Unusual Dimethyl Ketal from Cycloadduct of 4,6-O-Benzylidene-D-allal and Dichloroketene

摘要

Glycal derivatives are useful building blocks in organic synthesis as well as in carbohydrate chemistry.Cyclo-additions of dichloroketene to tri-O-benzyl or tri-O-acetyl-D-glucal and D-galactal were reported to produce alpha-oriented cyclobutanone ring,and the resulting bicyclic cyclo-butanones were converted into lactone compounds by oxidation.Although they showed high stereo and regioselectivity with moderate yield,this methodology has not been widely applied for synthetic purpose.For the last few years,we have studied the cycloaddition of ketene to glucal,galactal,and allal derivatives and their application for the synthesis of C-glycoside derivatives and modified nucleosides.Even though,in case of allal,beta-oriented cyclobutane ring formation is reported,no direct evidence supporting the face selectivity of dichloroketene cycloaddition to allal derivatives has been reported yet.Thus herein we report the X-ray structure of an unusual ketal from allal-derivative as the evidence that the facial selectivity of cycloaddition to glycal is controlled by the stereochemistry of C-3 constituent and discuss the mechanism for the formation of the unsual dimethyl ketal.