Synthesis and stereochemistry of substituted bi- and tri-cyclic 4,5-dihydropyrazoles

Abstract

A series of bi- and tricyclic 4,5-dihydropyrazoles have been studied by ¹H n.m.r. and ¹³C n.m.r. spectroscopy and X-ray crystallography. They were synthesised by treating the corresponding mono-and dibenzylidenecycloalkanones and 2-benzylidene-3,4-dihydronaphthalen-1(2H)-one with semi-carbazide, which gave two diastereoisomers, and thiosemicarbazide, which gave only one. The ¹H n.m.r. parameters allowed an unambiguous identification of the diastereoisomers and the configuration of one of the isomeric pairs was confirmed by ¹³C n.m.r. spectroscopy and X-ray crystallography. The crystallographic analysis shows that the six-membered rings assume an intermediate shape between the chair, half-chair and envelope forms, while the five-membered rings have a perfect envelope conformation with C(5) on the flap.