Journal of Natural Products
Article
(3H, s, CH3), 0.94 (1H, m), 0.88 (3H, s, CH3), 0.76 (1H, m); 13C
NMR (126 MHz, CDCl3) δ 220.9 (C-17), 211.1 (C-3), 54.3 (C-2),
53.5, 53.2, 51.0, 50.5, 47.1, 46.9, 43.3, 35.2, 34.0, 30.1, 29.8, 27.6, 21.2,
20.3, 17.2, 13.3, 11.6; HRMS (APCI+) m/z 367.1321 [M + H]+ (calcd
for C19H2879BrO2 367.1273); anal. C 62.21, H 7.39%, calcd for
C19H27BrO2, C 62.13, H 7.41%.
was concentrated in vacuo, and separation of the residue (CC, 9:1 n-
hexane−EtOAc) led to 10 as a light tan, crystalline solid (1.9 g, 15%);
1
mp 39.4−39.9 °C; IR (neat) νmax 1738 cm−1; H NMR (CDCl3, 300
MHz) δ 7.63 (1H, dd, J = 2.7, 1.2 Hz, H-6), 7.32 (1H, dd, J = 9.9, 2.7
Hz, H-4), 6.28 (1H, dd, J = 1.2, 9.9 Hz, H-3); 13C NMR (CDCl3, 126
MHz) δ 171.0 (C-2), 146.7 (C-4), 133.8 (C-6), 120.9 (C-3), 100.6
(C-5).
2-Amino-3,17-dioxo-5α-androstan-1-ene (6b). To a stirred
solution of 5b (1.5 g, 4.08 mmol) in DMF (15 mL) at rt under Ar
were added NaN3 (2.70 g, 41.5 mmol) and NaI (cat), and the
suspension was heated to 60 °C for 2 h. After cooling to rt, the mixture
was diluted with H2O (15 mL) and extracted with ether (5 × 20 mL).
The combined organic phase was concentrated in vacuo, and the
residue was purified (CC, 1% TEA in 7:3 n-hexane−EtOAc) to yield a
yellow, amorphous solid. Crystallization from cyclohexane−acetone
gave 6b as light yellow needles (0.92 g, 75%): mp 135.3−175.9 °C; IR
5-(Trimethylstannyl)-2H-pyran-2-one (11) (ref 10). To a stirred
solution of pyrone 10 (0.70 g, 4.00 mmol) in THF (7 mL) at rt under
Ar were added Pd(PPh3)4 (0.20 g, 0.173 mmol) and hexamethylditin
(5.0 g, 15.3 mmol), and the solution was heated to reflux for three
days. The black mixture was cooled to rt and concentrated in vacuo.
Separation of the residue (CC, 3:17 EtOAc−n-hexane) yielded 11 as a
1
colorless oil (0.73 g, 70%): IR (neat) νmax 1736 cm−1; H NMR (300
MHz, CDCl3) δ 7.23 (1H, dd, J = 2.4, 9.3 Hz, H-4), 7.17 (1H, dd, J =
1.8, 2.4 Hz, H-6), 6.26 (1H, dd, J = 1.8, 9.3 Hz, H-3), 0.25 (9H, s,
(CH3)3); 13C NMR (126 MHz, CDCl3) δ 171.0 (C-2), 146.7 (C-4),
134.6 (C-6), 120.9 (C-3), 111.8 (C-5), −2.1 (3 × −CH3).
1
(neat) νmax 1734, 1700 cm−1; H NMR (300 MHz, CDCl3) δ 6.07
(1H, s, H-1), 3.45 (2H, bs, NH2), 2.45 (3H, m, H-4α, H-16β, H-16α),
2.11 (1H, H-16β), 1.94−1.16 (14H), 1.00 (3H, s, CH3), 0.94 (1H, m),
0.88 (3H, s, CH3), 0.76 (1H, m); 13C NMR (126 MHz, CDCl3) δ
220.1 (C-17), 195.0 (C-3), 137.4 (C-1), 125.4 (C-2), 50.9, 50.8, 47.3,
44.3, 39.6, 37.7, 35.3, 34.6, 31.0, 29.7, 26.6, 21.1, 20.2, 13.4, 11.9;
HRMS (APCI+) m/z 302.2120 [M + H]+ (calcd for C19H28NO2,
302.2120); anal. C 75.42, H 9.13, N 4.56%, calcd for C19H27NO2, C
75.71, H 9.03, N 4.65%.
Bis(5α-bufa-16,20(21),22-trienolide-[2,3-b:2′,3′-e])pyrazine (12).
To a stirred solution of triflate 8 (0.90 g, 1.08 mmol) and pyrone
11 (0.64 g, 2.45 mmol) in THF (10 mL at rt under Ar) were added
LiCl (0.68 g, 16.0 mmol) and Pd(PPh3)4 (0.220 g, 0.190 mmol), and
the mixture was heated under reflux for three days. After the solution
had cooled to rt, brine (10 mL) and DCM (10 mL) were added
successively, and the aqueous phase was extracted with DCM (3 × 10
mL). The combined organic phase was concentrated in vacuo, and the
residue was separated (CC, 1:20 acetone−n-hexane) to afford 12 as an
amorphous solid. Crystallization from cyclohexane−acetone provided
bis-bufadienolide 12 as colorless needles (0.24 g, 31%): mp 242 °C
(dec); IR (neat) νmax 1722 cm−1; 1H NMR (126 MHz, CDCl3) δ 7.25
(1H, dd, J = 2.4, 9.3 Hz, H-22), 7.19 (1H, dd, J = 1.8, 2.4 Hz, H-21),
6.46 (1H, dd, J = 1.8, 9.3 Hz, H-23), 5.87 (1H, m, H-16), 2.92 (1H, d,
J = 8.8 Hz, H-1α), 2.70 (1H, dd, H-4α), 2.49 (2H, m, H-1β, H-4β),
1.84−1.11 (16H), 0.95 (3H, s, CH3), 0.91 (1H, m), 0.85 (3H, s,
CH3); 13C NMR (126 MHz, CDCl3) δ 171.1, 146.9, 145.3, 135.1,
129.7, 127.7, 123.8, 120.9, 113.1, 53.9, 53.1, 45.5, 44.1, 41.9, 35.7, 34.4,
32.5, 32.1, 29.6, 29.1, 28.6, 27.1, 19.1, 14.2, 11.5; HRMS (APCI+) m/z
725.4317 [M + H]+ (calcd for C48H57N2O4, 725.4318); anal. C 79.36,
H 7.92, N 3.79%, calcd for C48H56N2O4, C 79.52, H 7.79, N 3.86%.
Aldehyde 15. To a solution of alcohol 141 (120 mg, 0.183 mmol)
in 2-propanol (10 mL) was added MnO2 (0.4 g, 4.58 mmol), and the
mixture was stirred at rt. After 24 h the mixture was filtered through
Celite, which was washed with DCM. Removal of solvent from the
combined filtrate yielded the crude product as a solid. Purification
(CC, 30% DCM−MeOH) and crystallization from DCM−EtOAc
yielded 15 (80 mg, 0.122 mmol, 66%): mp 250 °C (dec); [α]25D +29.8
(c 0.48, DCM); IR (film) νmax 3368, 3275, 2915, 1666, 1400, 1153
cm−1; 1H NMR (300 MHz, CDCl3) δ 10.11 (2H, d, J = 8.7 Hz, H-21),
5.80 (2H, d, J = 8.7 Hz, H-20), 4.57 (2H, s, H-11), 3.13 (2H, d, J =
16.2 Hz, H-1β), 2.86 (2H, dd, J = 5.6, 18.2 Hz, H-4α), 2.74−2.58
(12H, m), 1.35 (6H, s), 1.13 (6H, s), 2.05−0.94 (22H, m); 13C NMR
(125 MHz, CDCl3) δ 190.7 (C-21), 179.1 (C-17), 148.6 (C-2/3),
148.5 (C-2/3), 123.4 (C-20), 67.8 (C-11), 56.94, 56.87, 48.1, 46.6,
45.0, 42.2, 35.9, 34.8, 33.1, 32.0, 30.7, 27.8, 23.8, 21.5, 14.4; HRMS
(APCI+) m/z 653.4236 [M + H]+ (calcd for C42H57N2O4, 653.4318);
anal. C 70.99, H 9.06, N 3.89%, calcd for C42H56N2O4·4CH3OH, C
70.74, H 9.29, N 3.59%.
Bis(17-oxo-5α-androstan[2,3-b:2′,3′-e])pyrazine (7b). To a stirred
solution of 6b (0.50 g, 1.66 mmol) in toluene (10 mL) was added 10%
Pd/C (0.28 g, 15 mol % Pd) in a heavy-walled hydrogenation flask.
The flask was purged with H2 (4×) before hydrogenation at 40 psi for
2 h. The black suspension was then removed by filtration and washed
with DCM (10 mL). The combined organic extract was concentrated
in vacuo to yield the 2α-amino ketone intermediate as a brown solid,
which was immediately used without further purification for
dimerization. The amine (1.66 mmol) was dissolved in EtOH (5
mL), and pTsOH (cat) added. After stirring for 48 h, the reaction
solution was filtered (vacuum) through a short pad of silica gel and
washed with EtOAc (100 mL). The solvent extract was concentrated
in vacuo, adsorbed onto silica gel, and subjected to CC (7:3 n-hexane−
EtOAc, 1% TEA) to give an amorphous solid. Crystallization from
cyclohexane−acetone provided 7b as colorless needles (0.19 g, 45%):
1
mp 235 °C (dec); IR (neat) νmax 1737 cm−1; H NMR (300 MHz,
CDCl3) δ 2.93 (1H, d, J = 8.8 Hz, H-1α), 2.68 (1H, dd, H-4α), 2.50
(3H, m, H-1β, H-4β, H-16β), 2.19 (1H, m, H-16α), 1.88−0.96 (16H),
0.88 (3H, s, CH3), 0.85 (1H, m), 0.82 (3H, s, CH3); 13C NMR (126
MHz, CDCl3) δ 220.4 (C-17), 133.6 (C-2), 127.1 (C-3), 53.3, 50.8,
47.1, 45.4, 41.9, 41.3, 35.3, 34.9, 34.4, 33.3, 32.6, 31.1, 29.9, 27.6, 24.2,
21.3, 20.0, 13.2, 11.5; HRMS (APCI+) m/z 569.4158 [M + H]+ (calcd
for C38H53N2O2, 569.4107); anal. C 80.31, H 9.11, N 4.86%, calcd for
C38H52N2O2, C 80.24, H 9.21, N 4.92%.
Bis(17-triflyloxy-Δ16-5α-androstan[2,3-b:2′,3′-e])pyrazine (8). To
a stirred solution of 7b (0.40 g, 0.70 mmol) and 2,6-di-tert-butyl-4-
methylpyridine (0.44 g, 2.14 mmol) in DCM (4 mL) at −10 °C under
Ar was added (dropwise) triflic anhydride (0.36 mL, 2.14 mmol). After
18 h, H2O (8 mL) and DCM (4 mL) were added, and the aqueous
phase was extracted with DCM (2 × 8 mL). The combined organic
phase was concentrated in vacuo, and silica gel separation (CC, 1:1 n-
hexane−EtOAc) yielded 8 as an amorphous, light yellow solid (0.49 g,
84%): mp 190 °C (dec); 1H NMR (300 MHz, CDCl3) δ 5.77 (1H, m,
H-16), 2.91 (1H, d, J = 8.8 Hz, H-1α), 2.71 (1H, dd, H-4α), 2.50 (2H,
m, H-1β, H-4β), 1.81−1.11 (16H), 1.00 (3H, s, CH3), 0.94 (1H, m),
0.84 (3H, s, CH3); 13C NMR (126 MHz, CDCl3) δ 158.7 (C-17),
152.0 (OSO2CF3), 135.6 (C-2), 129.1 (C-3), 113.9 (C-16), 53.6, 53.5,
45.1, 44.3, 41.4, 35.3, 34.9, 32.8, 32.2, 29.9, 29.6, 28.0, 27.5, 19.9, 14.7,
11.4; HRMS (APCI+) m/z 833.3094 [M + H]+ (calcd for
C40H51F6N2O6S2, 833.3093).
Bis-dihydro-4-pyrone 16. To a solution of aldehyde 15 (15 mg,
0.023 mmol) and Danishefsky’s diene (10 μL, 0.051 mmol)14 at −78
°C in DCM (10 mL) under Ar was added BF3·Et2O (12 μL, 0.012
mmol). The reaction mixture was stirred for 2 h at −78 °C and then
allowed to equilibrate to rt. After 48 h, no change was detectable by
TLC, and an additional 12 μL of BF3·Et2O was added. The reaction
mixture was stirred at rt for 14 days and then quenched with NaHCO3
(saturated aqueous) and extracted with EtOAc. Purification (CC, 7:3
5-Bromo-2H-pyran-2-one (10). To a stirred solution of coumalic
acid (9, 10.0 g, 71.4 mmol) in CH3CN−H2O (9:1; 400 mL) at rt were
added LiOAc (5.60 g, 84.9 mmol), NBS (19.0 g, 107 mmol), and
Bu4NI (1.0 g, 4 mol %). After the mixture was stirred for 10 days, H2O
(250 mL) and DCM (250 mL) were added, and the aqueous phase
was extracted with DCM (2 × 100 mL). The combined solvent extract
acetone−DCM) led to pyrazine 16 (3 mg, 17%): [α]25 +106.2 (c
D
0.10, DCM); IR (neat) νmax 3363 (OH), 1672, 1401 (pyrazine ring)
cm−1; 1H NMR (500 MHz, CDCl3) δ 7.38 (1H, d, J = 6.0 Hz, H-22),
7.35 (1H, d, J = 6.0 Hz, H-22), 5.42 (2H, dd, J = 1.0, 6.5 Hz, H-23),
5.20 (2H, m, J = 2.5, 8.5 Hz, H-20), 5.03−4.95 (2H, m, H-21), 4.55
1067
dx.doi.org/10.1021/np300069z | J. Nat. Prod. 2012, 75, 1063−1069