Asymmetric reactions of 2-methoxy-1,4-benzoquinones with styrenyl systems:Enantioselective syntheses of 8-aryl-3-methoxybicyclo [4.2.0]oct-3-en-2,5-diones, 7-aryl-3-hydroxybicyclo[3.2.1]oct-3-en-2,8-diones, 2-aryl-6-methoxy-2,3-dihydrobenzofuran-5-ols, and pterocarpans

Reactions of 2-methoxy-1,4-benzoquinones 2 and 3 with (E)-propenylbenzenes 1 promoted at -78 degrees C by Ti(IV)-TADDOLates prepared from diol-(+)- 4 afford (1R,6R,7R,8R)-8-aryl-3-methoxy-7methylbicyclo[4.2.0]oct-3-en-2,5-dio nes 5/8 or (1R,5R,6R,7R)-7-aryl-3-hydroxy-6-methylbicyclo-3-en-2,8-diones 7 /10 in good yield and high ee. (2S,3S)-2-Aryl-6-methoxy-3-methyl-2,3-dihydr obenzofuran-5-ols 6/9 are also found, but in slightly lower ee. Cyclobutane s 5/8 cleanly and efficiently rearrange to the dihydrobenzofurans 6/9 witho ut loss of enantiomeric purity upon treatment with the Ti-TADDOLates at hig her temperatures. Reactions of (Z)-propenylbenzene 17 and of indene with 2 and 3 give products in moderate enantiomeric purity. Products obtained from reactions of 1-anisylcycloalkenes with 2 differ significantly in yield and enantiomeric purity. In the latter reactions, the ee's of the cyclobutane products are consistently much higher than those of the dihydrobenzofuran p roducts. More significantly, products of different absolute configuration r esult from different cycloalkenes. With 1-anisylcyclopentene or 1-anisylcyc lohexene, all of the products are of similar configuration and are obtained in comparable yields and ee's. However, 1-anisylcycloheptene affords produ cts that are diastereomeric with those of the 1-anisylcyclopentene, and in lower ee's. A mechanistic model is proposed. Application of these reactions to the enantioselective synthesis of the pterocarpan class of isoflavonoid natural products is also reported.