Chemoenzymatic synthesis of enantiomerically enriched 2-oxobicyclo[m.1.0]alkan-3-yl acetate derivatives

Racemic α′-acetoxy α,β-unsaturated cyclopentanone and cyclohexanone have been resolved into the corresponding enantiomerically enriched α′-hydroxylated and acetoxylated compounds with 96–97% ee via PLE hydrolysis. Stereoselectivity in the palladium(II)-catalyzed reaction between the enantiomerically enriched α′-acetoxylated compounds and diazomethane has been investigated. In the α′-acetoxylated cyclopentenone, preferential cyclopropanation occurs in the anti-form, whereas α′-acetoxylated cyclohexenone affords both syn- and anti-products (syn:anti, 61:36%). The relative configuration of the products was determined by NOE experiments.     

Crystal structures of anti- and syn-9,10-di(1′-naphthyl)anthracene and isomerization in solid state

9,10-Di-(1′-naphthyl)anthracene is often used as electroluminescence materials in organic light-emitting diodes. Because of the hindered rotation about the σ-bond between naphthyl and anthracene chromophore, two possible stereoisomers can be isolated. HPLC, ¹H NMR, and ¹³C NMR spectra gave two different sets of peaks and the X-ray single crystal analysis confirmed the structures of the two isomers, anti and syn. syn was more soluble than anti in THF as well as toluene and the thermal properties of the two were quite different. Differential scanning calorimetry study and HPLC analysis showed that the isomerization between anti and syn in the solid state took place at >370 °C.     

Synthesis and spectroscopic studies of highly fluorescent,solvatochromic diastereomers with differentially stacked bithiophene-substituted quinoxaline rings

Diastereomeric C-shaped molecules containing closely stacked bithiophene-substituted quinoxaline rings were synthesized and characterized by NMR, UV–vis absorption, and fluorescence spectroscopy. The unique geometry of each diastereomer resulted in different degrees of π-overlap between the bithiophene-substituted quinoxaline ring chromophores, modulating their spectroscopic properties. The donor-acceptor nature of this chromophore gave rise to its positive solvatochromism. ¹H NMR and UV–vis absorption spectroscopy confirmed the existence of π-π interactions in the ground state between the quinoxaline rings in both molecules but between the bithiophene rings only in the syn isomer. They exhibited significant emission maxima bathochromic shifts, a strong, positive solvatochromism, increased band broadening, and larger Stokes shifts when compared to a compound with an unstacked chromophore. Additionally, the syn isomer consistently showed λ_max,em value red-shifts and larger band broadening and Stokes shifts compared to the anti isomer due to the greater π-overlap in the syn isomer.     

Remote substituent effect favoring the formation of syn-adducts in the chelation controlled radical reactions of γ-benzyloxy-α-methylenecarboxylic acid esters

The chelation controlled radical reactions of ethyl γ-benzyloxy-α-methylenecarboxylates bearing a bulky γ-substituent, such as CHMe₂, CHPh₂, c-C₆H₁₁ and CH(Ph)OTBDMS, with alkyl iodides gave the syn-adducts with high diastereoselectivities. However, the diastereoselectivity for the substrates bearing a γ-substituent CH(i-Pr)OTBDMS depended critically on the configuration of the substituent; the substrate bearing the OTBDMS group anti to the γ-benzyloxy group showed poor diastereoselectivity, but its diastereomer gave syn-adduct exclusively. The high syn-selectivitiy is referred to the H-atom transfer to the outside face of radical center in the sharply folded seven-membered chelate intermediate bearing the ethoxy group with Z-geometry. The corner flapping of the radical center atom of the global minimum energy conformer generates a local minimum conformer and the H-atom transfer to the outside face of the radical center of the newly formed structure gives the anti-adduct. The poor diastereoselectivity is due to the very small energy difference between the two conformers and consequently both the syn- and anti-adducts are yielded in nearly equal amounts.     

Chromium(II)-catalyzed diastereoselective pinacol type cross coupling: studies of substrate-controlled effects

Using 20 mol % of CrCl₂ as catalyst, manganese powder as reducing agent, and TMS-Cl as scavenger, various acroleins and aldehydes were coupled with moderate to high yields and diastereomeric excesses. Alkyl aldehydes usually favor syn configuration while aldehydes with functional groups containing chelating hetero atoms promote the formation of anti configuration. Using sterically demanding alkyl residues on the acrolein substrate, the syn configuration is definitely preferred.     

Détermination des configurations Z et E d'imines par RMP. Combinaison de l'étude directe et en présence d'Eu(fod)3

Analysis of NMR spectra of different cyclic and acyclic imines, together with those obtained with the lanthanide shift reagent Eu(fod)₃, allowed the characterisation of the syn-anti (Z-E) isomerism of these compounds. In one case, it was possible to demonstrate a syn-anti equilibration by adding Eu(fod)₃.     

Stereochemical course of acylation and aldol condensation in (4S)-4-methyl-2-benzyl-2,4-dihydro-1H-pyrazino[2,1-b]quinazoline-3,6-diones

Acylation of 1 leads to the syn derivatives 2. Traces of the anti isomers equilibrate to the syn isomers when stored in CHCl₃. Aldol condensation showed great diastereoselectivity, but racemization could not be avoided. The configuration of the new introduced stereogenic centres has been assigned on the basis of ¹H-NMR data and NOE measurements.     

Stereochemical course of the hydrogen migration in the boron trifluoride etherate-catalyzed rearrangement of 1,1-disubstituted epoxides

Mechanistic studies on the BF₃·Et₂O-catalyzed rearrangement of optically active, regioselectively deuterated 1,1-disubstituted epoxides to aldehydic products revealed that the two hydrogens migrate at the migration terminus with opposite stereochemical preferences, i.e. the hydrogen anti to the bulky substituent prefers to migrate with inversion of configuration, whereas the hydrogen syn to the bulky substituent prefers to migrate with retention of configuration.     

Pentadienyl transfer reagents based on zirconium: preparation and reactions with carbonyl compounds

A variety of 2,4-pentadienylzirconiums were generated by reacting pentadienyl ethers with zirconocene ‘Cp₂Zr’. These complexes underwent a highly γ-regioselective and anti-stereoselective in situ addition with carbonyl compounds to afford bis(homoallylic) alcohols in good yields. The reversal of anti vs syn selectivity was simply achieved with BF₃, thus expanding the synthetic potential of the reaction.     

Cephalosporines a chaines amino-2 thiazolyl-4 acetyles: Influence de la presence et de la configuration d'un groupe oxyimino sur l'activite antibacterienne

The syntheses of 2-(2-amino-4-thiazolyl) 2-(hydroxy or alkoxy)-imino acetic acid derivatives, with the anti (E) and syn (Z) configurations, are described. By means of these compounds, the acylation of the amino-group of 7β-amino cephalosporanic acid (7-ACA) has been achieved. The two resulting series of cephalosporin derivatives-anti and syn-are markedly different with respect to their antibiotic activity. Some of the syn compounds possess an antibacterial activity which is the highest ever observed.     

Highly stereoselective anti-aldol reactions catalyzed by simple chiral diamines and their unique application in configuration switch of aldol products

Chiral derivatives of trans-1,2-diaminocyclohexane with different N,N-dialkyl groups in well-defined orientations have been synthesized, and applied as catalysts for the asymmetric aldol reaction between a variety of aldehydes and ketones. Enantiomeric catalyst 1j catalyzed the reaction in ethanol and provided excellent diastereoselectivity and enantioselectivity. Significantly, simple replacement of organic solvents with water switched the products of the aldol reactions from anti to syn configuration. Such catalytic reactions led to the products with anti to syn diastereoselectivity up to 99:1 in ethanol, while in water gave the products with syn to anti diastereoselectivity up to 99:1.     

Regio-/stereoselective formation of monosulfoxides from thiacalix[4]arenes in all possible conformations

The partial S-oxidation of all four basic conformations of thiacalix[4]arene was carried out using NaBO₃·4H₂O as the oxidizing agent. It was found that despite the possible formation of many regio- and stereoisomers, the reactions leading to mono- or disulfoxides were highly regio- and stereoselective depending on the starting conformation. Our results clearly show that the sulfur atoms possessing syn-orientation of the appended phenolic units are remarkably more reactive than the sulfur atoms with anti-oriented aromatic subunits. Moreover, the most easily formed syn-oriented equatorial configuration is highly preferred over the corresponding axial arrangement which was never observed. As demonstrated by the resolution of racemic products, partial S-oxidation represents a very interesting tool to access novel inherently chiral building blocks based on thiacalix[4]arenes.     

Unusual sandwich platinum(II) complex: [Pt(phen)2]2+ cation between two Pt(phen)(OOCMe)2 molecules

New carboxylate platinum(II) complexes: syn and anti isomers of Pt(phen)(OOCMe)₂ molecular complex, [Pt(phen)(NCMe)₂](O₃SCF₃)₂, as well as unusual sandwich complex [Pt(phen)₂]²⁺ · 2syn-[Pt(phen)(OOCMe)₂] where [Pt(phen)₂]²⁺ cation is inserted between two syn-Pt(phen)(OOCMe)₂ molecules were synthesized and structurally characterized by X-ray diffraction analysis. As distinct from syn- and anti-Pt(phen)(OOCMe)₂ and [Pt(phen)(NCMe)₂](O₃SCF₃)₂ complexes with flat phenanthroline ligand, the phen ligands in [Pt(phen)₂]²⁺ cation have a curved configuration. Comparative DFT analysis of geometry of model structures phen, phen⁺, phenH⁺, and [Ptphen₂] ⁿ⁺ (n = 1, 2) showed that electron removal from phen molecule had no effect on its geometry in both free state and platinum(II) complexes.     

Synthetic utility of bowl-shaped tris(2,6-diphenyl-benzyl)silyl glyoxylate as a stable glyoxylate: application to highly diastereoselective aldol reactions

A stable glyoxylate can be successfully applied to both syn- and anti-selective aldol reactions by using two different kinds of ordinary Lewis acids. Thus, treatment of bowl-shaped tris(2,6-diphenylbenzyl)silyl glyoxylate 1 with enol silyl ether under the influence of BF₃·OEt₂ gave syn-aldol product, while the use of TiCl₄ afforded anti-aldol product with >97% selectivity.     

Study of the origin of enantioselectivity in cyclopropanation reactions using the chiral iron carbene complex [(η5-C5H5)(CO)2FeCH[(η6-o-MeOC6H4)Cr(CO)3]]+

During our low temperature NMR studies we observed two rotational isomers of the carbene complex [(η⁵-C₅H₅)(CO)₂FeCH[(η⁶-o-MeOC₆H₄)Cr(CO)₃]]+ (3) with the O–Me group either anti or anti to the Fp moiety. While the Cr(CO)₃ group very effectively shields one face of the carbene complex from attack by the olefin, the presence of anti and anti isomers allows for the formation of both R and S configuration on C-1 of the cyclopropane through a backside or a frontside ring closure mechanism. The reaction of olefin with anti R-3 can result in R-configuration of the cyclopropane carbon C-1 through a frontside closure mechanism, or in S-configuration if backside closure takes place. In a similar manner, anti R-3 may produce S-configuration through frontside closure or R-configuration through backside closure. We previously have shown by crystallography that reaction the R-isomer of 3 with 2-methyl-propene induces predominantly a R-configuration at C-1 of the resulting cyclopropane (RR-(−)-2,2 dimethyl-1-o-methoxyphenyl(tricarbonyl chromium)cyclopropane, whereas the S-carbene results in the corresponding SS isomer. These findings are consistent with cyclopropane formation from the syn isomer through a frontside closure mechanism or from anti isomer through a backside closure mechanism. In the case of [(η⁵-C₅H₅)(CO)₂FeCH[(η⁶-o-MeC₆H₄)Cr(CO)₃]]+ (4), only anti isomer is observed and optical rotation data indicate that the methylcarbene exhibits the same asymmetric induction (i.e., R-carbene yields R-cyclopropane C-1 and S-carbene yields S-cyclopropane C-1) as the methoxy analogue, and the assumption of the anti isomer being the reactive one then implies that the reaction proceeds through a backside closure mechanism rather a frontside mechanism. It is very likely that this preference is also valid for the methoxy substituted complex 4. Our results on 4 indicate that the enantioselectivity of the cyclopropanation reaction is not determined by the relative abundance of the isomers. As the syn isomer is the more abundant one, the anti isomer has to be the more reactive one compared to the syn isomer. Interchange of syn and anti isomers occurs fast compared to the rate of reaction of the carbene with olefin. The fast rate of interchange of syn and anti isomers relative to the rate of reaction with olefin precludes the direct observation of any differential reactivity form a change in the syn to anti ratio in the NMR spectrum. However, the in general lower ee values observed for 3 compared with 4 are consistent with the fact that the reactive isomer is less abundant in this case. Our data thus show that enantioselectivity of cyclopropanation with “chiral at carbene” complexes is controlled by the higher reactivity of the anti isomer and occurs through a backside ring closure mechanism.     

The cycloaddition reaction of 3,4-dimethyl-1-thio-1-phenylphosphole with tropone

3,4-Dimethyl-1-thio-1-phenylphosphole reacts, under elevated temperatures, with tropone to give two [4+2] isomeric adducts: Endo-syn-5,6-dimethyl-3-thio-3-phenyl-3-phosphatricyclo [5.3.2.0^2,6]dodec-4,8,11-trien-10-one (1) and Endo-anti-5,6-dimethyl-3-thio-3-phenyl-3-phosphatricyclo[5.3.2.0^2,6]dodec-4,9,11-trien-8-one (2). In addition to the endo-exo,syn-anti relationship the phosphorus configuration in 1, 2 and some derivatives was also elucidated by means of NMR-shift reagents and ²J_PH coupling constants.     

Synthesis of trihydroxylated pyrrolizidine and indolizidine alkaloids based on SmI2-induced reductive coupling of chiral nitrones with methyl acrylate

Synthesis of trihydroxylated pyrrolizidines, the enantiomer of 2-epihyacinthacine A₂ and indolizidine analogues of the natural alkaloids is reported. The key step of these syntheses is the stereoselective samarium diodide-induced coupling of the chiral nitrone prepared from d-ribose with methyl acrylate. The nitrone derived from d-ribose possessing C2/C3 syn configuration reacted with methyl acrylate in the presence of samarium diiodide in excellent yield and diastereoselectivity, with only the anti-diastereomer being detected.     

A MODEL FOR THE MOLECULAR STRUCTURE AND ORIENTATION OF THE CHROMOPHORE IN A DIMERIC PHYTOCHROME MOLECULE*

A model for the molecular structure and orientation of red-light absorbing form of phytochrome (P,) chromophores in a dimeric molecular model of P_r is proposed. A chromophore model with probable molecular structures was generated to reproduce the absorption spectrum produced by its π-electron conjugating system. The model has C₅-Z, syn, C₁₀-E, anti and C₁₅-Z, syn configurations and a protonation at a C-ring nitrogen. Orientation of the chromophore model in the dimeric phytochrome molecular was analyzed by displaying the atoms of the chromophore, the coordinates of which were converted into those with respect to the molecular axes to the dimeric molecule, on a 3-D graphic workstation. The conversions were performed by using the azimuthal angles between the Z axis of the dimeric molecule (axis of 2-fold rotational symmetry) and the dipole moments of the electronic transition at the blue- (384 nm) and red- (667 nm) absorbing bands of the chromophore, which were calculated as 55.5° and 59.3°, respectively, based on linear dichroism of the oriented phytochrome molecules. The result demonstrates that the long axis of the P, chromophore lies almost parallel to the Y axis of the molecular model, and that the tetrapyrrolic chromophore is well contained within the flat chromophoric domain without protruding from it, a configuration that assures that the chromophore is protected against aqueous environments. The model may explain the rotation angle of the transition moment of the red-absorbing band, induced by the phototransformation from P_r to P_rr which we measured as smaller than that measured in nonoriented preparations by a photoselection technique. The model also suggests a molecular basis for the polarotropic response of phytochrome.     

First report of syn isomers in the diastereoselective synthesis of highly functionalized piperidines catalysed by wet picric acid: factors influencing the syn-anti ratios

This is the first report of the formation of a syn-diastereoisomer in the diastereoselective synthesis of highly functionalized piperidines catalysed by wet picric acid via a one-pot, step-economic condensation of aromatic aldehydes, 1,3-dicarbonyl compounds and aromatic amines. DFT calculation shows that anti isomers are more stable compared to the syn. The syn-anti ratio of the products depends on the temperature and the nature of substitution on both the aromatic aldehydes and aromatic amines. Conditions for getting pure anti and pure syn compounds have been studied.     

Photocleavage of dimers of coumarin and 6-alkylcoumarins

The cleavage of four coumarin dimers, the syn-head-to-tail (ht) dimer of parent coumarin (syn-ht-CC1), the anti- and syn-hh dimers of 6-methylcoumarin (anti-hh-CC2 and syn-hh-CC2, respectively) and the anti-hh dimer of 6-dodecylcoumarin (anti-hh-CC3), was studied by UV–vis and IR spectroscopy and HPLC upon direct 254 nm irradiation as well as sensitized excitation. The quantum yield of dimer splitting is Φ_sp = 0.1–0.3 in various solvents and the effects of structure and solvent polarity are small. In certain solvents some of the dimers produced CO₂ along with the monomers in the splitting reaction. Electron transfer from dimers to the triplet state of sensitizers, such as benzophenone or 9,10-anthraquinone, was observed in acetonitrile.