Reaction conditions

(a) dry CH₂Cl₂, room temp., 24 h (reactant ratio: aldehyde/Meldrum's acid/EDDA: 1/1.1/0.4)

(b) CH₂Cl₂, room temp., sunlight

Products

Cr(CO)₃complexed cis-fused dihydropyran 1

yield 60% d.e.>98% [a]_D= -106 (c 0.17 CHCl₃)

cis-fused dihydropyran 2

yield 95% e.e.>98% [a]_D= +87.9 (c 0.5 CHCl₃)