FORMAL 2+2 AND 3+2 CYCLOADDITION REACTIONS OF 2H-CHROMENES WITH 2-ALKOXY-1,4-BENZOQUINONES - REGIOSELECTIVE SYNTHESIS OF SUBSTITUTED PTEROCARPANS

The titanium(IV)-catalyzed reaction of various 2H-chromenes and 2-alkoxy-l,4-benzoquinones stereoselectively yields the oxygenated pterocarpans 8 and/or the 2 + 2 adducts 7, depending upon reaction conditions. Cyclobutanes 7 rearrange to 8 upon treatment with protic acids. Syntheses of the pterocarpan phytoalexins (±)-homopterocarpin (2), (±)-pterocarpin (3), and (±)-9-0-benzy1-3-0-methylsophoropterocarpan A (4) are presented, which demonstrates the synthetic utility of these formal cycloaddition reactions. © 1990, American Chemical Society. All rights reserved.