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 -5-[(N- phenylsulfonyl)amino]2,3-dihydrobenzofurans 5-7 nearly exclusively, wh ereas promotion of the reactions by Ti4+ gives mixtures of the dihydro benzofurans and their alkoxy-2-aryl-5-hydroxy-3-methyl-2,3-dihydroindo le isomers 8-10, depending upon substituents present on the propenylbe nzene. However, reactions promoted with excess Ti4+, as mixtures of Ti Cl4:Ti(OiPr)(4), give the dihydroindoles as nearly the exclusive produ cts. Evidence for a mechanism involving initial 5 + 2 cycloaddition of the Lewis acid-bound quinone monoimide with the propenylbenzene is fo und in reactions of styrenes 1f/g with monoimide 3 in which hydroxy-6- methylbicyclo[3.2.1]oct-3-ene-2,8-diones 33 (5 + 2 adducts) are isolat ed. These reactions have been applied to stereoselective syntheses of pterocarpans bearing N-phenylsulfonyl groups, azapterocarpans and diaz apterocarpans. 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 syntheti c approach to 7-alkoxy-benzofuranoid neolignans, including (+/-)-licar in B and eupomatenoids-1 and -12 are reported.