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Selective reduction of 4H-1,3-thiazine-4-ones leads to new substituted 6H-1,3-thiazines. The isomerisation from 6-to 5-methoxycarbonyl-6H-1,3-thiazines via substituted 4-acetoxy-1-thia-3-aza-butadienes by a cycloreversion-cycloaddition process can be performed in one step.

New 2-aza-1,3-dienes bearing 1 and 3-donor substituents are prepared from N-thioacylacetamidines through deprotonation of N-ylidene acetamidinium iodides. The 2-aza-3-(dimethylamino)-1-(methylthio)-1-phenylbutadiene (3) is trapped in situ by the residual precursor salt acting as a heterodienophile to give the pyrimidine 5. Substituted 2-aza-1-(dimethylamino)-3-(methylthio) analogues react readily with a variety of electron-deficient dienophiles to yield pyridine or pyrimidine derivatives. The stereochemistry of the hetero Diels-Alder reaction in the cases of dimethyl fumarate and acrylonitrile has been assigned by X-ray diffraction analyses of the resulting tetrahydropyridines and corresponds to an 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 azabutadiene 8 are explained by an allylic rearrangement involving the 1,3-migration of dimethylamino group in the primary [4+2] adduct.

A new synthesis and the reactivity of stable selenaazadienes are discussed. Their dienic, electrophilic and nucleophilic character are used to prepare five and six membered selenoheterocycles. The easy access to selenazine, selenazinone, selenopyran, selenazoline, selenazole and selenophene heterocycles from N-selenoacylamidines is described. An interpretation of the reactivity of the selenaazadiene systems, experimentaly compared to their thia analogues, is based on their physico-chemical parameters and correlated to the theoretical calculations of their frontier molecular orbital energy levels.

The preparation of 2-phenyl-4-dimethylamino-1-aza-, 1-oxa-, 1-thia-, 1-selena-3-azabuta-1,3-dienes as well as their 4-methyl derivatives is described following a new heteroatom interchange reaction process. Heteronucleophilic attack at one particular reactive site of bis-electrophilic amidinium salts is the key feature of the process. In addition, we also disclose that the substituted 1-oxa-3-aza- and 1,3-diazabuta-1,3-dienes can be obtained by a reactional transformation cascade initiated by either silver acetate addition or tosyl azide [3+2] cycloaddition onto the CS or CSe double bonds of the 1-thia- and 1-selena-3-azabuta-1,3-diene analogues.

The synthesis of 3'-O2-(azaheterocycle)-thymidines is presented from 1-thia-3-aza- 1,3-butadiene precursors (N-thioacylamidines). A variety of heterocycles is accessible using the dienic, the electrophilic or the nucleophilic reactivity of these thia-azabutadiene systems. 3'-O2-(azaheterocycle)-thymidine analogues are regarded as potential substrates to interfere with the DNA-polymerization process.

(4+2)cyclocondensation reactions under high pressure between methylacrylate and 2-substitutes 1-thia 3-aza 4-aminobutadienes leads selectively to 2-substituted 5-methoxycarbonyl 6H-1,3-thiazines.