Structural, vibrational and electronic properties of α'-Ga2S3 under compression.

We report a joint experimental and theoretical study of the low-pressure phase of α'-Ga2S3 under compression. Theoretical ab initio calculations have been compared to X-ray diffraction and Raman scattering measurements under high pressure carried out up to 17.5 and 16.1 GPa, respectively. In addition, we report Raman scattering measurements of α'-Ga2S3 at high temperature that have allowed us to study its anharmonic properties. To understand better the compression of this compound, we have evaluated the topological properties of the electron density, the electron localization function, and the electronic properties as a function of pressure. As a result, we shed light on the role of the Ga-S bonds, the van der Waals interactions inside the channels of the crystalline structure, and the single and double lone electron pairs of the sulphur atoms in the anisotropic compression of α'-Ga2S3. We found that the structural channels are responsible for the anisotropic properties of α'-Ga2S3 and the A'(6) phonon, known as the breathing mode and associated with these channels, exhibits the highest anharmonic behaviour. Finally, we report calculations of the electronic band structure of α'-Ga2S3 at different pressures and find a nonlinear pressure behaviour of the direct band gap and a pressure-induced direct-to-indirect band gap crossover that is similar to the behaviour previously reported in other ordered-vacancy compounds, including ß-Ga2Se3. The importance of the single and, more specially, the double lone electron pairs of sulphur in the pressure dependence of the topmost valence band of α'-Ga2S3 is stressed.

Structural and vibrational properties of corundum-type In2O3 nanocrystals under compression.

This work reports the structural and vibrational properties of nanocrystals of corundum-type In2O3 (rh-In2O3) at high pressures by using angle-dispersive x-ray diffraction and Raman scattering measurements up to 30 GPa. The equation of state and the pressure dependence of the Raman-active modes of the corundum phase in nanocrystals are in good agreement with previous studies on bulk material and theoretical simulations on bulk rh-In2O3. Nanocrystalline rh-In2O3 showed stability under compression at least up to 20 GPa, unlike bulk rh-In2O3 which gradually transforms to the orthorhombic Pbca (Rh2O3-III-type) structure above 12-14 GPa. The different stability range found in nanocrystalline and bulk corundum-type In2O3 is discussed.

Gibberellins and antheridiogen on sex in Blechnum spicant L.

The role of gibberellins (GAs) in determining sex in the gametophyte of the fern Blechnum spicant L. was studied through (a) the effect of exogenous GA(4+7) and GA3 (b) quantitation of the endogenous levels of GA1, GA3, GA4, GA7, GA9, and GA20 in male and female gametophytes, and (c) the effect of flurprimidol, a GAs biosynthesis inhibitor of the steps of oxidation of ent-kaureno to ent-kaurenoic acid. Our results show that GA(4+7) had a slight effect of inducing either male or female sexual organs, antheridia and archegonia, respectively. The endogenous GAs content was not significantly different between sexes, but the GA4, GA7, and GA20 levels were raised above those of the other GAs in both sexes. Neither antheridiogen biosynthesis nor antheridia formation was inhibited by flurprimidol. Gametophytes regenerated from homogenized mature gametophytes (HG) show a different physiological behavior than spore-derived gametophytes. In the first case, gametophytes are males and synthesize antheridiogen before they attain maturity, in contrast to what occurs in spore-derived gametophytes which are females and synthesize antheridiogen when mature.

Exogenous and endogenous growth regulators on apogamy in Dryopteris affinis (Lowe) Fraser-Jenkins sp. affinis.

This work showed for the first time the relationship between the effect of exogenous auxins and gibberellins on apogamy in Dryopteris affinis (Lowe) Fraser-Jenkins sp. affinis and its endogenous contents during early apogamic events. The addition of NAA (0.53 and 5.37 microM) or GA(3) 2.8 microM to an MS solid medium significantly increased apogamous sporophyte formation. BA induced brown callus that regenerated sporophytes in a hormone-free medium. The endogenous contents of GA(1), GA(3), GA(4), GA(7), GA(9) and IAA were determined by GC-MS in gametophytes cultured on MS solid medium, before and during early stages of apogamous embryo development. The accumulation of both GA(9) and IAA before embryo development was evident as high levels of GA(4) in the earliest analysed stage of embryo development and high levels of GA(3) in elongating shoots were found. The role of gibberellins on apogamy was also supported by data showing a decrease in the percentage of gametophytes developing embryos because of the addition of flurprimidol to the culture medium.

Chemoenzymatic syntheses of homo- and heterodimers of AZT and d4T, and evaluation of their anti-HIV activity.

Homo- and heterodimers of AZT and d4T, possessing carbonate and carbamate linkers, have been synthesized with the aim to enhance the antiviral activity of their components. Homo- and heterodimer carbamates showed weak anti-HIV activity. On the other hand, dinucleoside carbonates showed marked antiviral activity.

Enzymatic resolution of trans-4-(4'-fluorophenyl)-3-hydroxymethylpiperidines, key intermediates in the synthesis of (-)-Paroxetine.

Two Candida antarctica lipases catalyze the enantioselective acylation of N-substituted trans-4-(4'-fluorophenyl)-3-hydroxymethylpiperidines in organic solvents. These two lipases show opposite stereochemical preference in these processes. Both enantiomers can be obtained in their optically pure forms. The (3S,4R) isomer, is an intermediate for the synthesis of (-)-Paroxetine.

Synthesis of new homo and heterodinucleosides containing the 2',3'-dideoxynucleosides AZT and D4T.

The synthesis of new dinucleosides of AZT and D4T is described.

Cal-B-catalyzed alkoxycarbonylation of a-ring stereoisomeric synthons of 1alpha,25-dihydroxyvitamin D3 and 1alpha,25-dihydroxy-19-nor-previtamin D3: a comparative study. first regioselective chemoenzymatic synthesis of 19-nor-A-ring carbonates.

A comparative study of alkoxycarbonylation processes of both 19-nor-A-ring and A-ring stereoisomers of 1alpha,25-dihydroxyvitamin D3 analogues catalyzed by Candida antarctica lipase B (CAL-B) has been described. The presence of the methyl group in the A-ring at C-2, as in 3-6, has a determining role in the regioselectivity of the biocatalysis, mainly allowing the hydroxyl group at C-5 position to react. For the 19-nor-A-ring stereoisomers 7-10, which lack the C-2 methyl group, the configurations at C-3 and C-5 have a high influence in the selectivity exhibited by CAL-B. Thus, each couple of enantiomers showed opposing regioselectivities depending on the C-3 configuration. When C-3 possesses an (S)-configuration, enzymatic alkoxycarbonylations took place at the C-5-(R) or C-5-(S) hydroxyl groups. However, if the chiral centers at C-3 are (R), CAL-B alkoxycarbonylated the C-3-(R) hydroxyl group independently of the configuration at C-5. The corresponding carbonates are useful A-ring precursors of 1alpha,25-dihydroxyvitamin D3 analogues, selectively modified at the C-1 or C-3 positions. In addition, an improved synthesis of cis A-ring synthons 5 and 6 is described using a Mitsunobu methodology.

Optically active dioxatetraazamacrocycles: chemoenzymatic syntheses and applications in chiral anion recognition

Two new C2 and D2 symmetrical dioxatetraaza 18-membered macrocycles [(R,R)-1 and (S,S,S,S)-2] are efficiently synthesized in enantiomerically pure forms by a chemoenzymatic method starting from (+/-)-trans-cyclohexane-1,2-diamine. The protonation constants and the binding constants with different chiral dicarboxylates are determined in aqueous solution by means of pH-metric titrations. The triprotonated form of (S,S,S,S)-2 shows moderate enantioselectivity with malate and tartrate anions (deltadeltaG=0.62 and 0.66 kcal mol(-1), respectively), being the strongest binding observed in both cases with the L enantiomer. Good enantiomeric discrimination is obtained with tetraprotonated (R,R)-1 and N-acetyl aspartate, the complex with the D-enantiomer being 0.92 kcalmol(-1) more stable than its diastereomeric counterpart. Despite the lack of enantioselectivity of tri- and tetraprotonated (R,R)-1 for the tartrate anion, a very good diastereopreference for meso-tartrate is found. All these experimental results allow us to propose a model for the host-guest structure based on coulombic interactions and hydrogen bonds.

6-s-cis locked analogues of the steroid hormone 1alpha, 25-dihydroxyvitamin D(3). Synthesis Of novel A-ring stereoisomeric 1, 25-dihydroxy-3-epi-19-nor-previtamin D(3) derivatives.

Efficient syntheses of A-ring synthons 24 and 32 are described from hydroxy ester 16, which is easily available on a preparative scale from (-)-quinic acid. Key features of the syntheses were (a) the ability to selectively perform desilylations in the presence of p-nitrobenzoate esters and (b) the excellent yield and complete stereospecificity with which the configuration of alcohols 16, 18, and 26 could be inverted under Mitsunobu conditions. Thus, A-ring synthons 24 and 32 were both prepared in 35-38% yield (eight steps) from the common precursor 16. The coupling of A-ring synthons 24 and 32 with the appropriate CD-ring/side chain fragment 7 provides access to novel 6-s-cis locked analogues of steroid hormone 1alpha, 25-dihydroxyvitamin D(3): 1alpha, 25-dihydroxy-3-epi-19-nor-previtamin D(3) (37) and 1beta, 25-dihydroxy-3-epi-19-nor-previtamin D(3) (38), which are unable to undergo rearrangement to the respective vitamin D form by virtue of the absence of the C-19 methyl group. Compounds 37 and 38 can be used as tools for studying the genomic and nongenomic mechanisms of action of the previtamin form of the hormone 1alpha, 25-dihydroxyvitamin D(3).

Non-conventional hydrolase chemistry: amide and carbamate bond formation catalyzed by lipases.

Biocatalysis in nonaqueous media is becoming increasingly important in organic synthesis. Lipases are the most used enzymes, especially in transesterification reactions. However, in the last years the amidation reaction catalyzed by lipases has also been shown to be a useful tool for the organic chemists. In this review, we discuss the possibilities of the enzymatic aminolysis and ammonolysis reactions for the preparation of different amides and for the resolution of esters, amines and aminoalcohols. The enzymatic alkoxycarbonylation of amines opens a new way for the synthesis of chiral carbamates.

Alcohol inhibition and specificity studies of lipase B from candida antarctica in organic solvents

Alcohol inhibition of the lipase B from Candida antarctica has been studied through two different approaches: using the same inhibitor (1-butanol) in different organic solvents and using different inhibitors (differing in chain length) in the same solvent. The competitive inhibition constant values obtained in each case correlate with the calculated activity coefficients of the substrate, suggesting that desolvation of the alcohol is the major force changed. Data dispersion observed using the second approach has been interpreted to come from contributions of enzyme-inhibitor interactions to the binding energy. On the other hand, deacylation has been found to be much less influenced by the solvent variation than the acylation step, despite of the fact that solvation of the substrate involved in this step (the alcohol) is expected to change more than for the ester. Concerning the specificity behavior of the enzyme, a bimodal pattern was observed for the deacylation rate dependence on the alcohol chain length, with the highest values for hexanol (C6) and decanol (C10). With regard to the ester specificity, ethyl caproate (C6) is the preferred one. These results have been confronted with those reported for the lipase from Candida rugosa. Copyright 1998 John Wiley & Sons, Inc.

Lipase-catalyzed transesterification in organic media: solvent effects on equilibrium and individual rate constants.

The kinetics of the immobilized lipase B from Candida antarctica have been studied in organic solvents. This enzyme has been shown to be slightly affected by the water content of the organic media, and it does not seem to be subject to mass transfer limitations. On the other hand, some evidence indicates that the catalytic mechanism of reactions catalyzed by this lipase proceeds through the acyl-enzyme intermediate. Moreover, despite the fact that the immobilization support dramatically enhances the catalytic power of the enzyme, it does not interfere with the intrinsic solvent effect. Consequently, this enzyme preparation becomes optimum for studying the role played by the organic solvent in catalysis. To this end, we have measured the acylation and deacylation individual rate constants, and the binding equilibrium constant for the ester, in several organic environments. Data obtained show that the major effect of the organic solvent is on substrate binding, and that the catalytic steps are almost unaffected by the solvent, indicating the desolvation of the transition state. However, the strong decrease in binding for hydrophilic solvents such as THF and dioxane, compared to the rest of solvents, cannot be easily explained by means of thermodynamic arguments (desolvation of the ester substrate). For this reason, data have been considered as an indication of the existence of an unknown step in the catalytic pathway occurring prior to formation of the acyl-enzyme intermediate.