Synthesis of the novel pi-(benzyloxymethyl)-protected histidine analogue of statine. Inhibition of penicillopepsin by pepstatin-derived peptides containing different statine side-chain derivatives

Abstract

The synthesis of aspartic proteinase inhibitors derived from a new histidine side-chain analogue of statine (Sta), (3S,4S)-4-amino-3-hydroxy-5-(imidazol-4-yl)pentanoic acid (HiSta, 20), is reported. Boc-HiSta(BOM)-OMe (7) was prepared in 16% overall yield from Boc-His(pi-BOM)-OH via formation of the tetramic acid derivative 11 and stereoselective cis reduction with NaBH4 to the 4-hydroxy lactam 12. Removal of the Boc group from ester 7 (enantiomeric purity ee = 88-90%) and coupling to the tripeptide segment Iva-Val-Val-OH (13) by the DCC/HOBt preactivation method followed by hydrogenolytic removal of the pi-BOM group over Pd(OH)2 on carbon gave Iva-Val-Val-HiSta-OMe (16). This new peptide 16 is a very potent inhibitor of the fungal aspartic proteinase penicillopepsin (Ki = 4.5 x 10(-9) M) that is 10 times more active than the comparable Sta-containing inhibitor 3 and 2-3 times more potent than the new (3S,4S)-4-amino-3-hydroxy-5-phenylpentanoic acid (AHPPA) analogue 17 (Ki = 1.5 x 10(-8) M). However, compound 16, which has an imidazole residue at the P1 position, is a significantly weaker inhibitor of the enzyme than the corresponding analogues with the lysine (5) and ornithine (6) side chains at P1. Considerations that led to the synthesis of 16 and the results of the enzyme kinetics are discussed in detail.