[3+2] Cycloadditions. Part XXXIII. Selective cycloadditions of C-(1-naphthyl)-N-methyl nitrone and C-phenyl-N-benzyl nitrone to α,β-unsaturated carbonyl compounds1,2

488-501[3+2] Cycloadditions [32CA] involving nitrones as 1,3-dipolar species to alkenes yield isoxazolidines, which on further transformations can be converted to naturally occurring bioactive compounds. Thus the 32CA route provides conversion of simple natural products to more complex natural occurring bioactive nitrogen heterocycles, and close analogues. The present work deals with 32CA between the nitrones C-(1-naphthyl)-N-methyl nitrone and C-phenyl-N-benzyl nitrone to α,β-unsaturated carbonyl compounds as dipolarophiles, viz. arylidene malonate esters, methyl cinnamate and benzylidene acetophenone (chalcone). Methyl cinnamate is a naturally occurring compound, while the chalcone scaffold is present in several natural products. Structure elucidation of the generated cycloadducts have been achieved by means of detailed spectroscopic and XRD studies. All the cycloadditions investigated occurr regioselectively to yield tetra/penta-substituted isoxazolidines, where the carbonyl group(s) are situated at the 4-position of the isoxazolidine ring. DFT computations including optimised geometries, FMO energies, electronic chemical potentials, chemical hardness, chemical softness and reactivity indices of a number of the reactants have been calculated at DFT/B3LYP/6-31++G(d,p) level of theory. The calculated reactivity indices have been used to analyse the 32CAs studied and to predict the regioselectivities; the predictions are in excellent accord with the experimental results

Structural analysis of urate oxidase in complex with its natural substrate inhibited by cyanide: Mechanistic implications

<p>Abstract</p> <p>Background</p> <p>Urate oxidase (EC 1.7.3.3 or UOX) catalyzes the conversion of uric acid and gaseous molecular oxygen to 5-hydroxyisourate and hydrogen peroxide, in the absence of cofactor or particular metal cation. The functional enzyme is a homo-tetramer with four active sites located at dimeric interfaces.</p> <p>Results</p> <p>The catalytic mechanism was investigated through a ternary complex formed between the enzyme, uric acid, and cyanide that stabilizes an intermediate state of the reaction. When uric acid is replaced by a competitive inhibitor, no complex with cyanide is formed.</p> <p>Conclusion</p> <p>The X-ray structure of this compulsory ternary complex led to a number of mechanistic evidences that support a sequential mechanism in which the two reagents, dioxygen and a water molecule, process through a common site located 3.3 Å above the mean plane of the ligand. This site is built by the side chains of Asn 254, and Thr 57, two conserved residues belonging to two different subunits of the homo-tetramer. The absence of a ternary complex between the enzyme, a competitive inhibitor, and cyanide suggests that cyanide inhibits the hydroxylation step of the reaction, after the initial formation of a hydroperoxyde type intermediate.</p

A new paradigm for macromolecular crystallography beamlines derived from high-pressure methodology and results

Macromolecular crystallography at high pressure (HPMX) is a mature technique. Shorter X-ray wavelengths increase data collection efficiency on cryocooled crystals. Extending applications and exploiting spin-off of HPMX will require dedicated synchrotron radiation beamlines based on a new paradigm

Adaptation of the base-paired double-helix molecular architecture to extreme pressure

The behaviour of the d(GGTATACC) oligonucleotide has been investigated by X-ray crystallography at 295 K in the range from ambient pressure to 2 GPa (∼20 000 atm). Four 3D-structures of the A-DNA form (at ambient pressure, 0.55, 1.09 and 1.39 GPa) were refined at 1.60 or 1.65 Å resolution. In addition to the diffraction pattern of the A-form, the broad meridional streaks previously explained by occluded B-DNA octamers within the channels of the crystalline A-form matrix were observed up to at least 2 GPa. This work highlights an important property of nucleic acids, their capability to withstand very high pressures, while keeping in such conditions a nearly invariant geometry of base pairs that store and carry genetic information. The double-helix base-paired architecture behaves as a molecular spring, which makes it especially adapted to very harsh conditions. These features may have contributed to the emergence of a RNA World at prebiotic stage

Functional relevance of the internal hydrophobic cavity of urate oxidase

AbstractUrate oxidase from Aspergillus flavus is a 135kDa homo-tetramer which has a hydrophobic cavity buried within each monomer and located close to its active site. Crystallographic studies under moderate gas pressure and high hydrostatic pressure have shown that both gas presence and high pressure would rigidify the cavity leading to an inhibition of the catalytic activity. Analysis of the cavity volume variations and functional modifications suggest that the flexibility of the cavity would be an essential parameter for the active site efficiency. This cavity would act as a connecting vessel to give flexibility to the neighboring active site, and its expansion under pure oxygen pressure reveals that it might serve as a transient reservoir on its pathway to the active site

X-ray crystallography of proteins and gas: methods and interpretation

CERVOxyInternational audienc

A convenient synthesis of C-22 and C-25 stereoisomers of cephalostatin north 1 side chain from spirostan sapogenins

3 pages, 2 schemes.-- PMID: 11950346 [PubMed].-- Printed version published Apr 18, 2002.-- Supporting information available at: http://pubs.acs.org/doi/suppl/10.1021/ol025580eA simple transformation of the eight-carbon side chain of a natural spirostan sapogenin into the cephalostatin north 1 spiroketal moiety is described. This methodology, based on an intramolecular hydrogen abstraction reaction promoted by alkoxy radicals, permits the synthesis of C-22 and C-25 stereoisomers of the dioxaspiro[4.4]nonane cephalostatin ring system. The acid-catalyzed isomerization of the spirocenter in the different isomers is studied.This work was supported by Investigation Programmes BQU2000-0650 and BQU2001-1665 of the Dirección General de Investigación, Spain. I.P.-M. thanks the Cooperativa Farmacéutica de Tenerife (COFARTE), Spain, for a fellowship

1,3-Dipolar cycloadditions. Part XXI: Catalytic effects of Lewis acids on 1,3-dipolar cycloaddition of <i style="">C</i>-(4-chlorophenyl)-<i style="">N</i>-phenyl nitrone to benzylidene acetophenone

835-842The normal electron demand reaction of C-chlorophenyl-N-phenyl nitrone with benzylidene acetophenone and the effects of mild Lewis acid catalysts like metal triflates and magnesium bromide on the reaction rate are reported. Frontier molecular orbital energies are calculated to rationalise the regioselectivity of the cycloaddition

Azide inhibition of urate oxidase

International audienc