Methyl 2-diazo-3-hydroxy-6,6-dimethyl-4-heptynoate was prepared by treating 4,4-dimethyl-2-pentyn-1-al with methyl diazoacetate at room temperature in the absence of catalysts. In the case of related aldehydes, RCCCHO (R = n-Bu, Me3Si, Et3Si, Et3Ge), this unusual reaction is partially or completely suppressed by a competing 1,3-cycloaddition process. The latter leads to a mixture of isomeric 3- and