Concise means for accessing an advanced precursor to 1-deoxypaclitaxel

A program directed toward a total synthesis of 1-deoxypaclitaxel is described. The most direct route consists of six steps from the previously described diketone 12 and proceeds in 18% overall yield. The transformations involved in reaching the target molecule 22 consist of stereoselective alpha-ketol generation, an EtAlCl(2)-catalyzed transannular hydride shift, regioselective monomesylation, and a Wagner-Meerwein 1,2-shift. The central issue of this synthesis is the sequential deployment of these highly controlled steps along the perimeter and across the interior gap of a nine-membered ring.     

Computer-determined kinetic parameters from TG curves. Part IX

By applying the restriction that α (conversion) = constant to non-isothermal TG (NITG), isothermal TG (ITG) and a combination of both (NITG/ITG), various corresponding expressions have been derived for the evaluation of the activation energy, E. By means of a computer, all the expressions developed were tested against data which was generated from theoretical equations. Also, changes were made in the number of significant figures (s.f.) of data in order to ascertain their effects on values of E. Data were generated and tested for the three theoretical mechanisms, A₂, F₁ and R₂.     

Divergent mechanisms of suicide inactivation for ethanolamine ammonia-lyase

Ab initio molecular orbital calculations have been used to study the mechanism of suicide inactivation of ethanolamine ammonia-lyase induced by three different substrate analogues. Analysis of the normal catalytic mechanism with 2-aminoethanol (ethanolamine) as substrate predicts that both the hydrogen-abstraction and hydrogen-reabstraction steps involving the B(12)-cofactor are likely to be exothermic. On the other hand, the proposed inactivation mechanism for the first substrate analogue, glycolaldehyde, leads to a highly stabilized radical that results in a very endothermic (by ca. 90 kJ mol(-)(1)) hydrogen-reabstraction step, which is thought to halt the normal function of the enzyme. Curiously, the energy requirements for a catalytically imposed mechanism in the case of the second substrate analogue, 2-hydroxyethylhydrazine (HEH), parallel those for the catalytic substrate, despite the fact that HEH is found to inactivate EAL experimentally. However, further analysis reveals the presence of a lower energy pathway for HEH that leads to the formation of the highly stabilized hydrazinium radical cation. In a manner similar to when glycolaldehyde is the substrate analogue, this results in an endothermicity for the hydrogen-reabstraction step that is prohibitively large. In contrast to these related inactivation mechanisms, the third substrate analogue, 2-aminoacetaldehyde, apparently accomplishes the inactivation of EAL in an entirely different manner. A pathway for the experimentally observed formation of acetic acid and ammonium cation has been identified and appears catalytic in the sense that 5'-deoxyadenosyl radical is regenerated. However, mechanisms to account for the subsequent formation of 4',5'-anhydroadenosine and degradation of the corrinoid ring of the cofactor have not been elucidated.     

Monte Carlo evidence of non-equilibrium effects for ising model in a random field

Simulating an Ising model, up to sizes 150×150×150 in three and 225×225 in two dimensions, in a random field of strength ±H, we find a magnetization slowly decreasing with time if initially all spins are parallel. This decay is extremely slow for small random fields, so that a stable magnetization cannot be excluded. Below some freezing temperature, the system remains a single domain, with a finite magnetization in both two and three dimensions. If instead the system is cooled down in a constant random field, from temperatures above to temperatures below this freezing transition, then at the lower temperature the magnetization increases with time; this increase is very slow for large systems, with similar results in two and three dimensions.     

Pyrazolo[3,4-e] [1,4] thiazepines: Synthesis and structure proof

Pyrazolo[3,4-e][1,4]thiazepine derivatives were obtained by reacting 5-amino-1,3-dimethylpyrazole with arylaldehydes or ketones and mercaptoacetic acid. The structure proof of these derivatives was carried out by identifying the benzylpyrazole products obtained by desulfurization and subsequent hydrolysis, and by comparison of the spectral data of a series of analogous pyrazolothiazepines. Treating the pyrazolothiazepines with sodium hydride and methyl iodide in dimethylformamide or dimethylsulfoxide resulted in a ring contraction with the elimination of sulfur, to yield the pyrazolopyridones in addition to the N-methylpyrazolothiazepines.     

Microwave spectra of [15n] and [13c] pyridines,quadrupole coupling constants,dipole moment and molecular structure of pyridine

The microwave spectra of two ring-substituted species of pyridine have been remeasured, and the spectra of the two remaining species investigated and assigned.¹⁴N quadrupole coupling splittings were analyzed and used to determine the quadrupole coupling constants, while the derived center frequencies were fitted to give improved values for the rotational constants.A complete substitution structure can now be presented.The dipole moment of pyridine was redetermined from Stark measurements in the spectrum of [¹⁵N]pyridine.     

Conversion of the enantiomers of spiro[4.4]nonane-1,6-diol into both epimeric carbaspironucleosides having natural C1' absolute configuration

[reaction: see text] The enantiomers of spiro[4.4]nonane-1,6-diol have been transformed by different reaction pathways into the two possible carbaspironucleoside epimers with natural C1' absolute stereochemistry.     

Schwingungsspektren von Alkoxostibanen

Vibrational Spectra of Alkoxostibanes The vibrational spectra of the ethoxostibanes Sb(OEt)₃ ( 1 ), Sb(OEt)₂Cl ( 2 ), SbOEtCl₂ ( 3 ), Sb(OEt)₂Br ( 4 ), SbOEtBr₂ ( 5 ), and Sb(OEt)₂I ( 6 ) were assigned with the aid of the known structures of 2 and 3 in the solid state. The structures of 5 and 6 , which were prepared by new methods, differ from those of the corresponding chloro compounds. The coordination number of antimony in 5 and 6 is probable 5 and 4 resp.     

Synthese und Struktur von Hexa-t-butyl-1,4-dichloro-1,4-distanna-2,3,5,6,7,8-hexaphosphabicyclo[2.2.2]octan – Eine neue Käfigverbindung mit dem Sn(P2)3Sn-Gerüst

Synthesis and Structure of Hexa-t-butyl-1,4-dichloro-1,4-distanna-2,3,5,6,7,8-hexaphosphabicyclo[2.2.2]octane – a New Cage Compound with the Sn(P₂)₃Sn Skeleton The reaction of the diphosphide K₂[(tBuP)₂] 1 with SnCl₄ leads by a redox process mainly to (tBuP)_3,4 and other sideproducts. However, at the same time a threefold [2 + 1]-cyclocondensation reaction takes place yielding the new cage compound hexa-t-butyl-1,4-dichloro-1,4-distanna-2,3,5,6,7,8-hexaphosphabicyclo[2.2.2]octane, ClSn(tBuP? PtBu)₃SnCl 2 . 2 could be obtained in a pure form and characterized ³¹P and ¹¹⁹Sn NMR spectroscopically; 2 was also characterized by a single crystal structure analysis.     

Structural and energetic predictions for anions from a modified Hartree—Fock procedure

The performance of a modifed Hartree—Fock (MHF) procedure which differs from conventional HF theory through the inclusion of continuum functions in t