tetrabenzo[a,c,g,i]fluorene(Tbf) derivatives as tagging reagents,suchas 8.70 (224), was reported for acids; each Tbf-tagged reagent is purified by specific adsorption on charcoal, typically stirring for 20 min in an ice bath, and subsequent washing with lipophilic solvents to recover pure intermediates, typically stirring with toluene aliquots at 40 °C until no more UV adsorbance is detected in the toluene aliquots.

These strategies provide high-quality products after simple filtration and washing, and a suitable scavenger could be applied to each combinatorial step in solution. A common feature of all the supported scavengers is their high loading (typically 1–4 mmol/g), so that their cost is significantly lower than classical PS-based SP supports with typical loadings of 0.2–0.6 mmol/g. Most of these scavengers are commercially available, and their use in combinatorial laboratories is becoming very popular. In general, they allow parallel, automated clean-up of discrete libraries and have a significantly higher throughput than a serial HPLC/MS separation. They provide chemical flexibility and typically require less assessment than a normal library of discretes prepared on SP, especially when small–medium size arrays are considered. They require only low-technology, solution-phase equipment and are becoming the preferred choice of purification method for this format of library. The area has been reviewed recently (139, 213, 225–230).

8.4.6 An Example: Synthesis and Purification of a Benzoxazinone Library

The synthesis of a 35-member, solution-phase benzoxazinone library L18 using a five-step synthetic scheme with intermediate purifications that employed supported

reagents, scavengers, and sequestration-enabling reagents has been described recently (231). The synthetic scheme (Fig. 8.36, top) started from the key intermediate 8.71 (225) and used the monomer sets M1 (five electrophiles including isocyanates, acyl chlorides, and chloroformates, Fig. 8.36) and M2 (seven electrophiles including

COCl

Figure 8.36 Synthesis of the solution-phase, discrete benzoxazinone libraryL18 and structure of the monomer sets M1–M2.

386 SYNTHETIC ORGANIC LIBRARIES: SOLUTION-PHASE LIBRARIES

isocyanates, acyl chlorides, sulfonyl chlorides, anhydrides, chloroformates, and the skip-Boc monomer M2,7, Fig. 8.36) to produce L18.

The first reaction (step a, Fig. 8.36) was driven to completion through the use of a 20% excess of M1 and by pyridine as a base with acyl chlorides or chloroformates M1,1, M1,3, and M1,5 (step a, Fig. 8.37). This protocol involved addition of the polyamine resin 8.58 to scavenge the HCl and also the excess M1,1, M1,3, and M1,5 upon completion of the reaction, thus freeing the volatile pyridine. After filtration and washing of the resin, evaporation of the solvents gave pure 8.72. Even two equivalents of isocyanates M1,2 and M1,4 and commercially available supported DMAP 8.50 in a more aggressive reaction protocol could not drive the reaction to completion (step b, Fig. 8.36). The same supported amine 8.58 was used after the second protocol, but first, an excess of the sequestration-enabling reagent 8.69 was added to transform unreacted 8.71 into an acid-tagged molecule. Supported 8.58 then scavenged the acid-tagged 8.71, the excess M1,2 and M1,4, and the excess of 8.69, leaving products 8.72 (Fig. 8.37) after filtration and washing.

Boc deprotection (step b, Fig. 8.36) was studied, but even the optimized conditions (step a, Fig. 8.38) produced a slight amount of deprotected carboxylic acid impurity together with hydrochlorides 8.73. Addition of 8.58 removed this side product (step b, Fig. 8.38). An aliquot of solution corresponding to monomer M2,7 (skip Boc) was not deprotected (step c, Fig. 8.38) and was carried forward to the ester deprotection. The following coupling with an excess M2 (step c, Fig. 8.36) was performed in pyridine (step d, Fig. 8.38), and addition of 8.58 scavenged the HCl and the excess of M2. The usual filtration, washing, and evaporation of solvent and pyridine gave pure products 8.74 (step e, Fig. 8.38).

The crude reaction mixture from the deprotection of the TMSE (trimethyl silyl ethanol) ester (as in step d, Fig. 8.36) contained the desired products as tetrabutylammonium salts as well as excess TBAF and the volatile trimethylsilyl fluoride and ethylene (step a, Fig. 8.39). This crude was purified by addition of a mixture of a sulfonic acid ion-exchange resin 8.76 and the same resin in its calcium sulfonate form 8.77 (206), which freed the products 8.75 from their ammonium salt and precipitated the excess fluoride as insoluble calcium fluoride (step b, Fig. 8.39). Filtration, washing, and evaporation gave pure acids 8.75, which were finally cyclized (as in step e, Fig. 8.36) using supported EDC 8.78, functioning here as both a supported reagent and a scavenger of unreacted 8.75 through formation of the corresponding adducts (step c, Fig. 8.39). Filtration and washing produced the array L18, whose components were characterized by HPLC. Table 8.4 (see Fig. 8.35 for monomer structures) indicates the overall good quality of the discrete library, considering the five-step reaction scheme.

8.4.7 An Example: Synthesis and Purification of a Library of Trisubstituted Amines

The SPS of a small array of trisubstituted amines L19 has been recently reported (232) building on work described in two previous papers (233, 234). These reported Michael addition onto an acrylate resin 8.79, reductive amination of amine 8.80, quaternization

Figure 8.37 Synthesis and purification of the solution-phase discrete benzoxazinone library L16: amine acylation and synthesis of 8.72.

388 SYNTHETIC ORGANIC LIBRARIES: SOLUTION-PHASE LIBRARIES

Figure 8.38 Synthesis and purification of the solution-phase discrete benzoxazinone library L18: Boc deprotection and amine functionalization, synthesis of 8.74.

of the tertiary amine 8.81, then cleavage of the ammonium salt 8.82 via Hoffman elimination, producing the library L19 (Fig. 8.40).

An excess of TEA in the elimination step (step f, Fig. 8.40) produced significant amounts of salts as contaminants, requiring either aqueous extractions (233) or SP biphasic columns (234) for its complete sequestration from the reaction mixture. Stoichiometric amounts of TEA resulted in lower quantities of salts, but the same work-up and purification protocols were nevertheless required. The authors then used catalytic amounts of TEA in the presence of an ion-exchange resin to regenerate the soluble base, scavenging the acid moiety and allowing the total cleavage of 8.82. This reduced the purification procedures to simple filtration, washing of the support, and evaporation of the solvent.

An intriguing finding came from the same reaction protocol with the ion-exchange resin 8.83 but in the absence of TEA. In this case, pure L19 was isolated, albeit with longer reaction times (Fig. 8.41, top). The mechanism reported in Fig. 8.41 was postulated. Thus, if the resin-bound ammonium salt 8.82 underwent a thermal elimination (step a), even in extremely small amounts, the ammonium salt released would be converted to the amine by the ion-exchange resin 8.83 (initiation, Fig. 8.41). The amine would then replace TEA as a soluble base, as depicted in the propagation steps (Fig. 8.41). To compare the efficiency of different eliminations, three cleavage protocols were tried in parallel on 8.82: stoichiometric TEA (A), an ion-exchange resin (Amberlite IRA-95, B), and a deprotected Rink amide resin (C). The yields are summarized in Table 8.5 (NMR purities were always 95% or better) and show that B was the best protocol, while PS–Rink resin was less satisfactory than either A or B. The structures of the library individuals L19a–n are reported in Fig. 8.42.

Even with SP cleavage reactions, supported reagents may be useful to directly perform or to promote clean cleavage processes for the production of high-quality libraries. Such an intriguing finding should be exploited further by other groups.

8.4.8 Resin Capture

A concept that has been repeatedly expressed is the complementarity, rather than the mutual exclusivity, of the various library formats; this also includes solutionand solid-phase libraries that are more or less suited to each planned synthetic scheme. It may well be, though, that the synthetic strategy contains steps that are more suited to homogeneous reactions and others that may benefit from the advantages of heterogeneous reactions. A hybrid approach in which the first steps are performed on SP and postcleavage combinatorial modifications in solution are used to produce the final library could be envisaged, but to date, this possibility remains largely unexploited. The opposite strategy, that is, the synthesis of advanced library intermediates in solution, their attachment onto SP, further SP transformations, and final cleavage to give a library, has been first reported by Armstrong and co-workers (235–237). This approach was named resin capture to stress the key event during the hybrid library synthesis. Another group has recently reported similar capture on the SP of advanced library intermediates, trapping bicyclic anhydrides derived from ring opening crossmetathesis with amine supports (238).

392 SYNTHETIC ORGANIC LIBRARIES: SOLUTION-PHASE LIBRARIES

Figure 8.41 Synthesis of the SP, discrete tertiary amine library L19: proposed cleavage mechanism.

TABLE 8.5 Hoffman β-Elimination on 8.82: Experimental Protocols

aL19a–n. bStoichiometric TEA. cIon-exchange resin.

dDeprotected Rink amide resin.