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 BF3 yield 7-alkoxy-2-aryl-3-methyl-5-[(N-phenylsulfonyl)amino]2,3-dihydrobenzofurans 5-7 nearly exclusively, whereas promotion of the reactions by Ti4+ 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 Ti4+, as mixtures of TiCl4: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.