Mechanistic studies of rearrangements during the ring expansions of cyclopropanated carbohydrates

Stereoselective Entry into α,α'-C-Oxepane Scaffolds through a Chalcogen Bonding Catalyzed Strain-Release C-Septanosylation Strategy

The utility of unconventional noncovalent interactions (NCIs) such as chalcogen bonding has lately emerged as a robust platform to access synthetically difficult glycosides stereoselectively. Herein, we disclose the versatility of a phosphonochalcogenide (PCH) catalyst to facilitate access into the challenging, but biologically interesting 7-membered ring α,α'-C-disubstituted oxepane core through an α-selective strain-release C-glycosylation. Methodically, this strategy represents a switch from more common but entropically less desired macrocyclizations to a thermodynamically favored ring-expansion approach. In light of the general lack of stereoselective methods to access C-septanosides, a remarkable palette of silyl-based nucleophiles can be reliably employed in our method. This include a broad variety of useful synthons, such as easily available silyl-allyl, silyl-enol ether, silyl-ketene acetal, vinylogous silyl-ketene acetal, silyl-alkyne and silylazide reagents. Mechanistic investigations suggest that a mechanistic shift towards an intramolecular aglycone transposition involving a pentacoordinate silicon intermediate is likely responsible in steering the stereoselectivity.

Reactions of 1,2-cyclopropyl carbohydrates

Addition of a carbene to a glycal is the prominent method for the synthesis of 1,2-cyclopropyl carbohydrates. This incorporation of a cyclopropane into a carbohydrate scaffold enables divergent reactivity, with the two main classes being ring expansion and cleavage to 2-C-branched carbohydrates. A wide variety of products are obtained depending on the functionality attached to the cyclopropane (none or ester or halogens) and the promoter (Lewis acid, Brønsted acid, halophile or base) used in the reaction. This article reviews progress in the synthesis and reactions of 1,2-cyclopropyl carbohydrates since 2000 and discloses efforts by our group in the area.

Synthesis and properties of septanose carbohydrates

Seven-atom ring sugars, called septanoses, are increasingly the focus of scientific inquiries because of their potential biological activities. This article details the synthesis, conformational analysis, and protein-binding properties of septanose carbohydrates. A distinction is drawn between septanoses that are substituted in the 6-position of the ring and those that are not. When a C-6 substituent is absent, the structure is essentially that of an aldohexose in its septanose, rather than furanose or pyranose, ring form; they may play as-of-yet unexplored roles in glycobiology. Septanoses having a hydroxymethyl group at C-6, on the other hand, are ring-expanded analogues of pyranoses. Syntheses have moved beyond the preparation of seven-membered ring monosaccharides to the development of septanosyl donors. These donors have been used in the synthesis of novel di- and trisaccharides that contain septanoses as well as a variety of glycoconjugates. Low-energy conformations adopted by septanoses have been organized based on ring substitution and stereochemistry. Instances where septanoses have been demonstrated to bind to natural proteins are presented and analyzed. The major conclusion drawn in the chapter is that advances in the synthesis of septanose carbohydrates now enable a detailed investigation of their activity in a number of biological contexts.