This review focuses on the use of dimethylformamide dimethyl acetal in the preparation of heterocyclic compounds via formylation of active methylene groups, methyl groups to give enamines, and formylation of amino groups to give amidines. These compounds are found to be useful intermediates in the formation and modification of heterocyclic compounds.

β-aminocrotononitrile (1) reacted with either cyanothioacetamide to give (3) or malononitrile to afford an anion (5). Pyridine-2(1H)-thione (4) was obtained by boiling of (3) in ethanol and Et 3 N or treatment of (5) with H 2 S, respectively. The reaction of anion 5 with isothiocyanates (6) gave N-substituted pyridine-2(1H)-thiones (7). N-Substituted pyridine-2(1H)-thiones (7) can be used for the preparation of pyrido[2,3-d]pyrimidines (8a-e) and (10a-e), or the preparation of pyrido[1,2-a]pyrimidines (12a-d). 1,8-Naphthyridine derivatives (14a-d) and (16a-e) can also be obtained from pyridine-2(1H)-thione (7). Finally, 1,8-naphthyridine derivatives (16a-e) can be used for the preparation of tetracyclic compounds 17a-c and 18a,b.

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Polysubstituted pyridines 11a,c and 12 were prepared by the reaction of benzoylacetone with dimethylformamide dimethyl acetal followed by treatment with cyanothioacetamide 9a, cyanoacetamide 9b and the anion of malononitrile dimmer 9c in dry DMF. When the reaction was carried out in ethanol as a solvent and piperidine as a base afforded 14a,b. Thienopyridines 16a,b were prepared by the reaction of pyridinethiones 11a and 14a with 2-chloro-N-p-tolyl-acetamide (15). Further reaction of thienopyridines 16a,b with either DMFDMA or nitrous acid to gave 17a,b and 18a,b respectively. The reaction of pyridine derivative 11c with hydrazine and phenylhydrazine afforded the tricyclic compounds 19a,b. J. Heterocyclic Chem., 44, 787 (2007).Formamide acetals are useful reagents; [3,4] their main application has been used for functional group transformations [5], but they may also be regarded as one-carbon synthons in the construction of carbon skeletons. One type of reaction, which is potentially valuable for the future purpose, is with 1,3-dicarbonyl compounds (1) to give enamines (2) [4].

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