Solid-Phase Synthesis and Combinatorial Technologies

9.1 PHARMACEUTICAL APPLICATIONS 443

Figure 9.17 SAR from N-capping modifications of 9.21: structures of the solution-phase peptidomimetic discrete library L12 and of hits 9.23–9.25 obtained from its screening.

displaced with M1 (phenols, thiophenols, anilines, and aminopyridines). While both thiophenols and anilines provided the desired compounds during the assessment, the reactivity of phenols and aminopyridines was not satisfactory. A 51-member discrete array L13 was prepared and confirmed after quality control by HPLC. The compounds were screened, confirming the importance of the dichloro-substituted phenyl ring. A few reprepared discretes showed the general potency order thiophenol > phenol > aniline (compounds 9.28–9.33, Fig. 9.18).

Modification of the butyl linker moiety was then studied on SP, treating 9.26 with M1 (symmetrical anhydrides or diacids, 10 representatives, Fig. 9.19) to give the resin-bound acids 9.34, which were pooled, reduced to alcohols 9.35, and brominated to give the alkyl bromides 9.36. The resin was split into 60 portions and treated with M2 (the same 60 thiophenol and aniline nucleophiles as for L13) to give the 600-member pool library L14 made of 60 pools of 10 individuals (Fig. 9.19). The library quality

446 APPLICATIONS OF SYNTHETIC LIBRARIES

Cl

9.37R1=H, CaPMI IC50 = >200 M

9.38R1=COOH, CaPMI IC50 = >500 M

9.39R1=COOMe, CaPMI IC50 = 34 M

9.40R1=CH2OH, CaPMI IC50 = 140 M

9.41R1=CONHMe, CaPMI IC50 = 34 M

9.42R1=CONHNH2, CaPMI IC50 = 40 M

9.43R1=CONHOH, CaPMI IC50 = 39 M

Figure 9.20 SAR from amide replacement in 9.21: structures of compounds 9.37–9.43.

Figure 9.21 SAR from amino indane replacement in 9.21: structures of the solution-phase peptidomimetic discrete library L15 and of selected library individuals 9.44–9.47.

9.1 PHARMACEUTICAL APPLICATIONS 447

was prepared following the SP strategy depicted in Fig. 9.22. The biological activity of these esters was completely lost, confirming the extremely strict structural requirements of the scaffold portion of these CaPMI inhibitors (compare with 9.39, Fig. 9.20).

The combined information acquired by the above-mentioned efforts produced the novel compound 9.53 (Fig. 9.23) as a moderately potent inhibitor of CaPMI with good physicochemical properties, modest selectivity versus the human enzyme, and some in vivo activity against several fungal strains.

ON H

O

a:piperidine, DMF, rt; b: benzophenone imine, AcOH, NMP, rt; c: NaHDMS, THF, -78°C to rt;

d:NH2OH.HCl, THF, rt; e: acid, HATU, DIPEA, DMF, rt; f: TFA, DCM, rt; g: Me3SiCHN2, THF, rt.

COOMe

Figure 9.22 SAR from amino indane replacement in 9.21: structures of compounds 9.52a–c.

448 APPLICATIONS OF SYNTHETIC LIBRARIES

S. cerevisiae MIC = 80 M

Figure 9.23 Structure and properties of the optimized lead compound 9.53.

9.1.11 An Example: Synthesis of a κ Opioid Receptor–Focused Piperidine

Library

Thomas et al. (57) recently reported the synthesis of a 288-member discrete solution library L16 of piperidines (Fig. 9.24), inspired by the structures of known piperidine-

Figure 9.24 Structures of the known opioid antagonists 9.54–9.56 and of the solution-phase discrete focused library L16 inspired by their structures.

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based opioid antagonists 9.54–9.56 (Fig. 9.24) (58, 59). These structures showed a subtype selectivity toward the µ opioid receptor. The authors intended to pursue the identification of selective opioid antagonists, possibly active on the κ receptor subtype, as novel and more effective agents in drug abuse therapy. Knowing that N-substitution did not alter the antagonistic nature of the piperidine-based derivatives, a synthetic scheme based on decoration of the N-unsubstituted scaffold 9.54 was designed (Fig. 9.25).

The scaffold was first coupled with monomer set M1 (11 N-protected α-amino acids, from which seven validated monomers were used for the library synthesis, Fig. 9.26) to give 9.57; then the N-protecting group was removed (steps a and b, Fig. 9.25). The amide bond was reduced (step c), and the intermediates 9.58 were purified by chromatography or by crystallization. Finally, monomer set M2 (171 carboxylic acids, from which 116 validated monomers were used for the library synthesis, Fig. 9.26) was coupled to 9.58 to give L16 (step d, Fig. 9.25). The monomers M1 were chosen to avoid µ-orienting cyclic lipophilic substituents at a distance of three carbon atoms from the piperidine nitrogen atom, while various aryl-, alkyl-, or alkenylaryl carboxylic acids were used as the M2 set to explore the substitution pattern allowed at the amidic nitrogen atom. The 288 confirmed library individuals contained 7 different M1 monomers (from 1 to 116 recurrences of the same monomer) and 116 different M2 monomers (from 1 to 7 recurrences of the same monomer). The unexplained imbalance in L16 composition between monomer representatives might be due to various factors, such as the successful characterization of final compounds (purity cutoff) or the decision to report only selected structures and biological data.

The 288 library individuals were tested as κ-opioid binders in a radioligand binding assay. The percent inhibition of the most relevant compounds at a concentration of 100 nM is listed in Table 9.1, and their structures are reported in Fig. 9.27. Among them, compound 9.66 displayed an extremely interesting in vitro κ-opioid binding activity

a: BOP, TEA, THF, rt; b: TFA, DCM, rt; c: BH3.SMe2, rt; d: BOP, TEA, THF, rt.

Figure 9.25 Synthetic scheme to the solution-phase discrete focused piperidine library L16.

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M2

171 representatives

4 alkyl amino acids (3 used in

L16 synthesis) such as

N

COOH

Figure 9.26 Monomer sets M1–M2 used in the synthesis of the solution-phase discrete focused piperidine library L16.

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TABLE 9.1 κ-Opioid Inhibition of Active Individuals from Screening of the Solution-Phase Discrete Focused Piperidine Library L16

aPercent inhibition of 100 nM.

N

HO

NH

9.66O

Figure 9.27 Active library individuals 9.59–9.66 from screening of L16.