Heterocyclic analogs of pleiadiene

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UDC 547.856.7.07:542.941.8 2,3-Dihydroperimidines are dehydrogenated smoothly by sodium pyrosulfite in refluxing aqueous alcohol, as a result of which the corresponding perimidines are formed in high yields. The reaction of 1,8-naphthalenediamine with the aldehydic forms of sugars gives 2-polyhydroxyalkyl-2,3-dihydroperimidines, which are also aromatized by sodium pyrosulfite to give the corresponding 2-polyhydroxyalkylperimidines in high yields. In 1965 Ridley and co-workers [2] found that the corresponding 2-substituted benzimidazoles and perimidines rather than the expected 2,3-dihydro derivatives are formed in high yields when o-phenylenediamine or 1.8-naphthalenediamine is heated briefly with the bisulfite adducts of aldehydes. It was assumed [2] that the intermediately formed benzimidazolines and 2,3-dihydroperimidines are oxidized during the reaction by the bisulfite ion to give the aromatized structures. Bisulfites are also of interest to biochemists [3], since they oxidize coenzyme NAD-H to NAD +. However, the dehydrogenating properties of the bisulfite ion have not yet been studied. We do not know of any studies in which bisulfites have been used for the preparative oxidation of genuine dihydro derivatives of heterocycles. With respect to the mechanism of the reaction, there are contradictory data on the conversion of bisulfite in the case of reduction by organic compounds to thiosulfate and dithionate ions [4] or dithionite ion [5]. It has also been assumed [3] that the reaction is a free-radical process that takes place with the participation of air oxygen and a superoxide anion radical, during which the bisulfite ion is actually a catalyst (an electron carrier). The aim of the present research was to study the dehydrogenating capacity of bisulfites in the 2,3-dihydroperimidine (I) series, since most of the methods for dehydrogenation of them (with sulfur [6], palladium on carbon [7, 8], and chloranil or manganese dioxide [9]) require relatively severe conditions and do not always proceed sufficiently smoothly. This makes it impossible to use them for the dehydrogenation of compounds that contain labile and easily oxidized substituents. In addition, the synthesis of only one perimidine derivative (2-phenylperimidine) [2] has been described, and the dehydrogenation of genuine 2,3-dehydroperimidines has not been studied at all. Instead of bisulfite we used sodium pyrosulfite (Na2S205, SPS), which is easier to use. The reaction was carried out by refluxing an aqueous alcohol solution of equimolar amounts of I and SPS. The results of experiments involving the dehydrogenation of various 2,3-dehydroperimidines with SPS are presented in Table I. Both 2,3-dihydroperimidine itself and its 2-substituted derivatives with alkyl, phenyl, or ~-surplus heteroaromatic (2-furyl, 2-thienyl, l-methyl-2-benzimidazolyl, etc.) groups as substituents undergo quantitative dehydrogenation in ~2 h under the indicated conditions.