A concise asymmetric synthesis of 5,8-disubstituted indolizidine alkaloids. Total synthesis of (−)-indolizidine 209B

A general access to 5,8-disubstituted indolizidine alkaloids has been developed, where the asymmetric addition of an optically active allenyltitanium to benzyl[4-(tert-butyldimethylsilyloxy)butylidene]amine (2) is the key reaction. As a typical example, (−)-indolizidine 209B was efficiently synthesized in 40% overall yield in a five-step reaction starting from readily available (S)-1-methyl-3-trimethylsilylprop-2-ynyl phosphate (1).