Enantioselective olefin epoxidation using homologous amine and iminium catalysts—a direct comparison

Homologous biphenyl and (diastereomeric) binaphthyl tertiary azepines and quaternary iminium salts were prepared from (+)-(S,S)-l-acetonamine. Both the amines and iminium ions behave as effective catalysts for the enantioselective epoxidation of unfunctionalized olefins (ee up to 83%).     

Enantioselective olefin epoxidation using novel biphenyl and binaphthyl azepines and azepinium salts

Homologous biphenyl and (diastereomeric) binaphthyl tertiary azepines and quaternary iminium salts were prepared from (S)- and (R)-3,3-dimethylbutan-2-amine. Both the amines and iminium ions behave as effective catalysts for the enantioselective epoxidation of unfunctionalized olefins (ee up to 87%).     

Organocatalysis of asymmetric epoxidation mediated by iminium salts under nonaqueous conditions

The first nonaqueous conditions are described for catalytic asymmetric epoxidation mediated by iminium salt organocatalysts, providing ee values of up to 67%.     

Enantioselective total synthesis of (+)-scuteflorin A using organocatalytic asymmetric epoxidation

We report the first enantioselective total synthesis of (+)-scuteflorin A in 14% overall yield, employing a chiral iminium salt to effect an organocatalytic asymmetric epoxidation of xanthyletin in >99% ee as the key step.     

On the enantioselective olefin epoxidation by doubly bridged biphenyl azepine derivatives--mixed tropos/atropos chiral biaryls

Diastereomeric doubly bridged biphenyl azepines, atropos at 20 degrees C and tropos at 80 degrees C, are precursors to effective iminium organocatalysts that are employed in the enantioselective epoxidation of prochiral olefins (up to 85% ee).     

Enantioselective iminium-catalyzed epoxidation of hindered trisubstituted allylic alcohols

The reactivity of diastereomeric biaryl iminium cations made of a (Ra)-5,5′,6,6′,7,7′,8,8′-octahydrobinaphthyl core and exocyclic appendages derived from (S)- or (R)-3,3-dimethylbutyl-2-amine was investigated with hindered trisubstituted allylic alcohols—a class of alkenes which had not been previously studied in detail in epoxidation reactions with cyclic iminium catalysts (ee up to 98%). Surprisingly, generally strong matched/mismatched effects are observed not only in terms of reactivity but also on the enantioselectivity of the reaction (Δee up to 16%). Also, for the most hindered substrates, two sets of reaction conditions were tested in a preliminary study and little advantage was found in running reactions in MeCN/water instead of CH₂Cl₂/water/18-C-6. In any case, the presence of the hydroxyl group did not reveal any anchimeric effect.     

Asymmetric epoxidation of olefins catalyzed by chiral iminium salts generated in situ from amines and aldehydes

[reaction: see text] A new approach for catalytic asymmetric epoxidation of olefins was developed that utilized chiral iminium salts, generated in situ from chiral amines and aldehydes, as catalysts. Epoxidation reactions can be conducted with 20 mol % of amines and aldehydes. The enantioselectivity of epoxides can be up to 65%. This modular approach obviates the difficulties inherent in the preparation and isolation of unstable exocyclic iminium salts.     

A cyclodextrin-modified ketoester for stereoselective epoxidation of alkenes

A beta-cyclodextrin-modified ketoester 2 was prepared by covalent attachment of a reactive ketone moiety to beta-cyclodextrin. Treatment of 2 with Oxone as terminal oxidant would produce CD-substituted dioxirane, which can effect stereoselective alkene epoxidation. The 2-mediated (S)-alpha-terpineol epoxidations proceeded to give terpineol oxides in high yields, and the stereoselectivities (i.e., cis-/trans-epoxide ratio) decreased from 2.5:1 to 1:1.2 with increasing steric bulkiness of the terpenes. This steric-dependent stereoselectivity can be understood based on different binding geometries of the 2/terpene inclusion complexes according to the (1)H NMR titration and 2D ROESY experiments. Enantioselective epoxidation of styrenes has also been achieved with 2 as catalyst (20-50 mol %) in aqueous acetonitrile solution, and up to 40% ee was obtained in 4-chlorostyrene epoxidation at 0 degrees C. Similar enantioselectivities were also obtained for the 2-mediated epoxidation of 1,2-dihydronaphthalene (37% ee), 4-chlorostyrene (36% ee), and trans-stilbene (31% ee).     

Asymmetric organocatalysis of epoxidation by iminium salts under non-aqueous conditions

Three iminium salts have been tested as catalysts for the asymmetric epoxidation of simple alkenes under non-aqueous conditions in various solvents, at varying temperatures, employing tetraphenylphosphonium monoperoxybisulfate (TPPP) as the stoichiometric oxidant. Enantiomeric excesses of up to 89% have been observed.     

醛和胺原位生成的手性亚胺盐催化的不对称环氧化

光学纯的环氧化物是合成许多药物和天然产物的重要中间体并构成不对称碳碳键的重要合成子 ,因此寻找不对称环氧化反应体系总是有机化学研究的重点和热点[1～ 3 ] .从 Sharpless[4 ] 到 Jacobsen[5] 的手性金属配合物催化体系 ,到杨丹 [6] 和施以安[7] 的手性有机酮催化体系 ,都取得了突破性的进展 ,并在天然产物、药物合成中得到了应用 [1,2 ,8] .但这些体系都有其局限性 ,每一类体系只能适用于某一类烯烃 ,因此开拓适用性更广的不对称环氧化催化剂 ,仍然是对有机化学工作者的挑战 .近年来 ,一种新的体系 ,即用手性亚胺盐或者手性亚胺盐氧…     

beta-peptides as catalysts: poly-beta-leucine as a catalyst for the Juliá-Colonna asymmetric epoxidation of enones

Poly-beta-leucines have been evaluated as catalysts for the Juliá-Colonna asymmetric epoxidation of enones; the beta 3-isomer was found to be an effective catalyst for the epoxidation of chalcone (70% ee) and some analogues.     

Organocatalytic asymmetric epoxidation reactions in water-alcohol solutions

The diastereo- and enantioselective organocatalytic epoxidation of alpha,beta-unsaturated aldehydes in aqueous solutions is presented. By the screening of the reaction conditions for the epoxidation of cinnamic aldehyde applying hydrogen peroxide as the oxidant and 2-[bis-(3,5-bis-trifluoromethyl-phenyl)-trimethylsilanyloxy-methyl]-pyrrolidine as the catalyst, a highly stereoselective reaction has been developed. The scope of the diastereo- and enantioselective organocatalytic epoxidation in aqueous solutions is documented by the asymmetric epoxidation of alpha,beta-unsaturated aldehydes with enantioselectivities up to 96% ee.     

Organocatalysis of asymmetric epoxidation mediated by iminium salts: comments on the mechanism

Non-aqueous conditions developed for catalytic asymmetric epoxidation mediated by iminium salt organocatalysts have allowed NMR spectroscopy to be carried out on the reaction mixtures for the first time.     

High enantioselectivities in an (E)-alkene epoxidation by catalytically active chromium salen complexes. Insight into the catalytic cycle

[structure: see text]. The epoxidation of (E)-beta-methylstyrene mediated by an oxochromium salen complex yields the epoxide in 92% ee in stoichiometric mode, the highest ee yet reported for a metal-mediated epoxidation of an (E)-alkene. The effect of added donor ligands, previously substantial, has reached a ceiling with this complex. In catalytic mode a slightly reduced ee and higher yield is obtained, indicating both the presence of a second oxidation cycle and that the major oxidant reacts with its reduced form.     

Titanium Salalen Catalyzed Enantioselective Benzylic Hydroxylation

The titanium complex of the cis-1,2-diaminocyclohexane (cis-DACH) derived Berkessel-salalen ligand is a highly efficient and enantioselective catalyst for the asymmetric epoxidation of terminal olefins with hydrogen peroxide (“Berkessel-Katsuki catalyst”). We herein report that this epoxidation catalyst also effects the highly enantioselective hydroxylation of benzylic C−H bonds with hydrogen peroxide. Mechanism-based ligand optimization identified a novel nitro-salalen Ti-catalyst of the highest efficiency ever reported for asymmetric catalytic benzylic hydroxylation, with enantioselectivities of up to 98 % ee, while overoxidation to ketone is marginal. The novel nitro-salalen Ti-catalyst also shows enhanced epoxidation efficiency, as evidenced by e.g. the conversion of 1-decene to its epoxide in 90 % yield with 94 % ee, at a catalyst loading of 0.1 mol-% only.     

Enantioselective manganese-porphyrin-catalyzed epoxidation and C-H hydroxylation with hydrogen peroxide in water/methanol solutions

The asymmetric epoxidation of alkene and hydroxylation of arylalkane derivatives by H(2)O(2) to give optically active epoxides (enantiomeric excess (ee) up to 68%) and alcohols (ee up to 57%), respectively, were carried out in water/methanol solutions using chiral water-soluble manganese porphyrins as catalysts.     

Asymmetric organocatalytic epoxidation of alpha,beta-unsaturated aldehydes with hydrogen peroxide

The first asymmetric organocatalytic epoxidation of alpha,beta-unsaturated aldehydes is presented. A chiral bisaryl-silyl-protected pyrrolidine acts as a very selective epoxidation organocatalyst using simple oxidation agents, such as hydrogen peroxide and tert-butyl hydroperoxide. The asymmetric epoxidation reactions proceed under environmental friendly reaction condition in, for example, water mixtures of alcohols, and the scope of the reaction is demonstrated by the formation of optically active alpha,beta-epoxy aldehydes in high yields and enantioselectivities >94% ee. Furthermore, the direct synthesis of the sex pheromone from an acaric mite by asymmetric epoxidation of citral is presented.     

A synthetic study of briarane-type marine diterpenoid, pachyclavulide B

Enantioselective preparation of the six-membered ring of pachyclavulide B, a briarane-type diterpenoid, was achieved. The key step is desymmetrization of an achiral symmetric substrate by enantiogroup- and diastereoselective epoxidation. The asymmetric epoxidation proceeded with up to 99% de and 94% ee.     

An N-aryl-substituted oxazolidinone-containing ketone-catalyzed asymmetric epoxidation with hydrogen peroxide as the primary oxidant

Asymmetric epoxidation of various olefins with an N-aryl-substituted oxazolidinone-containing ketone as catalyst and hydrogen peroxide as the primary oxidant has been investigated, and up to 96% ee was obtained.     

Enantioselective epoxidation of terminal olefins by chiral dioxirane

[see reaction]. This paper describes an enantioselective epoxidation of terminal olefins using chiral ketone 3 as catalyst and Oxone as oxidant. Up to 85% ee has been obtained.