Radical routes to indolizidines. Synthesis of (-)-slaframine
作者:Spencer Knapp、Frank S. Gibson
DOI:10.1021/jo00044a011
日期:1992.8
The synthesis of (-)-slaframine (5) was executed in 11 steps and 25% overall yield from resolved 3(S)-hydroxy-4-pentenamide (22). Two cyclization reactions were used to form the indolizidine skeleton and also to provide the necessary stereocontrol at C-8a and C-6 of the natural product. "Iodolactamization" of 22 gave selectively the cis-pyrrolidinone 21. Later in the synthesis, a silane-mediated radical cyclization of the phenylseleno lactam 33 gave selectively the 6-alpha-hydroxyindolizidinone 35a, an event predictable from model studies-such as 14c --> 15c. Replacement of hydroxy with azido and reduction of the lactam carbonyl gave "slaframine azide", 38, a stable and easily convertible immediate precursor to 5.