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Molecular modeling based on the X-ray crystal structure of the Tang-Ghosh heptapeptide inhibitor 1 (OM99-2) of BACE led to the design and synthesis of a series of constrained P1‘ analogues. A cyclopentane ring was incorporated in 1 spanning the P1‘ Ala methyl group and the adjacent methylene carbon atom of the chain. Progressive truncation at the P2‘−P4‘ sites led to a potent truncated analogue 5 with good selectivity over Cathepsin D. Using the same backbone replacement concept, a series of cyclopentane, cyclopentanone, tetrahydrofuran, pyrrolidine, and pyrrolidinone analogues were synthesized with considerable variation at the P and P‘ sites. The cyclopentanone and 2-pyrrolidinone analogues 45 and 57 showed low nM BACE inhibition. X-ray cocrystal structures of two analogues 5 and 45 revealed excellent convergence with the original inhibitor 1 structure while providing new insights into other interactions which could be exploited for future modifications.

Following earlier work on cystine-bridged peptides, cyclic phosphopeptides containing nonreducible mimics of cystine were synthesized that show high affinity and specificity toward the Src homology (SH2) domain of the growth factor receptor-binding protein (Grb2). Replacement of the cystine in the cyclic heptapeptide cyclo(CY*VNVPC) by d-α-acetylthialysine or d-α-lysine gave cyclo(Y*VNVP(d-α-acetyl-thiaK)) (22) and cyclo(Y*VNVP(d-α-acetyl-K)) (30), which showed improved binding 10-fold relative to that of the control peptide KPFY*VNVEF (1). NMR spectroscopy and molecular modeling experiments indicate that a β-turn conformation centered around Y*VNV is essential for high-affinity binding. X-ray structure analyses show that the linear peptide 1 and the cyclic compound 21 adopt a similar binding mode with a β-turn conformation. Our data confirm the unique structural requirements of the ligand binding site of the SH2 domain of Grb2. Moreover, the potency of our cyclic lactams can be explained by the stabilization of the β-turn conformation by three intramolecular hydrogen bonds (one mediated by an H2O molecule). These stable and easily accessible cyclic peptides can serve as templates for the evaluation of phosphotyrosine surrogates and further chemical elaboration.

The first successful preparation of mono- and disubstituted 3,7-dihydroxytropolone involves a four-step synthetic scheme. Thus, bromination of 3,7-dihydroxytropolone (8) followed by permethylation of the resultant products furnished gram quantities of intermediates 13−18. Single or double Suzuki coupling reactions between these permethylated monobromo- and dibromodihydroxytropolone derivatives and a variety of boronic acids delivered the expected products whose deprotection yielded the desired compounds 1a−u and 26a−n, usually in fair to good yields. Tropolones 1 and 26 were found to be potent inhibitors of inositol monophosphatase with IC50 values in the low-micromolar range. The results are discussed in the context of the recently described novel mode of inhibition of the enzyme by 3,7-dihydroxytropolones.