Stereospecific Lewis Acid-Promoted Reactions of Styrenyl Systems with 2-Alkoxy-(6-alkyl)-1,4-benzoquinones: Scope, Limitations, and Synthetic Applications

Article abstract of DOI:10.1021/jo00101a016

Titanium(IV)-promoted reactions of various (E)-1-propenylbenzenes with 2-methoxy- and 2-methoxy-6-methyl-1,4-benzoquinones produce trans 2-aryl-6-methoxy-3-(and 4-di)methyl-2,3-dihydro-5-benzofuranols (10-12), rel-(1S,6R,7R,8R)-3-methoxy-8-aryl-7-(and 1-di)methylbicyclo<4.2.0>oct-3-ene-2,5-diones (2 + 2 cycloadducts, 13-15) and/or rel-(1R,5R,6R,7R)-7-aryl-3-hydroxy-6-(and 4)-methylbicyclo<3.2.1>oct-3-ene-2,8-diones (5 + 2 cycloadducts, 16/17).In many cases, each of the three products can be obtained selectively in good yield by control of reaction conditions and/or by choice of substituents on the quinone or the propenylbenzene.The dihydrobenzofurans are formed stereoselectively, whereas the formation of the bicyclo<4.2.0> systems are stereospecific processes.Thus, reactions of (Z)-1-propenylbenzenes afford rel-(1R,6S,7R,8R)-8-aryl-3-methoxy-7-methylbicyclo<4.2.0>oct-3-ene-2,5-diones (24, 25).No bicyclo<3.2.1>systems are found in reactions of the (Z)-propenylbenzenes.The products all apparently result from a thermally allowed 2? + 4? (2 + 5) cycloaddition of the propenylbenzene with a 2-methoxy-4-oxo-2,5-cyclohexadienyl carbocation intermediate (26) formed by coordination of the Ti(IV) to the C-1 carbonyl oxygen of the quinone.In the cycloaddition, the aryl ring of the propenylbenzene occupies an endo position with respect to the pentadienyl carbocation moiety of 26 and the bicyclo<3.2.1> carbocation product of the cycloaddition (28/29) either undergoes dealkylation or rearrangement to yield the observed products.Treatment of the bicylo<4.2.0> systems with protic acid effects their rearrangement to the dihydrobenzofuranols.Reactions of 2-propenylbenzenes and arylcycloalkenes with the quinones regioselectively give dihydrobenzofuranols 43-45 and 49-54, respectively; a 2 + 2 cycloadduct is found in low yield in only one case.The 7-aryl-3-hydroxy-6-methylbicyclo<3.2.1>oct-3-ene-2,8-diones are produced exclusively in reactions of 2-((4-methoxybenzyl)oxy)-1,4-benzoquinones with various propenylbenzenes.Application of these reactions to the synthesis of (+/-)-obtusafuran, (+/-)-liliflol-B, (+/-)-kadsurenone, and (+/-) denudatin are reported.