Asymmetric synthesis of alpha-substituted beta-amino ketones from sulfinimines (N-sulfinyl imines). synthesis of the indolizidine alkaloid (-)-223A.
Of the possible four stereoisomers, addition of the lithium enolate of 4-heptanone to sulfinimines resulted in only the syn- and anti-alpha-substituted beta-amino ketones. The formation of the major syn-beta-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-alpha-substituted-beta-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.
Davis
Franklin A
FA
Department of Chemistry, Temple University, Philadelphia, PA 19122, USA. fdavis@temple.edu
Yang
Bin
B
D000588:Amines / Q000138:chemical synthesis*; D007097:Imines / Q000737:chemistry*; D007212:Indolizines / Q000138:chemical synthesis*; D007659:Ketones / Q000138:chemical synthesis*; D013441:Sulfinic Acids / Q000737:chemistry*
C104548:6,8-diethyl-5-propylindolizidine; D000588:Amines; D007097:Imines; D007212:Indolizines; D007659:Ketones; D013441:Sulfinic Acids