A diacetate ketone-catalyzed asymmetric epoxidation of olefins

Total Syntheses and Cytotoxicity Evaluation of Coryaurone A and Representative Analogues

The first total synthesis of coryaurone A, which was originally obtained from Psoralea corylifolia L., was achieved via an efficient route with the longest linear sequence of six steps from the commercially available 6-hydroxy-2H-benzofuran-3-one in 37% overall yield. A series of representative analogues were synthesized from the same starting material in 4-7 steps with overall yields of 27-56%. The cytotoxicities of these compounds against the leukemia cell line HL-60 and the colon cancer cell line SW480 were determined. Among them, compounds 12, 14, 21, and 27 exhibited different levels of cytotoxic activity, which were greater than the positive control cisplatin.

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.

Electrophilic Epoxidation of α,β-Unsaturated Oximes with Dioxiranes and Ring Opening of the Epoxides

α,β-Unsaturated oximes underwent electrophilic epoxidation with in-situ-generated dimethyldioxirane to give the corresponding epoxides in good yields. This reaction is an example of "carbonyl umpolung" by transformation of α,β-unsaturated ketones to their oximes. Nucleophilic ring-opening reactions of the epoxides afforded α-substituted products. Shi asymmetric epoxidation of the oximes proceeded with moderate enantioselectivity.

Studies on asymmetric total synthesis of (-)-β-hydrastine via a chiral epoxide ring-opening cascade cyclization strategy

Herein, facile and enantioselective approaches to synthesize the core phthalide tetrahydroisoquinoline scaffold of (-)-β-hydrastine via both a CF3COOH-catalyzed (86% ee) and KHMDS-catalyzed (78% ee) epoxide ring-opening/transesterification cascade cyclization from chiral epoxide under very mild conditions are described. The key elements include a highly enantioselective epoxidation using the Shi ketone catalyst and an intramolecular CF3COOH-catalyzed cascade cyclization in one pot, and a late-stage C-3' epimerization under MeOK/MeOH conditions as the key steps to achieve the first total synthesis of (-)-β-hydrastine (up to 81% ee).

Total Synthesis of the Marine Ladder Polyether Gymnocin B

Herein, we report the first total synthesis of marine ladder polyether gymnocin B (1) based on a two-phase strategy. In Phase I, inspired by the proposed biosynthesis, epoxide-opening cascades assemble 10 out of 15 cyclic ether rings making up the molecular core. In the subsequent Phase II, coalescence elevates the molecular complexity further by coupling of these subunits. Our two-phase synthetic approach significantly improved the step efficiency of the synthesis of this class of natural products.

Novel chiral Schiff base/Ti(iv) catalysts for the catalytic asymmetric epoxidation of N-alkenyl sulfonamides

A novel highly enantioselective Schiff base/Ti(iv) catalyst for the catalytic asymmetric epoxidation of a series of N-alkenyl sulfonamides under mild conditions was described.

Synthesis of Novel Spiro[Oxirane-Pyrrolizin] Derivatives by Epoxidation of α,β-Unsaturated Pyrrolizinone with Cyclohexylidenebishydroperoxide

A simple epoxidation of E-2-(arylmethylidene)-2,3-dihydro-1H-pyrrolizin-1-ones was accomplished using cyclohex ylidenebishydroperoxide as the oxygen source. A number of 3-arylspiro[oxirane-2,2′-pyrrolizin]-1′(3′H)-ones were obtained in moderate to good yields.

Efficient magnetic and recyclable SBILC (supported basic ionic liquid catalyst)-based heterogeneous organocatalysts for the asymmetric epoxidation of trans-methylcinnamate

A green heterogeneous catalytic alternative was developed for the asymmetric epoxidation of trans-methylcinnamate to (2R,3S)-phenyl glycidate.

Function-Oriented Investigations of a Peptide-Based Catalyst that Mediates Enantioselective Allylic Alcohol Epoxidation

We detail an investigation of a peptide-based catalyst 6 that is effective for the site- (>100:1:1) and enantioselective epoxidation (86% ee) of farnesol. Studies of the substrate scope exhibited by the catalyst are included, along with an exploration of optimized reaction conditions. Mechanistic studies are reported, including relative rate determinations for the catalyst and propionic acid, a historical perspective, truncation studies, and modeling using NMR data. Our compiled data advances our understanding of the inner workings of a catalyst that was identified through combinatorial means.

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.

Asymmetric synthesis of anti-β-amino-α-hydroxy esters via dynamic kinetic resolution of β-amino-α-keto esters

A method for the asymmetric synthesis of enantioenriched anti-α-hydroxy-β-amino acid derivatives by enantioconvergent reduction of the corresponding racemic α-keto esters is presented. The requisite α-keto esters are prepared via Mannich addition of ethyl diazoacetate to imines followed by oxidation of the diazo group with Oxone. Implementation of a recently developed dynamic kinetic resolution of β-substituted-α-keto esters via Ru(II)-catalyzed asymmetric transfer hydrogenation provides the title motif in routinely high diastereo- and enantioselectivity.

A new approach to the side chain formation of 24-alkyl-22-hydroxy steroids: application to the preparation of early brassinolide biosynthetic precursors

A new synthetic route to 22S-hydroxy-24R-methyl steroids has been developed and applied for the preparation of cathasterone, (22S)-hydroxycampesterol, and 6-deoxocathasterone, which are precursors in the early stages of the biosynthesis of brassinolide. The construction of the steroid side chain with the correct stereochemistry at C-24 is based on the use of Claisen rearrangement. The introduction of the 22-hydroxyl group has been achieved by epoxidation of the Δ(22)-double bond, nucleophilic opening of the intermediate mesyl epoxide with sodium sulfide, and desulfurization of the formed tetrahydrothiophenes with Raney nickel.

Iron-catalyzed asymmetric epoxidation of β,β-disubstituted enones

The combination of Fe(OTf)(2) and novel phenanthroline ligands enables the catalytic asymmetric epoxidation of acyclic β,β-disubstituted enones, which have been a heretofore inaccessible substrate class. The reaction provides highly enantioenriched α,β-epoxyketones (up to 92% ee) that can be further converted to functionalized β-ketoaldehydes with an all-carbon quaternary center.

Scalable and efficient synthesis of the mycolactone core

A highly efficient, scalable, and stereoselective synthesis of the mycolactone core is reported. The synthesis consists of 14 longest linear steps, with 19% overall yield.

Asymmetric epoxidation of fluoroolefins by chiral dioxirane. Fluorine effect on enantioselectivity

The asymmetric epoxidation of various fluoroolefins has been studied using chiral ketone catalyst, and up to 93% ee was achieved with fructose-derived ketone 1.

Asymmetric total synthesis of (-)-plicatic acid via a highly enantioselective and diastereoselective nucleophilic epoxidation of acyclic trisubstitued olefins

The first total synthesis of (-)-plicatic acid has been achieved by a concise and enantioselective route. In this synthesis, a conceptually new strategy featuring an asymmetric epoxidation-intramolecular epoxy-ring-opening Friedel-Crafts reaction sequence was developed for the stereoselective construction of the 2,7'-cyclolignane skeleton bearing contiguous quaternary-quaternary-tertiary stereocenters. The implementation of this strategy was enabled by the development of a modified protocol for the Seebach epoxidation with TADOOH, which affords an unprecedented, highly enantioselective and diastereoselective epoxidation with a range of alpha-carbonyl-beta-substituted acrylates 3.