Photoinduced molecular transformations. Part 144. One-carbon intercalation of γ- and δ-lactones involving the β-scission of alkoxyl radicals as the key step: synthesis of δ- and ε-lactones with α-substituents

Abstract

A new general method for a one-carbon intercalation of γ-lactones δ-lactones and δ-lactones to ε-lactones in three steps involving a selective β-scission of the alkoxyl radicals as the key step is described. The reactions of γ- and δ-lactones with lithioalkyl acetate gave an equilibrated mixture of alkyl (2-hydroxytetrahydrofuran-2-yl)acetates and alkyl (2-hydroxytetrahydropyran-2-yl)acetates, as well as their ring-opened isomers in 62–95% yields, respectively. The photolysis of the hypoiodites of these lactols in benzene containing mercury(II) oxide and iodine with Pyrex-filtered light resulted in a selective endocyclic β-scission of the corresponding alkoxyl radicals to give alkyl iodoalkyl propanedioates in 33–70% yields. Treatment of the iodoalkyl propanedioates with tetraethylammonium bromide and sodium hydride gave alkyl 3,4,5,6-tetrahydro-2-oxo-2H-pyran-3-carboxylates or alkyl 2,3,4,5,5a,6,7,8,9,9a-decahydro-2-oxobenz[1]oxepine-3-carboxylase in 61–81% yields. On the other hand, successive treatment of ω-iodoalkyl propanedioates with tetraethylammonium bromide–sodium hydride and then benzyl bromide gave a α-disubstituted δ-lactone, which gave a α-monosubstituted δ-lactone upon heating in trifluoroacetic acid under reflux. Cyclopentanone can similarly be transformed into 2-substituted cyclohexanone via a three-step procedure.