A CONVENIENT SYNTHESIS OF LINEAR 2-METHYLPYRANOCHROMONES

A convenient synthesis of linear 2-methylpyranochromones, viz., spatheliachromen(5-hydroxy-2,2,8-trimethyl-2H,6H-benzo[1,2-b:5,4-b′]dipyran-6-one), O-methylspatheliachromen (5-methoxy-2,2,8-trimethyl-2H,6H-benzo[1,2-b:5,4-b′]dipyran-6-one) (8) and O-methylallopteroxylin (5-methoxy-2,8,8-trimethyl-4H,8H-benzo[1,2-b:3,4-b′]dipyran-4-one) is described by blocking the eighth position of the corresponding 7-propargyl ether derivative 7-(1,1-dimethyl-2-propynyloxy)-8-iodo-5-methoxy-2-methyl-4H-1-benzopyran-4-one with iodine followed by thermal cyclization. The linear pyranochromones 2,2,8-trimethyl-2H,6H-benzo[1,2-b:5,4-b′]dipyran-6-one and 8 are also synthesized starting with the easily available appropriate 3,4-dihydro-2,2-dimethyl-2H-1-benzopyran-7-ols following the steps of Claisen condensation with ethyl acetate, cyclization with ethanolic sulfuric acid followed by dehydrogenation with NBS.