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 α-(4-trimethylsilyl-3-butynyl)-substituted senecio acid esters 7 was studied. Chiral alcohols ROH (9) were employed as auxiliaries to control the facial diastereoselectivity of the protonation step. The conversion of the four sugar alcohols diacetone-d-glucofuranose, diacetone-d-allofuranose, diacetone-d-gulofuranose, and diacetone-d-fructopyranose (9a−d) to the esters 7 was achieved in four steps employing 4-iodo-1-trimethylsilylbut-1-yne (3) as the alkylating agent (27−45% yield overall). Their photodeconjugation gave the corresponding β,γ-unsaturated (R)-esters 14a−d with moderate to excellent diastereomeric excess. The best results were achieved with diacetone-d-glucofuranose and diacetone-d-fructopyranose as the auxiliary (>95% de). To achieve the synthesis of the target compound 1 which has the (S)-configuration, the deconjugation was conducted 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 2-fold inversion of configuration 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 conducted 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).