Methylene-3,4-dihydrocoumarins

B. 3,4-Dihydro-2-methylene-2H-1-ben2opyran. Formation of the exomethylene enol ether with dihydrocoumarin is carried out as in the foregoing procedure except that the reaction solution is cooled with a dry ice-acetone bath before addition of the lactone. From 3.0 g (20 mmol) of dihydrocoumarin (Note 5) is obtained 1.85-1.97 g (63-67%) of the product (Note 12) as a pale yellow oil, after column chromatography on basic alumina (150 g) eluting with 5% diethyl ether in pentane. 

Reaction with the ester is relatively exothermic. Sensitivity of the substrate-product to heating varies and should be considered for each particular compound. Phenyl benzoate can be methylenated at room temperature with no significant decrease in product yield. By contrast, dihydrocoumarin (Preparation B) gives no product under these conditions and must be methylenated at -78°C to obtain a good yield. For most substrates it is satisfactory to carry out reactions at 0°C. 

Lactones are also alkenated with 4.85. For example, dihydrocoumarin (4.89) on treatment with a toluene solution of 4.85, prepared in situ in THF, afforded the 3,4-dihydro-2-methylene-2 H-l-benzopyran (4.90) in 76% yield. 

Among various a-methylene-8-lactones, derivatives with the S-lactone ring fused with the aryl ring having a structure of 3,4-dihydrocoumarins occupy prominent position and received substantial attention in the past few years. In 2007, Krawczyk and coworkers [76] demonstrated that the 3,4-dihydrocoumarin framework can be accessed by a means of Michael-type Friedel-Crafts reaction between 3-aryl-2-diethoxyphosphorylacrylic acids 181 and electron-rich hydroxyarenes 182... 

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