2-Halo Glycals as "Synthon" for 2-C-Branched Sugar: Recent Advances and Applications in Organic Synthesis

Glycals are unsaturated sugars having an enolic double bond present inside the ring and act as a versatile synthon for the synthesis of natural products and biologically important molecules. The lone pair of endocyclic ring oxygen which is in conjugation with the enolic double bond alters the reactivity of both unsaturated carbons of glycals and directs the selectivity of the incoming group at a particular center. Since the enolic double bond is very much prone to undergo various types of reactions such as epoxidation, addition, formylation, nitration halogenation, etc.; and such type of activated glycals or the derivatives of glycal have been extensively utilized as a precursor for the synthesis of branched sugars. 2-haloglycals have emerged as an important building block for the functionalization at C-2 position of glycals via metal-catalyzed cross-coupling reactions such as Heck, Suzuki, Sonogashira and carbonylative cross-coupling reactions. In this review, we have summarized the recent progress on the utilization of 2-haloglycals for the synthesis of 2-C-branched sugars and further transformation of branched sugars into annulated sugars, heterocycles, and glycoconjugates.