[1,2]- vs [2,3]-Wittig Rearrangement in Carbohydrate Derived Alkenyl Systems

Glycals as Chiral Synthons in Organic Synthesis of Privileged Molecular Scaffolds

Chirality is a vital characteristic of molecules and crucial for biological functioning. Glycals are unsaturated chiral sugars that contain an enolic double bond within the ring structure and they introduce the chirality in the molecular scaffolds. Since their discovery and synthesis by Fischer and Zach in 1913, they have been used as a flexible chiral synthon for synthesising natural products and biologically significant molecules. An important area of study in organic synthesis involves functionalizing glycals, with particular emphasis on creating novel molecular scaffolds with a common backbone using various topologies. This review discusses recent advancements in synthesizing chiral molecules, emphasizing the innovative use of glycals as chiral synthons, and covers literature from 2020 to the present.

Stereospecific Synthesis of 1,2,3-Trisubstituted Cyclopropanes from Pseudoglycals

Strategic design for constructing optically pure 1,2,3-trisubstituted cyclopropanes with three contiguous stereocenters represents a formidable challenge in synthetic organic chemistry. Herein, we report a simple and highly enantiospecific transformation of pseudoglycals into chiral 1,2,3-trisubstituted cyclopropanes, featuring three consecutive stereocenters, involving a stereospecific [1,4]-Wittig rearrangement. The effects of functional group orientation and conformational preferences were studied. Successful gram-scale preparations and subsequent derivatization reactions yielded various cyclopropane scaffolds with multiple stereocenters.

A Short Route to the Synthesis of Digoxose Trisaccharide Glycal Donor via Mislow-Evans Rearrangement

The Mislow-Evans rearrangement was used as a key reaction to construct digitoxose-derived glycals. The same rearrangement was iteratively performed on di- and trisaccharides to form the digoxose glycal donor component present in the cardenolides digitoxin, digoxin, and gitoxin. The scalability of the trisaccharide synthesis was shown by performing the reactions on a multigram scale. Glycosylation reactions were also performed between the synthesized digoxin glycal donor and aglycons digoxigenin and gitoxigenin to synthesize novel cardenolide derivatives.

Synthesis of Hexenuloses and a Library of Disaccharides Possessing 3-oxo-glycal Unit

An expeditious method for the synthesis of monosaccharides and disaccharides possessing 3-oxo-glycal units is revealed. Several monosaccharides and disaccharide-derived glycals are converted to the corresponding hexenuloses in three steps involving halo-alkoxylation, dehydrohalogenation, and ketalyzation reactions. A number of 3-oxo-glycals are synthesized to show the methodology's importance and generality. Further, the protocol is successfully applied to synthesize a rare-sugar disaccharide donor unit present as part of the trisaccharide moiety in the reported natural product, versipelostatin.

Experimental and Theoretical DFT Investigations in the [2,3]-Wittig-Type Rearrangement of Propargyl/Allyl-Oxy-Pyrazolones

Here we report the synthesis of interesting 3-alkyl-4-hydroxy-1-aryl-4-(propa-1,2-dienyl)1H-pyrazol-5(4H)-ones and 9-alkyl-7-aryl-1-oxa-7,8-diazaspiro[4.4]nona-3,8-dien-6-ones, starting from 1,2-diaza-1,3-dienes (DDs) and propargyl alcohol. The reaction proceeds through a sequence Michael-type nucleophilic attack/cyclization/[2,3]-Wittig rearrangement. In the same way, the reaction between the aforementioned DDs and allyl alcohol furnished 4-allyl-4-hydroxy-3-alkyl-1-aryl-1H-pyrazol-5(4H)-ones. A DFT study was also carried out, in order to have decisive clarifications about the mechanism.