Highly efficient Meinwald rearrangement reactions of epoxides catalyzed by copper tetrafluoroborate

Recent Advances in the Synthesis of Propargyl Derivatives, and Their Application as Synthetic Intermediates and Building Blocks

The propargyl group is a highly versatile moiety whose introduction into small-molecule building blocks opens up new synthetic pathways for further elaboration. The last decade has witnessed remarkable progress in both the synthesis of propargylation agents and their application in the synthesis and functionalization of more elaborate/complex building blocks and intermediates. The goal of this review is to highlight these exciting advances and to underscore their impact.

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.

Influence of the Support on Propene Oxidation over Gold Catalysts

The epoxidation of propene without forming a substantial amount of byproducts is one of the holy grails of catalysis. Supported Cu, Ag and Au catalysts are studied for this reaction and the activity of the supported metals is generally well understood. On the contrary, limited information is available on the influence of the support on the epoxide selectivity. The reaction of propene with equal amounts of hydrogen and oxygen was tested over gold nanoparticles deposited onto CeO2, TiO2, WO3, γ-Al2O3, SiO2, TiO2-SiO2 and titanosilicate-1. Several metal oxide supports caused further conversion of the synthesized propene oxide. Strongly acidic supports, such as WO3 and titanosilicate-1, catalyzed the isomerization of propene oxide towards propanal and acetone. Key factors for achieving high PO selectivity are having inert or neutralized surface sites, a low specific surface and/or a low density of surface -OH groups. This work provides insights and practical guidelines to which metal oxide support properties lead to which products in the reaction of propene in the presence of oxygen and hydrogen over supported gold catalysts.

Lewis acid catalyzed reactivity switch: pseudo three-component annulation of nitrosoarenes and (epoxy)styrenes

A Lewis acid catalyzed annulation reaction via arene functionalization of nitrosoarenes and C-C cleavage of (epoxy)styrene to provide arylquinolines is reported. The Lewis acid catalyst altered the annulation pattern providing arylquinolines instead of oxazolidines. The reaction with styrene resulted in a mixture of 2,4-diarylquinoline and 4-arylquinoline, while only 3-arylquinoline was formed from the reaction of epoxystyrene.

BF3·OEt2-promoted tandem Meinwald rearrangement and nucleophilic substitution of oxiranecarbonitriles

Introducing a cyano leaving group into epoxides results in BF₃·OEt₂-promoted tandem Meinwald rearrangement and nucleophilic substitution to synthesize arylacetic derivatives from vicinal arylcyanoepoxides.

One-pot synthesis of epoxides from benzyl alcohols and aldehydes

A one-pot synthesis of epoxides from commercially available benzyl alcohols and aldehydes is described. The reaction proceeds through in situ generation of sulfonium salts from benzyl alcohols and their subsequent deprotonation for use in Corey–Chaykovsky epoxidation of aldehydes. The generality of the method is exemplified by the synthesis of 34 epoxides that were made from an array of electronically and sterically varied alcohols and aldehydes.

Strobilurin biosynthesis in Basidiomycete fungi

Strobilurins from fungi are the inspiration for the creation of the β-methoxyacrylate class of agricultural fungicides. However, molecular details of the biosynthesis of strobilurins have remained cryptic. Here we report the sequence of genomes of two fungi that produce strobilurins and show that each contains a biosynthetic gene cluster, which encodes a highly reducing polyketide synthase with very unusual C-terminal hydrolase and methyltransferase domains. Expression of stpks1 in Aspergillus oryzae leads to the production of prestrobilurin A when the fermentation is supplemented with a benzoyl coenzyme A (CoA) analogue. This enables the discovery of a previously unobserved route to benzoyl CoA. Reconstruction of the gene cluster in A. oryzae leads to the formation of prestrobilurin A, and addition of the gene str9 encoding an FAD-dependent oxygenase leads to the key oxidative rearrangement responsible for the creation of the β-methoxyacrylate toxophore. Finally, two methyltransferases are required to complete the synthesis.

Strobilurins are fungal metabolites that inspired the creation of β-methoxyacrylate agricultural fungicides. Here, Nofiani et al. identify the strobilurin biosynthesis gene cluster, encoding a polyketide synthase as well as an FAD-dependent oxygenase for an oxidative rearrangement leading to β-methoxyacrylate formation.

A Computational Examination of the Uncatalyzed Meinwald Rearrangement of Monoterpene Epoxides

Epoxides are relatively reactive compounds and may undergo decomposition or rearrangement reactions at elevated temperatures, and gas chromatographic analysis of essential oils may cause thermal decomposition or rearrangement of epoxide components at gas chromatographic temperatures. Density functional theory (DFT) calculations were carried out using the B3LYP functional at the 6-311++G**//6-31G* level of theory on the Meinwald rearrangements of α-pinene oxide (two different mechanisms leading to trans-pinocamphone and α-campholenal), cis-limonene oxide (leading to trans-dihydrocarvone), trans-limonene oxide (leading to cis-dihydrocarvone), 6,7-epoxymyrcene (leading to 2-methyl-6-methylene-7-octen-3-one), and 1,2-epoxy-2,5-dimethyl-3-vinyl-4-hexene (two mechanisms leading to 5-methyl-3-vinyl-4-hexenal and 8-methyl-5,7-nonadien-2-one). Free energies of activation for the uncatalyzed Meinwald rearrangement of common monoterpene epoxides are relatively large, being of the order of 70 kcal/mol, and are, therefore, not predicted to be important reactions at gas chromatographic temperatures.

A Bioinspired Cyclization Sequence Enables the Asymmetric Total Synthesis of Dictyoxetane

We have developed the first synthesis of the unique oxetane containing diterpene (+)-dictyoxetane. Our retrosynthetic planning was guided by the putative biosynthesis of the unprecedented 2,7-dioxatricyclo[4.2.1.0(3,8)]nonane ring system. A bioinspired 4-exo-tet, 5-exo-trig cyclization sequence enabled the construction of the synthetically challenging dioxatricyclic framework. The overall synthesis proceeds in 15 linear steps from a known and readily available trans-hydrindane fragment. In addition, we were able to realize the first dyotropic rearrangement of an epoxide-oxetane substrate.

Efficient epoxide isomerization within a self-assembled hexameric organic capsule

The supramolecular organic nano-capsule formed by the self-assembly of six resorcin[4]arene units efficiently promotes the isomerization of epoxides (Meinwald isomerization) to the corresponding carbonyl compounds.

Meinwald-type rearrangement of monosubstituted epoxides to methyl ketones using an [Al porphyrin]+[Co(CO)4]−catalyst

A Meinwald-type rearrangement of monosubstituted epoxides to methyl ketones using a well-defined aluminum porphyrin catalyst is reported.

Selective rearrangement of terminal epoxides into methylketones catalysed by a nucleophilic rhodium–NHC–pincer complex

The highly nucleophilic complex 1 isomerises terminal epoxides exclusively to the respective methylketones in very high yield under mild conditions.

An air-stable cationic iridium hydride as a highly active and general catalyst for the isomerization of terminal epoxides

We describe the use of an air-stable iridium hydride catalyst for the isomerization of terminal epoxides into aldehydes with perfect regioselectivity.

Silicon-(thio)urea Lewis acid catalysis

We present a new class of catalysts based on the combination of N,N'-diaryl(thio)ureas and weak silicon Lewis acids (e.g., SiCl(4)). Such silicon-(thio)urea catalysts effectively catalyze the stereospecific rearrangement of epoxides to quaternary carbaldehydes.

Corey-Chaykovsky epoxidation of twisted amides: synthesis and reactivity of bridged spiro-epoxyamines

The first example of a Corey-Chaykovsky epoxidation employing amides as substrates is described. Medium-bridged twisted amides serve as precursors to a family of isolable aminoepoxides. The reactivity of bridged spiro-epoxyamines is also investigated.

Halonium ion-assisted deiodination of styrene-based vicinal iodohydrins followed by rearrangement through phenyl migration

Acid activation of bromate/bromide couple at 0-10 degrees C was found to trigger the deiodination of styrene-based vicinal iodohydrins. Violet coloration of the organic layer was ascribed to formation of IBr. Deiodination was followed by phenyl migration and deprotonation leading to formation of phenyl acetone and 2-phenylpropanal in good yields from 1-iodo-2-phenylpropan-2-ol and 2-iodo-1-phenylpropan-1-ol, respectively. Phenyl acetaldehyde--which was obtained in 92% GC yield from styrene iodohydrin--was also presumably formed in analogous manner. NBS and HOCl too were effective for transformation of styrene iodohydrin into phenyl acetaldehyde.

Epoxide ring-opening and Meinwald rearrangement reactions of epoxides catalyzed by mesoporous aluminosilicates

Mesoporous aluminosilicates efficiently catalyze the ring-opening of epoxides to produce beta-alkoxyalcohols in high yields under extremely mild reaction conditions. These materials also catalyze the corresponding Meinwald rearrangement in non-nucleophilic solvents to give aldehydes which can be trapped in situ to provide the corresponding acetals in an efficient tandem process.