STEREOSPECIFIC LEWIS ACID-PROMOTED REACTIONS OF STYRENYL SYSTEMS WITH2-ALKOXY-(6-ALKYL)-1,4-BENZOQUINONES - SCOPE, LIMITATIONS, AND SYNTHETIC APPLICATIONS

Titanium(IV)-promoted reactions of various (E)-1-propenylbenzenes with 2-methoxy- and 2-methoxy-6-methyl-1,4-benzoquinones produce trans 2-a ryl-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 c an be obtained selectively in good yield by control of reaction condit ions and/or by choice of substituents on the quinone or the propenylbe nzene. The dihydrobenzofurans are formed stereoselectively, whereas th e formation of the bicyclo[4.2.0] systems are stereospecific processes . Thus, reactions of (Z)-1-propenylbenzenes afford methoxy-7-methylbic yclo[4.2.0]oct-3-ene-2,5-diones (24, 25). No bicyclo[3.2.1]systems. ar e found in reactions of the (Z)-propenylbenzenes. The products all app arently result from a thermally allowed 2 pi + 4 pi (2 + 5) cycloaddit ion 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] carbo cation product of the cycloaddition (28/29) either undergoes dealkylat ion or rearrangment to yield the observed products. Treatment of the b icylo[4.2.0] systems with protic acid effects their rearrangement to t he dihydrobenzofuranols. Reactions of 2-propenylbenzenes and arylcyclo alkenes with the quinones regioselectively give dihydrobenzofuranols 4 3-45 and 49-54, respectively; a 2 + 2 cycloadduct is found in low yiel d in only one case. The hydroxy-6-methylbicyclo[3.2.1]oct-3-ene-2,8-di ones 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.