New mannose-derived ketones as organocatalysts for enantioselective dioxirane-mediated epoxidation of arylalkenes. Part 3: Chiral ketones from sugars

Stereoselective Shi-type epoxidation with 3-oxo-4,6-O-benzylidene pyranoside catalysts: unveiling the role of carbohydrate skeletons

Asymmetric epoxidation represents a hot topic in organic synthesis. In recent years, organocatalysts based on sugar skeletons have been exploited in asymmetric epoxidation to achieve enantiomeric pure epoxides. In this work, two different endocyclic ketones derived from glucose and galactose protected with a 4,6-O-benzylidene group have been prepared and exploited for Shi-type epoxidation. The two carbohydrates show an opposite preferential stereoselective epoxidation on various olefins, affording the epoxides in high conversions and modest enantioselectivities. DFT calculations disclosed the reasons behind the inversion of selectivity achieved by the two catalysts, showing that a delicate balance between the catalyst conformation, its protecting groups, and the secondary interactions with the substrate govern the final observed results.

Synthesis and Applications of Carbohydrate-Based Organocatalysts

Organocatalysis is a very useful tool for the asymmetric synthesis of biologically or pharmacologically active compounds because it avoids the use of noxious metals, which are difficult to eliminate from the target products. Moreover, in many cases, the organocatalysed reactions can be performed in benign solvents and do not require anhydrous conditions. It is well-known that most of the above-mentioned reactions are promoted by a simple aminoacid, l-proline, or, to a lesser extent, by the more complex cinchona alkaloids. However, during the past three decades, other enantiopure natural compounds, the carbohydrates, have been employed as organocatalysts. In the present exhaustive review, the detailed preparation of all the sugar-based organocatalysts as well as their catalytic properties are described.

Asymmetric organocatalytic epoxidations: reactions, scope, mechanisms, and applications

Chiral epoxides serve as versatile building blocks in the synthesis of complex organic frameworks. The high strain imposed by the three-membered ring system makes epoxides prone to a variety of nucleophilic ring-opening reactions. Since the development of the Sharpless epoxidation, there have been many important contributions and advances in this area. With the rapid development of the field of asymmetric organocatalysis, a wide range of organocatalysts is now able to catalyze the epoxidation of broad class of unsaturated carbonyl compounds. In this Minireview, recent progress in the development of organocatalytic asymmetric epoxidation methods, the proposed mechanisms of these reactions and their applications as intermediates is reported.