DETAILS ASSOCIATED WITH THE BIMOLECULAR 1,4-DIPOLAR CYCLOADDITION REACTION OF CROSS-CONJUGATED HETEROAROMATIC BETAINES

A series of 3,3-disubstituted bicyclic anhydro-4-hydroxy-2-oxo-1,3-thi azinium hydroxides are easily prepared from the reaction of 3H-thiolac tams with 1,3-bielectrophiles. These cross-conjugated heteroaromatic b etaines undergo regio- and diastereospecific 1,4-dipolar cycloaddition with electron-rich and electron-deficient pi-bonds to produce 1,4-cyc loadducts containing a carbonyl sulfide bridge. A representative betai ne dipole and a 1,4-cycloadduct were characterized by single crystal X -ray determinations. In certain cases, the initially formed cycloadduc t can be induced to lose COS on further heating. The frontier orbital coefficients of the thiazinium betaine were determined by semi-empiric al MOPAC calculations with the PM3 Hamiltonian. The HOMO of the 1,4-di pole is dominant for reactions with electron-deficient dipolarophiles such as N-phenylmaleimide, while the LUMO becomes important for cycloa ddition to more electron-rich species such as ynamines or vinyl ethers .