2‐Halo Glycals as “Synthon” for 2‐C‐Branched Sugar: Recent Advances and Applications in Organic Synthesis

Abstract

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.[a] Dr.

Deoxy sugars. General methods for carbohydrate deoxygenation and glycosidation

“…46,47 They include glycals, with the double bond between C-1 and C-2, which are particularly valuable starting materials of biologically relevant compounds and chiral building blocks. 48,49…”

Sonogashira cross-coupling as a key step in the synthesis of new glycoporphyrins

“…On the other hand, in recent years, continuous progress has been made in the recognition of 2-haloglycals as convenient starting materials for metal-catalyzed reactions. 13 New methodologies have been developed, focusing mostly on the creation of new C–C bonds via the Heck 14 and Suzuki–Miyaura reactions, 15 cyanation, 16 or other transformations. 17 More recently, carbonylative equivalents have been put in the spotlight, 18 as well as reactions leading to new C-heteroatom bonds.…”

Synthesis of chiral azides from C-2 substituted glycals and their transformation to C3-glycoconjugates and α-triazolo-naphthalene polyol

“…21,22 The double bond between the C1/C2 positions of glycals can be easily transformed to generate different linkages at the C1/C2 position. 23,24 The reactivity of glycals at the C1/C2 position is quite different, as the C1 position could easily trap any external nucleophiles, while the reverse is the case for the C2 position. Recently, we and other researchers have installed various functionalities at the C2 position of glycals via metal-catalyzed C-H functionalization and cross-coupling approaches.…”

“…Recently, we and other researchers have installed various functionalities at the C2 position of glycals via metal-catalyzed C-H functionalization and cross-coupling approaches. 23,25 In general, the Ferrier reaction is the most studied glycosylation reaction in glycals to synthesize different glycosides. 21,26 A handful of protocols are available in the literature that disclose the reactivity of unsubstituted glycals with azide precursors under the influence of an activator, providing the azido glycal derivatives via Ferrier glycosylation (Scheme 1A).…”