NMR structural study of cycloadducts obtained by hetero-Diels-Alder reaction of chiral 1-oxa-1,3-butadienes

Chiral pyran derivatives were prepared by hetero-Diels-Alder reaction betwe en ethyl vinyl ether and 1-oxa-1,3-butadienes bearing a chiral substituent in position 4 to obtain more information about the stereoselectivity of thi s reaction. The structure of reaction products was studied by one-dimension al NMR methods based on a double pulsed field gradient spin echo. Two novel NMR pulse sequences based on a combination of COSY and NOESY/ROESY are des cribed, relieving the problem of heavy overlaps of target resonances in NOE experiments. The NMR studies in combination with further synthetic transfo rmations allowed us to assign the configurations of the isolated cycloadduc ts. The results indicate that the configuration of the chiral substituent i n position 4 is not the principal factor which controls the approach of the ethyl vinyl ether to the planar 1-oxa-1,3-butadiene system. Copyright (C) 2000 John Wiley & Sons, Ltd.