Effect of an 8-Hydroxymethyl Substituent on the Base-Catalyced Ring Opening of 7,8-Epoxyiridoid Glucosides

Article abstract of DOI:10.1021/jo00165a014

Base-catalyzed ring opening of 7,8-epoxyiridoids has been found to be dependent on the substitution pattern at C-8.Epoxyiridoids 5, 6, and 10 (CH3-8) react in barium or sodium hydroxide solutions through a normal S_N2 process, yielding the expected 7α,8β-diols while for iridoids 3 and 4 (CH2OH-8) the stereochemistry of the final diol function is reversed (7β,8α).This abnormal S_N2 process is explained by a preliminary "epoxide migration" from carbons 7,8 to 8,10 followed by normal oxirane cleavage.Participation of the CH2OH-8 substituent in the cleavage reaction is proved by the behavior of the hexa-O-methyl derivative 3 which gives the normal 7α,8β-diol.Four new nonnatural iridoid glucosides (9, 11, 14, 16) are also described and spectroscopically (1H and 13C NMR) characterized.