syn ‐Allylstannation of N ‐Acyliminium Intermediates by Tributyl[γ‐(silyloxy)allyl]stannanes: A Key Reaction for the Diastereoselective Synthesis of Polyhydroxypiperidines and Polyhydroxyazepanes

The allylstannation of N ‐alkenyl N ‐acyliminium intermediates by tributyl[γ‐(silyloxy)allyl]stannanes afforded the expected adducts with a high syn ‐selectivity (up to 99:1). Ensuing ring‐closing metathesis afforded dehydropiperidines or dehydroazepanes which were engaged in a stereoselective dihydroxylation reaction leading to polyhydroxypiperidines or polyhydroxyazepanes in good yields. This sequence was applied to a diastereoselective synthesis of (±)‐1‐deoxy‐gulonojirimycine.