Synthesis,chiroptical properties and absolute configuration of α-phenylglycidic acid

The (+)- and (-) enantiomers of potassium α-phenylglycidate, an irreversible inhibitor of the enzyme mandelate racemase, were synthesized by resolution of the diastereomeric esters with R-(-)-2-octanol. Base-catalyzed ring-opening of the resolved α-phenylglycidate esters gave the enantiomers of 2,3-dihydroxy-2-phenylpropanoic acid, also obtained by resolution of the racemic dihydroxy acid using ephedrine. A comparison of the chiroptical properties of the esters of α-phenylglycidic and 2,3-dihydroxy-2-phenylpropanoic acids with those of the structurally similar atrolactic and mandelic acids and their 2-methoxy-derivatives showed that the (-)-methyl 2,3-dihydroxy-2-phenylpropanoate corresponding to the (+)-enantiomer of potassium α-phenylglycidate, as well as the esters of α-phenylglycidic acid derived from the same (+)-potassium salt, were all configurationally related to S-(+)-atrolactic and mandelic acids. The configurational assignments made on the basis of the chiroptical data were confirmed by lithium aluminum hydride reduction of the (-)-2-octyl S- and R-α-phenylglycidates, which led exclusively to the R-(-)- and S-(+)-2-phenyl-1, 2-propanediols, respectively, previously related configurationally to R-(-)- and S-(+)-atrolactic acids.     

Enantiocomplementary preparation of (S)- and (R)-mandelic acid derivatives via α-hydroxylation of 2-arylacetic acid derivatives and reduction of α-ketoester using microbial whole cells

Forty one microorganisms belonging to different taxonomical groups were used to carry out the enantioselective reduction of methyl benzoylformate to afford the corresponding (R)-methyl mandelate, with moderate to high ee. In contrast, the monooxygenase enzyme in Helminthosporium sp. CIOC3.3316 catalyzed the hydroxylation of methyl 2-phenylacetate to (S)-methyl mandelate. This combination of oxidation and reduction biotransformations thus provides a method for preparing the enantiomers of chiral α-hydroxy acid derivatives.     

Stereoselective synthesis of pinane-based β- and γ-amino acids via conjugate addition of lithium amides and nitromethane

Michael addition of dibenzylamine to (?)- and (+)-tert-butyl myrtenate, (?)-2 and (+)-2, derived from (?)- and (+)-myrtenal, furnished monoterpene-based β-amino acid derivatives in highly stereospecific reactions. The resultant amino esters (?)-3 and (+)-3 were transformed to unsubstituted, mono- and disubstituted and Fmoc-protected amino acids (?)-6-11 and (+)-6-11, which are promising building blocks for the synthesis of β-peptides and 1,3-heterocycles. The microwave-assisted conjugate addition of nitromethane to α,β-unsaturated esters (?)-12 and (+)-12 likewise resulted in nitro esters (?)-13 and (+)-13 in highly stereospecific reactions. Compounds (?)-13 and (+)-13 were successfully transformed into γ-amino acids (?)-16 and (+)-16 in two steps.     

Chromatographic separation of diastereoisomeric esters-II Mandelates and lactates of 2-methylbutanol-1, 3-methylbutanol-2, pentanol-2 and hexanol-2

The racemic alcohols, 2-methylbutanol-l, 3-methylbutanol-2, pentanol-2 and hexanol-2 have been esterified with optically active mandelic or lactic acid, and the resultant mixtures of diastereoisomeric esters separated chromatographically with an optically inactive sulphonated polystyrene resin as the stationary phase and water or dilute aqueous sodium sulphate as the mobile phase. Lactate esters were more easily separated than mandelate esters. The Chromatographic separation of 26 mmole of (+/-)-3-methyl-2-butyl(+)-lactate and subsequent hydrolysis of the fractions containing only the ester of the (+)-alcohol yielded 1-2 mmole of pure 3-methyl-2-butanol.     

D(−)- and L(+)-γ-Carboxyglutamic Acid (Gla): Resolution of Synthetic Gla Derivatives

The chemical resolution of γ,γ′-di-t-butyl DL -N-benzyloxycarbonyl-γ-carboxy-glutamate is described in detail (preliminary account see [1]). The D (?)-derivative was obtained as a crystalline quinine salt, and the L (+)-derivative as a crystalline salt with (?)-ephedrine in yields of 44 and 70%, respectively. Physical data are indicated for the enantiomers of γ,γ′-di-t-butyl N-benzyloxycarbonyl-γ-carboxyglutamate, γ,γ′-di-t-butyl γ-carboxyglutamate, and γ-carboxyglutamic acid. The absolute configurations and optical purities of the γ,γ′-di-t-butyl (+)- and (?)-N-benzyloxycarbonyl-γ-carboxyglutamates were determined by removal of the protecting groups and decarboxylation to optically active glutamic acid.     

Conformational Studies of Marine Polyhalogenated α-Chamigrenes Using Temperature-Dependent NMR Spectra. Cyclohexene-ring flipping and rigid-chair cyclohexane ring in the presence of equatorial halogen atoms at C(8) and C(9)

Temperature-dependent NMR spectra indicate that the α-chamigren-3-ones (?) -11 , (+) -12 , (+) -14 (?) -15 , (+) -16, 18 , and 19 bearing equatorial halogen atoms at C(8) and C(9) undergo slow conformational flipping of the envelope-shaped enone ring, while the cyclohexane ring is maintained in the chair conformation. The α-chamigren-3-ols (+) -20 and (+) -21 , obtained by hydride reduction of (+) -12 , behave similarly, with slow half-chair inversion of the cyclohexenol ring. In each case, both conformers are about equally populated and detectable by NMR, except in the case of (+) -15 , where repulsive interactions between Br? C(2) and H_eq?C(7) make the population of the conformer 15b with Me—C(5) faced to H_ax?C(10) so low that it escapes direct ¹H-NMR detection. The energy barriers to these conformational motions are viewed to arise mainly from repulsive interactions between Me—C(5) and the axial H-atoms at C(8) and C(10), while, contrary to previous beliefs, no twist-boat conformations of the cyclohexane ring intervene. Similar conclusions hold for the 4,5-epoxides of both (?) -6 and (+) -7 . Clean Jones oxidatio of (?) -2 to 17 , where the CH₂?C(5) bond is maintained, and acid dehydration-isomerization of the α-chamigrene (+) -21 to the β-chamigrene (+) -24 , reflect the special stability of β-chamigrenes, providing a reason for their frequent occurrence in nature.     

The use of chiral interaction reagents in the separation of D(−)-and L(+)-ascorbic acid by ion interaction reversed-phase HPLC

Summary An HPLC reversed-phase ion interaction reagent method is presented, which makes use of chiral compounds as the interaction reagents. By employing the optical isomeric forms of malic, tartaric and mandelic acids as the interaction reagents, a good separation of D(−) and L(+) ascorbic acid has been achieved. The method has also been applied to the identification of vitamin C in some medical formulations. The separation of te enantiomeric forms of DL malic acid has been attempted and a shor discussion is presented about the elution sequence in chromatographic separation of D- and L-enantiomers.     

Polymerization in the crystalline state. VIII. Polymerization in N-carboxy anhydrides of γ-benzyl glutamate, γ-methyl glutamate,and ϵ-carbobenzoxylysine

The kinetics of the solid-state polymerization of the N-carboxy anhydrides (NCA) of the L - and racemic forms of γ-benzyl glutamate (BG), γ-methyl glutamate (MG), and ?-carbobenzoxylysine (CL) were studied as a function of temperature and aqueous vapor pressure. The reaction of the L -forms of BG and MG was characterized by an induction period, while the CL derivative reached its maximum polymerization rate at the outset of the reaction. Water vapor had only a minor effect in accelerating the reaction and reducing the chain length of the polypeptides formed. The racemic monomers were found to have different crystal structures from those of the L -isomers and the racemic MG and CL derivatives polymerized much more slowly than the corresponding optically active crystals. All polymers gave diffuse x-ray diffraction patterns. Infrared spectra of the L -polypeptides showed that they were largely in the α-helical form. The polymer derived from the racemic BG–NCA had a content of α-helical material which suggested that it consisted of polypeptides with long blocks of D and L residues.     

The mandelamide keto-enol system in aqueous solution. Generation of the enol by hydration of phenylcarbamoylcarbene

Flash photolysis of diazophenylacetamide in aqueous solution produced phenylcarbamoylcarbene, whose hydration generated a transient species that was identified as the enol isomer of mandelamide. This assignment is based on product identification and the shape of the rate profile for decay of the enol transient, through ketonization to its carbonyl isomer, as well as by the form of acid-base catalysis of and solvent isotope effects on the decay reaction. Rates of enolization of mandelamide were also determined, by monitoring hydrogen exchange at its benzylic position, and these, in combination with the ketonization rate measurements, gave the keto-enol equilibrium constant pK(E) = 15.88, the acidity constant of the enol ionizing as an oxygen acid, pQ(E)(a)= 8.40, and the acidity constant of the amide ionizing as a carbon acid pQ(K)(a)= 24.29. (These acidity constants are concentration quotients applicable at ionic strength = 0.10 M.) These results show the enol content and carbon acid strength of mandelamide, like those of mandelic acid and methyl mandelate, to be orders of magnitude less than those of simple aldehydes and ketones; this difference can be attributed to resonance stabilization of the keto isomers of mandelic acid and its ester and amide derivatives, through electron delocalization into their carbonyl groups from the oxygen and nitrogen substituents adjacent to these groups. The enol of mandelamide, on the other hand, again like the enols of mandelic acid and methyl mandelate, is a substantially stronger acid than the enols of simple aldehydes and ketones. This difference can be attributed to the electronegative nature of the oxygen and nitrogen substituents geminal to the enol hydroxyl group in the enols of mandelic acid and its derivatives; in support of this, the acidity constants of these enols correlate well with field substituent constants of these geminal groups.     

A Formal Total Synthesis of (+)-Methyl Trachyloban-18-oate and (+)-Methyl 16-Oxo-17-norkauran-18-oate: Regio- and Diastereoselective Preparation of Methyl (13S)-13-Hydroxyisoatisiren-18-oate from (−)-Abietic Acid

A novel preparation of methyl (13S)-13-hydroxyisoatisiren-18-oate ( 4 ), a key-intermediate in a synthesis of (+)-methyl trachyloban-18-oate ((+)- 1 ), from (?)-abietic acid, is described. Since (?)- 1 has been previously converted into (?)-methyl 16-oxo-17-norkauran-18-oate ((?)- 16 ), our preparation of 4 constitutes also a formal total synthesis, from (?)-abietic acid, of (+)- 16 . Key steps in this approach were the allene photoaddition to podocarp-8(14)-en-13-one ( 5 ) and the conversion of the endo-toluene-4-sulfonate 11 into the exo-benzoate 12b .     

Chiral calix[4]arenes bearing α-hydroxy amide units as membrane carriers for amino acid methyl esters

Novel chiral calix[4]arene derivatives functionalized at the lower rim have been prepared from the reaction of p-tert-butylcalix[4]arene diamine or acylhydrazine derivative with mandelic acid or hydroxyisovaleric acid. The structures of these receptors were characterized by FTIR, ¹H, ¹³C and 2D COSY NMR spectroscopy and elemental analysis. The transport of amino acid derivatives (phenylalanine, phenylglycine and tryptophan methyl ester hydrochlorides) was studied through bulk liquid membrane in the presence of chiral calix[4]arene derivatives. The receptors have been found to act as carriers for transport of aromatic amino acid methylesters from the aqueous source phase to the aqueous receiving phase. The transport rate and L/D selectivity of amino acid esters studied depend strongly upon the structure of the chiral receptors and guests. The best enantioselectivity was obtained in the case of phenylglycine methyl ester for all chiral carriers.     

Enantioselective Artificial Receptors Formed by the Spreader‐Bar Technique

Chiroselective binding sites have been created on thin gold films by application of the spreader‐bar approach. Impedometric techniques and surface plasmon resonance were applied to detect binding. (R)‐(+)‐1,1′‐Binaphthyl‐2,2′‐diol (R‐BNOH) and (S)‐(?)‐1,1′‐binaphthyl‐2,2′‐diol (S‐BNOH) were used as model analytes. The artificial receptors were prepared by co‐adsorption of 16‐mercaptohexadecane (matrix) with a thiol‐modified chiral selector (template). The conjugates of D ,L ‐thioctic acid and (R)‐(+)‐ or (S)‐(?)‐1,1′‐binaphthyl‐2,2′‐diamine were used as templates. Different concentration ratios of the matrix and template were tested. No chiral selectivity of surfaces formed by either the matrix or the template alone was observed. The use of alkylthiols shorter than 16‐mercaptohexadecane led to the formation of surfaces with no chiral selectivity. The gold electrodes coated by the spreader‐bar technique displayed an enantioselectivity of up to 4.76 or up to 2.55 as measured by the capacitive and SPR methods, respectively.     

Biosynthese der Verrucarine und Roridine. Teil 5. Synthese von zwei Diastereoisomerenpaaren des [1,8-14C]-α-Bisabolols und Versuche zu deren Einbau in Verrucarol Verrucarine und Roridine, 32. Mitteilung [1]

The diastereoisomeric (+)-[1,8-¹⁴C]-(1'R,6R, S)-α-bisabolol ( 2a ) and (?)-[1,8-¹⁴C]-(1′S, 6R, S)-α-bisabolol ( 2b ) were synthesized by reaction of the Grignard compound of [1,6-¹⁴C]-5-bromo-2-methyl-2-pentene ( 12 ) with (+)-(R)- and (?)-(S)-4-acetyl-1-methyl-1-cyclohexene, ( 6a ) and ( 6b ) respectively. For the preparation of compound 12, cyclopropyl methyl ketone was treated with [¹⁴C]-methyl magnesium iodide to form the carbinol 11, which was cleaved by HBr. Compounds 6a and 6b were synthesized from (+)-(R)- and (?)-(S)-limonene, ( 4a ) and ( 4b ), via the derivatives 5a , 6a and 5b , 6b respectively. - This synthesis established the absolute configuration at C(1′) of the natural α-bisabolols: (R) for (+)-α-bisabolol and (S) for (?)-α-bisabolol. - Feeding experiments with cultures of Myrothecium roridum and radioactive (+)-(1′R, 6R, S)- and (?)-(1′S, 6R, S)-α-bisabolol ( 2a ) and ( 2b ) gave negative results. These findings indicate that bisabolane derivatives are not intermediates in the biosynthesis of verrucarol (3).     

Synthetic analogues of polynucleotides—XIII: The resolution of dl-β-(thymin-1-yl)alanine and polymerisation of the β-(thymin-1-yl)alanines

dl-β-(Thymin-1-yl)alanine has been resolved into d(+) and l(?) forms. The pure d(+) form was obtained by fractional crystallisation of the (+)α-methylphenylethylamine salts of the α-N-formyl derivatives. The pure l(?) isomer was obtained on a small scale by chromatography of the same salts. The optically active amino acids and the dl-mixture were polymerised by the mixed anhydride procedure to give polymers which showed no evidence of base stacking or of interaction with polyadenylic acid. The molecular weights of the polymers were in the range 2–4 × 10³. These were determined by end group assay which involved the synthesis of α-N-(2,4-dinitrophenyl)-dl-β-(thymin-1-yl)alanine as a standard.     

Application de reactions d'hydroalumination en synthese organique: obtention d'α-alkyl α-hydroxyesters par addition enantioselective de tetraalkylaluminates AU phenylglyoxalate de (−) menthyle.

Using tetraalkylaluminates prepared via hydroalumination in THF of various vinyl compounds resulted in selective addition to the keto group of phenylglyoxylic acid (?) menthyl ester. Not only simple alkyl groups but also functionalized alkyl groups were added. By this way, a variety of α-substituted mandelic acid (?) menthyl esters were obtained with diastereomeric excesses close to 70%.     

α/β-Selectivity in Hydrolyses of α- or β- Naphthyl Acetates in the Presence of Cycloamyloses.

Cyclohexaamylose hydrolyzed α-naphthyl acetates more rapidly than the corresponding β-isomers, a phenomenon termed “α-selectivity”, while cyclooctaamylose showed “β-selectivity” and the selectivity of cycloheptaamylose fell between those of cyclohexa- and cyclooctaamyloses.     

Improved stability of organic field-effect transistor performance in oligothiophenes including β-isomers

Dialkyl quarter- and quinquethiophenes end-capped with β-connected thiophenes are prepared, and the field-effect transistor (FET) properties are investigated. Molecular orbital calculation as well as the redox and optical measurements indicate that the β-isomers have low-lying HOMO levels and large energy gaps compared with the α-isomers. Molecular packing of the dihexylquaterthiophene with β-isomers consists of a typical herringbone motif analogous to the α-isomers. In the single crystal, the alkyl chains are extending in the tilted directions from the core molecular plane, but straightly extending in the thin films, resulting in even more perpendicular molecular arrangement to the substrate than the α-isomers. These β-isomers show p-type FET performance comparable to the corresponding α-isomers, whereas all new oligothiophenes show air stability better than the corresponding α-isomers. In particular, dihexylquinquethiophene with β-isomers has shown significantly improved air stability maintained over 270 days. This stabilization effect is ascribed to the low-lying HOMO level and the dense packing realized by perpendicular molecular arrangement.     

Substituent effects and aromaticity of six-membered heterocycles

Structure, aromaticity, relative stability, and conformational flexibility of nitro and amino substituted monoheterocyclic analogous of benzene were studied by ab initio quantum-chemical method at MP2/aug-cc-pvDZ level of theory. Amino derivatives were found to be slightly less aromatic than nitro derivatives. Strong push–pull interactions were found in α- and γ-aminochalcogenopyrylium cations and, in less extent, in α- and γ-aminopyrydines. These molecules are less aromatic but more stable, as compare to their β-isomers. All heterocycles with 3rd and 4th row heteroatoms reveal C–NO₂ bond elongation accompanied by C–Heteroatom bond shortening in β-nitro isomers, and strong inequality of two endocyclic C–Heteroatom bonds in α-amino isomers.     

Recherches dans la serie des metallocenes—XXVII: Stereochimies comparees de ferrocenylcyclohexenones chirales portant un carbone asymetrique quaternaire en position α ou β. Puretes optiques par dilution isotopique

The two diastereoisomeric pairs of α-phenyl α-methyl and β-phenyl β-methyl ferrocenyl cyclohexenones were prepared in optically active series from a single chiral precursor, (+) phenyl-2 methyl-2 succinic acid, the S configuration of which was established by chemical correlation. The chiral precursor and the four ketones were shown to be optically pure by isotopic dilution with tritium labelled racemates. We infer that diastereogenic cyclisation of the γ-ferrocenyl butyric acids into ketones is under pure kinetic control and leads mainly to the isomer bearing the bulkiest group in an endo position.     

Über die Stereospezifität der α-Alkylierung von β-Hydroxycarbonsäureestern. Vorläufige Mitteilung

About the Stereospecific α-Alkylation of β-Hydroxyesters It was found, that dianions derived from β-hydroxyesters with lithium diisopropylamide (LDA) at ?50 to ?20° were alkylated stereospecifically (Scheme 1). The stereospecificity was 95–98%, the threo-compound (threo -2, -3 and -4) being the main product. This was proved for threo -2 and -3 by preparing the β-lactones 7 and 8 , respectively, which were pyrolyzed to trans-1, 4-hexadiene (9) and trans-1-phenyl-2-butene (10) , respectively (Scheme 2). Moreover, the acid threo -6 from threo -3 was converted by dimethylformamide-dimethylacetal to cis-1-phenyl-2-butene (11) (s. footnote 6). The alkylation of α-monosubstituted β-hydroxyesters also turned out to be stereospecific. Reduction of 16 and 18 with actively fermenting yeast furnished (+) -17 and (+) -2. respectively (Scheme 4), which were each mixtures of the (2R, 3S)- and the (2S, 3S)-isomers. Alkylation of (+) -17 with allyl bromide yielded after chromatography (2S, 3S) -19 and of (+) -2 with methyl iodide (2R, 3S) -19 , the oxidation of which finally gave (S)-(?) -20 and (R)-(+) -20 , respectively.