The absolute stereochemistry of blumenol C glucoside and byzantionoside B was revised here as (6R,9S)-and (6R,9R)-9-hydroxymegastigman-4-en-3-one 9-O-b b-D-glucopyranosides, respectively, by modified Mosher's method. The empirical rules of 13 C-NMR chemical shift to determine the absolute stereochemistry of C-9 of 9-hydroxymegastigmane 9-O-b b-D-glucopyranoside were also discussed.Key words blumenol C glucoside; byzantionoside B; megastigmane glucoside Blumenol C was first isolated from the leaves of Podocarpus blumei in 1972.1) The absolute stereochemistry of blumenol C was then determined as (6R,9R) by chemical conversion of related compound.2) Its glucoside, i.e. blumenol C glucoside (1), was then isolated from the aerial parts of Epimedium grandiflorum var. thunbergianum and the structure was determined by comparison of the spectral data of its aglycone, derived from enzymatic hydrolysis of 1, with those of blumenol C.3) Recently, byzantionoside B (2) was isolated from the aerial parts of Stachys byzantina as the C-9 epimer of blumenol C glucoside.4) Modified Mosher's method is widely used recently for determination of the absolute stereochemistry of chiral secondary alcohol. 5) In this study, the absolute configurations of these compounds (1, 2) were reinvestigated by this reliable method.
Results and DiscussionBlumenol C glucoside (1) and byzantionoside B (2) were identified by comparison of the literature data at first (Tables 1, 2).3,4) The aglycones (1a, 2a, respectively) were prepared by enzymatic hydrolysis of the glucosides. Their (R)-or (S)-MTPA esters were then prepared by the conventional procedure (1b and 1c from 1a, 2b and 2c from 2a, see Experimental). The distribution patterns of Dd S-R values for 1b and 1c, and 2b and 2c clearly demonstrated that 1 and 2 possess 9S and 9R configurations, respectively (Figs. 1, 2). In addition, the application of the b-D-glucosylation-induced shift-trend rule 6) also supported this result (Table 1). The absolute stereochemistry of C-6 was also confirmed by CD spectra. The CD data of 1 and 2 were essentially identical to that of sedumoside H of which the absolute stereochemistry was determined precisely by catalytic hydrogenation and application of CD octant rule, and also chemical conversion to related compound. 7,8) Therefore, the actual structures of 1 and 2 must be (6R,9S)-9-hydroxymegastigman-4-en-3-one 9-O-b-D-glucopyranoside and (6R,9R)-9-hydroxymegastigman-4-en-3-one 9-O-b-D-glucopyranoside, respectively (Fig. 1).Thus, blumenol C glucoside (1) isn't the glucoside of blumenol C, but the glucoside of 9-epi-blumenol C in fact, and in other word, byzantionoside B is the really glucoside of blumenol C.Aasen et al. reported the specific optical rotation value of blumenol C as [a] D ϩ54°(cϭ0.22, CHCl 3 ).2) Thus, blumenol C glucoside (1) was first misidentified as a glucoside of blumenol C, probably due to the moderate positive optical rotation value of aglycone [[a] D 22 ϩ112.5°(cϭ0.52, CHCl 3 )].3) However, the aglycones (1a, 2a) prepared in this study from...