LEWIS ACID-DIRECTED REACTIONS OF QUINONES WITH STYRENYL SYSTEMS - THECASE OF 2-METHOXY-3-METHYL-1,4-BENZOQUINONE

Reactions of 2-methoxy-3-methyl-1,4-benzoquinone with various (E)-1-pr openylbenzenes promoted by 1 equiv of Ti(IV), as a 1:1 mixture of TiCl 4-Ti(O-i-Pr)(4), produce rel-(1S, 6R, 7R, methyl-3-methoxybicyclo[4.2. 0]oct-3-ene-2,5-diones 3 and 3,7-dimethyl-6-methoxy-2,3-dihydro-5-benz ofuranols 5 as the major products. Reactions promoted by 2 equiv of Ti (IV) as a 1:1 mixture of TiC1l(4)-Ti(O-i-Pr)(4) or 1 equiv of either T iCl4, SnCl4, or BF3 . OEt2 give regioisomeric rel-(1R, 6S, 7R, methyl- 3-methoxybicyclo[4.2.0]oct-3-ene-2,5-diones (4) and/or 3,6-dimethyl-7- methoxy-2,3-dihydro-5-benzofuranols (6). A mechanism involving regiose lective coordination of the various Lewis acids to the quinone is used to explain the formation of the products. These reactions demonstrate the effective regiocontrol exerted over the reactions by the nature o f the Lewis acid promoters. Cyclobutanes 3 and 4 cleanly undergo rearr angement to the corresponding benzofuranols 5 and 6 on treatment with protic acid. In contrast, reactions of 2-methoxy-1,4-benzoquinone prom oted by either BF3 . OEt2 or Ti(IV), as 1 equiv or excess amounts of T iCl4 or 1:1 TiCl4-Ti(O-i-Pr)(4), all afford the same regioisomeric pro ducts, i.e., rel-(1S, 6R, 7R, -methyl-3-methoxybicyclo[4.2.0]oct-3-ene -2,5-dione 25 and/or aryl-3-methyl-6-methoxy-2,3-dihydro-5-benzofurano l 27.