PHOTOINDUCED MOLECULAR-TRANSFORMATIONS .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

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-1-hydroxy-3-alkyl (or 3-aryl)-1H-2-benzopyrans and their ring-opened isomers in 42-94% yield. Photolysis of mixtures in chloroform with Pyrex-filtered 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 beta-scission of the alkoxyl radicals generated from the ring-opened isomer of the lactones. The formation mechanisms of all the products are discussed.