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DOI: 10.1055/s-0030-1259521
Studies towards the Total Asymmetric Synthesis of the Pentacyclic Indole Alkaloid Arboflorine: Asymmetric Synthesis of a Key Intermediate
Publikationsverlauf
Publikationsdatum:
27. Januar 2011 (online)
Abstract
The synthesis of a plausible key intermediate for a biomimetic asymmetric synthesis of indole alkaloid arboflorine is described. The method featured the use of Ellman’s sulfinamide chemistry for the establishment of the first chiral center, and the Polonovski-Potier reaction for the formation of the α-aminonitrile moiety.
Key words
indole alkaloids - biogenetic pathway - intermediate - Polonovski-Potier reaction - Ellman’s sulfinamide
- Supporting Information for this article is available online:
- Supporting Information
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References and Notes
All new compounds gave satisfactory
analytical and spectral data.
Experimental
Procedure for the Synthesis of the Key Intermediate 28: A solution
of MCPBA (72%, 84 mg, 0.35 mmol) in CH2Cl2 (1
mL) was added dropwise to a solution of compound 25 (138
mg, 0.25 mmol) in CH2Cl2 (1.5 mL) at 0 ˚C.
After stirring at 0 ˚C for 1 h, K2CO3 (70
mg, 0.5 mmol) was added. After stirring for an additional 1 h at
0 ˚C, the mixture was filtered through celite. The residue
was purified by flash column chromatography on silica gel (R
f
0.18, eluent:
CH2Cl2-MeOH, 15:1) to give N-oxide 26 (140
mg, 98%), which was dissolved in anhyd CH2Cl2 (2.0
mL) and cooled to 0 ˚C. TFAA (0.07 mL, 0.5 mmol) was added.
After being stirred for 30 min at 0 ˚C, an aqueous solution
(0.5 mL) of KCN (65 mg, 1.0 mmol) was added, and the solution was
buffered to pH 4 by addition of AcOH and NaOAc. After stirring for
1 h at 0 ˚C, the mixture was basified with K2CO3 and
extracted with CH2Cl2 (3 × 3 mL).
The combined extracts were successively washed with H2O
(2 mL) and brine (1 mL), dried over anhyd Na2SO4,
filtered and concentrated in vacuo. The residue was purified by
flash column chromatography on neutral Al2O3 (R
f
= 0.4,
eluent: EtOAc-n-hexane, 1:4)
to give compound 28 (108 mg, 74% from 25) as a white amorphous solid. IR (film):
3370, 2978, 2934, 2216 (w, CN), 1730 (s), 1511, 1458, 1392, 1367,
1328, 1248, 1169, 1133 cm-¹. ¹H
NMR (400 MHz, CDCl3; diastereomeric mixture and rotamers): δ = 1.25
(m, 3 H, CH2CH
3),
1.33 (m, 3 H, CHCH
3), 1.44 [m,
9 H, C(CH3)3], 1.65 [s,
9 H, C(CH3)3], 2.08-2.18
(m, 1 H, C=CHCH
2), 2.26-2.44
(m, 1 H, C=CHCH
2),
2.53-2.63 (m, 1 H, C=CHCH2CH
2), 2.70-2.95 (m,
5 H, C=CHCH2CH
2, ArCH
2CH
2N),
4.03 (s, 2 H, CH2CO2), 4.16 (m, 2 H, CH
2CH3), 4.27 (s,
1 H, CHCN), 4.22-4.38 (m, 1 H, CHCH3), 4.57
(br s, 1 H, NH), 5.86, 5.89 (br s, 1 H, C=CH), 7.24 (t,
J = 7.3 Hz, 1 H, ArH), 7.29 (t, J = 7.3 Hz, 1 H, ArH), 7.53 (d, J = 7.8 Hz, 1 H, ArH), 8.09 (d, J = 7.8 Hz, 1 H, ArH). ¹³C NMR (100 MHz, CDCl3; diastereomeric mixture and rotamers): d = 14.2, 22.4, 22.5, 25.1, 25.2, 28.1, 28.25, 28.29, 28.33, 28.7, 29.6, 33.17, 33.2, 45.6, 45.65, 47.5, 52.4, 55.2, 55.3, 60.8, 79.8, 80.1, 84.0, 115.7, 115.9, 116.1, 118.0, 118.1, 118.2, 122.5, 124.2, 124.449, 124.458, 129.2, 129.7, 129.8, 134.08, 134.14, 135.8, 150.4, 155.3, 170.12, 170.18. HRMS: m/z [M + Na+] calcd for C32H44N4NaO6: 603.3159; found: 603.3153.