A Model Study of Intramolecular Asymmetric Radical Cyclizations of α-Ester and α-Amide Radicals
作者:Ru-Long Yeh、Weir-Tom Jiaang、Yeun-Min Tsai
DOI:10.1002/jccs.199700039
日期:1997.6
AbstractStarting from malonate, a practical route was developed for the synthesis of α‐phenylthio acid 3. Several chiral compounds including (‐)‐menthol, (‐)‐8‐pbenylmenthol and a camphor based oxazolidinone 8 reacted with 3 to give α‐phenylthio esters or amide. These sulfides cyclized efficiently when reacted with tributyltin hydride. Among the chiral auxiliaries used, 8‐phenylmenthyl group displayed moderate asymmetric induction (64% ee for cis‐product and 40% ee for trans‐product). Based on this results, a transition state model was proposed to explain the observed stereoselectivity. In this model, due to π,π‐orbital overlap of the phenyl ring and the carbonyl, the si‐face of the most stable conformer of the radical was shielded. This controlled the carbon‐carbon bond formation to occur from the re‐face.