Synthesis of hexa(methyl/phenyl) substituted pyrazolines and thermolysis to hexasubstituted cyclopropanes

Addition of methyllithium to the 3‐position of 4,5‐dihydro‐3,4,4,5,5‐pentasubstituted‐ N ‐tosyl‐1H‐pyrazoles [ 1a 4,4,5,5‐tetramethyl‐3‐phenyl; 1b 4,4,5‐trimethyl‐3,5‐diphenyl; 1c 4,4‐dimethyl‐3,5,5‐triphenyl] produced the corresponding hexasubstituted pyrazolines, 2a , 2b , 2c , as the only isolable products. For 2b , the 3,5‐phenyl groups were found to be exclusively cis , indicative of facial specificity for the addition reaction . The reaction of phenyllithium with 1b yielded 2c as the minor product. For phenyllithium addition , direct attack on sulfur of the tosyl group with subsequent loss of phenyl p ‐tolylsulfone was the major pathway vs. the S N 2i attack at carbon‐3. Thermolysis of pyrazolines 2a , 2b , 2c , at 200°C, smoothly produced the hexasubstituted cyclopropanes [ 3a 1,1,2,2,3‐pentamethyl‐3‐phenylcyclopropane; 3b cis‐1,1,2,3‐tetramethyl‐2,3‐diphenylcyclopropane; 3c 1,1,2‐trimethyl‐2,3,3‐triphenylcyclopropane] in excellent yield. J. Heterocyclic Chem., (2010).