the relative reactivities (k OH /k add ) being 0.3-0.5. In marked contrast, the «carboxylate» carbene generated from the sodium salt of the diazoacetate under the same conditions produces mostly the OH insertion product at the expense of the cyclopropanes, k OH /k add being >100. The marked effect of the carboxylate group is nicely explained in terms of the participation by the neighboring carboxylate