Lewis Acid-Controlled Regioselectivity in Reactions of Styrenyl Systems with Benzoquinone Monoimides: New Regioselective Syntheses of Substituted 2-Aryl-2,3-dihydrobenzofurans, 2-Aryl-2,3-dihydroindoles, and 2-Arylindoles

Reactions of 4-( N -phenylsulfonyl)-2-alkoxy-1,4-benzoquinone monoimines 2 − 4 with electron-rich propenylbenzenes promoted by BF 3 yield 7-alkoxy-2-aryl-3-methyl-5-[( N -phenylsulfonyl)amino]-2,3-dihydrobenzofurans 5 − 7 nearly exclusively, whereas promotion of the reactions by Ti 4+ gives mixtures of the dihydrobenzofurans and their N -(phenylsulfonyl)-6-alkoxy-2-aryl-5-hydroxy-3-methyl-2,3-dihydroindole isomers 8 − 10, depending upon substituents present on the propenylbenzene. However, reactions promoted with excess Ti 4+, as mixtures of TiCl 4:Ti(OiPr) 4, give the dihydroindoles as nearly the exclusive products. Evidence for a mechanism involving initial 5 + 2 cycloaddition of the Lewis acid-bound quinone monoimide with the propenylbenzene is found in reactions of styrenes 1f/g with monoimide 3 in which 7-aryl-3-hydroxy-6-methylbicyclo[3.2.1]oct-3-ene-2,8-diones 33 (5 + 2 adducts) are isolated. These reactions have been applied to stereoselective syntheses of pterocarpans bearing N -phenylsulfonyl groups, azapterocarpans and diazapterocarpans. In addition, DDQ oxidation of derivatives of several of the 2-aryl-2,3-dihydroindoles afford the corresponding 2-arylindoles in good yield. Finally, the experimental details of a general synthetic approach to 7-alkoxy-benzofuranoid neolignans, including (±)-licarin B and eupomatenoids-1 and −12 are reported.