Synthesis of pyrazolone derivatives—XXI : On the reduction of 1-methyl-2-phenyl-1,2,3,10-tetrahydro-4H-pyrazolo[3,4-c][1] benzothiepin-3,4-dione with sodium borohydride

The selective reduction of α,β-unsaturated ketones and CC double bonds of pyrazolo3,4-c]1]benzothiepins with sodium borohydride was studied. The reduction of 1-methyl-2-phenyl-1,2,3,10-tetrahydro-4H-pyrazolo3,4-c]1]benzot (1) with sodium borohydride in refluxing methanol gave 1-methyl-2-phenyl-1,2,3,10-tetrahydro-4H-pyrazolo3,4-c]1]benzothiepin-3-one (2). The mechanism of this unusual reaction in which the heterocyclic ketone was reduced to the corresponding methylene grouping with such a reagent was elucidated by the isolation of the following intermediates: 1-methyl-2-phenyl-1,2,3,3a, 10, 10a-hexahydro-4H-pyrazolo3,4-c]1]benzothiepin-3,4-dione (6) and 1-methyl-2-phenyl-1,2,3,3a,10,10a-hexahydro-4H-pyrazolo3,4-c]1]benzothiepin-3-one (4).