2-AZA-1,3-DIENES WITH ELECTRON-RELEASING SUBSTITUENTS AT THE 1,3-POSITION - REAGENTS FOR THE CONSTRUCTION OF PYRIDINE AND PYRIMIDINE-DERIVATIVES

New 2-aza-1,3-dienes bearing 1 and 3-donor substituents are prepared f rom N-thioacylacetamidines through deprotonation of N-ylidene acetamid inium iodides. The 3-(dimethylamino)-1-(methylthio)-1-phenylbutadiene (3) is trapped in situ by the residual precursor salt acting as a hete rodienophile to give the pyrimidine 5. substituted 2-aza-1-(dimethylam ino)-3-(methylthio) analogues react readily with a variety of electron -deficient dienophiles to yield pyridine or pyrimidine derivatives. Th e stereochemistry of the hetero Diels-Alder reaction in the cases of d imethyl fumarate and acrylonitrile has been assigned by X-ray diffract ion analyses of the resulting tetrahydropyridines and corresponds to a n exo selectivity. The number and nature of cycloadducts in the cases of dimethyl acetylenedicarboxylate and phenyl isothiocyanate depend on C-4 substitution. The results obtained from the C-4 unsubstituted aza butadiene 8 are explained by an allylic rearrangement involving the 1, 3-migration of dimethylamino group in the primary [4 + 2] adduct. Copy right (C) 1996 Elsevier Science Ltd