Search PubMed⌕ Search

Biomedical subjects

A Riera

Publications and source records attributed to A Riera.

At least 19 recordsLinked to original sources
PubMed2000

Low-energy pathway for pauson-khand reactions: synthesis and reactivity of dicobalt hexacarbonyl complexes of chiral ynamines

A family of dicobalt hexacarbonyl complexes of 1-(dialkylamino)-2-(trimethysilyl)acetylenes (3a-f) derived both from achiral and chiral amines [a, morpholine; b, (S)-2-methoxymethylpyrrolidine; c, (2R,5R)-2, 5-bis(methoxymethyl)pyrrolidine; d, (+/-)-trans-2, 5-bis(benzyloxymethyl)pyrrolidine; e, (2R,5R)-2, 5-dimethylpyrrolidine; f, (S)-(alpha-methylbenzyl)benzylamine] has been prepared by a one-stage process from dichloroacetylene. The methanolysis at room temperature of these complexes (MeOH/K(2)CO(3)) induces the selective cleavage of the carbon-silicon bond, leading to the thermally unstable terminal ynamine complexes 12a-f. The Pauson-Khand reaction of 12a-f with strained olefins (norbornadiene and norbornene) takes place at unprecedentedly low temperatures (-35 degrees C) in the absence of chemical promoters. Diastereoselectivities of up to 94:6 are recorded in these reactions with the ynamine complexes derived from C(2) symmetrical chiral auxiliaries (12c,d). The high reactivity depicted by terminal ynamines in the Pauson-Khand reaction has been analyzed by theoretical semiempirical procedures [PM3(tm)] and with density functional theory (VWN/B88), and appears to reflect an easy CO loss from the ynamine-dicobalt hexacarbonyl complexes.

Journal Article↗
PubMed2000

Highly enantioselective addition of diethylzinc to diphenylphosphinoyl imines under dual amino Alcohol/Halosilane mediation

Arylethylene-derived, enantiomerically pure amino alcohols have been evaluated as ligands for the dual-catalyzed (amino alcohol/halosilane) enantioselective addition of diethylzinc to diphenylphosphinoyl imines. Among them, the conformationally restricted 9-fluorenone-derived ligand 4c provides the highest enantioselectivities so far reported over a range of substrate imines.

Journal Article↗
PubMed2000

A concise enantioselective entry to the synthesis of deoxy-azasugars.

[reaction: see text] A concise enantioselective preparation of oxazolidinylpiperidine 4, a key intermediate in the synthesis of glycosidase inhibitors such as 1-deoxymannojirimycin or 1-deoxygalactostatin, has been developed. Sharpless catalytic asymmetric epoxidation of (E)-2,4-pentadienol followed by treatment with allyl isocyanate afforded epoxy carbamate 8. Regioselective intramolecular ring opening promoted by sodium bis(trimethylsilyl)amide and ring-closing metathesis provided the bicyclic intermediate 4 in high enantiomeric purity. The four-step sequence takes place in 51% overall yield.

1-Deoxynojirimycin↗
PubMed1991

Vitamin B-12 abnormalities in HIV-infected patients.

A prospective study of 60 consecutively admitted patients with HIV infection was performed to document the prevalence, etiology and manifestations of low serum vitamin B-12 in such patients. Low serum B-12 levels were found in 10 patients (16.7%). In 6, vitamin B-12 absorption was impaired and hog intrinsic factor addition did not improve it. Patients with low vitamin B-12 levels showed lower hemoglobin, leukocytes, lymphocytes, CD4 lymphocytes and CD4/CD8 lymphocyte ratio than HIV patients with physiological serum vitamin B-12 levels. However, bone marrow megaloblastosis was found in only 3 low vitamin B-12 patients and the deoxyuridine suppression test was pathological in only 1 case. In 7 patients, parenteral treatment was begun with variable response despite serum vitamin B-12 correction. In conclusion, low serum vitamin B-12 is often found in HIV-infected patients and it could be related to malabsorption, but clear megaloblastic abnormalities and treatment response could not be demonstrated. A decreased concentration of the serum binders due to disturbances in the leukocytes and related immunocompetent cell may play an additional role.

Adult↗