Synthetic methods for ring contraction and expansion in carbohydrate chemistry

Owing to the vast repertoire of biologically active carbohydrates featuring medium-sized ether motifs, organic synthesis researchers have focused intensely on engineering innovative strategies to access these challenging architectures. Carbohydrates display an extraordinary degree of structural diversity, encompassing highly intricate, ring-expanded or ring-contracted heterocyclic and carbocyclic scaffolds. This review provides a comprehensive exploration of such ring-size alterations across the spectrum of carbohydrate synthesis. We critically examine the current state-of-the-art chemical methodologies designed to construct atypical carbon skeleta, achieved either via de novo cyclization or subsequent oxidative tailoring. This literature overview is systematically divided into two key segments: the first highlights groundbreaking strategies for sugar ring contraction, while the second delineates advanced synthetic pathways driving sugar ring expansion.