The conjugateaddition of magnesium cuprates to 2-silyloxycyclopentenecarboxylates leads, after desilylation, to 5-substituted-2-hydroxycyclopentanecarboxylates in high yields and diastereoselectivities. The 2-silyloxy derivatives avoid the formation of the SN2′ reaction by-products. No loss of selectivity is also observed when a catalytic amount of cuprous salt is used.