Advances in the chemistry of β-lactam and its medicinal applications

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Coordination chemistry of group 14 metalloles: VI. Crystal structure and dynamic stereochemistry of cis-dicarbonyl-bis(φ4-1,1,3,4-tetramethylsilole)molybdenum and related complexes

The crystal structure of cis-Mo(CO)₂(η⁴-1,1,3,4-tetramethylsilole)₂ (1) shows the presence of only the uu isomer, as an enantiomeric pair (Δ, Λ). Variable temperature NMR measurements reveal the presence in solution of the Δ and Λ enantiomers (presumably uu) for the four cis-MT(CO)₂(η⁴-silole)₂ complexes, M_T  Mo (1, 6), M_T  W (2) and cis-Mo(CO)₂(η⁴-germole)₂ (3), but also the diastereoisomers (ΔΛ uu and trans ou) in the case of cis-Mo(CO)(PPh₃)(η⁴-silole)₂ (5). IR spectroscopy confirms the presence of diastereoisomers for all the complexes and ³¹P NMR also shows this in the case of 5. The structures of the diastereoisomers are assigned on the basis of the multiplicity of the ¹H NMR signals. Both rotations of the η⁴ ligands and classical twists, of the type usually associated with octahedral chelate complexes, are necessary to explain this dynamic behaviour fully. The activation parameters have been calculated and the values of 30–40 kJ mol⁻¹ for ΔG* at the coalescence temperature are comparable to those obtained for similar systems.     

Synthesis of polycyclic alkenesvia reductive elimination of β-dicyano derivatives: a facile preparation ofAnti-sesquinorbornene and related molecules

Anti-Sesquinorbornene1b, the hitherto unknown related unsaturated molecules1a,a and the dienes13 and14 are among the endocyclic alkenes which can be prepared via reductive elimination of the respective, readily available β-dicyano derivatives.     

Catalytic phosphorylation of polyfluoroalkanols. 17. Synthesis and the stereochemistry of tris(α-trifluoromethylbenzyl) phosphates

Catalytic phosphorylation of -trifluoromethylbenzyl alcohols with POCl₃ taken in a ratio of 3 : 1 under particular temperature conditions afforded predominantly symmetrical tris(-trifluoromethylbenzyl) phosphates. The latter were obtained as mixtures of two diastereomers with a statistical ratio of the components.     

Inner and outer-sphere reductive elimination during decomposition of the platinum(IV) β-ammonioethyl complex in aqueous solutions

The -ammonioethyl complex of platinum (IV), KPt(CH₂CH₂NH₃)Cl₅, undergoes two types of transformations in aqueous chloride solutions,i.e., inner- and outer-sphere reductive elimination, depending on the pH of the medium. In acidic solutions outer-sphere decomposition, which consists of an S_N2 attack by the chloride ion of the -carbon atom of the complex anion [trans-Pt(CH₂CH₂NH₃)Cl₄(H₂O)]^– (or at the complex with a vacant coordination site in thetrans-position relative to the -CH₂CH₂NH₃ group; these two variants are kinetically indistinguishable) occurs to yield -chloroaminoethane and PtCl₄ ^2–. In alkaline solutions decomposition occurs after deprotonation of the ammonium group of the starting complex as an intramolecular nucleophilic attack of the amino group at the -carbon atom in [trans-Pt(CH₂CH₂NH₂)Cl₄(H₂O)]^– (or in the complex with a vacant coordination site in thetrans-position relative to the -aminoethyl group). In this case aziridine (Az) and PtCl₄ ^2– are the primary products. Aziridine is coordinated with the unreacted starting complex to give a compound that is stable with respect to reductive elimination. Cyclometallation of the latter species gives the uncharged complex insoluble in water.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 518–524, March, 1994.     

Kinetics and mechanism of thermal gas-phase elimination of α- and β- (N-arylamino)propanoic acid: experimental and theoretical analysis

2-(N-Phenylamino)propanoic acid 1a and 3-(N-phenylamino)-propanoic acid 2a together with four of their aryl analogues were pyrolysed in the gas-phase. The reactions were homogeneous and free from catalytic and radical pathways. Analysis of the pyrolysate of 1 showed the elimination products to be carbon monoxide, acetaldehyde and aniline, while the pyrolysate of 2 reveals the formation of acrylic acid in addition to aniline. Theoretical study of the pyrolysis of 2 using an ab initio SCF method lend support to a reaction pathway involving a 4-membered cyclic transition state.     

Chemoselective Removal of Acyloxy in 1-(Benzotriazole-1-yl)alkyl Esters and Its Application in the Preparation of β-(Benzotriazole-1-yl)alcohols

Mediated by samarium diiodide, 1-(benzotriazole-1-yl)alkyl esters 1 underwent cross-coupling with aldehydes or ketones to afford β-(benzotriazole-1-yl)alcohols 2 exclusively with the selective removal of acyloxy over benzotriazolyl.     

A new approach to (±)-heritonin. The preparation of β-tetralones from allylsilanes and acid chlorides

A new method for the preparation of 4-alkyl-β-tetralones is described, by reaction of arylacetic acid chlorides with allylsilanes. Employing β-tetralone 5, the synthesis of (±)-heritonine and (±)-epi-heritonine, natural piscicides isolated from Heritiera littoralis, was achieved in four steps and 22% overall yield. The key step of this synthesis involved the selenocarbenium ion-mediated elaboration of the butenolide ring of the natural product.     

Rhodium catalyzed deuteroformylation of styrene: (E)- and (Z)-β-deuterostyrene and β,β-dideuterostyrene formation via selective β-hydride elimination from the branched alkylrhodium intermediate

Deuteroformylation of styrene in the presence of Rh₄(CO)₁₂ as a catalytic precursor was carried out at 160 atm of CO and D₂ 1/1 at two temperatures (20 and 90°C) and for times yielding partial or complete conversion. Compounds recovered from the mixture produced by reaction and partial conversion at 90°C include unlabeled styrene, (E)- and (Z)-β-deuterostyrene, C₆H₅CHCHD, and β,β-dideuterostyrene, C₆H₅CHCD₂, whereas at room temperature the styrene does not take up deuterium. These results indicate that under hydroformylation conditions the branched alkylrhodium intermediate, which affords the branched aldehyde, in part dissociates into rhodium hydride and deuterated olefin. By contrast the linear alkyl intermediate does not dissociate under the same conditions, but instead yields almost completely the corresponding aldehyde.     

Switching the photoinduced processes in host-guest complexes of β-cyclodextrin-substituted silicon(IV) phthalocyanines and a tetrasulfonated porphyrin

Porphyrins and phthalocyanines are two attractive classes of functional dyes for the construction of artificial light harvesting and charge separation molecular systems. The assembly of these components by supramolecular approach is of particular interest as this provides a facile route to build multi-chromophoric arrays with various architectures and tuneable photophysical properties. We report herein a series of host-guest complexes formed between a tetrasulfonated porphyrin and several silicon(IV) phthalocyanines substituted axially with two permethylated β-cyclodextrin units via different spacers. As shown by electronic absorption and fluorescence spectroscopic methods, the two components bind spontaneously in a 1:1 manner in water with large binding constants in the range of 1.1 × 10(7) to 3.5 × 10(8) M(-1). The photophysical properties of the resulting supramolecular complexes have also been studied in detail using steady-state and time-resolved optical spectroscopic methods. It has been found that two major photoinduced processes, namely fluorescence resonance energy transfer and charge transfer are involved which are controlled by the spacer between the β-cyclodextrin units and the silicon centre of phthalocyanine. Despite the fact that charge transfer is a thermodynamically favourable process for all the complexes, only the ones with a tetraethylene glycol or oxo linker exhibit an efficient charge transfer from the excited phthalocyanine to the porphyrin entity. The lifetimes of the corresponding charge-separated states have been determined to be 200 and 70 ps by picosecond pump-probe experiments.     

On the clathrate form of elemental silicon,Si136: preparation and characterisation of NaxSi136 (x→0)

The clathrate form of silicon, Si₁₃₆ (otherwise known as Si₃₄), having a residual sodium content as low as 35 ppm (i.e., x∼0.0058 in Na_xSi₁₃₆), has been prepared by thermal decomposition of NaSi under high vacuum, followed by several other treatments under vacuum, and completed by repeated reactions with iodine. The residual amount of sodium has been determined by a combination of analytic and spectroscopic methods involving XRD, electron probe microanalysis, atomic absorption, NMR and EPR. This latter technique proved to be very appropriate to the characterisation of very diluted sodium atoms in such clathrate structure and to the quantitative determination of its residual concentration.     

Reactions of Direct Electrocarboxylation of 1-Phenylethylbromide and 1-(4-Isobutylphenyl)ethyl Chloride in the Presence of β-Cyclodextrin

Effect of supramolecular systems, in particular -cyclodextrin, in the reaction of direct electrochemical carboxylation of -bromoethylbenzene and 1-(4-isobutylphenyl)ethyl chloride is studied. It is shown that there is an in-principle possibility to perform enanthioselective synthesis in the reaction of direct reduction of aryl bromide with subsequent chemical reaction of electrophilic attachment of carbon dioxide. Effect of the substrate nature on the yield of enanthiomers in the electrocarboxylation conditions is demonstrated. It is found that -cyclodextrin has no impact on the enanthiomer composition of the carboxylation products of aryl chloride, which undergoes reduction at higher cathodic potentials.     

(R)- and (S)-α-Methoxy-(1-naphthyl) acetic acids: resolution by fractional crystallization and use for the NMR stereochemical analysis of alkylsulfoxides

Both enantiomers of α-methoxy-(1-naphthyl) acetic acid (1-NMA) were conveniently obtained via fractional crystallization using the enantiomers of 1-(1-naphthyl) ethylamine. 1-NMA was shown to be very powerful for differentiating the enantiomeric signals of quasi-symmetrical aliphatic sulfoxides.     

Enantiomerically pure chiral pyridino-crown ethers: synthesis and enantioselectivity toward the enantiomers of α-(1-naphthyl)ethylammonium perchlorate

Seven new enantiomerically pure chiral pyridino-crown ethers (S,S)-4–(R,R)-10 were prepared. Three of them [(S,S)-4, (S,S)-7 and (R,R)-10] contain one, and two of them [(S,S)-5 and (S,S)-8] contain two linker chains with a terminal double bond. These linker chains were connected to the carbon atom at position 9 (opposite the pyridine moiety) of the macrocycle. The terminal double bond of the linker makes it possible to attach these ligands to silica gel to obtain chiral stationary phases (CSPs). The enantioselectivity of the new ligands toward the enantiomers of α-(1-naphthyl)ethylammonium perchlorate (NEA) was also determined by a titration ¹H NMR method.     

Zinc catalyst recycling in the preparation of (all-rac)-α-tocopherol from trimethylhydroquinone and isophytol

Recycling of ZnCl₂ as a catalyst in the cyclocondensation of trimethylhydroquinone and isophytol to all-rac-α-tocopherol was studied. ZnCl₂ was recycled at over 98 % efficiency in required purity by repeated extraction of the reaction streams with water, followed by switching of solvent to butyl acetate and then back to water. Recycled ZnCl₂ had no negative effect on the yield of all-rac-α-tocopherol (maintaining over 90 %) at nearly total conversion of isophytol and only a 3 % molar excess of trimethylhydroquinone.     

Cyclization of α- and β-amino ketones. (Review)

In this review data on the synthesis of three-, four-, five-, and six-membered heterocycles, as well as of condensed heterocyclic compounds from -amino ketones and Mannich bases are classified for the first time.Samara State Technical University, Samara 433100, Russia. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 723–735, June, 1999.     

Synthesis of the disaccharide CD fragment found in everninomicin-C and -D,avalamycin-A and -C and curamycin-A: stereochemistry at the spiro-ortholactone center

The two isomers at the spiro-ortholactone center of the disaccharide CD fragment of orthosomycins have been synthesized. Their mild acidic hydrolysis was under stereoelectronic control with each isomer leading to only one ester. It is therefore possible, from these results, to deduce the absolute configuration of the spiro-center in natural antibiotics.