Unsaturated carbohydrates. Part 26. Alkenes from 4-bromohexofuranose esters; reactions of 5-deoxyald-4-enofuranose derivatives in the presence of mercury(II) ions
Removal of hydrogen bromide from mixed 1-O-acetyl-2,3,5,6-tetra-O-benzoyl-4-bromo-β-D-gluco- and galacto-furanose with 1,5-diazabicyclo[5.4.0]undec-5-ene gives the endocyclic 3-ene (2), whereas treatment with a zinc–copper couple in aqueous acetic acid affords a mixture of 1-O-acetyl-2,5,6-tri-O-benzoyl-3-depxy-β-D-erythro-hex-3-enofuranose (4) and the exocyclic (E)- and (Z)-5-deoxyhex-4-enofuranose isomers (3). The latter compounds and methyl 2,3-di-O-benzoyl-5-deoxy-α-D-threo-pent-4-enofuranoside (9) in the presence of mercury(II) salts do not give cyclopentanone derivatives. Instead, the C-5-mercury intermediates undergo elimination or hydrolysis reactions. This resistance to ring closure under conditions in which 6-deoxyhex-5-enopyranose derivatives readily give cyclohexanones is consistent with Baldwin's rules for ring closure.