General,Highly Selective Synthesis of 1,3‐ and 1,4‐Difunctionalized Building Blocks by Regiodivergent Epoxide Opening

We describe a regiodivergent epoxide opening (REO) featuring a catalyst‐controlled synthesis of enantiomerically and diastereomerically highly enriched or pure syn‐ and anti‐ 1,3‐ and 1,4‐difunctionalized building blocks from a common epoxide precursor. The REO is attractive for natural product synthesis and as a branching reaction for diversity‐oriented synthesis with epoxides.     

General,Highly Selective Synthesis of 1,3‐ and 1,4‐Difunctionalized Building Blocks by Regiodivergent Epoxide Opening

We describe a regiodivergent epoxide opening (REO) featuring a catalyst‐controlled synthesis of enantiomerically and diastereomerically highly enriched or pure syn‐ and anti‐ 1,3‐ and 1,4‐difunctionalized building blocks from a common epoxide precursor. The REO is attractive for natural product synthesis and as a branching reaction for diversity‐oriented synthesis with epoxides.     

Stereoselective [4+2] Cycloaddition of Singlet Oxygen to Naphthalenes Controlled by Carbohydrates

Stereoselective reactions of singlet oxygen are of current interest. Since enantioselective photooxygenations have not been realized efficiently, auxiliary control is an attractive alternative. However, the obtained peroxides are often too labile for isolation or further transformations into enantiomerically pure products. Herein, we describe the oxidation of naphthalenes by singlet oxygen, where the face selectivity is controlled by carbohydrates for the first time. The synthesis of the precursors is easily achieved starting from naphthoquinone and a protected glucose derivative in only two steps. Photooxygenations proceed smoothly at low temperature, and we detected the corresponding endoperoxides as sole products by NMR. They are labile and can thermally react back to the parent naphthalenes and singlet oxygen. However, we could isolate and characterize two enantiomerically pure peroxides, which are sufficiently stable at room temperature. An interesting influence of substituents on the stereoselectivities of the photooxygenations has been found, ranging from 51:49 to up to 91:9 dr (diastereomeric ratio). We explain this by a hindered rotation of the carbohydrate substituents, substantiated by a combination of NOESY measurements and theoretical calculations. Finally, we could transfer the chiral information from a pure endoperoxide to an epoxide, which was isolated after cleavage of the sugar chiral auxiliary in enantiomerically pure form.     

Stereoselective synthesis of hydroxylated β-aminocyclohexanecarboxylic acids

A simple synthetic approach has been developed for the regio- and diastereoselective synthesis of hydroxylated 2-aminocyclohexanecarboxylic acid stereoisomers from 1,4-cyclohexadiene by the reductive opening of appropriate epoxide intermediates derived from the corresponding bicyclic β-lactams. This method has been extended to the synthesis of these hydroxylated β-amino acids in enantiomerically pure form.     

A stereoselective synthesis of (+)-L-733,060 from ethyl (R)-(+)-2,3-epoxypropanoate

An asymmetric synthesis of neurokinin substance P receptor antagonist (+)-L-733,060 starting from enantiomerically pure ethyl (R)-(+)-2,3-epoxypropanoate (ethyl glycidate) is described. The synthesis relies on a diastereoselective reductive amination, regioselective intramolecular epoxide opening, and in situ cyclization as the key steps.     

Catalytic thiolysis of chemoenzymatically derived vinylepoxides. Efficient synthesis of homochiral phenylthioconduritol F

The chemoenzymatic synthesis of enantiomerically pure phenylthioconduritol F obtained in 44% overall yield is described. The key step of the synthesis is the Yb(III) thiolysis of a vinyl epoxide, which was studied in depth. The methodology is amenable to scale up and expandable to the preparation of other thiocyclitols.     

Improved synthesis of structural analogues of (−)-epicatechin gallate for modulation of staphylococcal β-lactam resistance

The high-yielding synthesis of enantiomerically pure epicatechin gallate analogues where the A and/or B-ring hydroxylation is reduced or altered has been achieved by optimising routes to the catechin stereochemistry. The B-ring analogues were synthesised by using an electrophilic ring closure onto an enantiomerically enriched epoxide as a key step. The A and B-ring hydroxyl-deleted analogues were synthesised through a Mitsunobu cyclisation. For the B-ring analogues, the anti- (catechin) stereochemistry was converted to the syn- (epicatechin) stereochemistry by a known oxidation/reduction protocol. Absolute stereochemistry was derived from either a Sharpless epoxidation or asymmetric dihydroxylation.     

One pot synthesis of amino acid derived chiral disubstituted morpholines and 1,4-oxazepanes via tandem aziridine/epoxide ring opening sequences

A new one-pot synthetic strategy is described for the synthesis of enantiomerically pure cis-3,5-disubstituted morpholines and 3,6-disubstituted 1,4-oxazepanes via tandem aziridine/epoxide ring opening sequences. This new strategy describes how epoxy alcohols could act as both a nucleophile and an electrophile in a tandem fashion and undergo intermolecular regioselective ring opening of chiral aziridines for the first time.     

A flexible enantioselective synthesis of the isofurans

Recently, the isolation of a new class of human arachidonic acid tetrahydrofuran oxidation products, the isofurans (IsoF's), was reported. These new compounds are available from natural sources only in microgram quantities as mixtures. The enantioselective preparation of a versatile epoxide intermediate and its conversion to the enantiomerically pure isofurans SC-Delta(13)-9-IsoF and 15-epi-SC-Delta(13)-9-IsoF are described. This synthesis will make these metabolites available for physiological evaluation.     

Direct catalytic synthesis of enantiopure 5-substituted oxazolidinones from racemic terminal epoxides

A venerable scaffold for asymmetric synthesis and drug development, chiral 5-substituted oxazolidinones are obtained in almost enantiomerically pure form (up to 99.9% ee) starting from racemic terminal epoxides. The salient features of this process include the very simple and convenient experimental protocol and the employment of a readily accessible catalyst and inexpensive, easily handled starting materials. An enantioconvergent approach for the total conversion of racemic epoxide into a single stereoisomeric oxazolidinone is also described.     

A convenient synthesis of new enantiomerically pure trihydroxypyrrolizidines using l-erythrose glycosylhydroxylamine as a masked acyclic chiral nitrone

A route has been developed for the synthesis of enantiomerically pure trihydroxylated pyrrolizidines starting from l-erythrose glycosylhydroxylamine. The latter acts as a masked acyclic nitrone and reacts diastereoselectively from its Re-face with methyl acrylate to give the corresponding isoxazolidines, which after reductive N-O cleavage are recyclized to trihydroxypyrrolizidines via a Mitsunobu condensation.     

A new bicyclic dipeptide isostere with pyrrolizidinone skeleton

The synthesis of a new conformationally constrained Gly-(s-cis)Pro Turn Mimetic (GPTM) in both racemic and enantiomerically pure forms and their incorporation into peptides 18, 21, and 24 are reported. The synthetic strategy adopted to assemble the bicyclic pyrrolizidinone skeleton is based on the 1,3-dipolar cycloaddition of the cyclic nitrone 4a derived from proline and acrylamide, followed by a reductive cleavage/cyclization domino process. The enantiomerically pure GPTMs are obtained by synthesis and separation of diastereomeric intermediates containing (1R)-1-phenylethylamine as chiral auxiliary. Analysis of pseudotripeptides 18, 21, and 22 by FT-IR and NMR shows that the amide proton of GPTM derivatives 21 is intramolecularly hydrogen bonded in CDCl(3), while DMSO was shown to disrupt this hydrogen bond.     

Syntheses of (+)-(S,S)-(cis-6-Methyltetrahydropyran-2-yl)acetic acid and of (−)-(R,R)-Didesoxy-pyrenophorine Using a New d5-Reagent. Preliminary communication

The Li/K-derivative 6 is used to synthesize the title compounds ( 3a and 4a ) in enantiomerically pure form from (?)-(S)-propylene epoxide. The C,C bond forming key step leading to the hydroxyketone 7 is followed by cyclization (→, 8 ), Beckmann cleavage (→ 9b ) and hydrolysis to 3a (recently isolated from civet). Base treatment of 3a opens the ring ( 10 ) to give the hydroxyacid 1 which is cyclized to the macrolide 4a . The synthetic usefulness of the highly nucleophilic doubly reduced dienone system 6 as d⁵-reagent (see synthons 2) is thus demonstirated.     

Enantiospecific synthesis of norcoronamic acids

The synthesis of enantiomerically pure norcoronamic acids, starting from enantiomerically pure 1,2-propanediols, is described.     

Conformationally constrained dipeptides. Obtention of enantiomerically pure 6‐acetamido‐5‐oxo‐1,2,3,5,6,7‐hexahydro‐3‐indolizine carboxylic acid

The separation of a stereoisomeric mixture of esters 6, and the synthesis of the new enantiomerically pure unsaturated 6,5‐fused bicyclic lactam 2 are described. The key‐step involves trimethylsilyl iodide cleavage, without racemization, of a vinylogous carbamate. The cis N‐ acetylamino acid 2 is a new scaffold for the synthesis of rigid β‐turn mimetics.     

Stereocontrolled entry to 1-oxapenams and 1-oxacephems from carbohydrates

N-Benzyl-2-carboxy-2-deoxypento and hexopyranosylaminolactams readily available via addition of trichloroacetyl isocyanate to substituted glycals, were transformed into enantiomerically pure N-benzyl-4-alkoxy-3-hydroxymethylazetidinones-2. The transformation involved the glycolic cleavage which was followed by sodium borohydride reduction. The above compounds can serve as starting materials for the synthesis of 1-oxapenams and 1-oxacephems.     

Synthetic studies toward the cytotoxic norditerpene (+)-harringtonolide: setting up key-stereogenic centers of the cyclohexane ring D

The pivotal stereogenic centers of the asymmetric cycle D of (+)-harringtonolide were installed by functionalization of an enantiomerically pure IMDA cycloadduct, constructed from the chiral pool. The chiral 1,3-dioxane template used to direct the IMDA reaction was unraveled in an acidic medium, through spectacular hydrolysis of the acetal and concomitant lactone ring contraction. The central cyclohexene was selectively epoxidized either on the β- or on the α-side depending on the substitution pattern. The reactivity of several epoxide intermediates was challenged toward the construction of the oxygenated bridges of harringtonolide. We found one of them suitable for an access to another natural product, tetrodecamycin, which shares a similar substitution pattern as harringtonolide. Alternatively, functionalization led to set up key-stereocenters, en route to the asymmetric total synthesis of harringtonolide. The reactivity of the epoxide intermediates gave helpful insight for future work on this total synthesis.     

Vitamin D: a concise synthesis of the C19 hydroxylated enyne A-ring, an interesting precursor for the preparation of C19 substituted vitamin D analogues

A new C₁₉ hydroxylated enyne 15, as potential A-ring building block of vitamin D analogues, was synthesized in enantiomerically pure form in nine steps from (−)-(S)-limonene. This short synthesis involved ozonolyzis of 1,2-limonene oxide followed by a Criegee rearrangement, epoxide trans diaxial ring opening by lithium acetylide, elimination, epoxidation and syn β-elimination of the resulting homopropargylic oxirane.     

Regioselective ring cleavage of chiral β-trichloromethyl-β-propiolactone with organoaluminum compounds for the synthesis of optically active intermediates

A novel alkylating ring cleavage reaction of enantiomerically pure β-trichloromethyl-β-propiolactone as a chiral building block with organoaluminum compounds provided ring-opened products with a chiral trichloromethyl carbinol moiety. A product was demonstrated to be used as an effective chiral synthon for the synthesis of chiral bioactive derivatives such as ipsdienol and sodium cilastatin.     

Synthesis of enantiomerically pure tertiary 1,2-aminoalcohols by the highly diastereoselective reductive ring opening of oxazolidines

A number of enantiomerically pure 1,2-aminoalcohols containing tertiary nitrogen atoms bearing chiral substituents have been prepared by highly diastereoselective reductive ring cleavage of oxazolidines derived from ketones and pseudoephedrine or ephedrine. The ring cleavage occurs with retention of configuration.