Photochemical deconjugation of chiral 3-methyl-2-butenoates derived from carbohydrate-based alcohols: The influence of the sugar backbone on the facial diastereoselectivity

The photodeconjugation of the alpha-(4-trimethylsilyl-3-butynyl)-substitute d senecio acid esters 7 was studied. Chiral alcohols ROH (9) were employed as auxiliaries to control the facial diastereoselectivity of the protonatio n step. The conversion of the four sugar alcohols diacetone-D-glucofuranose , diacetone-D-allofuranose, diacetone-D-gulofuranose, and diacetone-D-fruct opyranose (9a-d) to the esters 7 was achieved in four steps employing 4-iod o-1-trimethylsilylbut-1-yne (3) as the alkylating agent (27-45% yield overa ll). Their photodeconjugation gave the corresponding beta,gamma -unsaturate d (R)-esters 14a-d with moderate to excellent diastereomeric excess. The be st results were achieved with diacetone-D-glucofuranose and diacetone-D-fru ctopyranose as the auxiliary (> 95% de). To achieve the synthesis of the ta rget compound 1 which has the (Si-configuration, the deconjugation was cond ucted with the diacetone-L-fructopyranose (ent-9d) derived ester ent-7d. L- Fructose (20) was prepared from L-sorbose (15) in a modified procedure that allowed for the isolation of intermediates. The a-fold inversion of config uration worked nicely, and the fructofuranose 19 was obtained in 19% yield from L-sorbose. The conversion of L-fructose to the ester ent-7d was conduc ted in full analogy to the synthesis of its enantiomer 7d. Deconjugation of ester ent-7d yielded the product 2d (70% yield), which was reduced to the alcohol 1 (85% yield).