Cyclic Dipeptides. 3. Synthesis of Methyl (R)-6-[(tert-Butoxycarbonyl)amino]-4,5,6,7-tetrahydro-2-methyl-5-oxo-1,4-thiazepine-3-carboxylate and Its Hexahydro Analogs. Elaboration of a Novel Dual ACE/NEP Inhibitor

Synthetic routes to highly functionalized 1,4-thiazepinones 2 and 3 have been developed. S-Ac-NBOC-L-Cys-D(L)-ThrOMe 7a,b have been prepared and, after transformation into the corresponding mesylates, used as the cyclization substrates. Treatment of these compounds with LiA1H(OMe)3 in THF results in mesylate elimination and thiolacetate reduction, giving rise to both a Michael acceptor (α,β-unsaturated ester) and Michael donor (thiol anion), which undergo in situ intramolecular conjugate addition leading to the stereoselective formation of only two of the four possible stereoisomers of 2. On the other hand, PCC/CaCO3 oxidation of 7a gave in 80% yield the corresponding ketone 11, which was in turn transformed into the enol triflate 15. Cleavage of the thiolacerate moiety, simultaneous elimination of trifluoromethanesulfonic acid to generate an allene system, and addition of the thiol group to the central carbon of the allene to provide the enantiomerically pure cyclic compound 3 in 85% yield was effected via a one- pot reaction by means of MeONa/MeOH. Thiazepinone 3 is an interesting intermediate for the preparation of conformationally restricted dipeptide mimetics, and its elaboration into the dual ACE/NEP inhibitor 4 is reported.