Abstract
A direct functionalization of the anomeric carbon of unprotected, hydroxy group–abundant
carbohydrates is desirable, as it circumvents the need to overcome protecting group
interventions and enables the formation of glycosides in a step- and atom-economical
manner. The primary, secondary hydroxy, and hemiacetal functionalities encounter reaction
selectivities to permit the formation of glycosides selectively, without affecting
the remaining sites. The advantage lies in the reactivity differences that provide
a manner by which to achieve the desired selective functionalization, namely, that
of the hemiacetal in the present context. This article provides a comprehensive overview
of recent advancements in the protecting group-free synthesis of glycosides from the
unprotected, hydroxy functionality–rich carbohydrates, primarily, mono- and disaccharides.
The discussion highlights topical advancements made with particular emphasis on the
concept and the mechanistic rationale. The article is organized with a brief introduction;
discussions on the (i) direct synthesis of O-glycosides; (ii) base-mediated-; and (iii) direct anomeric functionalization of unprotected
carbohydrates; (iv) formation of glycosyl esters; (v) glycosyl phosphates; (vi) N-glycosyl amides; and (vii) C-glycosides that are formed from protecting groups-free carbohydrates. An outlook
of the present status of developments is given in the conclusion.
Keywords
Carbohydrates - Hemiacetal - Glycosylation - Glycoside - Protecting group
