Tether-controlled cycloadditions for the asymmetric synthesis of decalins: increased selectivity in acetonitrile solvent.

[reaction: see text] The beneficial influence of cis-isopropylidene acetal tether control groups, to facilitate the asymmetric synthesis of substituted decalins by intramolecular Diels-Alder reactions, is described. Compared to trans-acetonides, these cases proceed under milder conditions to afford the cis-fused adducts from an endo transition state. An unusual acetonitrile solvent effect exerts a dramatic influence on the diastereoselectivity. This strategy leads to the chiral nonracemic bicyclo[4.4.0]decane core of diverse natural products.