Regio- and stereoselective multisubstituted enol ester synthesis

Regio- and stereoselective cohalogenation of alkynes with NXS (X = Br, I) was achieved, and the stereoselectivity of the resulting alkenes was dependent on the substituent on the alkyne. Cohalogenation and successive cross-coupling gave multisubstituted enol esters in a one-pot process.     

Highly stereoselective synthesis of anti-N-protected-alpha-amino epoxides

A simple and efficient method for the synthesis of anti-N-protected amino epoxides from carbamate-protected amino acids is described. The two key steps are the monobromination of a beta-ketoester and chelation-controlled reduction of a bromomethyl ketone intermediate. Good overall yields, high diastereoselectivity, and excellent functional group compatibility are characteristic.     

Enantioselective synthesis of epoxides by alpha-deprotonation--electrophile trapping of achiral epoxides

Enantioselective alpha-deprotonation of achiral epoxides 1, 21, and 26 using organolithiums in the presence of (-)-sparteine 2 and subsequent electrophile trapping gives access to enantioenriched trisubstituted epoxides 9-17, 22, 23, 27 and 28 (in up to 86% ee).     

Enantioselective enol lactone synthesis under double catalytic conditions

[reaction: see text] The reaction of dimedone with 1-(2-alkenoyl)-4-bromo-3,5-dimethylpyrazoles in THF, catalyzed by catalytic amounts of both DBFOX/Ph-nickel(II) perchlorate trihydrate and 2,2,6,6-tetramethylpiperidine, in the presence of acetic anhydride in THF produces the corresponding enol lactones in high enantioselectivities through enantioselective Michael additions followed by cyclization with removal of the pyrazole auxiliary. Other related nucleophile precursors can be successfully applied in the enantioselective enol lactone synthesis under the double catalytic conditions.     

Regio- and stereoselective synthesis of aziridino epoxides from cyclic dienes

Two different routes for the regio- and stereoselective synthesis of aziridino epoxides from cyclic dienes have been explored. The first strategy involves regiospecific aziridination of cyclic diene derivatives and subsequent epoxidation with m-CPBA to yield cis-aziridino epoxides as major products. The second strategy utilizes regiospecific epoxidation of cyclic diene derivatives followed by Sharpless aziridination to provide exclusively trans-aziridino epoxides. Synthesis of both enantiomers of cis-aziridino epoxides from (R)-(-)- and (S)-(+)-carvones are also reported.     

Chiral Lewis acid catalyzed resolution of racemic enol ester epoxides. Conversion of both enantiomers of an enol ester epoxide to the same enantiomer of acyloxy ketone

This paper describes an efficient kinetic resolution of racemic enol ester epoxides via a chiral Lewis acid catalyzed rearrangement. Both enantiomerically enriched enol ester epoxides and alpha-acyloxy ketones can be obtained through this resolution. A positive nonlinear effect is observed in this process. By taking advantage of the mechanistic duality in acid-catalyzed enol ester epoxide rearrangement, we can completely convert a racemic enol ester epoxide into an enantiomerically enriched alpha-acyloxy ketone by treatment with a catalytic amount of a chiral Lewis acid followed by a catalytic amount of an achiral protic acid.     

Enantioselective synthesis of preclavulone A and its methyl ester

A highly enantioselective synthesis of the (8S,12S)-enantiomer of preclavulone A and its methyl ester is described featuring the Julia protocol for installing the (Z)-double bond in the lower chain. This procedure is suitable for the preparation of labeled preclavulone analogues for biosynthetic studies on marine clavulones.     

Phosphonyl radical addition to enol ethers. The stereoselective synthesis of cyclic ethers

Addition of diethyl phosphite or diethyl thiophosphite to enol ethers, in the presence of a radical initiator, results in the regioselective synthesis of organophosphonate or phosphonothioate derivatives, respectively, under mild conditions. This method can be applied to the stereoselective formation of substituted tetrahydrofurans and tetrahydropyrans, on cyclisation of vinyl ethers bearing unsaturated side chains.     

Enantioselective hydrolyses with Yarrowia lipolytica: a versatile strain for esters,enol esters,epoxides, and lactones

Racemic secondary esters 1–3, γ-lactones 8–9, and styrene oxide 7 are kinetically resolved via hydrolysis with Yarrowia lipolytica YL2 strain. The enantioselective hydrolysis of prochiral enol esters 4–6 to the corresponding homochiral carbonyl compounds 13–15 is also described. Subsequent reduction of the ketone 13 and of the aldehyde 15 can be avoided using lyophilised cells.     

Alkylation epoxide rearrangement. Application to stereoselective synthesis of chiral pheromone epoxides.

An approach is described for the stereospecific conversion of threo and erythro 1,2-epoxy-3-alkanol tosylates to cis and trans internal epoxides, respectively. The method is illustrated by the synthesis of chiral epoxides, including insect pheromones.     

Enantioselective RH-catalyzed hydrogenation of enol acetates and enol carbamates with monodentate phosphoramidites

[reaction: see text] Monodentate phosphoramidites, in particular PipPhos and its octahydro analogue, are excellent ligands for the rhodium-catalyzed asymmetric hydrogenation of aromatic enol acetates, enol carbamates, and 2-dienol carbamates up to 98% ee. These latter substrates were hydrogenated selectively to the carbamates of the allyl alcohol.     

Solvolytic hydroperoxide rearrangements V. A stereoselective synthesis of trans-fused butyrolactones.

Stereoselective rearrangement of cis-5-(4-methoxybenzyl)-spiro[5,2]-octan-4-ol cis-1 in acidified THF-90% H₂O₂ affords 9-oxabicyclo[6.2.1]nonyl hydroperoxides 7 and 8 which are converted to medium-ring, keto-butyrolactones 2 and 3.     

Enantioselective total synthesis of both diastereomers of preclavulone-A methyl ester

The enantioselective total synthesis of preclavulone-A methyl ester and its diastereomer was achieved from enantiomerically pure 5 in a stereocontrolled manner. The absolute stereochemistry of naturally occurring preclavulone-A methyl esters was determined by comparison of the [α]_D value.     

Enantioselective ester synthesis in the presence of optically active polymers

Enantiomer-differentiating ester syntheses from acid salts, chlorides, and an anhydride were studied in the presence of optically active polyamines. Enantiomer selection occurred in reactions of a racemic salt with an achiral alkyl bromide and vice versa, which is good evidence of the importance of activation of the carboxylate group by complexation of the counterion and activation of alkyl bromide by the formation of a quaternary salt with polyamine under solid–liquid, phase-transfer reaction conditions. Only small optical activity was observed in the products of acid anhydride or chloride with alcohol.     

Total synthesis of capsanthin and capsorubin using Lewis acid-promoted regio- and stereoselective rearrangement of tetrasubsutituted epoxides

The synthesis of capsanthin 1 and capsorubin 2 was accomplished via the C(15)-cyclopentyl ketone 6 prepared by Lewis acid-promoted regio- and stereoselective rearrangement of the epoxy dienal 5.     

Ethyl alpha-fluoro silyl enol ether: stereoselective synthesis and its aldol reaction with aldehydes and ketones

Ethyl alpha-fluoro silyl enol ether is stereoselectively synthesized in high yield from inexpensive chlorofluoroacetate and Mg (or Zn) in DMF (or HMPA). Lewis acid promoted aldol reaction of this enol ether with aldehydes and ketones gives alpha-fluoro-beta-hydroxy esters in good to excellent yields.     

Ether-directed, stereoselective aza-Claisen rearrangements: synthesis of the piperidine alkaloid, alpha-conhydrine

[reaction: see text] A new approach for the stereoselective synthesis of the piperidine alkaloid (+)-alpha-conhydrine and its pyrrolidine derivative has been developed using a palladium(II)-catalyzed, MOM-ether-directed aza-Claisen rearrangement and ring-closing metathesis to effect the key steps.     

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).