Stereoselective synthesis of cis- and trans-2,6-disubstituted 5,6-dihydro-2H-pyrans based on 1,5-asymmetric induction in reactions between allylstannanes and aldehydes promoted by tin(IV) chloride

Abstract

The allyltin trichloride formed by treating [(4S)-5-(tert-butyldimethylsilyloxy)-4-(2-trimethylsilylethoxy methoxy)]pent-2-enyl(tributyl)stannane 9 with tin(IV) chloride, reacts stereoselectively with 2-oxoethyl toluene-p-sulfonate to give the syn-6-(2-trimethylsilylethoxymethoxy)hept-4-en-2-ol 10 which was converted into the epoxide 11 by treatment with potassium carbonate in methanol. After replacing the tert-butyldimethylsilyl ether by a benzyl ether, removal of the 2-trimethylsilylethoxymethoxy group using trifluoroacetic acid, gave the 2,6-cis- and 2,6-trans-disubstituted 5,6-dihydro-2H-pyrans 14 and 15, ratio 80:20, which were separated as their tert-butyldimethylsilyl ethers 16 and 17 and characterised as their acetates 18 and 19. The 5,6-dihydro-2H-pyrans 24 and 26, ratio 80:20, were similarly prepared from (4S)-4-(2-trimethylsilylethoxymethoxy)pent-2-enyl(tributyl)stannane 21. A complementary route to the trans-2,6-disubstituted 5,6-dihydro-2H-pyran 15 was developed from 2-oxoethyl benzoate which gave the syn-hydroxy ether 28 under the usual conditions. Mesylation and ester saponification gave the anti-epoxide 30 and, after replacing the tert-butyldimethylsilyl ether by a benzyl ether, treatment with trifluoroacetic acid gave the 5,6-dihydro-2H-pyrans 14 and 15, in favour of the trans-isomer, ratio 14:15= 20:80. Improved stereoselectivity in these dihydropyran syntheses was obtained if the stannane 9 was treated with butyl glyoxylate in the first step. The cis-2,6-disubstituted 5,6-dihydro-2H-pyran 16 was converted into the tetrahydropyran-3-one 37 by hydroboration–oxidation, and into the tetrahydropyran-4-one 43 by treatment with bromine water, reductive debromination and oxidation.