Ideaux primitifs de certains produits croises

Sans résumé     

Capto-dative substituent effects. 121 - New ketene equivalents for diels-alder cycloadditions

We describe the use of “capto-dative” olefins $\text{1}$ as novel ketene equivalents which behave as efficient dienophiles in the Diels-Alder process.     

Inhibitor Ionization as a Determinant of Binding to 3-Dehydroquinate Synthase

Phosphinomethyl and carboxymethyl monoacids along with succinyl, malonyl ether, malonyl, and hydroxymalonyl diacids were substituted for phosphorylmethyl, phosphonoethyl, and phosphonomethyl groups in carbocyclic inhibitors of DHQ synthase. All but one of the carbocyclic inhibitors were synthesized via intermediacy of a 2,3-butane bisacetal-protected 3-dehydroquinic acid. Carbaphosphinate (K(i) = 20 x 10(-)(6) M) was a modest competitive inhibitor of DHQ synthase, while carbaacetate was a linear mixed-type inhibitor (K(i) = 3 x 10(-)(6) M, K(i)' = 20 x 10(-)(6) M). Carbasuccinate (K(i) = 5 x 10(-)(6) M), carbamalonate ether (K(i) = 7 x 10(-)(6) M), carbamalonate (K(i) = 0.7 x 10(-)(6) M), and carbahydroxymalonate (K(i) = 0.3 x 10(-)(6) M) were all competitive inhibitors. Carbaacetate was the only inhibitor that was not oxidized by DHQ synthase. On the basis of these data, carbocyclic inhibitors with malonyl and hydroxymalonyl groups are apparently bound by DHQ synthase as tightly as carbocyclic inhibitors possessing phosphorylmethyl and phosphonoethyl moieties.     

Synthesis and Antagonistic Activity of Two Covalently Linked Dimers of Adrenocorticotropin-(11-24)-tetradecapeptide

The chemical synthesis of two dimers of the sequence 11 to 24 of adrenocorticotropin is described. The two monomers were covalently linked through their N-or C-termini, respectively, using N-acetylglutamic acid or lysine amide as spacers. Compared to the monomer, the first dimer showed moderately, the second strongly increased inhibitory potency upon steroidogenesis in isolated adrenal cells.     

Stereoselective synthesis of pyrrolidines from N-allyl oxazolidines via hydrozirconation-cyclization

[reaction: see text] A new diastereoselective synthesis of pyrrolidines from readily available chiral N-allyl oxazolidines is presented. The construction of the pyrrolidine ring is achieved via a tandem hydrozirconation-stereoselective Lewis acid mediated cyclization sequence.     

High pressure phase equilibria in methane + waxy systems: 1. Methane + heptadecane

Fluid–fluid and fluid–solid phase equilibrium were experimentally determined under pressure on the system methane + heptadecane using a full visibility cell. Measurements were performed using the synthetic method on mixtures ranging from pure heptadecane to 99% of methane. The liquid–solid phase transitions were investigated up to 90 MPa and fluid phase boundary was studied in the temperature domain from 293 to 373 K. The appearance of a minimum in the three phase (V–L–S) equilibrium curve is discussed and it is shown that the difference in the solid phase structure and the presence of a solid–solid phase transition do not affect significantly the phase diagram determined.     

Regioselective allylic alkylation and etherification catalyzed by in situ generated N-heterocyclic carbene ruthenium complexes

Benzimidazolium halides are used for the first time as ligand precursors in ruthenium-catalyzed substitution of allylic carbonates and chlorides by carbon nucleophiles and phenols, respectively. After generation of diaminocarbene species upon deprotonation by tBuOK, their association with [Cp*Ru(MeCN)₃]PF₆ induces a very high regioselectivity in favor of the branched isomers when cinnamyl derivatives are used as starting substrates. They also provide good regioselectivities for the allylation of phenols by unsymmetrical aliphatic allylic substrates such as 3-chloro-4-phenylbut-1-ene, and thus provide a straightforward access to new allylic phenyl ethers.     

Acclimation of Haslea ostrearia to light of different spectral qualities - confirmation of 'chromatic adaptation' in diatoms

The marine diatom Haslea ostrearia was cultured under light of different qualities, white (WL), blue (BL), green (GL), yellow (YL), red (RL), and far-red (FRL) and at two irradiance levels, low and high (20 and 100 micromolphotonsm(-2)s(-1), respectively). The effects of the different light regimes were studied on growth, pigment content, and photosynthesis, estimated by the modulated fluorescence of chlorophyll, as relative electron transport rate (rETR). For all the light qualities studied, growth rates were higher at high irradiance. Compared to the corresponding WL controls, growth was higher in BL and lower in YL at low irradiance, and lower in YL and GL at high irradiance. Except for YL, almost all the pigment contents of the cells were lower at high irradiance. At low irradiance, cell pigment contents (chlorophyll a and c, fucoxanthin) and pigment ratios (in function of chlorophyll a) were lower in YL, RL, and FRL. Whatever the irradiance level, the maximum PSII quantum efficiency (F(v)/F(m) remained almost constant for WL, BL, and GL. Other fluorescence parameters (photochemical quenching, rETR(max), and alpha, the maximum light utilization coefficient) were lower in GL, YL, RL, and FRL, at low irradiance. Although not statistically significant, BL caused an increase in these fluorescence parameters. These findings are interpreted as evidence that inverse chromatic acclimation occurs in diatoms.