In common with other a, p-unsaturated alde- Methacrylaldehyde dimer is an aldehyde which hyde~ 3~ 8 bvc and ketones, 3b9d, e methacrylaldehyde has no a-hydrogen. Accordingly, the Cannizzaro (methylpropenal, methacrolein) undergoes a reaction was clean-cut. 6b The expected unsatu-Diels-Alder type4 of reaction with itself to pro-rated acid6b and alc~ hol~~,~(111) isomerized readily duce a dimer5 (I), 3, 4-dihydr0-2, 5-dimethyl-l, 2H-into a ...