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).     

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.     

Enantioselective synthesis of hydroxylated pyrrolidines via Sharpless epoxidation and olefin metathesis

The enantioselective synthesis of polyhydroxylated pyrrolidines from enantiomerically pure 2,3-epoxy-pent-4-en-1-ol 5 is described herein. The epoxy alcohol, readily available in any configuration by Sharpless epoxidation, was submitted to regioselective C-3 ring-opening with allyl amine, Boc-protection and ring-closing metathesis to yield dehydropyrrole derivative 7. From this key intermediate, 1,4-dideoxy-1,4-imino-d-ribitol (+)-3 and 1,4-dideoxy-1,4-imino-d-allitol (+)-4 were prepared in high yields. The enantiomers of these compounds can be obtained by the same sequence starting from an epoxy alcohol with the opposite configuration.     

Enantioselective synthesis of chiral carbocyclic pyrimidine nucleosides via asymmetric cyclopropanation

An efficient method to construct chiral cyclopropyl pyrimidine carbocyclic nucleoside analogues bearing a quaternary center was developed via asymmetric Michael-initiated cyclopropanation. The axis chirality was observed in cyclopropyl pyrimidine carbocyclic nucleoside analogues for the first time, which was caused by the rotationally restricted NC bond in N-COPh moiety. Using (DHQD)₂AQN as the organocatalyst, diverse cyclopropyl pyrimidine carbocyclic nucleoside analogues were generated in 76–93% yields and 73–96% ee.     

Enantioselective synthesis of gamma-aryl-gamma-butyrolactones by sequential asymmetric epoxidation, ring expansion, and Baeyer-Villiger oxidation

This paper describes an enantioselective synthesis of gamma-butyrolactones, using the N-tolyl-substituted oxazolidinone-containing ketone as catalyst and Oxone as oxidant via a sequential asymmetric epoxidation of benzylidenecyclopropanes, ring expansion, and Baeyer-Villiger oxidation. Up to 91% ee was obtained. Optically active cyclobutanones can also be obtained by suppressing the Baeyer-Villiger oxidation with use of more ketone catalyst and less Oxone.     

Enantioselective synthesis of quaternary stereocenters via a catalytic asymmetric Stetter reaction

A new electron-deficient chiral triazolium salt has been shown to catalyze the formation of quaternary stereocenters by an asymmetric intramolecular Stetter reaction. Pentafluorophenyl substitution on an aminoindanol-derived catalyst affords tertiary ether, thioether, and quaternary stereocenters in typically greater than 90% ee and very good chemical yield. Aromatic and aliphatic aldehydes are equally competent substrates for this reaction. The reaction proceeds at room temperature under mild conditions to provide quaternary stereocenters with functional group relationships that are particularly difficult to access by other methods.     

Enantioselective synthesis of 1-vinyltetrahydroisoquinolines via Pd-catalyzed intramolecular asymmetric allylic amination reactions

1-Vinyltetrahydroisoquinolines serve as versatile intermediates for the synthesis of a variety of naturally occurring isoquinoline alkaloids. 1-Vinyl-6,8-dimethoxytetrahydroisoquinoline 4 and 1-vinyl-5,6,7-trimethoxytetrahydroisoquinoline 6 with >90% ee by means of Pd-catalyzed intramolecular asymmetric allylic amination reactions, using MPN and BOP ligands, developed in our laboratory. The fine-tuning capability of the MPN and BOP ligands has played a significant role in the optimization of enantioselectivity. Interesting substituent effect as well as solvent effect on the product selectivity and enantioselectivity was observed. Plausible mechanisms are proposed, which can accommodate various findings, including the critical importance of the activation of the trifluoroamide moiety through its coordination to the Lewis acidic Pd²⁺ metal center.     

Enantioselective synthesis of a chiral fluoropiperidine via asymmetric hydrogenation of a vinyl fluoride

Asymmetric hydrogenation of a vinyl fluoride derivative gives efficient access to enantioenriched 1,3,4-trisubstituted piperidine 1 with a stereogenic alkyl fluoride center. Extensive catalyst screening across transition metals and chiral ligands identified only one catalyst, a Rh/Walphos complex, that gives high conversion, enantioselectivity and chemoselectivity for olefin reduction over defluorination. The presence of acid additives in the hydrogenation exerts a profound effect on reaction outcome. The results of deuterium labeling studies demonstrate that significant olefin isomerization accompanies the undesired defluorination side-reaction.     

Enantioselective synthesis of 2-alkyl-2-aryl cyclopentanones by asymmetric epoxidation of tetrasubstituted cyclobutylidene olefins and epoxide rearrangement

This letter describes a highly enantioselective epoxidation of tetrasubstituted benzylidenecyclobutanes using glucose-derived ketone as catalyst and oxone as oxidant. The Et₂AlCl promoted rearrangement of the resulting epoxides provides 2-alkyl-2-aryl cyclopentanones with high ees.     

Stereodivergent total asymmetric synthesis of polyhydroxylated pyrrolidines via tandem allylic epoxidation and intramolecular cyclization reactions

Epoxidation of the allylic alcohols 10 and 11 using the VO(acac)₂/t-BuOOH system followed by an intramolecular 5-exo cyclization of the resulting δ-epoxycarbamates 12, 13, 18, and 19 has been shown to provide a general and efficient solution for the asymmetric synthesis of polyhydroxy pyrrolidines. The requisite vicinal amino alcohol functionality was enantio-/regio-selectively installed by the Os-catalyzed asymmetric aminohydroxylation reaction of the designed achiral olefin 6.     

Synthesis of (+)-ambrisentan via chiral ketone-catalyzed asymmetric epoxidation

The synthesis of optically pure (+)-ambrisentan has been achieved from 3,3-diphenylacrylate in four steps with 53% overall yield and >99% ee at the >100 g scale without column purification. The chiral epoxide intermediate was prepared via asymmetric epoxidation with a fructose-derived diacetate ketone as catalyst.     

Enantioselective hydrovinylation via asymmetric counteranion-directed ruthenium catalysis

The first Ru-catalyzed enantioselective hydrovinylation has been realized by using an asymmetric counteranion-directed catalysis strategy. Styrene derivatives react with ethylene in excellent yields and promising enantioselectivity using this approach.     

Enantioselective synthesis of (3-indolyl)glycine derivatives via asymmetric Friedel-Crafts reaction between indoles and glyoxylate imines

Chiral phosphoric acid-catalyzed enantioselective Friedel-Crafts reaction of indoles with ethyl glyoxylate imines was developed. With 10 mol % of the catalyst, the Friedel-Crafts reactions between a wide range of indoles and imines have been carried out, affording optically active (3-indolyl)glycine derivatives with excellent yields and high enantioselectivities (up to 87% ee).     

Enantioselective synthesis of bridgehead hydroxyl bicyclo[2.2.2]octane derivatives via asymmetric allylindation

A recent new strategy for the transformation of mono-dioxolane protected 1,3-cyclohexadione into bridgehead hydroxyl bicyclo[2.2.2]octane derivatives, based on allylindation followed by ozonolysis and intramolecular aldol addition, was modified to include asymmetric allylindation. This enabled the first enantioselective synthesis of (1R,4R,6S)-endo-4-(tert-butyl-dimethyl-silyloxy)-6-hydroxy-bicyclo[2.2.2]octan-2-one and (1S,4S,6R)-endo-4-(tert-butyl-dimethyl-silyloxy)-6-hydroxy-bicyclo[2.2.2]octan-2-one in high enantiomeric excess. Issues concerning the non-reproducibility of the asymmetric allylindation were also addressed.     

C-Galactosylceramide diastereomers via Sharpless asymmetric epoxidation chemistry

C-Glycoside analogs of α-galactosylceramide (KRN7000) were synthesized in 19 linear steps with Sharpless asymmetric epoxidation as a key reaction. Opening of a hydroxy epoxide with sodium azide provided an anti vicinal azido diol with inversion of configuration at the azide-bearing carbon while opening with Ti(O-i-Pr)₂(N₃)₂ gave syn vicinal azido diol with retention. The latter, unusual outcome could be rationalized either by invoking Ti-catalyzed intramolecular double S_N2 inversion or by epoxide opening/intramolecular delivery of azide from the Ti complex.     

Enantioselective total synthesis of idesolide via NaHCO3-promoted dimerization

The enantioselective total synthesis of idesolide has been accomplished in 20% overall yield from a known allylic alcohol by a nine-step sequence involving the Sharpless asymmetric epoxidation as the source of chirality and an efficient NaHCO₃-promoted dimerization of the monomeric form of idesolide as the key transformation.     

Enantioselective synthesis of Garner's aldehyde by asymmetric hydroformylation

Both enantiomers of Garner's aldehyde (3) are prepared from the same alkene 4 by catalytic asymmetric hydroformylation.     

Enantioselective synthesis of (S)-salmeterol via asymmetric reduction of azidoketone by Pichia angusta

An efficient enantioselective route to (S)-salmeterol involving asymmetric reduction of an azidoketone intermediate to an azido alcohol by Pichia angusta is described.