DEVELOPMENT OF METHODOLOGY FOR THE SYNTHESIS OF STEREOCHEMICALLY PURE PHE-PSI[CH2N]PRO LINKAGES IN HIV PROTEASE INHIBITORS

One of the strategies currently being pursued for the design of potential HIV protease inhibitors involves the replacement of the cleaved amide bond in a minimum peptide substrate with an aminomethylene unit. A commonly used method for the synthesis of these compounds involves a reductive alkylation of an amine with an aldehyde in the presence of sodium cyanoborohydride under acidic conditions. Accordingly, BOC-phenylalaninal (4) was reacted with the peptide-resin ProIleSer(OBzl)OResin (5) in the presence of acetic acid and sodium cyanoborohydride. The resulting product was found to consist of a mixture of diastereomers, which may result from the fact that the proline residue, which contains a secondary amine, reacts with the aldehyde to form an enamine 9 with loss of chirality at the modified Phe residue. Subsequent reduction of the iminium ion 10 would then result in production of the observed two diastereomers. In order to circumvent this problem, BOCPheProOBzl (12b) was synthesized and the central amide bond was reduced selectively with diborane. Hydrogenolysis of the benzyl protecting group gave BOCPhe-PSI[CH2N]Pro (14a), which was coupled manually to the peptide resin IleSer(OBzl)OResin to give BOCPhe-PSI[CH2N]ProIleSer(OBzl)OResin (6). Subsequent addition of amino acid residues to 6 and cleavage from the resin gave a series of stereochemically defined potential HIV protease inhibitors as single diastereomers. The most potent of these substances was ThrLeuAsnPhe-PSI[CH2N]ProIleSer (1), which displayed an IC50 of 1.1-mu-g/mL (1.4-mu-M) when tested for inhibition of HIV-1 protease. However, the epimer of 1 having the opposite configuration at the reduced Phe residue was inactive. A minimum length of seven amino acid residues appears to be necessary for effective recognition of the inhibitor by the enzyme. Further increase in chain length did not result in greater inhibitory potency.