André A Pavia

BiBr3, an efficient catalyst for the benzylation of alcohols: 2-phenyl-2-propyl, a new benzyl-type protecting group

Abstract The benzylation of aliphatic alcohols with various benzylic alcohols has been achieved in the presence of BiBr3 under mild conditions. 2-Phenylpropan-2-ol proved to be the most efficient and can be considered as a novel protecting group.

Stereoelectronic control of oxazolidine ring-opening: structural and chemical evidences

Abstract Ring opening of oxazolidines derived from tris(hydroxymethyl)aminomethane, l -serine and l -threonine was investigated. It was shown that n(N)→σ∗(C–O) electron delocalization (endo-anomeric effect) occurring in the five-membered ring plays a major role in the cleavage of the intracyclic C–O bond. The present work establishes that when the nitrogen lone pair is conjugated with a carbonyl group (n(N)→π(CO) delocalization) as happens in N-acyloxazolidines, both hydrolysis and reductive ring-opening become much more difficult as a consequence of a concomitant decrease of oxygen basicity and of an increase of the intracyclic C–O bond strength.

Stereochemical study of 1,3-N,X-heterocycles derived from α-aminoacids and formaldehyde. Structural evidence for the existence of the anomeric effect

Abstract We have shown that condensation of l -cysteine methyl ester ( 8 ) and l -threonine methyl ester ( 11 ) with formaldehyde provides a convenient and efficient access to novel heterocyclic 2:3 adducts. Depending on the amino acid, the condensation leads to either the N,N′-methylenebis(thiazolidine) ( 10 , l -cys) or -(oxazolidine) ( 13 , l -thr) derivative or to its bicyclo[4.4.1]undecane isomer ( 5 , l -ser) as the major product of the reaction. The structure of 10 , 12 and 13 , was unambiguously confirmed by diffraction analysis and/or NMR spectroscopy. The latter proved to be a powerful tool to discriminate between the two possible isomers. X-Ray data emphasized the contribution of stereoelectronic effects to the structure of the above compounds.

Anomeric effects in non-carbohydrate compounds: conformational differences between the oxazolidine rings of a cis-fused bicyclic system

Tris(hydroxymethyl)aminomethane (Tris) can react with benzaldehyde (1:2 molar ratio) to produce cis-2,8-diphenyl-5-hydroxymethyl-1-aza-3,7-dioxabicyclo[3.3.0]octa ne, the structure of which has been confirmed by nuclear magnetic resonance spectroscopy and X-ray crystallography. The crystal structure showed that both oxazolidine rings A and B are puckered in opposite directions. Ring A exists in an E3 envelope form with 0-3 noticeably down (0.65 A) the plane of the remaining atoms, whereas ring B adopts the 7E envelope conformation with the 0-7 atom displaced up from the mean reference plane by 0.70 A. Comparison of bond angles and bond distances showed that both oxazolidine rings A and B exhibit cross endo-anomeric effects resulting from electron delocalization over the bond sequence O-3-C-2-N-1-C-8-O-7.

New fluorinated biocompatible non-ionic telomeric amphiphiles bearing trishydroxymethyl groups

Abstract New perfluoroalkylated telomeric surfactants derived from tris(hydroxymethyl)aminomethane were synthesized in two steps and 50% overall yield. These surfactants display significantly better surface activity and enhanced capacity to emulsify fluorocarbons compared to Pluronic F-68®. Preliminary in vitro and in vivo evaluation (cell cultures, hemolysis, LD50 i.v. in mice) indicates that this type of compounds is remarkably non-toxic, and could find applications in preparations for biomedical uses including injectable fluorocarbon emulsions. New perfluoroalkylated telomeric surfactants derived from tris(hydroxymethyl)aminomethane were synthesized in 50% overall yield. These surfactants display significantly better surface activity, enhanced capacity to emulsify fluorocarbons compared to Pluronic F-68®, and are not hemolytic.

Contribution of the anomeric effect to the solution and crystal structure of [1S,2S,6S,7S]-1,6-diaza-4,9-dioxa-2,7-dimethoxycarbonylbicyclo[4.4.1]undecane, a condensation product of l-serine methyl ester with formaldehyde

The reaction of L-serine methyl ester hydrochloride (1) with paraformaldehyde (2) in dichloromethane in the presence of triethylamine afforded a novel compound: [lS,2S,6S,7S]-1,6-diaza-4,9-dioxa-2,7-dimethoxycarbonylbicyclo[4.4.1]undecane (4) as a 2:3 adduct of 1 with 2. 1H and 13C NMR spectroscopy were unable to discriminate between two possible symmetrical structures. The latter was unambiguously proved by X-ray crystallography. The crystal structure established: (i) the existence of two identical seven-membered rings each containing a N-C-O grouping; (ii) the existence of a long C-O-C-N-C-N-C-O-C sequence in which each nitrogen belongs simultaneously to a N-C-O (oxazolidine) and to a N-C-N (aminal) motifs; (iii) the existence of a chair-like conformation for both seven-membered rings; (iv) the antiperiplanar geometry of pN-C-O and consequently the manifestation of a strong anomeric effect in both N-C-O groupings, whereas anomeric effect was virtually absent in the N-C-N sequence, as corroborated by bond distances and bond angles. Chemical shifts, coupling constants and NOE effects confirm that the conformational features of 4 are preserved in solution.

Synthesis of precursors of iodine-labeled multifunctional ligands containing 2-nitroimidazole for the detection of hypoxic tissues and/or tumors

Abstract Biocompatible multifunctional ligands with telomeric structure containing 2-nitroimidazole as affinity marker and tosylate as precursor of 123I radioactive tracer were prepared. Telomerization of appropriate monomers derived from tris(hydroxymethyl)-acrylamidomethane was performed in the presence of dodecanethiol, and either AIBN or tert-butylperoxide. These conjugates are designed to target hypoxic tissues and tumors.

Light Effects on α-amylase Activity and Carbohydrate Content in Relation to Lipid Mobilization during the Seedling Growth of Sunflower

The changes in α-amylase activity and in starch and free sugar content were investigated in correlation with lipid mobilization inHelianthus annuus during the first 15 days of seedling growth in discontinuous light and in darkness. Throughout the seedling development α-amylase activity increased more significantly in light than in darkness. It was always lower in cotyledons than in other tissues of the embryo axis. In both culture conditions, most of the transitory carbohydrates accumulated in germinating cotyledons were very likely synthesized by gluconeogenesis from the stored lipid breakdown. Nevertheless, in light-grown cotyledons, photosynthesis contributes to increase the carbohydrate levels. The study of several soluble sugars indicates that 1) sucrose stored in cotyledons of mature seeds was used at the onset of seedling growth, more rapidly in light than in darkness, 2) galactose and xylose, both involved as precursors of some cell-wall polysaccharides, remained at a very low level throughout the 15 days and 3) glucose, fructose and maltose accumulated in old etiolated cotyledons in contrast to what occurred in the light.

Preparation et proprietes solubilisantes de micelles polymerisees d'un tensioactif glycolipidique : le poly (N- (undecene - 10-yl) lactobionamide)

Resume We have investigated the micellar properties of an amphiphilic carbohydrate - containing molecule, the N-(undecen-10- yl ) lactobionamide ( 1 ) as well as those of its corresponding polymer 2 obtained by γ - irradiation Comparative studies showed a better efficiency of the polymer 2 compared to the monomer, especially for the solubilization of highly hydrophobic compounds and with respect to the enhancement of the rate of base - catalyzed hydrolysis of hydrophobic esters.

Telomers of tris(hydroxymethyl)-acrylamidomethane, a new class of non-ionic surfactants : synthesis and assessment of their properties

Abstract A new class of amphiphilic molecules suitable for biological purposes, especially extraction of membrane proteins and/or glycoproteins, is reported. These compounds were prepared by telomerization of a Tris-(Hydroxymethyl)-acrylamidomethane monomeric unit in the presence of an alkyl thiol as transfer reagent and of α,α′-(azobis)-isobutyronitrile (AIBN) as initiator. These compounds show similar or better detergent properties than commercially available surfactants.