Synthesis of bicyclic carbamates as precursors of Sedum alkaloid derivatives

Synthesis of a N-Boc-protected piperidin-2-yl phosphine oxide starting from piperidine in three steps, followed by olefination using a variety of α,β-unsaturated aldehydes resulted in tert-butyl 2-(2′-alkenylidene)piperidine-1-carboxylates in high yields. A novel type of intramolecular cyclization of these enamides furnished a new family of 3-alkyl-4,6,7,8-tetrahydro-3H-pyrido[1,2-c][1,3]oxazin-1-ones as useful substrates for further elaboration. Subsequent reduction of these unsaturated bicyclic carbamates using NaCNBH3 or NaBH4 afforded the corresponding 3-alkylhexahydropyrido[1,2-c][1,3]oxazin-1-ones in a highly stereoselective way. Reductive ring opening of two representatives furnished the corresponding Sedum alkaloid derivatives in good yields.