Publisher Summary Acetal groups are largely used as protecting groups in synthesis and as model compounds for studying various chemical reactions. With the advent and general accessibility of new physical-chemical methods of analysis, particularly, nuclear magnetic resonance (n.m.r.) spectroscopy and mass spectrometry (m.s.), considerable progress has been made in knowledge of the stereochemistry of these fused-ring systems and of aspects such as diastereoisomerism. Cyclic acetal moieties occur widely in bacterial polysaccharides. The formation of cyclic acetals occurs by condensation of a diol with an aldehyde or ketone, the carbonyl compound often serving as the solvent. The oxonium ion reacts rapidly with the nearest hydroxyl group to give the first product, which can subsequently rearrange to give the thermodynamically most stable product. The reaction can be largely divided into a kinetic and a thermodynamic phase and the product of each of these can differ considerably from each other. Whether or not the products isolated are true equilibrium products depends on the stage at which the reaction is terminated, and on factors such as the solubility and the temperature.

Cyclic Acetals of The Aldoses and Aldosides Highlights of the Literature Since 1964, and A Supplement to the Tables

Synthesis and oxidative activation of an oxabicyclo[7.2.1]enediyne

Synthesis of an oxabicyclo[7.2.1] enediyne from a furanoside derivative

Iod-tris(trifluoracetat) (1)-oxidiert den cyclischen Ether 2 zu einem Gemisch (3), das aus den vier diastereomeren 3-Desoxypentofuranosen A–D besteht. Das Hauptisomere A, ein 3-Desoxy-threo-pentofuranose-Derivat, kristalliert aus dem Gemisch und ist somit in einem einzigen Syntheseschritt zuganglich, wahrend bisher mindestens sechs Stufen erforderlich waren. – Die 1H- und 13C-NMR-Spektren der vier Diastereomeren wurden analysiert (Abb. 1 und 2). – Laut 1H-NMR-Spektren konnen auch γ-Effekte zur Konfigurationsbestimmung dieser Funfringe herangezogen werden: γ-Substituenten, die von der trans- in die cis-Position wechseln, verursachen eine Tieffeldverschiebung des Protons zwischen 0.04 und 0.22 ppm.



Oxidation of Cyclic Ethers by Iodine Tris(trifluoroacetate). Synthesis and NMR Analysis of Some Diastereomeric 3-Deoxypentofuranose Derivatives



Iodine tris(trifluoroacetate) (1) oxidizes the cyclic ether 2 to yield a mixture (3) consisting of the four diastereomeric 3-deoxypentofuranoses A–D. The main isomer A, a derivative of 3-deoxy-threo-pentofuranose, crystallizes out of the mixture and is thus available in a single step, contrary to six steps necessary before. – The 1H and 13C NMR spectra of the four diastereomers were analysed (fig. 1 and 2). According to the 1H NMR spectra γ-effects may also be used for determining the configuration in these five-membered rings: γ-substituents changing over from the trans-to the cis-position cause a low field shift of the proton between 0.04 and 0.22 ppm.

Monte J. Withee

Papers

3-deoxy-2,5-DI-O-p-nitrobenzoyl-α-D-threo-pentofuranosyl bromide, a starting material for the synthesis of nucleosides☆