Chemoselectivity and Conformational Analysis of the Reaction of 2-Acetylcycloalkanones with Benzohydrazide: Synthesis and Reduction of 1-Aroylcycloalkapyrazole Derivatives

From the reaction of 2‐acetylcyclopentanone and 2‐acetyl‐2‐methylcyclopentanone with benzohydrazide, the 1‐benzoyl‐6a‐hydroxycyclopentapyrazole derivatives 2a and 2b were obtained as the only reaction products, whereas from the reaction of 2‐acetylcyclohexanone an epimeric cis/trans mixture of the 2‐benzoyl‐3‐hydroxy‐2H‐indazole derivative 3c was formed. The dehydration of the isolated compounds 2a and 3c, as well as the NaBH4 and NaBH3CN reduction products of 2a were studied. The structural assignments of the compounds derived were established by analysis of their NMR spectra (1H, 13C, DEPT, COSY, NOESY, HETCOR CH, and COLOC CH). The chemoselectivity of the reactions of 1a and 1c with benzohydrazide was studied by conformational analysis with MM2 and semiempirical (AM1 and PM3) MO calculations.