Asymmetric Synthesis of α-Substituted β-Amino Ketones from Sulfinimines (N-Sulfinyl Imines). Synthesis of the Indolizidine Alkaloid (−)-223A

Abstract

Of the possible four stereoisomers, addition of the lithium enolate of 4-heptanone to sulfinimines resulted in only the syn- and anti-α-substituted β-amino ketones. The formation of the major syn-β-amino ketone was rationalized in terms of addition of the E-enolate to the C−N double bond of the sulfinimine via a six-member chelated chairlike transition state. The enolates of 4-heptanone were generated using LiHMDS in THF where a 1:2.5 E:Z enolate ratio was noted. In diethyl ether the E:Z ratio was 15:1 in favor of the E-enolate and explained in terms of Ireland's transition state model. Here increased steric interactions between the ethyl group and the carbonyl-LiN(TMS)₂ moiety destabilize the transition state leading to the Z-enolate in the poorly coordinating diethyl ether solvent. This new synthesis of syn-α-substituted-β-amino ketones was applied to the concise enantioselective total synthesis of indolizidine (−)-223A, a 5,6,8-trisubstituted alkaloid isolated from the skin of the dendrobatide frog.