Cycloadditions of diazoesters to α,β-unsaturated aldehydes

The reaction of α,β-unsturated aldehydes with diazoesters primarily gives unstable 1,3-dipolar adducts (1-pyrazolines) which evolve according to their substitution pattern: whenever there is a proton geminal to the formyl group, conjugation with the latter is highly favoured, yielding 3-formyl-2-pyrazolines which subsequently polymerize. On the other hand, if no proton is available for conjugation with the formyl group, two competitive reactions take place: tautomerisation into an ester conjugated pyrazoline and loss of nitrogen with formation of cyclopropanes, the latter reactionnal pathway being favoured with electronwithdrawing substituents. © 1978.