Synthesis of linear 3-phenylpyranocoumarins: synthesis of robustin, robustic acid, and their methyl ethers

The linear 3-phenylpyranocoumarins, viz., robustin (5b; R4= H), its methyl ether (5b; R4= Me), robustic acid (5c; R4= H), its methyl ether (5c; R4= Me), 4-methoxy-3-phenylxanthyletin (5a; R4= H), and its methyl ether (5a; R4= Me), have been synthesised starting from the corresponding 7-(1,1-dimethylprop-2-ynyloxy)-coumarin derivatives having the 8-position blocked with an easily introduceable and removable group like iodine. The subsequent step consists of heating the prop-2-ynyl ethers in NN-dimethylaniline, which resulted in cyclization to linear pyranocoumarins, accompanied by the removal of iodine and also partial demethylation of the methoxy-group at the 4-position. The compounds obtained were identical with the natural samples and were different from authentic samples of the corresponding angular isomers.