An Efficient One Pot Protocol to the Annulation of Face “d” of Benzazepinone Ring with Pyrazole, Isoxazole, and Pyrimidine Nucleus through the Corresponding Oxoketene Dithioacetal Derivative

A highly facile single step approach to the annulation of face “d” of benzazepinone nucleus with pyrazole, isoxazole, and pyrimidine ring has been described. The annulation proceeded smoothly on the reaction of oxoketene dithioacetal derivative 3 with (i) NH2–NH2·H2O, (ii) NH2OH·HCl, (iii) acetamidine hydrochloride, (iv) guanidine nitrate, (v) urea, and (vi) thiourea which yielded the pyrazolo, isoxazolo, and pyrimido annulated analogues of benzazepinone 4–9, respectively, in acceptable yields. The 4-ketene dithioacetal analogue of 7-fluorobenzo[b]azepine-2,5-dione (3) was in turn obtained from the reaction of 7-fluoro-3,4-dihydro-1H-benzo[b]azepine-2,5-dione (2) (with CS2 + CH3I in presence of t-BuOK). 7-Fluoro-3,4-dihydro-1H-benzo[b]azepine-2,5-dione (2) resulted from the acylation of p-fluoroaniline with succinyl chloride followed by cyclocondensation of the later with polyphosphoric acid (PPA). 1. Introduction Koch et al. [1] have carried out a quantitative analysis of physiologically active natural products and showed that compound molecules with two or three rings were most often found in active natural products. The interest in the various facets of the chemistry and biology of small bicyclic and tricyclic (carbocyclic and heterocyclic) systems has expanded exponentially thereafter. Since then the development of small molecule libraries with potential biological activities has been a major focus of research in the area of chemical biology and medicinal chemistry. In view of this, the development of efficient methodologies to access small molecules of medicinal utility has been currently of special interest, with particular emphasis on the preparation of compound libraries from the privileged medicinal structures or from those structures akin to these. Benzodiazepine framework in general has been recognized to belong to the family of “privileged medicinal structures,” by virtue of their ability to provide ligands to a number of functionally and structurally discrete biological receptors. A derivative of benzazepine “the mirtazapine,” being in close analogy to its activity, with 1,4-benzodiazepines, has emerged as an active antidepressant for the treatment of moderate to severe depression [2, 3] (Figure 1). This discovery provided optimism towards the development of other novel agents from benzazepine class of compounds to find if there were others too, to show a higher level of medicinal efficacy. This search resulted in the discovery of 7-phenyl sulfonyl-tetrahydro-3-benzazepine derivative for its use as antipsychotic agent [4] and