Solvent-Free Approaches in Carbohydrate Synthetic Chemistry: Role of Catalysis in Reactivity and Selectivity

Abstract

Owing to their abundance in biomass and availability at a low cost, carbohydrates are very useful precursors for products of interest in a broad range of scientific applications. For example, they can be either converted into basic chemicals or used as chiral precursors for the synthesis of potentially bioactive molecules, even including nonsaccharide targets; in addition, there is also a broad interest toward the potential of synthetic sugar-containing structures in the field of functional materials. Synthetic elaboration of carbohydrates, in both the selective modification of functional groups and the assembly of oligomeric structures, is not trivial and often entails experimentally demanding approaches practiced by specialized groups. Over the last years, a large number of solvent-free synthetic methods have appeared in the literature, often being endowed with several advantages such as greenness, experimental simplicity, and a larger scope than analogous reactions in solution. Most of these methods are catalytically promoted, and the catalyst often plays a key role in the selectivity associated with the process. This review aims to describe the significant recent contributions in the solvent-free synthetic chemistry of carbohydrates, devoting a special critical focus on both the mechanistic role of the catalysts employed and the differences evidenced so far with corresponding methods in solution.

Microwave-Assisted, Solid-State Procedure to Covalently Conjugate Hyaluronic Acid to Curcumin: Validation of a Green Synthetic Protocol

“…It is worth remembering that the activation–esterification protocol employing DMAP is reported as very effective, even in the absence of any solvent. , In major detail, DMAP activates the carboxylic function, thanks to its great nucleophilicity, by reacting with the O -acylisourea intermediate to form the acylated pyridinium ion (Route 1), as electrophilic species are very sensitive to promptly react with the alcoholic function of CUR. On the other hand, the activation of the hyaluronan carboxylic functions as O -acylisourea species was obtained in the presence of K 2 CO 3 (Route 2) or in the absence of base (Route 3).…”

Use of Novel Homochiral Thioureas Camphor Derived as Asymmetric Organocatalysts in the Stereoselective Formation of Glycosidic Bonds

“…Solvent-free methodologies have been developed recently in stereoselective glycosylation reactions [ 32 ]; however, these procedures are uncommon in carbohydrate chemistry and have only directed very few applications to glycosylation reactions [ 33 ]. Solvent-free reactions avoid the use of polluting, high-boiling, toxic, and commonly used organic solvents; such approaches allowed a great simplification and improvement [ 34 , 35 ].…”

Catalytic, Regioselective Sulfonylation of Carbohydrates with Dibutyltin Oxide under Solvent-Free Conditions

“…Over the last few years, there has been a growing interest in the development of solvent-free approaches focused on synthetic carbohydrate chemistry [29], and numerous contributions were provided by our laboratory, with the proposal of non-conventional and experimentally simple procedures for the regioselective protection of sugars with benzyl [30][31][32][33], allyl [31], silyl [34,35], trityl [35,36] and acetal groups [37], and the possible sequential application thereof in one-pot procedures [34,36,38]. In this frame, the mentioned benzylation/allylation procedure [31] represented the first example at the use of catalytic dibuyltin oxide for stannylene-mediated alkylations, after about 30 years of stoichiometric applications.…”