“…[10] Enantioselective formation of 2,5-dihydrooxepine was first examined with a-bromoacrolein and g-methyl-a-vinyl diazoester in the presence of COBI catalyst 3a.W hen the reaction was carried out at À78 8 8Cinpropionitrile,chiral 2,5dihydrooxepine 2a was formed in 50 %y ield with 58 % ee ( Table 1, entry 1). Use of dichloromethane or toluene as the solvent was found to be less effective than using propionitrile (entries [4][5]. Interestingly,while there are some reports that high temperature is required for retro-Claisen rearrangement, [4c] otherwise mixtures of cis-FVC and 2,5-dihydrooxepine form, [6b-c, 9b-c] cis-FVC 1awas not observed with these reaction conditions.I ta ppeared that all of the cis-FVC 1a was immediately transformed into 2,5-dihydrooxepine 2athrough retro-Claisen rearrangement at low temperature (path a).…”