Antonio Alemany

The conformation of some 1,2-O-alkylidene-β-l-lyxo- and -β-l-arabino-pyranoses in the solid state and in solution

Abstract The structures of 3,4-di- O -acetyl-1,2- O -( S )-(1-cyanoethylidene)-β- l -lyxo-pyranose ( 1 ) and its R -isomer ( 2 ) have been determined from X-ray diffraction data. The respective space groups are P 2 1 with Z = 4 (two independent molecules) and P 2 1 2 1 2 1 with Z = 4. The pyranoid ring has the following conformations: 3,o B in 1a , o S 2 in 1b , and 1 C 4 in 2 . The conformations of the dioxolane rings are E o-2 for 1a and 1b , and E 1 for 2 . 1 H- and 13 C-N.m.r. experiments and molecular mechanics calculations have been performed in order to investigate the conformation in solution for both the pyranoid and dioxolane rings in 1 , 2 , and their arabinopyranose analogues.

Conformation of 1,2-O-(S)-(1-aminomethylethylidene)-α-d-glucopyranose and 1,2-O-(R)-(1-tert-butoxyethylidene)-α-d-glucopyranose

Abstract The crystal structures of 1,2- O -( S )-(1-aminomethylethylidene)-α- d -glucopyranose ( 1 ) and 1,2- O -( R )-(1- tert -butoxyethylidene)-α- d -glucopyranose ( 2 ) have been analysed by X-ray diffraction methods. The pyranoid rings in 1 and 2 have flattened 4 C 1 conformations, the dioxolane ring in 1 has a half-chair 1 T O-1 conformation, and that in 2 a 2 T O-2 conformation distorted towards 2 E . The 1 H-n.m.r. spectra of 1 and 2 have been analysed by computer simulation. The computed best values of the vicinal coupling constants and the n.O.e. measurements can be interpreted in terms of a preponderant, distorted 4 C 1 conformation in solution, although long-range H,H coupling and low-temperature 13 C-n.m.r. spectra suggest the presence of a conformational equilibrium.

The solid state and solution conformation of 1,2-o-alkylidene-α-D-ribopyranoses. Chirality assessment of pentoses from crystallograph

Abstract The structure of 3,4-di- O -acetyl-1,2- O -( S )-(1-cyanoethylidene)-α- D -ribopyranose ( 1 ) has been determined from X-ray diffraction data and refined to a conventional R value of 0.050 for the 1259 observed reflexions (3 σ (I) criterion). The compound packs in the P212121 space group with Z = 4 and lattice parameters of a = 20. 0157 (7), b = 10.0552 (2), and c =7.0870(1) a. The conformation of the pyranoid ring in the solid state can be described as chair 1C4 ( D ) and that of the dioxolane ring, as envelope E 1. The use of configurational angles for characterization of pentoses is presented. NMR studies have been carried out to determine the major conformation in solution of 1 and its analogues 2 - 5 . The results seem to indicate in all cases the presence of a conformational equilibrium between the chair-like, 1C4, and the boat-like, ° S 2 conformation, in agreement with the results obtained from molecular mechanics calculations.

The solid-state and solution conformation of some 1,2-O-alkylidene xylopyranoses. Application of heteronuclear long-range coupling constants to the conformational analysis of dioxolane rings

The molecular structures of 3,4-di-O-acetyl-1,2-O-[(R)-1-cyanoethylidene]-α-D-xylopyranose (1) and its (S)-isomer (2) have been determined from X-ray diffraction data. The pyranoid ring adopts a 1C4 chair conformation for both compounds and the dioxolane ring presents E1 and 2E distorted envelope conformations for (1) and (2) respectively. The solution conformation of (1) and (2), and the ethylidene analogues (3) and (4), have been determined by n.m.r. spectroscopy. Proton–proton coupling constants, variable-temperature and n.O.e. experiments, and molecular mechanics calculations indicate that the major conformation of the pyranoid ring can be described as 1C4 for (1), ∘S2 for (2) and (4), and as an equilibrium among 4C1, 1C4, and ∘S2 forms for (3). Long-range carbon–proton coupling constants can be interpreted in terms of E1 and EO2 conformations for the dioxolane ring of (1) and (2) respectively.

Stereoselective Synthesis of 3-C-Branched-Chain Sugars by Aldol Reaction of Furanos-3-Uloses with Acetone

Abstract Aldol reaction of 1,2-O-isopropylidene-5-O-tertbutyl-dimethylsilyl-α-D-erythro-pentofuranos-3-ulose (1) with acetone in the presence of aqueous K2CO3 afforded 3-C-acetonyl-1,2-O-isopropylidene-5-O-tertbutyl-dimethylsilyl-α-D-ribofuranose(2). Similar reaction of 1,2:5, 6-di-o-isopropylidene- α-D-ribo-hexofuranos-3-ulose (3) afforded 3-C-acetonyl-1,2:5, 6-di-o-isopropylidene- α-D-allofuranose (4) and (1R, 3R, 7R, 8S, 10R)-perhydro-8-hydroxy-5,5,10-trimethyl-2,4,6,11,14-pentaoxatetracyclo[8,3,1,01,8,03,7] tetradecane. The stereochemistry of the new chiral centers were determined by 1H NOE experiments.

Tautomerism of substituted 2H-1,2,6-thiadiazin-3-one 1,1-dioxides: 1H, 13C, and 15N nuclear magnetic resonance studies

Tautomerism of 2,5-disubstituted (4), 4,5,6-substituted (5), and 5-substituted 2H-1,2,6-thiadiazin-3-one 1,1-dioxides (6) has been studied by u.v., 1H, 13C and natural-abundance 15N n.m.r. spectroscopies. The effect of solvent concentration, temperature, and the nature of the substituents on the KT variation has been investigated. Comparison between the tautomeric behaviour of 2H-1,2,6-thiadiazin-3-one derivatives and the related pyrazol-5-ones provides some conclusions about the role of aromaticity on the stability of the different tautomers.