Theoretical study on the selectivity of asymmetric sulfur ylide epoxidation reaction

[structure: see text] We report the first theoretical studies on the asymmetric sulfonium ylide epoxidation reaction using a chiral sulfide that successfully reproduces the experimentally determined high enantiomeric excess. Calculations at the DFT level suggest that the transition states for the addition of the sulfonium ylide to benzaldehyde have energies which account for the observed enantioselectivity.     

Chiral fluoro ketones for catalytic asymmetric epoxidation of alkenes with oxone

Two structurally dissimilar, chiral fluoro ketones have been prepared and their potential as enantioselective catalysts for asymmetric epoxidation with Oxone has been evaluated. The tropinone-based ketone (-)-5 was easily prepared and showed excellent reactivity but only modest enantioselectivity. The biphenyl-based ketone (-)-6 was prepared in a somewhat lengthy synthesis (along with its monofluoro and geminal fluoro analogues). This ketone exhibited only modest reactivity; 30 mol % of (-)-6 was needed to bring about complete conversion in a reasonable time. The enantioselectivity of this catalyst was generally much higher, but again very substrate dependent.     

Stereocontrolled aziridination of imines via a sulfonium ylide route and a mechanistic study

The reaction of N-diphenylphosphinoyl imines 1 with [3-(trimethylsilyl)allyl]dimethylsulfonium bromide (5) in the presence of NaH at room temperature predominantly gave trans-vinylaziridines 4. On the other hand, cis-vinylaziridines 4 were the main products when the preformed ylide prepared from the reaction of [3-(trimethylsilyl)allyl]diphenylsulfonium perchlorate (6) was reacted with the same imines 1 at low temperature. trans-Aziridines were also obtained when imines 1 and sulfinimines 9 were reacted with N,N-dimethylacetamide-2-dimethylsulfonium bromide (7) in the presence of a base, respectively. A mechanistic study showed that the stereochemistry of these reactions was controlled by the reactivity of the imines and ylides. A higher reactivity of imines and ylides favors the formation of cis-aziridines, whereas a lower reactivity leads to trans-products.     

Diastereoselective rhodium(II)-catalyzed sulfonium ylide formation-[2,3]-sigmatropic rearrangement reaction of chiral non-racemic allylic sulfides

The tandem sulfonium ylide formation-[2,3]-sigmatropic rearrangement reaction of chiral non-racemic secondary allylic sulfides, (E)-9 and (Z)-10, is found to proceed with high diastereocontrol. The C-5 stereocenter bearing the sulfide group is essential for high diastereoselectivity in the reaction. Transition state conformers are proposed to explain the high diastereoselectivity in the formation of the diastereomeric products, 18a and 18b. The method is applied to the synthesis of (R)-4-(4-chlorophenyl)-2-butyrolactone. Modest enantioselectivity (63% ee) was achieved and this is attributed to partial racemization during the formation of the secondary allylic sulfide 22.     

A catalytic asymmetric synthesis of chiral glycidic acid derivatives through chiral dioxirane-mediated catalytic asymmetric epoxidation of cinnamic acid derivatives

A novel and practical asymmetric synthesis of chiral glycidic acid derivatives involving methyl (2R,3S)-3-(4-methoxyphenyl)glycidate ((2R,3S)-2a), a key intermediate for diltiazem hydrochloride (1), was developed. Treatment of methyl (E)-4-methoxycinnamate ((E)-3a) with chiral dioxirane, generated in situ from a catalytic amount (5 mol %) of an 11-membered C(2)-symmetric binaphthyl ketone (R)-7a, provided (2R,3S)-2a in 92% yield and 80% ee. Other cinnamic acid esters and amides were epoxidized by the use of the same procedure to give the corresponding chiral glycidic acid derivatives with up to 95% yield and 92% ee. Higher enantioselectivities in the asymmetric epoxidation of (E)-cinnamates than that of (E)-stilbene derivatives were observed and were proposed to be attributed to a dipole-dipole repulsion between oxygen atoms of an ester group in the cinnamates and those of the lactone moieties in the binaphthyl dioxirane.     

Chiral ligand-controlled catalytic asymmetric epoxidation of α,β-unsaturated carbonyl compounds with peroxide

Asymmetric epoxidation reaction of α,β-unsaturated carbonyl compounds with alkylperoxide was catalyzed by an external chiral tridentate aminodiether-lithium peroxide giving epoxides with good enantiomeric excess. Slow addition of alkylhydroperoxide was beneficial for a catalytic asymmetric reaction. Lone pair electron-differentiating coordination of a carbonyl oxygen to lithium is another critical factor for high enantioselectivity.     

Highly efficient catalytic asymmetric epoxidation of allylic alcohols by an oxovanadium-substituted polyoxometalate with a regenerative TADDOL-derived hydroperoxide

The oxovanadium(IV) sandwich-type POM catalyzes the chemo-, regio-, and stereoselective epoxidation of allylic alcohols by chiral hydroperoxides with very high catalytic efficiency (up to 42 000 TON), a potentially valuable oxidation for the development of sustainable processes. By using the sterically demanding, TADDOL-derived hydroperoxide TADOOH as the chiral oxygen source, enantiomeric ratios (er) of up to 95:5 have been achieved.     

Enantioselective syntheses of aeruginosin 298-A and its analogues using a catalytic asymmetric phase-transfer reaction and epoxidation

We developed a versatile synthetic process for aeruginosin 298-A as well as several attractive analogues, in which all stereocenters were controlled by a catalytic asymmetric phase-transfer reaction and epoxidation. Furthermore, drastic counteranion effects in phase-transfer catalysis were observed for the first time, making it possible to three-dimensionally fine-tune the catalyst (ketal part, aromatic part, and counteranion).     

Arabinose-derived ketones as catalysts for asymmetric epoxidation of alkenes

[reaction: see text] Readily available arabinose-derived ketones, containing a tunable butane-2,3-diacetal as the steric blocker, displayed increasing enantioselectivity (up to 90% ee) with the size of the acetal alkyl group in catalytic asymmetric epoxidation of trans-disubstituted and trisubstituted alkenes. The stereochemical communication between our ketone catalysts and the alkene substrates is mainly due to steric effect, and electronic effect involving pi-pi interaction between phenyl groups of substrate and of catalyst did not appear to be operative in our system.     

Practical synthesis of Shi's diester fructose derivative for catalytic asymmetric epoxidation of alkenes

[reaction: see text] A practical synthesis of Shi's diester 3 for catalytic asymmetric epoxidations has been developed. The catalyst has been prepared in multigram quantities from D-fructose in four steps with a 66% overall yield. Efficiency, cost, and selectivity aspects of the reagents involved for its preparation have been taken care of during its preparation. The workup procedures have been simplified to the bare minimum, rendering a very practical preparation method. The well-known high efficiency of this catalyst 3 in the epoxidation of alpha,beta-unsaturated carbonyl compounds has also proved to be high in unfunctionalized alkenes.     

Synthesis of quinine and quinidine using sulfur ylide-mediated asymmetric epoxidation as a key step

The epoxidation of meroquinene aldehyde with a chiral sulfur ylide as the key step in the synthesis of quinine and quinidine is described. The epoxidation reactions proceed under reagent control with high selectivity and good yield. The effect of sulfide and ylide substituents on the stereochemical outcome of the reaction is discussed.     

催化量的氧化钙-硅胶对Sharpless烯丙醇不对称环氧化的影响

当在Sharpless试剂中加入催化量的金属氢化物和硅胶后, 烯丙醇的不对称环氧化反应时间大为缩短, 而化学和光学产率不受影响。着重讨论了氢化钙和硅胶对Sharpless烯丙醇不对称环氧化的作用。     

Enantioselective synthesis of 2,2-disubstituted terminal epoxides via catalytic asymmetric Corey-Chaykovsky epoxidation of ketones

Catalytic asymmetric Corey-Chaykovsky epoxidation of various ketones with dimethyloxosulfonium methylide using a heterobimetallic La-Li(3)-BINOL complex (LLB) is described. The reaction proceeded smoothly at room temperature in the presence of achiral phosphine oxide additives, and 2,2-disubstituted terminal epoxides were obtained in high enantioselectivity (97%-91% ee) and yield ( > 99%-88%) from a broad range of methyl ketones with 1-5 mol% catalyst loading. Enantioselectivity was strongly dependent on the steric hindrance, and other ketones, such as ethyl ketones and propyl ketones resulted in slightly lower enantioselectivity (88%-67% ee).     

Catalytic asymmetric deprotonation of phosphine boranes and sulfides as a route to P-stereogenic compounds

A comparison between phosphine boranes and sulfides in their catalytic asymmetric deprotonation using organolithiums and sub-stoichiometric amounts of (-)-sparteine has revealed superior catalytic efficiency in the phosphine sulfide deprotonation.     

Catalytic asymmetric epoxidation of alkenes with arabinose-derived uloses

Four l-erythro-2-uloses were readily prepared from l-arabinose via a reaction sequence involving Fischer glycosidation, acetalization and oxidation. Bulky steric sensors at the anomeric center could enhance the stereoselectivity of the dioxirane epoxidation and one of the uloses performed with good enantioselectivity towards trans-stilbene (up to 90% ee). However, the catalysts decomposed during the epoxidation and the maximum chemical yield was only 13% under the basic conditions. Three l-threo-3-uloses could overcome the decomposition problem based on the electron withdrawing effect of the ester group(s) α to the ketone functionality. The best chemical yield was up to 93% using a ketone with two flanking ester groups. One of the improved uloses displayed moderate enantioselectivity towards trans-disubstituted and trisubstituted alkenes (40-68% ee).     

Zinc-mediated asymmetric epoxidation of nitro alkenes with oxygen

The asymmetric epoxidation of nitro alkenes using oxygen in the presence of diethylzinc and N-methyl pseudo-ephedrine as a chiral additive is reported. This method provides an access to 3-substituted trans-2-nitro oxiranes of excellent diastereomeric purity (de≥98%) and with medium to good enantiomeric excesses (ee=36–82%).     

Design and synthesis of chiral poly(binaphthyl salen zinc complex) and application of the asymmetric field based on its helical conformation to a catalytic asymmetric reaction

According to a rational design of helices (i.e., fusing C₂ chiral binaphthyl units and metallosalen complexes inevitably results in the formation of helical polymers), chiral poly(binaphthyl salen zinc complex)es ( 3 -Zn) were synthesized from an (R)-3,3′-diformylbinaphthol derivative, α,ω-diamines, and zinc acetate or diethylzinc. Their helical structures were well supported by their infrared, ultraviolet, and circular dichroism spectra, in addition to MM calculations and a CPK model study. The catalysis of 3 -Zn during the asymmetric addition of diethylzinc to aldehydes was investigated. In the presence of 5 unit mol % 3 -Zn, diethylzinc reacted with benzaldehyde to yield 1-phenyl-1-propanol in high yields (∼100%) and with high enantioselectivity (∼95%). The asymmetric yield of 1-phenyl-1-propanol increased significantly as the temperature was lowered, whereas both the asymmetric yield and the absolute configuration were drastically changed as the structure of the diamine unit of 3 -Zn was varied. Several aromatic aldehydes were converted into their corresponding alcohols with high enantioselectivity. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4693–4703, 2004