Lewis acid-directed reactions of quinones with styrenyl systems: The case of 2-methoxy-3-methyl-1,4-benzoquinone

Reactions of 2-methoxy-3-methyl-1,4-benzoquinone with various (E)-1-propenylbenzenes promoted by 1 equiv of Ti(IV), as a 1:1 mixture of TiCl4-Ti(O-i-Pr)(4), produce rel-(1S, 6R, 7R, 8R)-8-aryl-4,7-dimethyl-3-methoxybicyclo[4.2.0]oct-3-ene-2,5-diones 3 and trans-2-aryl-3,7-dimethyl-6-methoxy-2,3-dihydro-5-benzofuranols 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 TiCl4, SnCl4, or BF3 . OEt2 give regioisomeric rel-(1R, 6S, 7R, 8R)-7-aryl-4,8-dimethyl-3-methoxybicyclo[4.2.0]oct-3-ene-2,5-diones (4) and/or trans-2-aryl-3,6-dimethyl-7-methoxy-2,3-dihydro-5-benzofuranols (6). A mechanism involving regioselective 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 of the Lewis acid promoters. Cyclobutanes 3 and 4 cleanly undergo rearrangement to the corresponding benzofuranols 5 and 6 on treatment with protic acid. In contrast, reactions of 2-methoxy-1,4-benzoquinone promoted by either BF3 . OEt2 or Ti(IV), as 1 equiv or excess amounts of TiCl4 or 1:1 TiCl4-Ti(O-i-Pr)(4), all afford the same regioisomeric products, i.e., rel-(1S, 6R, 7R, 8R)-8-aryl-7-methyl-3-methoxybicyclo[4.2.0]oct-3-ene-2,5-dione 25 and/or trans-2-aryl-3-methyl-6-methoxy-2,3-dihydro-5-benzofuranol 27.