Novel protocol for the synthesis of octahydropyrano- and thiopyrano[4,3-a]carbazole derivatives via Prins/Friedel–Crafts cyclization

A novel Prins/Friedel–Crafts cyclization of (Z)-6-(1-methyl-1H-indol-3-yl)hex-3-en-1-ol with aldehydes has been achieved using a catalytic amount of BF3·OEt2 under mild reaction conditions to produce the corresponding octahydropyrano[4,3-a]carbazole derivatives in good yields with high diastereoselectivity. The cross-coupling of (Z)-6-(1-methyl-1H-indol-3-yl)hex-3-ene-1-thiol with aldehydes affords the corresponding octahydrothiopyrano[4,3-a]carbazole derivatives under similar conditions.