Triterpenoid saponins from B. grandi¯orum
2093
Constituent 2
with CHCl3. The solvent was evaporated under vac-
uum to give a crude, crystalline product. CC with
CHCl3±MeOH (9:1) yielded 2,3-O-isopropylidene-b-
D-apiofuranose as a white powder (220 mg, 61% of
2-(hydroxymethyl)-2,3:4,5-di-O-isopropylidene-D-
threo-pentitol). A soln of this compound (220 mg)
in H2O (10 ml) was treated with Dowex 50 W resin
(H+ form, 400 mg) for 5 h at 708C. The resin was
removed, washed with H2O and the aq. extracts
and ®ltrate evaporated at oil pump vacuum to give
D-apiose as a syrup (120 mg); [a]2D5+5.458 (H2O, c
1.1), lit. [11] [a]D25 +5.68 (H2O, c 1.1). NMR analy-
sis of D-apiose showed that a solution of this pro-
duct in D2O contained the four possible isomeric
forms (a±d) of D-apiose. 13C NMR (75.42 MHz,
D2O, 258C); (a) d 106.1 (d, C-1), 84.3 (s, C-3), 83.0
(d, C-2), 76.7 (t, C-4), 64.7 (t, C-5); (b) d 104.4 (d,
C-1), 81.6 (s, C-3), 79.9 (d, C-2), 75.7 (t, C-4), 66.1
(t, C-5); (c) d 100.5 (d, C-1), 83.7 (s, C-3), 78.2 (d,
C-2), 75.0 (t, C-4), 64.8 (t, C-5) and (d) d 99.0 (d,
C-1), 80.2 (s, C-3), 75.5 (t, C-4), 74.0 (d, C-2), 66.7
(t, C-5).
L-Apiose. L-Apiose (122 mg) was prepared by the
same procedure starting with L-xylose; [a]2D5 5.458
(H2O, c 1.1), lit. [11] [a]2D5 5.08 (H2O, c 1.1).
According to an NMR analysis of L-apiose a sol-
ution of this product in D2O contained the four
possible isomeric forms of L-apiose. 13C NMR
(75.42 MHz, D2O, 258C); (a) d 106.1 (d, C-1), 84.3
(s, C-3), 83.0 (d, C-2), 76.7 (t, C-4), 64.7 (t, C-5);
(b) d 104.4 (d, C-1), 81.6 (s, C-3), 79.9 (d, C-2), 75.7
(t, C-4), 66.1 (t, C-5); (c) d 100.5 (d, C-1), 83.7 (s,
C-3), 78.2 (d, C-2), 75.0 (t, C-4), 64.8 (t, C-5) and
(d) d 99.0 (d, C-1), 80.2 (s, C-3), 75.5 (t, C-4), 74.0
(d, C-2) 66.7 (t, C-5).
D-Rhamnose. Methyl 2,3-O-isopropylidene-a-D-
mannopyranoside was prepared as follows from
methyl a-D-mannopyranoside [12]. Methyl a-D-man-
nopyranoside (10 g, 0.05 mol) was added to a soln
of 2,2-dimethoxypropane (100 ml) and p-toluenesul-
fonic acid (2 g) in Me2CO (100 ml) and the reaction
mixture was stirred until the solid had dissolved
(ca. 7 h) [Rf 0.22 (unreacted methyl a-D-mannopyra-
noside), 0.62 (methyl 2,3-O-isopropylidene-a-D-man-
nopyranoside, traces), 0.85 (methyl 2,3:4,6-di-O-
isopropylidene-a-D-mannopyranoside, main pro-
duct)]. The desired product, methyl 2,3-O-isopropy-
lidene-a-D-mannopyranoside, was prepared in good
yield from the main product, methyl 2,3:4,6-di-O-
isopropylidene-a-D-mannopyranoside by selective
hydrolysis, by adding H2O (200 ml) to the reaction
A white amorphous powder. Rt 19.8 min. [a]D20
28.338 (MeOH, c 1.2). TOF-MALDI m/z: 1262.4
[M + Na]+. 1H NMR (499.9 Hz, DMSO-d6): see
Table 1, 13C NMR (125.703, DMSO-d6): see
Table 2.
Reference compounds
D-Apiose and L-apiose as well as D-rhamnose are
not commercially available and were synthesized.
Reactions were monitored by TLC (EtOAc±hexane,
3:2).
D-Apiose. D-Apiose was synthesized according to
the literature [10] with a few modi®cations. p-Tolu-
enesulfonic acid hydrate (40 mg) was added with
stirring to an ice-cold solution of D-xylose (10 g,
0.067 mol) and 2-methoxypropene (14.4 g, 0.2 mol)
in DMF (130 ml). After 12 h at 0±58C, the xylose
had reacted and three reaction products were evi-
dent by TLC (Rf: 0.61, 0.3, 0.28). The acid catalyst
was neutralized by stirring with dried Amberlite
IRA-400 resin (OH form). The resin was ®ltered
o and the ®ltrate was evaporated at oil pump vac-
uum (1 mm, <408C) to give a syrup (17.15 g) that
was thoroughly extracted with dry pentane. Vac-
uum evaporation of the pentane-soluble fraction
gave the aldehyde, 2,3:4,5-di-O-isopropylidene-D-
xylose (6.15 g, 40%, Rf 0.61). This product was im-
mediately subjected to Cannizzaro reduction with
formaldehyde. Formaldehyde (5 ml of 35% aqueous
soln, 0.056 mol) and NaOH (2.35 g in 33 ml of
H2O) were added in succession to a stirred soln of
the aldehyde (5 g, 0.02 mol) in EtOH (50 ml). After
stirring for 27 h at room temp, all of the starting
material (Rf 0.49) had been converted into product,
2-(hydroxymethyl)-2,3:4,5-di-O-isopropylidene-D-
threo-pentitol (Rf 0.24). The reaction mixture was
neutralized with HCO2H (2 ml, 96%), the EtOH
removed under red. pres. and the residual water
layer thoroughly extracted with CHCl3. The
combined extracts were dried (MgSO4) and the
solvent removed under vacuum to give crystalline
2-(hydroxymethyl)-2,3:4,5-di-O-isopropylidene-D-
threo-pentitol, which was recrystallized from
CHCl3±hexane (1:1) to give the product as white
needles (2.1 g, 37%). This compound (500 mg) was
then hydrolyzed with 65% HOAc (70 ml) for 20 h
at room temp until the starting material (Rf 0.52)
had been converted into the glycol (Rf 0.1). HOAc
and H2O were removed at oil pump vacuum
(<408C) to give a syrup which was dissolved in
H2O (45 ml). The pH was adjusted to 7 with
Na2CO3 and the aq. soln was treated with NaIO4
(675 mg in 45 ml of H2O) at room temperature for
1.5 h. The excess of periodate was decomposed by
adding a few drops of ethylene glycol and the reac-
tion mixture was stirred for another hour at room
mixture and stirring it for another 3 h [Rf
0
(unreacted methyl a-D-mannopyranoside), 0.29
(methyl 2,3-O-isopropylidene-a-D-mannopyranoside,
main product), 0.92 (methyl 2,3:4,6-di-O-isopropyli-
dene-a-D-mannopyranoside, traces)].
soln (40 ml, 1 M) was added to neutralize the cata-
A NaHCO3
temp. The reaction mixture was evaporated to dry- lyst and the reaction mixture was dried at oil pump
ness at oil pump vacuum and thoroughly extracted vacuum. The residue was dissolved in H2O (300 ml)