STEREOSELECTIVE SYNTHESIS OF CIS-2,6-DISUBSTITUTED 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

The allyltin trichloride formed by treating t-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 yn-6-(2-trimethylsilylethoxymethoxy)hept-4-en-2- ol 10 which was converted into the epoxide 11 by treatment with potass ium 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-dis ubstituted 5,6-dihydro-2H-pyrans 14 and 15, ratio 80:20, which were se parated as their tert-butyldimethylsilyl ethers 16 and 17 and characte rised 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- ylsilylethoxyme thoxy)pent-2-enyl(tributyl)stannane 21. A complementary route to the t rans-2,6-disubstituted 5,6-dihydro-2H-pyran 15 was developed from 2-ox oethyl benzoate which gave the syn-hydroxy ether 28 under the usual co nditions. Mesylation and ester saponification gave the anti-epoxide 30 and, after replacing the tert-butyldimethylsilyl ether by a benzyl et her, treatment with trifluoroacetic acid gave the 5,6-dihydro-2H-pyran s 14 and 15, in favour of the trans-isomer, ratio 14:15 = 20:80. Impro ved stereoselectivity in these dihydropyran syntheses was obtained if the stannane 9 was treated with butyl glyoxylate in the first step. Th e cis-2,6-disubstituted 5,6-dihydro-2H-pyran 16 was converted into the tetrahydropyran-3-one 37 by hydroboration-oxidation, and into the tet rahydropyran-4-one 43 by treatment with bromine water, reductive debro mination and oxidation.