Epimerization of Glycal Derivatives by a Cyclopentadienylruthenium Catalyst: Application to Metalloenzymatic DYKAT

Abstract

Epimerization of a non‐anomeric stereogenic center in carbohydrates is an important transformation in the synthesis of natural products. In this study an epimerization procedure of the allylic alcohols of glycals by cyclopentadienylruthenium catalyst 1 is presented. The epimerization of 4,6‐O‐benzylidene‐D‐glucal 4 in toluene is rapid, and an equlibrium with its D‐allal epimer 5 is established within 5 min at room temperature. Exchange rates for allal and glucal formation were determined by 1D 1H EXSY NMR experiments to be 0.055 s−1 and 0.075 s−1, respectively. For 4‐O‐benzyl‐L‐rhamnal 8 the epimerization was less rapid and four days of epimerization was required to achieve equilibration of the epimers at room temperature. The epimerization methodology was subsequently combined with acylating enzymes in a dynamic kinetic asymmetric transformation (DYKAT), giving stereoselective acylation to the desired stereoisomers 12, 13, and 15. The net effect of this process is an inversion of a stereogenic center on the glycal, and yields ranging from 71 % to 83 % of the epimer were obtained.

In Situ Structural Determination of a Homogeneous Ruthenium Racemization Catalyst and Its Activated Intermediates Using X‐Ray Absorption Spectroscopy

“…[20] Dicarbonyl(pentaphenylcyclopentadienyl)ruthenium chloride (1a)i sf ormallya transfer hydrogenation catalyst, which has found numerous applicationsa sr acemization catalysti nd ynamic kinetic resolution (DKR) of aw ide range of alcohols. [21][22][23][24][25][26][27] The mechanism for the hydride transfer of alcohols with catalyst 1 and related complexes has long been at opic of discussion and dispute. [28][29][30][31][32][33][34][35][36][37] The current proposed mechanism starts with activation of the Ru chloride complex 1 by potassium tert-butoxide, to give tert-butoxide complex 3 (Scheme 1).…”

“…Metal‐carbonyls are very common in modern coordination and organometallic chemistry, and they are frequently found as reagents or catalysts in organic synthesis . Dicarbonyl(pentaphenylcyclopentadienyl)ruthenium chloride ( 1 a ) is formally a transfer hydrogenation catalyst, which has found numerous applications as racemization catalyst in dynamic kinetic resolution (DKR) of a wide range of alcohols . The mechanism for the hydride transfer of alcohols with catalyst 1 and related complexes has long been a topic of discussion and dispute .…”

DFT Investigation of the η6 ⇌ η6-Inter-ring Haptotropic Rearrangement of the Group 8 Metals Complexes [(graphene)MCp]+ (M = Fe, Ru, Os)

“…No data on the dependence of activation parameters on a metal (the difference from Fe via Ru to Os) have been obtained as well as no effects of solvents on the activation parameters of the metal complex HRs. We believe that knowledge of these data will compose a very important part of the organometallic chemistry of graphene which is well-known to be a highly competitive, rapidly developing, and promising area of research. − The strong interest in obtaining these data can be clearly understood from the fact of numerous examples of using cyclopentadienyl complexes of the group 8 metals as catalysts (Fe, − Ru ,− ) or valuable precursors for different organometallic compounds (Os). − Motivated by these examples, it would be of high interest to elucidate potential catalytic activity and chemical reactivity in general of the group 8 OMG complexes on the graphene surface. It should be noticed that, in a more general way, in the course of an inter-ring haptotropic rearrangement, as was mentioned earlier, the metal encompasses unsaturation and thus becomes prone to reaction with various incoming species. − ,− Furthermore, it is very important to mention that large PAHs such as graphene and nanotubes were shown to have intrinsic anticancer activity. − Also, numerous arene complexes of Fe, , Ru, ,,,− and Os have been proven to possess anticancer activity.…”

“Eco‐Friendly” Epimerization of Lutein to 3′‐Epilutein Under Solvent‐Free Mechanochemical Conditions by Using a Strongly Acidic Cation‐Exchange Resin

“…Reports of allylic alcohol epimerizations are abundant in the literature, [61][62][63][64][65] however, to the best of our knowledge, no mechanochemical processes effecting these reactions involving sulfonic acid resins have been reported. Thus, herein we disclose the development of a solvent-free mechanochemical epimerization protocol for converting naturally occurring luteincontaining extracts into 3′-epilutein-enriched mixtures by using strongly acidic cation-exchange resins as "eco-friendly" catalysts.…”