Photochemical [2 + 2] cycloaddition of acetylene to chiral 2(5H)-furanones. 
The [2 + 2] photocycloaddition of acetylene to chiral 2(5H)-furanones was investigated.  The influence on the chemical yield and facial diastereoselectivity of the substituent at the stereogenic center and also the effect of a 4-methyl group were evaluated.  A mechanistic proposal based on a simple theoretical conformational analysis is presented.  Using a C(2)-symmetric bis(lactone) as the substrate, a diastereomeric excess higher than 98% was found.