Stereoselective Synthesis of Stannylated Dehydropiperidines and Dehydroazepanes

An efficient allylation and butenylation of N-alkoxycarbonyl-2-tributylstannyl-1,3-oxazolidines derived from (S)-vinylglycinol or (S)-styrylglycinol is described. After conversion of the stannylated azadienes into stannylated dienyl oxazolidinones, a ring-closing metathesis generates dehydropiperidine or dehydroazepane; both are interesting scaffolds for the synthesis of polyfunctionnalized piperidines or azepanes. Whereas the dehydropiperidine synthesis was found to be selective regardless of the Grubbs catalyst used, we found that Grubbs II catalyst induced partial double bond isomerization in the dehydroazepane series. In addition, when allyltrimethylsilane was used in the ring-opening reactions of N-alkoxycarbonyl-2-tributylstannyl-1,3-oxazolidines, cyclopropyl derivatives were selectively obtained.