The evolution of transition objects between the asymptotic giant branch (AGB) stage and the planetary nebulae (PN) phase is known to be the crucial moment where very efficient mass loss processes determine the basic morphological, kinematical, and chemical layout of the newly born planetary nebulae. The keystone objects to the study of such evolution are the protoplanetary nebulae CRL2688, CRL618, and the extremely young planetary nebula NGC7027. We propose to carry out a deep study of the chemical abundance, excitation, and radial distribution of a few "key" molecules in these three prominent objects. These molecular observations will trace the chemical evolutionary path from the AGB to the PN stage. In particular, the effects of the shock waves associated to the high velocity winds blown up from the central stars will be unvealed by the proposed observations. In addition, the chemical abundances and excitation conditions in the high velocity winds (HVWs) will be determined. The results of these observations will constitute a major contribution to the understanding of the last stages of stellar evolution.