Heating methyl 4-O-benzoyl-2,6-dideoxy-3-O-triflyl-β-D-lyxo-hexopyranoside (1) at 100 °C in aqueous pyridine opens the pyranoid ring to form 3-O-benzoyl-2-O-formoyl-4-penten-2,3-diol (2) as the primary product. [Under the reaction conditions 2 is partially hydrolyzed to 3-O-benzoyl-4-penten-2,3-diol (3).] Mechanistic study of this reaction indicates that the carbocation 6 and the hemiacetal 7 are both intermediates in the reaction process. The importance of stereochemistry at C1 and C3 (the two chiral centers involved in the ring-opening reaction) was demonstrated by studying two additional triflates, methyl 4-O-benzoyl-2,6-dideoxy-3-O-triflyl-β-D-ribo-hexopyranoside (11) and methyl 4-O-benzoyl-2,6-dideoxy-3-O-triflyl-α-D-lyxo-hexopyranoside (14). Compound 11 experienced no ring-opening but rather exclusive triflate displacement by pyridine. For 14 the major process was E2 elimination to give methyl 4-O-benzoyl-2,3,6-trideoxy-α-D-glycero-hex-3-enopyranoside (15), while ring-opening was only a minor reaction pathway.
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