Inter‐ and Intramolecular Hetero Diels‐Alder Reactions, XXV. The Tandem Knoevenagel Hetero Diels‐Alder Reaction with a Formylacetic Acid Equivalent. Synthesis of Dihydropyrancarboxylates

Abstract

Abstract The tandem Knoevenagel hetero Diels‐Alder reaction of 4,4,4‐trichloro‐3‐oxobutanal ( 9 ) with the aldehydes 10 as well as 16 – 21 and the enol ethers 6 as well as 28 – 32 followed by base‐catalyzed methanolysis yields the methyl dihydro‐2 H ‐pyran‐5‐carboxylates 15 , 22 – 27 , and 33 – 37 . In this reaction 9 is used as an equivalent of the non‐stable formylacetic acid, since the trichloromethylcarbonyl moiety can be transformed into an alkoxycarbonyl group by solvolysis with an alcohol. As an example, in a three‐component reaction 9 was condensed with propanal ( 10 ) in the presence of potassium fluoride to give the activated oxadiene 11 as an intermediate. This reacts with ethyl vinyl ether ( 6 ), which is present as one component in the reaction mixture, to afford the cycloadduct 13 after acidic workup. Methanolysis of 13 in the presence of DBU yields the methyl dihydro‐2 H ‐pyran‐5‐carboxylate 15 in 50% overall yield.