A facile synthetic route to 2 H-chromenes: reconsideration of the mechanism of the DBU-catalyzed reaction between salicylic aldehydes and ethyl 2-methylbuta-2,3-dienoate.

Reactions of salicylaldehydes with ethyl buta-2,3-dienoate or penta-3,4-dien-2-one catalyzed by a catalytic amount of potassium carbonate produce the corresponding 2 H-chromene products in moderate to good yields under mild conditions. A plausible reaction mechanism is discussed in the light of the results of an 18O-labeling experiment. In addition, in view of these findings, the catalytic function of DBU in reactions of this kind is reconsidered.