Meinwald Rearrangement Research Articles

Review on Catalytic Meinwald Rearrangement of Epoxides.

The past few decades have witnessed tremendous development within epoxides. Among the many known reactions involving epoxide, Meinwald rearrangements represent one of the most important and attractive approaches, which can transform epoxides into versatile carbonyl compounds. Given the high efficiency of this protocol, substantial efforts have been made by researchers by utilizing multiple catalyst systems. This review provides an overview of recent advances in the Meinwald rearrangement (from 2014 onward), along with detailed discussions on mechanistic insights. This review aims to highlight the importance and value of these methodologies, thereby promoting further investigation and application.

On the Origin of Enantioselectivity in Chiral Zeolite Asymmetric Catalyst GTM-3: Host-Guest Transfer of Chirality.

Knowledge of how extra-large-pore chiral zeolite asymmetric catalysts based on the -ITV framework imprint their chirality during a catalytic reaction is crucial in order to spread the scope for the catalytic enantioselective production of chiral compounds of interest. In this work, we have carried out a combined experimental and computational study on the catalytic activity of antipode GTM-3 catalysts during the ring-opening of trans-stilbene oxide with 1-butanol. Identification of the enantiomers of all the chiral species unraveled a surprising catalytic behavior: these chiral catalysts promote the transformation of one enantiomer of trans-stilbene oxide in the corresponding unlike product (with inversion of configuration of the attacked C) via an SN2 mechanism, and at the same time, the transformation of the other enantiomer of trans-stilbene oxide via an SN1-like mechanism into the like (with retention of configuration) and secondary products (diphenylacetaldehyde via Meinwald rearrangement and derived products). A computational study based on DFT + D methods suggested a potential explanation for this catalytic behavior, associated with a different orientation of trans-stilbene oxide enantiomers bound on the Ge(T7) positions in d4r units, which is stabilized by the development of intraframework H-bonds between the interrupted T7OH adjacent positions characteristic of this framework. Calculations suggest that each enantiomer of trans-stilbene oxide follows a different reaction pathway, one favoring the SN2 route by addition of butanol from the opposite side to form unlike-products, while the different orientation of the antipode enantiomer disfavors such SN2 route mainly by steric repulsions and at the same time favors the reaction toward the SN1 mechanism to give like- and secondary-products. Our study suggests that the strong enantioselectivity of GTM-3 catalysts for this reaction is associated with the particular orientation adopted by the chiral reactants within the chiral nanospace provided by the -ITV framework, similarly to what occurs with enzymes, and such preferential orientation is directly controlled by the asymmetric cavities where the reaction takes place, by the particular features of the Ge active sites in adjacent interrupted positions and by the presence of several framework OH groups in the nearby nanospace that interact with guest species. The experimental observations and the reaction mechanism proposed suggest that GTM-3 catalysts prepared from the (1S,2S) enantiomer of the N,N-ethyl-methyl-pseudoephedrinium organic agent should be enriched in the P4332 enantiomorphic space group of the -ITV framework and GTM-3 prepared from the (1R,2R) enantiomer in the antipode P4132. Interestingly, resolution of the absolute configuration of GTM-3 materials from 3D-electron diffraction data has been accomplished and confirms such an assignment, giving an average 82% enantio-enrichment in the corresponding chiral polymorph. Structure-solution of the location of the chiral structure-directing agents indicates that the transfer of chirality from the molecular component to the zeolite polymorph is governed by the development of strong H-bonds between the molecular hydroxyl group and the interrupted T(7)OH framework positions.

Engineering Styrene Oxide Isomerase Towards Enhanced Chemoenzymatic Synthesis of (S)‐Flurbiprofen

AbstractThe conventional synthesis of (S)‐flurbiprofen, a valuable non‐steroidal anti‐inflammatory drug (NSAID), involves inefficient chiral resolution with low‐yielding multiple reaction steps, or asymmetric synthesis with low enantioselectivity and/or regioselectivity. Here, we developed high‐yielding, enantioselective, and sustainable synthesis of (S)‐flurbiprofen via two chemoenzymatic approaches from a simple alkene: (A) enzymatic epoxidation‐isomerization‐oxidation cascade and Pd‐catalyzed arylation; and (B) chemical epoxidation, enzymatic isomerization‐oxidation cascade, and Pd‐catalyzed arylation. Styrene oxide isomerase (SOI)‐catalyzed enantio‐retentive Meinwald rearrangement was the key step in the two pathways, but SOI suffers from low activity. SOI was engineered via two rounds of iterative saturation mutagenesis and screening with a three‐enzyme cascade assay, generating a double‐mutant SOIDM (N99A/D95A) with 5.5‐fold improvement in catalytic efficiency (kcat/KM). In Pathway A, cascade biotransformation of 3‐fluoro‐4‐bromo‐α‐methylstyrene with engineered E. coli strain expressing SOIDM, styrene monooxygenase (SMO) and aldehyde dehydrogenase (ALDH), and subsequent Pd‐catalyzed arylation afforded (S)‐flurbiprofen in 98% yield and 91% ee. In Pathway B, chemical epoxidation (Sharpless dihydroxylation and ring‐closing), cascade conversion using an engineered E. coli strain expressing SOIDM, ALDH and NADH oxidase (NOX), and Pd‐catalyzed arylation produced (S)‐flurbiprofen in 83% yield and 98% ee. Compared to conventional synthesis, the developed methods reduced reaction steps, minimized waste generation and improved enantioselectivity or overall yield.

Copper(II) Triflate Catalyzed Rearrangement of Amino 2,3-Epoxides to α-Amino Ketones

Abstractα-Amino ketones were synthesized by a Meinwald rearrangement of biomass-based amino epoxides using copper(II) triflate as a catalyst. The regioselectivity of the rearrangement can be rationalized in terms of the reaction proceeding via the most stable carbocationic intermediate to give various α-amino α′-aryl ketones in moderate to good yields. This is an attractive method to prepare α-amino ketones using a benign and inexpensive catalyst.

Structural basis of the Meinwald rearrangement catalysed by styrene oxide isomerase

Membrane-bound styrene oxide isomerase (SOI) catalyses the Meinwald rearrangement—a Lewis-acid-catalysed isomerization of an epoxide to a carbonyl compound—and has been used in single and cascade reactions. However, the structural information that explains its reaction mechanism has remained elusive. Here we determine cryo-electron microscopy (cryo-EM) structures of SOI bound to a single-domain antibody with and without the competitive inhibitor benzylamine, and elucidate the catalytic mechanism using electron paramagnetic resonance spectroscopy, functional assays, biophysical methods and docking experiments. We find ferric haem b bound at the subunit interface of the trimeric enzyme through H58, where Fe(III) acts as the Lewis acid by binding to the epoxide oxygen. Y103 and N64 and a hydrophobic pocket binding the oxygen of the epoxide and the aryl group, respectively, position substrates in a manner that explains the high regio-selectivity and stereo-specificity of SOI. Our findings can support extending the range of epoxide substrates and be used to potentially repurpose SOI for the catalysis of new-to-nature Fe-based chemical reactions.

Sequential Paired Electrochemical Transformation of Styrene Oxide via Anodic Meinwald Rearrangement and Cathodic Nitro­methylation in an Electrochemical Flow Reactor with Catalytic Electrical Input

Sequential Paired Electrochemical Transformation of Styrene Oxide via Anodic Meinwald Rearrangement and Cathodic Nitro­methylation in an Electrochemical Flow Reactor with Catalytic Electrical Input

Synthesis of Ferrocenyl Boranes and their Application as Lewis Acids in Epoxide Rearrangements

AbstractA series of substituted ferrocenyl boron derivatives was synthesized. The oxidation of the ferrocenyl unit resulted in a significant increase of the boron‐centered Lewis acidity. The neutral and cationic Lewis acids were characterized by NMR spectroscopy, crystal structure analysis and by computational methods. The new Lewis acids were then applied in the Meinwald rearrangement of epoxides, predominantly furnishing aldehydes as the kinetic products.

Discovery and Bioinspired Synthesis of Salpratone A.

Salpratone A (1), a novel abietane diterpenoid containing a unique cis-fused A/B ring, was isolated from Salvia prattii. Bioactivity studies showed that 1 has potent activity in inhibiting platelet aggregation induced by multiple agonists as well as antithrombotic efficacy in the FeCl3-induced rat in vivo thrombosis model. Furthermore, a bioinspired synthesis of 1 from the abundant natural product ferruginol was achieved in 6 steps with a 22% overall yield. The key steps include a stereoselective allyl oxidation and a subsequent regioselective Meinwald rearrangement.

Organocatalytic, Chemoselective, and Stereospecific House–Meinwald Rearrangement of Trisubstituted Epoxides

AbstractWe present a novel method for the chemoselective House–Meinwald rearrangement of trisubstituted epoxides under mild conditions with the use of simple perfluorinated disulfonimides as Brønsted acid catalysts. We isolated the α-quaternary aldehyde products in yields of 27–97% using catalyst loadings as low as 0.5 mol% on a scale of 1 mmol. In addition, we show the stereospecific rearrangement using an enantioenriched substrate, which makes this method suitable for applications in total synthesis of natural products.

Kinetics and reactor modelling of a complex three-phase system: Carbonation of epoxides on grafted catalysts

Fatty acid epoxides are obtained by epoxidation of fatty acids and esters with hydrogen peroxide. Carbonation of fatty acid epoxides with CO2 is an important topic in the development of environmentally friendly technologies for the production of chemical intermediates, polymers and fuel components. Very precise kinetic and mass transfer studies on the carbonation of a tall-oil model component, epoxidized methyl oleate were carried out in an autoclave operating at 20–60 bar CO2 and 100–160 °C. SBA-15(0.12)-4PPI was used as the grafted heterogeneous catalyst. The reaction times were 0–23 h and the stirring rates were varied between 200 and 1000 rpm to study the gas–liquid mass transfer of CO2. The main reaction product was the carbonate, while some amounts of ketone were formed as a side product via the Meinwald rearrangement. Five rival reaction mechanisms were considered, based on the concept of interaction between the Lewis and nucleophilic sites on the catalyst surface. A multiphase reactor model including the interaction of intrinsic kinetics and gas–liquid mass transfer effects was developed to determine the parameters from the experimental data. The model reproduced the experimental data very well, giving a perspective for process scale-up.

One‐Carbon Homologation of α,β‐Unsaturated Aldehydes: Access to α‐Tertiary β,γ‐Unsaturated Aldehydes

AbstractAn efficient protocol for the synthesis of enolizable α‐substituted β,γ‐unsaturated aldehydes is reported. The developed strategy involves two steps, epoxidation and Meinwald rearrangement, to result in a one‐carbon homologation of α,β‐unsaturated aldehydes enabling the insertion of a CHR unit.

Coupling Titanium and Chromium Catalysis in a Reaction Network for the Reprogramming of [BH4 ]- as Electron Transfer and Hydrogen Atom Transfer Reagent for Radical Chemistry.

We describe a unique catalytic system with an efficient coupling of Ti- and Cr-catalysis in a reaction network that allows the use of [BH4 ]- as stoichiometric hydrogen atom and electron donor in catalytic radical chemistry. The key feature is a relay hydrogen atom transfer from [BH4 ]- to Cr generating the active catalysts under mild conditions. This enables epoxide reductions, regiodivergent epoxide opening and radical cyclizations that are not possible with cooperative catalysis with radicals or by epoxide reductions via Meinwald rearrangement and ensuing carbonyl reduction. No typical SN 2-type reactivity of [BH4 ]- salts is observed.

Coupling Titanium and Chromium Catalysis in a Reaction Network for the Reprogramming of [BH4]− as Electron Transfer and Hydrogen Atom Transfer Reagent for Radical Chemistry

AbstractWe describe a unique catalytic system with an efficient coupling of Ti‐ and Cr‐catalysis in a reaction network that allows the use of [BH4]− as stoichiometric hydrogen atom and electron donor in catalytic radical chemistry. The key feature is a relay hydrogen atom transfer from [BH4]− to Cr generating the active catalysts under mild conditions. This enables epoxide reductions, regiodivergent epoxide opening and radical cyclizations that are not possible with cooperative catalysis with radicals or by epoxide reductions via Meinwald rearrangement and ensuing carbonyl reduction. No typical SN2‐type reactivity of [BH4]− salts is observed.

Enantioselective Divergent Syntheses of Cephalotaxus Alkaloids: (−)-Cephalotaxine, (−)-Cephalotine B, and (−)-Fortuneicyclidins A and B

A new strategy focusing on the last-stage asymmetric assembly of the ring D, which inherently possesses the densest part of stereogenic centers and functional groups in the A/B/C/D ring system of (-)-cephalotaxine, has been developed, in which a novel Rh-catalyzed asymmetric (2 + 3) annulation of tertiary enamides with enoldiazoacetates is designed and explored for enantioselective construction of the crucial cyclopentane ring D bearing a unique spirocyclic aza-quaternary stereocenter. Based on the expeditious access of chiral functionalized building block with the tetracyclic A/B/C/D ring system, a concise enantioselective total synthesis of (-)-cephalotaxine starting from readily available homopiperonyl alcohol has been achieved in nine steps with only two column chromatography purifications. Following the tactical introduction of the Meinwald rearrangement, enantioselective divergent syntheses of (-)-cephalotine B with an additional C3-O-C11 oxo-bridged bond (14 steps), (-)-fortuneicyclidin B with an unprecedented C3-C10 bond (14 steps), and its 2-epimer (-)-fortuneicyclidin A (16 steps) have been also accomplished for the first time.

Styrene Oxide Isomerase-Catalyzed Meinwald Rearrangement in Cascade Biotransformations: Synthesis of Chiral and/or Natural Chemicals.

Styrene oxide isomerase (SOI) catalyzes the Meinwald rearrangement of aryl epoxides to carbonyl compounds via a 1,2-trans-shift in a stereospecific manner. A number of cascade biotransformations with SOI-catalyzed epoxide isomerization as a key step have been developed to convert readily available substrates into valuable chiral chemicals. Cascade conversion of terminal or internal alkenes into chiral acids, alcohols or amines was achieved, which involved SOI-catalyzed enantio-retentive isomerization of terminal epoxides via 1,2-H shift, or internal epoxides via 1,2-methyl shift. SOI-involved cascades were also developed to convert racemic epoxides into chiral acids or amines via dynamic kinetic resolution. Additionally, combining SOI-catalyzed isomerization with enantioselective C-C bond forming enzymes enabled the synthesis of chiral amino acids or amino alcohols from racemic epoxides. Finally, integration of SOI-involved cascades with biosynthesis pathways allowed for the direct utilization of renewable substrates for the sustainable synthesis of high-value natural chemicals such as alcohols, acids, and esters.

Sterically Hindered and Deconjugative α-Regioselective Asymmetric Mannich Reaction of Meinwald Rearrangement-Intermediate.

Compared to γ-addition, the α-addition of α-branched β,γ-unsaturated aldehydes faces larger steric hindrance and disrupts the π-π conjugation, which might be why very few examples are reported. In this article, a highly diastereo- and enantioselective α-regioselective Mannich reaction of isatin-derived ketimines with α-, β- or γ-branched β,γ-unsaturated aldehydes, generated in situ from Meinwald rearrangement of vinyl epoxides, is realized by using chiral N,N'-dioxide/ScIII catalysts. A series of chiral α-quaternary allyl aldehydes and homoallylic alcohols with vicinal multisubstituted stereocenters are constructed in excellent yields, good d.r. and excellent ee values. Experimental studies and DFT (density functional theory) calculations reveal that the large steric hindrance of the ligand and the Boc (tButyloxy carbonyl) protecting group of imines are critical factors for the α-regioselectivity.

Sterically Hindered and Deconjugative α‐Regioselective Asymmetric Mannich Reaction of Meinwald Rearrangement‐Intermediate

AbstractCompared to γ‐addition, the α‐addition of α‐branched β,γ‐unsaturated aldehydes faces larger steric hindrance and disrupts the π–π conjugation, which might be why very few examples are reported. In this article, a highly diastereo‐ and enantioselective α‐regioselective Mannich reaction of isatin‐derived ketimines with α‐, β‐ or γ‐branched β,γ‐unsaturated aldehydes, generated in situ from Meinwald rearrangement of vinyl epoxides, is realized by using chiralN,N′‐dioxide/ScIIIcatalysts. A series of chiral α‐quaternary allyl aldehydes and homoallylic alcohols with vicinal multisubstituted stereocenters are constructed in excellent yields, good d.r. and excellenteevalues. Experimental studies and DFT (density functional theory) calculations reveal that the large steric hindrance of the ligand and the Boc (tButyloxy carbonyl) protecting group of imines are critical factors for the α‐regioselectivity.

Total synthesis of (±)-pterosin M and (±)-onitisin

Total synthesis of phenolic sesquiterpene pterosin M has been achieved from known indanone via bromination, Suzuki-Miyaura cross coupling, oxone mediated epoxidation, Meinwald rearrangement, BBr3 mediated demethylation and decarboxylation in one go and chemoselective reduction as the key steps. We also synthesized the known indanone via a different route comprising of Knoevenagel condensation of aldehyde derivative with Meldrum’s acid, subsequent reduction, methylation, PPA mediated intramolecular Friedel-Crafts acylation and β-ketoester formation. Hydroxymethylation reaction of pterosin M completed the first total synthesis of onitisin.

Continuous-flow Fe-zeolite-catalyzed temperature-directed synthesis of bioactive tetraketones and xanthenes using epoxides and cyclic-1,3-diketones via a Meinwald rearrangement

The continuous flow Fe-zeolite-catalyzed direct transformation of epoxides and cyclic-1,3-diketones to tetraketones and their subsequent catalytic cyclization to xanthene derivatives are described.

Synthesis of functionalised isochromans: epoxides as aldehyde surrogates in hexafluoroisopropanol.

The oxa-Pictet-Spengler reaction is arguably the most straightforward and modular way to construct the privileged isochroman motif, but its scope is largely limited to benzaldehyde derivatives and to electron-rich β-phenylethanols that lack substitution along the aliphatic chain. Here we describe a variant of this reaction starting from an epoxide, rather than an aldehyde, that greatly expands the scope and rate of the reaction (<1 h, 20 °C). Besides facilitating the initial Meinwald rearrangement, the use of hexafluoroisopropanol (HFIP) as a solvent expands the electrophile scope to include partners equivalent to ketones, aliphatic aldehydes, and phenylacetyl aldehydes, and the nucleophile scope to include modestly electronically deactivated and highly substituted β-phenylethanols. The products could be easily further derivatised in the same pot by subsequent ring-opening, reductions, and intra- and intermolecular Friedel-Crafts reactions, also in HFIP. Finally, owing to the high pharmacological relevance of the isochroman motif, the synthesis of drug analogues was demonstrated.