Structural Revision of Some Recently Published Iridoid Glucosides

Structures of six different iridoid glucosides have been revised. Three compounds isolated from Eremostachys glabra and designated 6,9-epi-8-O-acetylshanziside (1), 5,9-epi-penstemoside (2), and 5,9-epi-7,8-didehydropenstemoside (3) have been shown to be identical to the known iridoids barlerin (4, 8-O-acetylshanziside), penstemoside (5), and 7,8-didehydropenstemoside (6), respectively. Another compound named harpagoside-B, isolated from Scrophularia deserti and proposed to be 9-epi-6-O-methyl-harpagoside (11), was demonstrated from the spectral data given to be the known harpagoside (10b). Finally, two alleged iridoid galactosides from Buddleja crispa named buddlejosides A and B (12a and 12b) have been shown to be the corresponding glucosides; the former is identical to agnuside (13a) while the latter is 3,4-dihydroxybenzoylaucubin (13b), an iridoid glucoside not previously published. It has again been shown that care should be taken with the interpretation of NOEs involving bridgehead protons in iridoid structures because they can be capricious and lead to mistaken structural assignments