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.