HIGHLY LIKE-SELECTIVE [4+2]-CYCLOADDITIONS OF CHIRAL DIENOLS - THE IMPORTANCE OF 1,3-ALLYLIC STRAIN IN THE HYDROXY-DIRECTED STEREOCONTROL

The chiral open-chain dienols 3a,b, which possess 1,3-allylic strain due to the presence of a cis substituent, give with the dienophiles maleic anhydride (MA), N-phenyl maleimide (NPM), 4-phenyl- (PTAD), and 4-methyl-1,2,4-triazoline-3,5-dione (MTAD) the corresponding [4 + 2] cycloadducts in very good yields and with high like selectivity. In contrast to previous reports on dienols without 1,3-allylic strain, the sense of diastereoselectivity does not vary with the dienophile type. The cis substituent aligns preferred conformations in the ground and transition states for which the hydrogen atom is placed in the sterically most biased inside position. Sterically and electronically controlled dienophile attack on these rotamers leads to the observed like stereochemistry. The participation of electronic features, most prominently hydroxy-directed stereocontrol, is substantiated by solvent effects in the triazolinedione cycloadditions, i.e., lower pi-facial selectivities are observed in polar solvents. The present results demonstrate the efficacy of 1,3-allylic strain in promoting conformational preferences in [4 + 2] cycloadditions with asymmetric dienols.