Photoinduced molecular transformations. Part 137. New general synthesis of 3-substituted 3,4-dihydro-1H-benzo[2]pyran-1-ones (3,4-dihydroisocoumarins)via radical and photochemical fragmentations as the key step

Abstract

A new general method for a two-step synthesis of 3-substituted 3,4-dihydro-1H-benzo[2]pyran-1-ones (3,4-dihydroisocoumarins) is described. This method involves either regioselective alkoxyl radical fragmentation or regioselective photochemical fragmentation as the key step. 2-Hydroxyalkylation of a lithiated o-tolyl tert-butyl ketone with aromatic and aliphatic aldehydes and ketones gave equilibrated mixtures of 1-tert-butyl-3,4-dihydro-l -hydroxy-3-alkyl (or 3-aryl)-1H-2-benzopyrans and their ring-opened isomers in 42–94% yield. Photolysis of mixtures in chloroform with Pyrexfiltered light gave 3-alkyl (or 3-aryl)-3,4-dihydro-1H-benzo[2]pyran-1-ones (3,4-dihydroisocoumarins) in 27–64% yield as exclusive isolable products. On the other hand, photolysis of hypoiodites of the equilibrated mixture in benzene containing mercury(II) oxide and iodine gave the 3-substituted dihydroisocoumarins in 37–64% yield with an accompanying formation of phthalide, arising from a radical cascade process triggered by β-scission of the alkoxyl radicals generated from the ringopened isomer of the lactones. The formation mechanisms of all the products are discussed.