Some reactions of unsaturated a-diazo-a-(trimethylsily1) acetates with aldehydes or acetone under the catalytic action of rhodium (I1) perfluorobutyrate or copper (1) triflate have been investigated. Thus, the allylic ester 6a affords 1, 3-dioxolan-4-ones 8 under Rh (I1) catalysis;