Seven membered ring sugars, septanoses, are synthetic analogs of pyranoses where one carbon atom has been added to the ring. Septanosides have shown activity as ligands of lectins as well as substrates for glycosidase enzymes. Glycosylations involving 1, 2-anhydroseptanoses, thio-septanosides and septanosyl fluorides have been demonstrated in the past. Drawing inspiration from the ubiquitous use of per-O-acetyl pyranoses in carbohydrate synthesis, here we introduce a new method to synthesize septanosides using per-O-acetyl septanoses as key starting material. Synthesis of the per-O-acetyl septanoses started with addition of vinyl magnesium bromide to benzyl protected pyranose lactols followed by ozonolysis to obtain the corresponding hydroxy-aldehydes. This product was then trapped in the cyclic form by acetylation. A deprotection-reprotection sequence removed the benzyl groups and replaced them with acetates to give the per-O-acetyl sugars. These per-O-acetyl sugars are very stable precursors for a variety of synthetic targets and a large scale synthesis of these using the above strategy has been demonstrated. Per-acetate septanoses were treated with HBr in acetic acid to give septanosyl bromides which acted as donors in glycosylation reactions. The donors were activated in the presence of various nucleophiles to yield their respective glycosides. The resulting per-O-acetate protected septanosyl glycosides displayed 1, 2-trans stereochemistry selectively with respect to anomeric and C2 substituents. The feasibility as well as efficiency of this synthesis strategy is so far the best amongst all the other routes.

New old favorite: Glycosylations using septanosyl bromides as donors

251st National Meeting (2016)

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