Studies on regioselective addition of benzylic organometallics to alpha-oxoketene dithioacetals in our aromatic annelation protocol

The alpha -oxoketene dithioacetals derived from various acyclic and cyclic active methylene ketones are shown to undergo either 1,2- or sequential 1,4 - and 1,2-addition with various substituted benzylic Grignard reagents (or benzyl lithium) and 1-/2-(naphthylmethyl) Grignard reagents to give carbino l adducts which on facile BF3.Et2O induced cycloaromatization afford a vari ety of substituted naphthalenes, phenanthrenes and other condensed aromatic s. This methodology has emerged as a versatile tool for the regioselective construction of aromatic ring via [3 + 3] annulation from readily available acyclic precursors. The various substituted benzylic Grignard reagents (or benzyl lithium) and the corresponding 1- and 2-(naphthylmethyl) reagents d isplay diverse regioselectivity in these reactions via either 1,2-addition or sequential 1,4- and 1,2-addition depending on the substituents in the be nzylic moiety, nature of the metal and the substrate oxoketene dithioacetal s. The benzylic organocopper reagents undergo regioselective conjugate addi tion-displacement to give beta -benzyl-beta -methylthiomethylene ketones wh ile the corresponding alkoxy substituted benzyl organocopper reagents gave unexpected results under these conditions. An attempt has been made to gene ralize some of these observations in terms of charge control 1,2- and orbit al control 1,4-addition, although theoretical studies are needed to throw f urther light for better understanding of these results. (C) 2001 Elsevier S cience B.V. All rights reserved.