Highly Stereospecific Dimerization of 5-Formyl-5-methyl-l-pyrazolines. Preparation and Characterization of Stable Carbinolamines (Amino Hemiacetals)

The unstable 5-methyl-5-formyl-2-pyrazolines 3, generated in situ by a 1, 3-dipolar additon of α-methylpropenal (methacrolein) to α-diazo esters, dimerize in a highly specific way to meso-4, which are stable carbinolamines. Surprisingly, the latter show no equilibrium with the monomers (pyrazolines) in solution, even at 90 °C in Me2SO, but they are cleanly transformed into the aminals 5 by a variety of nucleophiles. The conversion of 4 to 5 occurs with retention of configuration at the reacting center, as established by x-ray diffractometry. © 1977, American Chemical Society. All rights reserved.