A Concise Synthesis of (−)-Indolizidines 209D and 209B

A general stereoselective synthetic route to 5‐substituted and 5,8‐disubstituted indolizidine alkaloids has been developed starting from commercially available L‐proline. (−)‐Indolizidines 209D and 209B were efficiently synthesized in 9.8% and 14.8% overall yields in seven and five‐step reactions from readily available aldehyde 3 and ketone 10, respectively. The key steps of this synthesis involve a substrate‐induced asymmetric addition of ethyl lithiopropiolate to aldehyde 3 or methyl ketone 10, and a two‐step one‐pot hydrogenation/cyclization sequence to construct the bicyclic skeleton. (−)‐Indolizidine 209D and 209B were efficiently synthesized in very short steps using readily available L‐proline as starting material. The key feature of the approach resides in the addition reaction of ethyl lithiopropiolate to the key intermediates aldehyde 3 or ketone 10.