Search results for "Racemi"

Brewster angle microscopy and X-ray GID studies of morphology and crystal structure in monolayers of N-tetradecyl-γ,δ-dihydroxypentanoic acid amide

1997

First-order phase transitions from the low-density fluidlike phase to the condensed phase of monolayers of N-tetradecyl-γ,δ-dihydroxypentanoic acid amides at the air−water interface have been studied by π−A isotherms, Brewster angle microscopy (BAM), and synchrotron X-ray grazing incidence diffraction (GID). The thermodynamic differences between enantiomeric and racemic monolayers are too small to be measured. However, chiral discrimination is observable in the macroscopic domain morphology by BAM. The domains of the condensed phase grow dendritically. The characteristic shapes of domains of the pure S and R enantiomers are mirror images of each other. The domains of the racemic mixture are…

Ring-opening polymerization of lactides using heterobimetallic yttrocene complexes

1999

Structurally characterized, chiral heterobimetallic yttrocene derivatives Li[Y(η 5 :η 1 -C 5 R 4 Si-Me 2 NCH 2 CH 2 OMe) 2 ] (R = Me, H) have been shown to be active in the controlled ring-opening polymerization of L-lactide to give poly(L-lactide)s with high molecular weights and moderately narrow molecular weight distributions (M w /M n < 1.50). Both transesterification and racemization appear to be less prominent. 'H NMR spectroscopic tetrad analysis of copolymers prepared using a mixture of L- and D-lactide demonstrates the absence of any preference for one enantiomer during the polymerization.

Strategie microbiologiche per la produzione di vini base ad elevata acidità da racemi della cultivar Grillo per l’ottenimento di spumanti siciliani

Sempre maggiore è l’importanza che stanno acquisendo gli spumanti Italiani nel mondo, ed anche la produzione vitivinicola in Sicilia è aumentata nell’ultimo decennio. Gli spumanti di qualità, però, sono caratterizzati da un’elevata acidità che non sempre è raggiunta nei vini base e negli spumanti prodotti nelle aree con climi caldo-temperati come quello siciliano. A tale scopo, si sono svolte delle fermentazioni su un mosto di racemi di Grillo, per l’ottenimento del vino di base spumante, avviando numerose attività di natura microbiologica, molecolare, chimico-fisica e sensoriale. I lieviti utilizzati come starter nelle fermentazioni, sono stati isolati da uve racemi raccolte nelle vendemmi…

Chiral Inversion of Thiolate-Protected Gold Nanoclusters via Core Reconstruction without Breaking an Au-S Bond

2019

On the basis of density functional theory computations of the well-known chiral Au38(SR)24 nanocluster and its Pd- and Ag-doped derivatives, we propose here a mechanism for chiral inversion that does not require the breaking of a metal-sulfur bond at the metal-ligand interface but features a collective rotation of the gold core. The calculated energy barriers for this mechanism for Au38 and Pd-doped Au38 are in the range of 1-1.5 eV, significantly lower than barriers involving the breakage of Au-S bonds (2.5 eV). For Ag-doped Au38, barriers for both mechanisms are similar (1.3-1.5 eV). Inversion barriers for a larger chiral Au144(SR)60 are much higher (2.5-2.8 eV). Our computed barriers are…

Enzyme Promiscuity in Enolase Superfamily. Theoretical Study of o-Succinylbenzoate Synthase Using QM/MM Methods

2015

The promiscuous activity of the enzyme o-succinylbenzoate synthase (OSBS) from the actinobacteria Amycolatopsis is investigated by means of QM/MM methods, using both density functional theory and semiempirical Hamiltonians. This enzyme catalyzes not only the dehydration of 2-succinyl-6R-hydroxy-2,4-cyclohexadiene-1R-carboxylate but also catalyzes racemization of different acylamino acids, with N-succinyl-R-phenylglycine being the best substrate. We investigated the molecular mechanisms for both reactions exploring the potential energy surface. Then, molecular dynamics simulations were performed to obtain the free energy profiles and the averaged interaction energies of enzymatic residues wi…

Peptide Metal–Organic Frameworks for Enantioselective Separation of Chiral Drugs

2017

We report the ability of a chiral Cu(II) 3D MOF based on the tripeptide Gly-L-His-Gly (GHG) for the enantioselective separation of metamphetamine and ephedrine. Monte Carlo simulations suggest that chiral recognition is linked to preferential binding of one of the enantiomers as result of either stronger or additional H-bonds with the framework that lead to energetically more stable diastereomeric adducts. Solid phase extraction (SPE) of a racemic mixture by using Cu(GHG) as extractive phase permits isolating more than 50% of the (+)-ephedrine enantiomer as target compound in only four minutes. To the best of our knowledge, this represents the first example of a MOF capable of separating ch…

Two strained hexahelicenophanes

2000

The crystal structures of the [6]­helicenes 4,13-(1,10-deca­methyl­ene­dioxy)­hexahelicene, C36H34O2, (I), and 4,13-(1,8-octa­methyl­ene­dioxy)­hexahelicene, C34H30O2, (II), show strong steric interactions between the terminal benzene rings and the poly­methyl­ene­dioxy chains. The shortest ring A and F distances amount to 2.941 (3) and 2.902 (3) A, respectively. The increased steric energy of the ground state is responsible for a significantly lower racemization barrier of (I) and (II) in comparison to the unsubstituted [6]­helicene.

The proton complex of a diaza-macropentacycle: structure, slow formation, and chirality induction by ion pairing with the optically active 1,1'-binap…

2008

The protonation of a sterically crowded [N2S6] macropentacycle (1) with 1 equiv of CF3SO3H in CDCl3 is slow and gives the singly (oo(+) [1 x H](+)) and doubly (o(+)o(+) [1 x 2H](2+)) protonated forms as kinetic products, the i(+)o form of [1 x H](+) being the thermodynamic product. i(+)o [1 x H](+) is C3 helically chiral in the solid state and in solution. The barrier to racemization (DeltaG(double dagger)) of the [1 x H](+) propeller is71 kJ mol(-1). The ammonium proton is encapsulated in the tetrahedral coordination sphere provided by the endo (i) nitrogen bridgehead atom and the three proximal thioether sulfurs, which makes [1 x H](+) a proton complex. Use of the optically active acid (R…

ChemInform Abstract: Two Strained Hexahelicenophanes.

2000

The crystal structures of the [6]­helicenes 4,13-(1,10-deca­methyl­ene­dioxy)­hexahelicene, C36H34O2, (I), and 4,13-(1,8-octa­methyl­ene­dioxy)­hexahelicene, C34H30O2, (II), show strong steric interactions between the terminal benzene rings and the poly­methyl­ene­dioxy chains. The shortest ring A and F distances amount to 2.941 (3) and 2.902 (3) A, respectively. The increased steric energy of the ground state is responsible for a significantly lower racemization barrier of (I) and (II) in comparison to the unsubstituted [6]­helicene.

Efficient Synthesis of β-Halogeno Protected L-Alanines and Their β-Phosphonium Derivatives.

2003

Abstract Ring opening of oxazolines, prepared from l -serinates, with trimethylsilyl halides (TMSX) led to β-halogeno-N-benzoyl-α-amino esters in good to excellent yields. Quaternization of triphenylphosphine by the β-bromo or -iodo amino esters gave the corresponding β-phosphonium salts in overall yields of up to 93% and with e.e. >96%. Hydrolysis of the ester function afforded the phosphonium salt bearing an N-benzoyl-α-amino acid substituent, with partial racemization. However, the reaction of the TMSX with the carboxylic salt, prepared by saponification of the starting oxazoline ester, furnished the corresponding β-halogeno-N-benzoyl-α-amino acids in 70–95% yields. Quaternization of tri…