1,721,169 research outputs found
ACID-CATALYZED AND ENZYMATIC-HYDROLYSIS OF TRANS-2-METHYL-3,4-EPOXYTETRAHYDROPYRAN AND CIS-2-METHYL-3,4-EPOXYTETRAHYDROPYRAN
No full textWhereas the acid-catalyzed hydrolysis of trans- and cis-3,4-epoxy-2- methyltetrahydropyran gives the corresponding diols by opening both at C-4 and C-3 (64% attack at C-4 in the trans-epoxide, 86% in the cis-epoxide), the hydrolysis of the same substrates catalyzed by rabbit liver microsomal epoxide hydrolase is entirely regiospecific and involves in both cases exclusive attack at C-4. The racemic cis-epoxide reacts at a faster rate than the trans. The 2R,3R,4S-enantiomer of the latter epoxide reacts at a much faster rate than its antipode to yield the (-)-(2R,3R,4R)-diol which is isolated at least 98% optically pure up to almost 50% conversion when starting from the racemic substrate. A reference sample of optically pure (+)-(2S,3S,4S)-diol was prepared from L-rhamnose. The enantiomeric excess of the (-)-(2R,3R,4R)-diol was also determined more precisely through g.l.c. analysis of diastereoisomeric MTPA esters. The present results confirm previous hypotheses on the topology of the hydrolase active site and emphasize the overriding importance of the orientation of a lipophilic substituent near the oxirane ring
Glycal of Common Monosaccharide: Useful Intermediates for the Synthesis of Rare Bioactive Carbohydrates
No full textA review with refs. In this article we will discuss the more relevant results of three not recently reviewed specific aspects of the aldopyranose glycal chem.; their prepn. from simple aldoses and their use for the stereoselective synthesis of 2-deoxy- and 2-deoxy-2-aminoglycosides, two classes of sugars constituent of the glycidic portion of important types of biol. relevant substance
AN IMPROVED PREPARATION OF 3,4-O-ISOPROPYLIDENE DERIVATIVES OF ALPHA-D-GALACTOPYRANOSIDES AND BETA-D-GALACTOPYRANOSIDES
No full text
Synthesis of some 4-keto-D-glucose derivatives and their transformation in 1-deoxyazafuranoses
No full textSintesi regioselettiva di 4-desossi- e 2-ammino-2,4-dideossi-beta-D-treo-3-esenopiranosidi a partire da beta-D-galattipiranosidi.
No full textLactose as an inexpensive starting material for the preparation of aldohexos-5-uloses: synthesis of L-ribo and D-lyxo derivatives
No full textSUMMARY: Partially protected derivatives of L-ribo- and D-lyxo-aldohexos-5-ulose have been prepared starting from triacetonlactose dimethyl acetal derivatives. Key steps of the synthetic sequences are a) the synthesis of 4'-deoxy-4'-eno- and 6'-deoxy-5'-eno lactose derivatives, and b) the epoxidation-methanolysis of the above enol ethers to give 1,5-bis-glycopyranosides, masked form of the target 1,5-dicarbonyl hexoses
CONFORMATIONAL-ANALYSIS OF 3,4-EPOXYTETRAHYDROPYRAN BY NMR-SPECTROSCOPY
No full textThe conformational equilibrium of the title compound has been determined by correlating its n.m.r. parameters with those of its 2,2,6,6-tetradeuterio derivative and trans- and cis-2-tert-butyl-4,5-epoxytetrahydropyran. The high preference (ΔG ∼ -0.8 kcal/mol) for the half-chair conformation in which the pyranoid oxygen is furthest from the oxirane oxygen atom can be interpreted in terms of electrostatic interactions between the two oxygen atoms
Enzymatic preparation and absolute configuration of (+)-2-(hydroxymethyl)cyclohexanone
No full textLipase mediated treatment of racemic 2-(hydroxymethyl)cyclohexanone, (+/-)-1, with vinyl acetate allows enantioselective acetylation to give the acetate (R)-(+)-4 and unreacted (S)-(+)-1 in satisfactory yield and enantiomeric purity. The absolute configuration of (S)-(+)-1 (and therefore that of (R)-(+)-4 as well) was determined by the analysis of the n-->pi* Cotton effect at 290 nm present in the CD spectrum of (S)-(+)-1 by means of the octant rule and CNDO/S calculations. Furthermore the optical rotatory power of optically pure (S)-(+)-1 and (R)-(+)-4 have been determined
- …
