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DOI: 10.1055/s-2008-1078250
Synthesis of Ptaeroxylin (Desoxykarenin): An Unusual Chromone from the Sneezewood Tree Ptaeroxylon obliquum
Publication History
Publication Date:
05 August 2008 (online)
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Abstract
The first synthesis of the oxepinochromone ptaeroxylin (also known as desoxykarenin) isolated from the sneezewood tree is described, in which the key steps are a Claisen rearrangement and a ring-closing metathesis to form the seven-membered oxygen heterocyclic ring.
Keywords
natural products - chromones - Claisen rearrangement - ring-closing metathesis reaction
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References and Notes
Synthesis of 6-Allyl-5,7-dihydroxy-2-methyl-4
H
-chromen-4-one (
11)
5-Allyloxy-7-hydroxy-2-methyl-4H-chromen-4-one (8, 0.15
g, 0.65 mmol) was heated in N,N-dimethylaniline (1.5 mL) and Ac2O
(1.5 mL) at 200 ˚C for 1.5 h in a microwave reactor
(300 W). The mixture was poured into HCl (6 M, 40 mL) and then extracted
with EtOAc (3 × 30 mL). Chromatography (CH2Cl2-EtOAc,
95:5) gave a mixture of 6-allyl-5-hydroxy-2-methyl-4-oxo-4H-chromen-7-yl acetate
(9, 0.06 g, 34%) and 6-allyl-2-methyl-4-oxo-4H-chromene-5,7-yl diacetate (10, 0.12 g, 58%).
To
the above mixture (0.13 g) was added a solution of K2CO3 (0.15
g) in MeOH (2.5 mL). The solution was heated under reflux for 30
min. The solvent was removed in vacuo and H2O (10 mL)
was added. The solution was then acidified with HCl (6 M) and the
resulting solid filtered to give the title compound 11 as
a colourless solid (0.09 g, 87%); mp 232-234 ˚C.
HRMS: m/z calcd for C13H13O4:
233.0808; found [MH+]: 233.0811.
IR (CHCl3): νmax = 3007,
1657, 1634, 1587, 1456, 1152 cm-¹. ¹H
NMR (400 MHz, CDCl3): δ = 13.08 (1
H, s, OH), 10.78 (1 H, s, OH), 6.40 (1 H, s, H-8), 6.15 (1 H, s,
H-3), 5.91-5.81 (1 H, m, =CH), 4.96-4.90 (2
H, m, =CH2), 3.25 (2 H, d, J = 6.0
Hz, CH2), 2.33 (3 H, s, Me). ¹³C
NMR (100 MHz; CDCl3): δ = 183.3 (C),
167.9 (C), 162.3 (C), 159.1 (C), 156.2 (C), 136.1 (CH), 115.1 (CH2), 109.3
(C), 108.3 (CH), 103.6 (C), 93.4 (CH), 26.4 (CH2), 20.3
(Me).
Synthesis of 5-Hydroxy-6,9-dihydro-2,8-dimethyl-4 H -oxepino[3,2- g ]chromen-4-one (Ptaeroxylin, 2) A solution of 6-allyl-5-hydroxy-2-methyl-7-[(2-methyl-prop-2-en-1-yl)oxy]-4H-chromen-4-one (12, 0.035 g, 0.135 mmol) in CH2Cl2 (65 mL, 0.002M) was treated with a single portion (0.021 g, 20 mol%) of bis(tricyclohexylphosphine)benzylidene ruthenium(IV) dichloride. The solution was heated under reflux for 18 h. The solvent was removed in vacuo and the brown residue filtered through a short pad of Celite using cyclohexane-EtOAc (9:1) as eluent. The solvent was then removed and the residue purified by chromatography (cyclohexane-EtOAc, 9:1) to give the title compound 2 as a colourless solid (0.020g, 64%); for analytical data, see ref. 9.
9(9) Table 1 Comparison of Data for Natural and Synthetic Ptaeroxylin |
|
Natural product (ref. [¹] ) |
Natural product (ref. [²] ) | Synthetic sample | |||||||||||||||||
|
|||||||||||||||||||
Mp | 135-138 ˚C | 133-135 ˚C | 126-128 ˚C | ||||||||||||||||
¹H NMR | CDCl3 | CDCl3 | CDCl3 | ||||||||||||||||
OH | 13.02 (1 H, s) | ||||||||||||||||||
H-11 | 6.51 (1 H, s) | 6.56 (1 H, s) | |||||||||||||||||
H-3 | 6.01 (1 H, s) | 6.06 (1 H, s) | |||||||||||||||||
H-7 | 5.6 (1 H, t) |
5.67 (1 H, t, J = 6 Hz) | 5.73-5.70 (1 H, br t, J = 4.4 Hz) | ||||||||||||||||
H-9 | 4.4 (2 H, s) | 4.53 (2 H, s) | 4.53 (2 H, s) | ||||||||||||||||
H-6 | 3.4 (2 H, m) |
3.48 (2 H, d, J = 6 Hz) | 3.60 (2 H, d, J = 4.4 Hz) | ||||||||||||||||
2-Me | 2.34 (3 H, s) | 2.37 (3 H, s) | |||||||||||||||||
8-Me | 1.54 (3 H, s) | 1.60 (3 H, s) | 1.62 (3 H, s) | ||||||||||||||||
IR | Nujol (cm-¹) | CHCl3 (cm-¹) | CHCl3 (cm-¹) | ||||||||||||||||
ca. 2970 | 3011 | ||||||||||||||||||
1650 | 1656 | 1655 | |||||||||||||||||
1610 | 1627 | 1626 | |||||||||||||||||
1590 | 1595 | 1592 | |||||||||||||||||
UV | EtOH (nm) | EtOH (nm) | MeCN (nm) | ||||||||||||||||
206 (log ε 4.20) | 209 (log ε 4.23) | ||||||||||||||||||
230 (log ε 4.17) | 232 (log ε 4.27) | 232 (log ε 4.2) | |||||||||||||||||
241 (log ε 4.22) | |||||||||||||||||||
255 (log ε 4.15) | 255 (log ε 4.24) | 254 (log ε 4.1) | |||||||||||||||||
284 (log ε 3.68) | |||||||||||||||||||
320 (log ε 2.60) | 321 (log ε 3.59) | 322 (log ε 3.5) | |||||||||||||||||
|