Three methods for the generation of pyranosyl nitrile oxide have been utilised: firstly isocyanate-induced dehydration of pyranosylnitromethanes (the Mukaiyama approach); secondly hypochlorite-mediated oxidation of pyranosyl aldoximes, and finally base-catalysed dehydrohalogenation of pyranosyl hydroxyimoyl chlorides. The pyranosyl aldoximes derived from D-mannose, D-glucose, D-xylose and L-fucose were synthesised via stannate (II) mediated reduction of the corresponding pyranosylnitromethane (80-90%), whilst direct chlorination of the aldoximes gave the hydroxyimoyl chlorides (80-95%). The efficiency of nitrile oxide generation by all three methods was illustrated by the high levels of 3,4-(dipyranosyl)-1,2,5-oxadiazole-2-oxides (furoxans) obtained by dimerisation on generation in the absence of a dipolarophile. For example, 3,4-(2,3,4,6-tetra-<i>O-</i>acetyl-β-D-mannopyranosyl)-1,2,5-oxadiazole-2-oxide was prepared from 2,3,4,6-tetra-<i>O</i>-acetyl-β-D-mannopyranosylnitromethane in a 90% yield. The D-glucose, D-xylose and L-fucose derived analogues were synthesised in high yields (79-96%) utilising all three methods. Where compatible, the Mukaiyama approach gave cycloadducts in good yields (>75%) (e.g. on reaction with methylenecyclohexane and styrene). However for low boiling or thermally unstable dipolarophiles or for dipolarophiles with functional groups incompatible groups incompatible with the isocyanate dehydrating agent (e.g. dimethyl acetylenedicarboxylate, allyl alcohol), cycloadditions were carried out successfully using pyranosyl aldoximes and pyranosyl hydroximoyl chlorides as precursors. The cycloaddition reactions with achiral alkenes proceeded with low levels of diastereoselectivity (d.e. < 10%), in contrast to the cycloaddition of chiral alkenes where significant p-facial selectivity was observed (d.e. 40-80%). The chemistry of the dipyranosyl furoxans has also been investigated. Hydrogenation of 3,4-(2,3,4-tri-<i>O</i>-acetyl-β-D-xylopyranosyl)-furoxan in the presence of Raney nickel yielded the corresponding dipyranosyl-1,2-dioxime in 77% yield. Preliminary studies of the chemistry of this class of dioxime have been carried out, for example dehydration to the 1,2,5-oxadiazole (furazan) and re-oxidation to the dipyranosyl furoxan. Attempts at complexation of these novel chiral ligands with nickel show promising results.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:641233 |
| Date | January 2002 |
| Creators | Baker, Kenneth William John |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/10709 |
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