Abstract
The 1,3-dipolar cycloaddition of 2-diazopropane 1 to propargylic alcohols is regioselective and leads to 3H-pyrazoles 3 in good yield. The surprising formation of the tetrasubstitued pyrazolenine 4c from HC=CCH
2
OH and 1 can be explained via the 1,3-dipolar cycloadduct intermediate followed by a second cycloaddition of 1 to the C=C double bond and loss of dinitrogen. The photolysis of the antibacterial pyrazolenines 3 and 4 selectively gives α and β dimethylcyclopropenyl alcohols 5 and 6.