Synthesis, resolution, and absolute stereochemistry of (1R, 2S)-(+)-cis-1-methoxycarbonyl-2-methylcyclobutane

Racemic cis-1-methoxycarbonyl-2-methylcyclobutane uncontaminated with the trans isomer was prepared efficiently in five steps; the corresponding amides from (R)-(-)-phenylglycinol were separated. An X-ray crystallographic structure determination of the amide from (+)-cis-1-methoxycarbonyl-2-methylcyclobutane showed that it was of (1R,2S) absolute stereochemistry, a revision of configurational assignment.     

Efficient preparation of enantiomerically pure cyclic aminoalditols,total synthesis of 1-deoxynojirimycin and 1-deoxymannojirimycyn

Expeditious new syntheses of the title compounds, which are potent glycosidase inhibitors, have been developed based on a high-yield, ring-forming aminomecuration.     

Synthesis,stereochemical characterisation and absolute configuration of enantiomerically pure complexes of sexidentate ligands

Summary The optically active sexidentate Schiff bases [H₃ L ⁵] and [H₃ L ⁶] were prepared from tris[(S)-2-aminopropyl]amine and 5-chlor- and 5-isopropylsalicylaldehyde, respectively. Reaction of rhodium trichloride with [H₃ L ⁶] gave an enantiomerically pure [RhL ⁶] complex with an absolute configuration of for the octahedral arrangement. The chiroptical properties are given.

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Synthese, stereochemische Charakterisierung und Absolutkonfiguration enantiomerenreiner Komplexe sechszähniger Liganden (Kurze Mitt.)

Zusammenfassung Ausgehend von Tris[(S)-2-aminopropyl]amin und 5-Chlor- beziehungsweise 5-Isopropylsalicylaldehyd wurden die optisch aktiven sechszähnigen Schiffschen Basen [H₃ L ⁵] und [H₃ L ⁶] hergestellt. Die Umsetzung von Rhodiumtrichlorid mit [H₃ L ⁶] ergab einen enantiomerenreinen Komplex [RhL ⁶] mit einer Absolutkonfiguration von für die oktaedrische Anordnung des Liganden. Die chiroptischen Eigenschaften werden angegeben.

     

Synthesis,resolution, and absolute stereochemistry of (-)-blestriarene C

A naturally occurring 1,1'-biphenanthrene, blestriarene C (1), was prepared in 13 steps and 30% overall yield. The key steps are the ester-mediated nucleophilic aromatic substitution on 2,6-di-tert-butyl-4-methoxyphenyl 5-isopropoxy-2-methoxybenzoate (4) by 2-methoxy-4-methoxymethoxy-6-methylphenylmagnesium bromide (5) and a novel intramolecular cyclization of the resulting 4-isopropoxy-2'-methoxy-4'-methoxymethoxy-6'-methylbiphenyl-2-carboxylic ester 14 to 7-isopropoxy-4-methoxy-2-(methoxymethoxy)phenanthren-9-ol (15). The racemic blestriarene C was optically resolved by chiral HPLC on a preparative scale to give several 10-mg yields of both the enantiomers in up to 95% ee. The absolute stereochemistry was determined to be S(a)-(-) by the axial chirality recognition method, which was based on the stereospecific formation of a 12-membered cyclic diester containing two biaryl-o,o'-diyl unites joined by ester -CO(2)- linkages. The validity of the method was confirmed by an X-ray crystallographic analysis and ab initio conformational analyses of such 12-membered cyclic diesters. It was found that blestriarene C and its 7,7'-diisopropyl ether 2 underwent rapid photoracemization even under ambient light exposure.     

Studies on epoxidation of enantiomerically pure 2,3-dideoxy hex-2-enitols: a convenient access to highly functionalized enantiomerically pure tetrahydrofuran derivatives

A detailed comparative study on the epoxidation of enantiomerically pure allylic alcohols 8–14 derived from their respective glycals with Sharpless, m-CPBA and Camp’s reagents was carried out in order to obtain 2,3-epoxy alcohols keeping in view the versatility of these synthons in synthetic chemistry for the preparation of various molecules of biological importance by suitable chemical transformations. During the course of this study, the Sharpless asymmetric epoxidation reaction was found to be an unprecedented alternative, due to its mild reaction conditions, for synthesizing highly functionalized enantiomerically pure tetrahydrofuran derivatives. A detailed mechanistic pathway of their formation has also been studied.     

A route to enantiomerically pure 5-(2′-hydroxyethyl)cyclopent-2-en-1-ol and its absolute configuration by Mosher esters

The (±)-5-(2′-hydroxyethyl)cyclopent-2-en-1-ol 1 was prepared in a one-pot procedure, and was resolved using lipase AK and vinyl acetate with high ee. This provides a readily available chiral synthon for the synthesis of a wide variety of biologically interesting molecules. Further, the absolute configuration of diol 1 was confirmed directly by the Mosher ester method.     

N,N-acetals as N-acyliminium ion precursors: synthesis and absolute stereochemistry of epiquinamide

A stereoselective synthesis of (+)-epiquinamide is presented in combination with determination of the absolute configuration of the natural product. Key steps in the sequence involved chemoenzymatic formation of an enantiomerically pure cyanohydrin, reductive cyclization to the corresponding cyclic N,N-acetal, and subsequent conversion into a suitable N-acyliminium ion precursor to enable construction of the second ring.     

Lipase-catalysed resolution of 3,3′-bi-indolizines: The first preparative access to enantiomerically pure samples

The lipase-catalysed kinetic resolution of the axially chiral 3,3′-bis[1-(2-hydroxyethyl)-2-phenylindolizine] [(±)-1a] and the corresponding 3-hydroxypropyl derivative (±)-1b by acylation with vinyl acetate in the presence of lipases from different origins has been investigated. For the first time, enantiomerically pure 3,3′-biindolizine derivatives were obtained on a preparative scale by careful monitoring of the conversion.     

Synthesis of enantiomerically pure (E)-1,1,3,3,6,6-hexamethyl-1-sila-4-cycloheptene and its absolute configuration

The title compound 1, a highly strained (E)-cycloalkene, was prepared in enantiomerically pure form from the corresponding trans-1,2-diol 4 via the thionocarbonate 5. The racemic 4 was separated by enantioselective HPLC on an amylose tris(3,5-dimethylphenylcarbamate) column. The absolute configuration of 1 was determined by circular dichroism spectroscopy in connection with theoretical calculations; the (+)-enantiomer has the (S)- and the (−)-enantiomer the (R)-configuration.     

Applications of enzymatic and non-enzymatic methods to access enantiomerically pure compounds using kinetic resolution and racemisation

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Different methods to access enantiomerically pure compounds.     

Enantioselective synthesis of 2-hydroxy-1-indanone,a key precursor of enantiomerically pure 1-amino-2-indanol

Enantiomerically pure (R)- or (S)-2-hydroxy-1-indanone was synthesized by enzymatic kinetic resolution of racemic 2-acetoxy-1-indanone through hydrolysis or transesterification.     

Stereochemical studies on marine cyclic peroxides : an unequivocal alignment of absolute stereochemistry by asymmetric synthesis

Absolute stereochemical assignments for norterpene cyclic peroxides from marine sponges, previously determined by application of the Horeau procedure of asymmetric esterification, have been independently confirmed by degradation and asymmetric synthesis.     

Synthesis of enantiomerically pure cyclohex-2-en-1-ols: development of novel multicomponent reactions

Multicomponent reactions of aldehydes, dienophiles, and alcohols or carboxylic acid anhydrides have been developed for the first time. In situ generation of 1-acyloxy- and 1-alkoxy-1,3-butadiene derivatives in toluene in the presence of electron-deficient dienophiles provides selective and efficient access to functionalized cyclohex-2-ene-1-ols in good yields. Subsequent enzyme-catalyzed kinetic resolution gave the corresponding enantiomers with high enantioselectivity.     

Aziridinium from N,N-dibenzyl serine methyl ester: synthesis of enantiomerically pure beta-amino and alpha,beta-diamino esters

[reaction: see text] Reaction of N,N-dibenzyl-O-methylsulfonyl serine methyl ester with a variety of heteronucleophiles (sodium azide, sodium phthalimide, amines, thiols) and carbanions (sodium malonate) gave, via an aziridinium intermediate, the corresponding beta-amino or alpha,beta-diamino ester in good to excellent yield. A short synthesis of orthogonally protected and enantiomerically pure 2,3-diamino propionate (Dap) is described.     

Efficient lipase-catalyzed enantioselective desymmetrization of prochiral 2,2-disubstituted 1,3-propanediols and meso 1,2-diols using 1-ethoxyvinyl 2-furoate.

An efficient lipase-catalyzed desymmetrization of prochiral 2,2-disubstituted 1,3-propanediols was developed using 1-ethoxyvinyl 2-furoate 1b, for which the well-known method using vinyl or isopropenyl acetate has had limited success due to low reactivity and easy racemization of the products through acyl group migration. The reagent 1b is highly reactive and converts various prochiral 1,3-diols to the monoesters having a chiral quaternary carbon center with 82-99% ee. These products were stable against racemization under acidic conditions, and their furoyl groups were compatible with oxidative conditions. Prolonging the reaction time led to the kinetic resolution of the monoesters resulting in an increase of their optical purity. The similar desymmetrization of meso cis-1,2-cycloalkanediols gave the monoesters with 82-97% ee without racemization.