Showing papers on "Ferrier rearrangement published in 2021"

Insight into the Ferrier Rearrangement by Combining Flash Chemistry and Superacids

Abstract: The transformation of glycals into 2,3-unsaturated glycosyl derivatives, reported by Ferrier in 1962, is supposed to involve an α,β unsaturated glycosyl cation, an elusive ionic species that has still to be observed experimentally. Herein, while combination of TfOH and flow conditions failed to observe this ionic species, its extended lifetime in superacid solutions allowed its characterization by NMR-based structural analysis supported by DFT calculations. This allyloxycarbenium ion was further exploited in the Ferrier rearrangement to afford unsaturated nitrogen-containing C-aryl glycosides and C-alkyl glycosides under superacid and flow conditions, respectively.

A General Strategy for the Stereoselective Synthesis of Pyrrole-Fused Chiral Skeletons: [3+2] Cycloaddition with 2-Nitro-2,3-Unsaturated Glycosides

Abstract: In this study, a two‐step methodology was developed for the synthesis of chiral pyrroles from 2‐nitroglycals via Ferrier rearrangement and Barton‐Zard reaction under mild conditions without transition metal catalysts. The Ferrier rearrangement reaction of 2‐nitro‐glycals and a series of O‐nucleophiles proceeded smoothly in the presence of N‐heterocyclic carbene (NHC) catalyst and K2CO3 which allowed the highly stereoselective synthesis of the diverse 2‐nitro‐2,3‐unsaturated glycosides in excellent yields. Subsequently, the rearrangement products were conveniently transformed into the desired chiral pyrroles by [3+2] cycloaddition (Barton‐Zard reaction) with isocyanoacetate in the presence of Cs2CO3. One‐pot strategy was also successfully demonstrated for the gram‐scale synthesis of one chiral pyrrole. This is the first report of stereoselective conversion from 2‐nitroglycals to chiral pyrrole.

From Glycals to Nitrogen Heterocycles and Carbocycles via "Cleavage-Intramolecular Recombination Strategy".

Abstract: Glycals (carbohydrate enol-ethers) have enjoyed profound applications in organic synthesis for more than a century. They not only serve as versatile glycosyl donors or as substrates for Ferrier rearrangement, but also find extensive synthetic applications especially as a "chiral pool" for accomplishing the synthesis of a variety of natural and biologically important compounds. As cyclic enol ethers, they demonstrate high reactivity and are among the most and variously transformable monosaccharide derivatives. The uniqueness of the reactivity of glycals is that they can be synthetically tuned to get a library of derivatives through stereo- and regioselective introduction of a variety of functional groups at C1, C2, C3 as well as C4 carbons of the sugar. We have developed a practical approach for stereoselective mono- and diamination of glycals and over the years utilized these scaffolds for the synthesis of a variety of biologically important nitrogen heterocycles and carbocycles through a "Diversity Oriented Approach". Our synthetic strategy in this direction mainly relied on the cleavage of ring O-C bond of the sugar followed by an "intramolecular recombination" reaction. Utilizing this strategy, we have accomplished the synthesis of several biologically important natural products, their analogues and related unnatural derivatives. Examples of such compounds reported from our group include polyhydroxypyrrolidines, DMDP, anisomycin, steviamine, pochonicine, conduramines, bulgecinine, aminocyclitols, azepanes, 4-hydroxy-D-proline, azanucleosides and their analogues. A personal account highlighting these syntheses is presented here.