Hydroformylation of glycals
Creators: name: Abson, Derek
Publisher: name: University of British Columbia
Year: 2011

description: The reaction of 3,4-di-0-acetyl-D-xylal with 3 moles of synthesis gas (CO + 2H₂) under oxo conditions gave predominantly two isomeric 2,3-di-0-acetyl-1,5-anhydro-4-deoxy-hexitols, by addition of a hydroxymethyl group at C-1 of the glycal. The structures of the two polyols, obtained by deaeetylation of the reaction product and fractionation by paper partition chromatography, were completely established. Formation of a pair of enantiomeric triol ethers by periodate cleavage and sodium borohydride reduction of each polyol showed that they were l,5-anhydro-4-deoxy-hexitols, having unbranched carbon skeletons, this also being shown by the proton resonance positions and intensities in the n.m.r. spectra of the polyols. One of the enantiomeric triol ethers, having the L-configuration, was prepared from a carbohydrate of known structure, 1,4-anhydro-5-deoxy-D-arabino-hexitol, thereby establishing the configurations at C-5 of the two isomeric 1,5-anhydro-4-deoxy-hexitols. Assignments of the D-arabino and L-xylo- configurations to the two isomers conflicted with results of Gorin⁸², who had previously assigned the D-arabino- configurations to a 1,5-anhydro-4-deoxy-hexitol which did not resemble either of our compounds. That these were the D-arabino- and L-xylo- isomers of 1,5-anhydro-4-deoxy-hexitol was proved by their conversion into a pair of isomeric 1,5-anhydro-4,6-dideoxy-hexitols which were identical with those obtained by the reaction of 3,4-di-0-acetyl-2-deoxy-D-xylopyranosyl chloride with methyl magnesium bromide, both series of reactions allowing no possibility of configurational inversions. The polyol described by Gorin was subsequently shown to be the alternative trans isomer, 1,5-anhydro-4-deoxy-D-xylo-hexitol. A concurrent study of the structures of the two anhydrodeoxyhexitols was made by nuclear magnetic resonance, and the stereochemistry of the L-xylo- isomer could be assigned from the multiplicities of the C-4 proton signals. The single C-4 proton in the deuterated analogue of the L-xylo- isomer (prepared by reacting 3,4-di-0-acetyl-D-xylal with carbon monoxide and deuterium) was shown to be equatorial by its resonance position, and its multiplicity on deuterium-hydrogen decoupling, this providing evidence for cis- addition to the double bond of the glyeal on hydroformylation. The oxo reaction of 3,4,6-tri-0-acetyl-D-galactal has been reinvestigated, and found to be entirely analogous to those of other glycals, giving, on deacetylation, a mixture of 2,6-anhydro-3-deoxy-D-galacto- and D-talo-heptitols. These were isolated and characterised, and their stereochemistry established by correlation with the D-gluco-isomer, whose structure has been proved by X-ray analysis. The reaction of 3,4-di-0-acetyl-D-xylal under hydro-formylation conditions, leading to the formation of aldehydes rather than alcohols, has been investigated. From the reaction of the glycal with 2 moles of synthesis gas, two isomeric 4,5-di-0-acety1-2,6-anhydro-3-deoxy-aldehydo-hexoses were isolated as their crystalline 2,4-dinitro-phenylhydrazones. These were identified by conversion of one of them, 4,5-dl-0-acety1-2,6-anhydro-3-deoxy-aldehydo-D-lyxo-hexose, to 1,5-anhydro-4-deoxy-D-arabino-hexitol, whose structure had been established previously. The two aldehydo-hexoses were also obtained when a mixture of 2,3-di-0-acetyl-1,5-anhydro-4-deoxy-D-arablno- and L-xylo-hexitols were reacted with dimethylsulphoxide and N,N'-dicyclohexylcarbodiimide.