Transformation of 6 into the azido derivative 96 involved
removal of the silyl group, O-tosylation, and nucleophilic
displacement with NaN3 in DMF (Scheme 1).
diastereomers (12 and 13). Separation of this diastereomeric
mixture at this juncture was difficult. Therefore, the mixture
as such was subjected to catalytic reduction over Pd/C
followed by reaction with 4-bromo-2-(trichloroacetyl)pyrrole
in DMF at room temperature, which gave a mixture of 2
and 3 (Scheme 3).
Scheme 1a
Scheme 3a
a Reagents and conditions: (a) ref 4; (b) NaH, CS2, MeI, THF,
rt, 2 h (98%); (c) Bu3SnH, toluene AIBN, reflux, 5 h, (84%); (d)
1 M Bu4NF, THF rt, 4 h, (81%); (e) p-Ts-Cl, Et3N, CH2Cl2, rt,
(92%); (f) NaN3, DMF, 85 °C, 12 h, (85%).
a Reagents and conditions: (a) urea, 40% aq HF, MeOH, rt
(62%); (b) 10% Pd-C, H2, MeOH, 1 atm, 2 h (100%); 4-bromo-
2-trichloroacetyl-pyrrole, DMF, rt, 16 h, (83%); (c) chromatography.
Our next goal was to introduce an imidazolidine ring
system across the C1-C2 segment of 9. For this endeavor,
the isopropylidene group was cleaved under acidic conditions
and the resulting diol was selectively silylated with TBSCl-
imidazole to give 10. Swern oxidation7 of 10 provided the
2-ulose derivative (11) whose 1H NMR spectrum showed a
downfield shift of protons located at C-1 and C-3 (Scheme
2).
Silica gel chromatography conveniently provided slagenin
B (2) and C (3) as pure products. Slagenins B and C were
fully characterized by 1H and 13C NMR spectroscopic data,9
which were found to be identical to reported values.1 The
observed optical rotation of synthetic slagenin B (2) was [R]D
+ 36 (c 0.2, MeOH) [lit.1 [R]D + 33 (c 0.2, MeOH)], and
that of slagenin C (3) was [R]D -39 (c 0.2, MeOH) [lit.1
[R]D -35 (c 0.2, MeOH)]. In summary, a carbohydrate-based
synthesis of naturally occurring slagenins B and C have been
reported starting from L-arabinose.
Scheme 2a
Acknowledgment. S.B. thanks CSIR (New Delhi) for
financial assistance in the form of a Senior Research
Fellowship.
a Reagents and conditions: (a) TBSCl, imidazole, Et3N, CH2Cl2,
45 min (72%); (b) (COCl)2, DMSO, Et3N, 2 h (72%).
Supporting Information Available: 1H NMR and 13C
NMR spectra for 10, 11, 2, and 3. This material is available
Treatment8 of 11 with urea in the presence of 40% aqueous
HF in methanol at room temperature gave a mixture of
OL020112Q
(6) Selected spectroscopic values for 9: 1H NMR (CDCl3, 200 MHz):
δ 1.26 (s, 3 H), 1.50 (s, 3 H), 1.97 (dd, 1 H, J ) 2.9, 15.1 Hz), 2.11 (ddd,
1 H, J ) 5.7, 7.5, 15.1 Hz), 3.16 (dd, 1 H, J ) 6.8, 12.4 Hz), 3.61 (dd, 1
H, J ) 7.5, 12.4 Hz), 4.23 (m, 1 H), 4.67 (m, 1 H), 5.61 (d, 1 H, J ) 4.2
Hz). 13C NMR (CDCl3, 50 MHz): δ 25.8, 27.0, 34.4, 54.72, 79.6, 80.5,
106.7, 112.3. Ms: 184 (M+ - 15); IR: 2100 cm-1 (N3). Anal. Calcd for
C8H13N3O3: C, 48.24; H, 6.53. Found: C, 47.9; H, 6.56.
(9) NMR data. Slagenin B (2): 1H NMR (CDCl3, 500 MHz) δ 1.76 (t,
1 H, J ) 11.9 Hz), 2.17 (dd, 1 H, J ) 3.9, 11.9 Hz), 3.15 (s, 3 H), 3.48 (m,
2 H), 4.04 (m, 2 H), 5.19 (s, 1 H), 6.86 (d, 1 H, J ) 1.6 Hz), 6.96 (d, 1 H,
J ) 1.6 Hz), 7.50 (s, 1 H), 8.42 (t, 1 H, J ) 5.7 Hz); 13C NMR (CDCl3,
125 MHz) δ 41.5, 41.6, 50.3, 76.1, 88.5, 94.8, 97.9, 112.0, 121.3, 126.8,
155.5, 159.8. Slagenin C (3): 1H NMR (CDCl3, 200 MHz) δ 1.90 (dd, 1
H, J ) 6.4, 12.8 Hz), 2.29 (dd, 1 H, J ) 6.7, 12.8 Hz), 3.17 (s, 3 H), 3.40
(m, 2 H), 4.19 (m, 1 H), 5.12 (s, 1 H), 6.87 (s, 1 H), 6.97 (s, 1 H) 7.65 (s,
1 H), 7.69 (s, 1 H), 8.24 (t, 1 H, J ) 5.1 Hz), 11.87 (s, 1 H); 13C NMR
(CDCl3, 50 MHz) 40.8, 42.9, 49.8, 76.2, 89.5, 95, 97.3, 111.8, 121.2, 126.8,
159.4, 159.7.
(7) Omura, K.; Swern, D. Tetrahedron 1978, 34, 1651.
(8) (a) Grillon, E.; Gallo, R.; Pierrot, M.; Boileau, J.; Wimmer, E.
Tetrahedron Lett. 1998, 29, 1015. (b) Gautam, S.; Katcham, R.; Nematullahi,
J. Synthetic Comm. 1979, 9, 863.
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Org. Lett., Vol. 4, No. 21, 2002