2-Substituted-1,3,2-dithioborolans as Chiral Lewis Acid Catalysts

1,3,2-Dithioborolans substituted at the 2-position with various homochiral groups can act as chiral Lewis acids in the Diels-Alder reaction between crotonaldehyde or methacrolein with cyclopentadiene. The endo:exo selectivities obtained were good although the enantiomeric excesses were low to moderate