Process for the dehydration of dihydroxypiperidinedicarboxylates

A process for dehydrating a dialkyl 2,6-bis(trifluoromethyl)-2,6-dihydroxy-4-(alkyl)-3,5-piperidinedicarboxyla te to prepare the corresponding dialkyl 2,6-bis(trifluoromethyl)dihydro-4-(alkyl)-3,5-pyridinedicarboxylates comprising contacting the dihydroxypiperidine starting material with concentrated sulfuric acid, in a molar ratio between about 1.5 and about 2.2 moles of the acid per mole of dihydroxypiperidine, at a temperature at which the dihydroxypiperidine or mixture containing it is substantially liquid, and essentially in the absence of a solvent for the dihydroxypiperidine.