An asymmetric Kharasch reaction mediated by d-xylose: long range diastereocontrol

An asymmetric Kharasch-type atom transfer reaction was studied under 13 different sets of reaction conditions in order to obtain the highest levels of diastereoselection. The highest ratio of 12.5:1 (71%) was observed at low temperatures (−78°C) using the Lewis acid Eu(OTf)3. Because the new asymmetric center is so distant from the d-xylose, we propose a transition state involving two sugars. The conversion of the major product to a naturally occurring chiral γ-butyrolactone, under basic conditions, is described which also confirmed the diastereoselectivity and absolute stereochemistry of the Kharasch reaction.