Synthesis of 3-Benzyl- or 3-Benzoyl-7,8-dihydro-6H-chromene Derivatives Starting from Baylis-Hillman Adducts

Abstract

acetate 1 and cyclohexane-1,3-dione (2a) in the presence of K2CO3 in DMF gave 3a in 68% yield. Conversion of 3a into the corresponding lactone derivative 4a was conducted by refluxing 3a in p-xylene to give 4a in 59% yield. The exodouble bond of 4a could be isomerized in its endo-position by treatment with DMAP (4,4-dimethylaminopyridine) in refluxing p-xylene to give 5a in 89% yield. As easily expected, the reaction of 3a in the presence of DMAP in refluxing p-xylene gave 5a in 75% yield directly. In addition, compound 5a was synthesized directly from the reaction of 1 and 2a in 78% yield without separation of the intermediate 3a as also shown in Scheme 1. As a next trial, we examined the allylic oxidation of 4a and 5a, and we found that the reaction of 4a and PCC (pyridinium chlorochlomate) produced the 3-benzoyl derivative 6a in 63% yield. 11 However, the oxidation of 5a with PCC showed no reaction. Like this we found efficient syn