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