Enantioselective Synthesis of 1,2-Dihydro­naph­thalene-1-carbaldehydes by Addition of Boronates to Isochromene Acetals Catalyzed by Tartaric Acid

Tartaric acid is an ideal asymmetric
catalyst as it is abundant,
cheap, and environmentally friendly. (+)-Tartaric acid was found to
catalyze a novel enantioselective [4 + 2] cycloaddition of isochromene
acetals and vinylboronates. A variety of substituted isochromene acetals
were tolerated, furnishing the desired dihydro­naph­thalenes
and dihydro­benzo­fluorene products in good yields. High
enantiomeric ratios (up to 98.5:1.5) and excellent diastereoselectivities
(all >99:1) were observed employing 10 mol % of (+)-tartaric acid
as the catalyst, in combination with 5 mol % of Ho­(OTf)3.