Enantioselective desymmetrisation of achiral epoxides

Enantioselective Ring Opening of meso-Epoxides by Aromatic Amines Catalyzed by Lanthanide Iodo Binaphtholates

[reaction: see text] Lanthanide iodo binaphtholates are efficient enantioselective catalysts for the ring opening of meso-epoxides by various aromatic amines. The study of the influence of temperature on the ring opening of cyclohexene oxide by o-anisidine catalyzed by the samarium complex shows an isoinversion effect with the maximum enantiomeric excess at -40 degrees C. Reactions of aniline, o-anisidine, or p-anisidine with five- or six-membered ring epoxides at this temperature allow the preparation of beta-amino alcohols with enantiomeric excesses up to 93%.

Discrete Metal-Based Catalysts for the Copolymerization of CO2 and Epoxides: Discovery, Reactivity, Optimization, and Mechanism

Most synthetic polymers are made from petroleum feedstocks. Given the non-renewable nature of these materials, there is increasing interest in developing routes to polymeric materials from renewable resources. In addition, there is a growing demand for biodegradable polymeric materials. Polycarbonates made from CO(2) and epoxides have the potential to meet these goals. Since the discovery of catalysts for the copolymerization of CO(2) and epoxides in the late 1960's by Inoue, a significant amount of research has been directed toward the development of catalysts of improved activity and selectivity. Reviewed here are well-defined catalysts for epoxide-CO(2) copolymerization and related reactions.

Double Deprotonation of Acetylenic Oxiranes: Synthesis of Allenic Ketones through Dilithio Ynenolates

[reaction: see text] The double deprotonation of acetylenic oxiranes gives allenic ketones through the 1,2-H or 1,2-Ar shift on the transient oxiranyl dianion intermediates. The resulting dilithium ynenolates give allenic ketones upon hydrolysis or can be quenched with various electrophiles.

Cyclododecatetraene tetraepoxide: highly diastereofacial epoxidation of all-(Z)-1,4,7,10-cyclododecatetraene and selective synthesis of bridged bis-oxocanes

Epoxidation of all-(Z)-1,4,7,10-cyclododecatetraene has been investigated with use of m-CPBA and dimethyldioxirane in anhydrous solvent. The diastereoselectivity of multiple epoxidation steps is complete affording only exo,exo,exo,endo-1,4,7,10-tetraepoxide. To understand this result, the step-by-step epoxidation reaction was investigated and each step was found to be highly regio- and stereoselective. Finally, a Lewis acid-catalyzed ethanolysis or treatment with HBr/KBr of the tetraepoxide gave rearranged diepoxy-oxabicyclo[5.5.1]tridecane (bridged bis-oxocanes) in which eight stereocenters were controlled.

2,3-Anhydro sugars in glycoside bond synthesis. Highly stereoselective syntheses of oligosaccharides containing alpha- and beta-arabinofuranosyl linkages

The ever-increasing discovery of biologically important events mediated by carbohydrates has generated great interest in the synthesis of oligosaccharides and the development of new methods for glycosidic bond formation. In this paper, we report that 2,3-anhydrofuranose thioglycosides (1, 5) and glycosyl sulfoxides (2, 6), in which the hydroxyl groups C-2 and C-3 are "protected" as an epoxide, glycosylate alcohols with an exceptionally high degree of stereocontrol. The predominant or exclusive product of reactions with this fundamentally new class of glycosylating agent is that in which the newly formed glycosidic bond is cis to the epoxide moiety. We further demonstrate that subsequent nucleophilic opening of the epoxide moiety proceeds under basic conditions to give products in high yield and with good to excellent regioselectivity. The major ring-opened products possess the arabino stereochemistry, and thus this methodology constitutes a new approach for the synthesis of arabinofuranosides. In the epoxide opening reactions of glycosides with the 2,3-anhydro-beta-D-lyxo stereochemistry (e.g., 73), the addition of (-)-sparteine (78) to the reaction mixture dramatically enhanced the regioselectivity in favor of the arabino product. This represents the first example of the use of 78 to influence the regioselectivity of an epoxide ring opening reaction with a non-carbon nucleophile. We have demonstrated the utility of this methodology through the efficient synthesis of an arabinofuranosyl hexasaccharide, 7, which is a key structural motif in two mycobacterial cell wall polysaccharides.

Enantiocontrolled synthesis of 2,6-disubstituted piperidines by desymmetrization of meso-eta-(3,4,5)-dihydropyridinylmolybdenum complexes. application to the total synthesis of (-)-dihydropinidine and (-)-andrachcinidine

A conceptually new approach to the enantiocontrolled synthesis of 2,6-disubstituted piperidines was achieved by desymmetrization of meso-2,6-dimethoxy-eta-(3,4,5)-dihydropyridinylmolybdenum complexes. After protection of the piperidine nitrogen as a urethane derived from (+)- or (-)-trans-2-(alpha-cumyl)cyclohexyl (TCC), a sequential, one-pot methoxide abstraction/nucleophilic addition/methoxide abstraction/nucleophilic addition generated good yields of 2,6-disubstituted-eta-(3,4,5)-dihydropyridinylmolybdenum complexes. This sequence proceeds by way of a highly diastereoselective methoxide abstraction (>40:1). High yielding protodemetalation and N-deprotection provided a simple and enantiocontrolled synthetic entry to a variety of 2,6-disubstituted piperidines. This new method was used for the total synthesis of (-)-dihydropinidine and (-)-andrachcinidine.

Enantioselective synthesis of epoxides by α-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 deprotonation of meso-cycloheptanone derivative: application to the synthesis of a potential intermediate for pseudomonic acid B

[reaction: see text] A novel synthetic path to a potential intermediate for the synthesis of pseudomonic acid B was established by employing enantioselective deprotonation of a meso-cycloheptanone derivative bearing hydroxy groups at the 3,4,5,6-positions with lithium (S,S')-alpha,alpha'-dimethyldibenzylamide as a key step.

Synthesis of allylic alcohols from oxazolinyloxiranes

Lithium diisopropylamide (LDA) (or s-BuLi/TMEDA) in diethyl ether promotes smooth ring opening of oxazolinyl alkyl oxiranes to give oxazolinyl allylic alcohols, which are masked Baylis-Hillman adducts, in good to excellent yields. An E2-E1cb-like syn-beta-elimination is proposed to explain the easy base-promoted isomerization of the studied oxiranes.