BiBr3 initiated cyclization–addition reactions: effect of π-nucleophile on oxocarbenium ion addition and total syntheses of (+)-(S,S)-(cis-6-methyltetrahydropyran-2-yl)acetic acid and its trans-diastereomer

For BiBr3 initiated tandem cyclization–additions of very reactive silyl ketene acetal nucleophiles with δ-silyloxy aldehydes to afford 2,6-disubstituted THP products, diastereoselectivities range from 5–6:1 (trans-/cis-). The selectivity for axial attack on the intermediate oxocarbenium ion is inversely proportional to π-nucleophilicity. We have utilized this chemistry to convert a common starting material to both cis- and trans-diastereomers of an acid first isolated from the secretions of Viverra civetta.