Photooxidation of beta-asarone
by: DB Saxena and SK Mukerjee. Indian Journal of Chemistry 24B (1985) 683-684
Abstract: Exposure of beta-cis-asarone (1) as neat oil to sunlight for a month gives five photoproducts characterised as beta-trans-asarone (2), (E)-(3)- and (Z)-(4)-3-(2,4,5-trimethoxyphenyl)-propan-2-als, asaraldehyde (5) and acoradin (6). All the products have been identified on the basis of their spectral data. A reaction mechanism leading to the formation of photoproducts is suggeste.
Article: Acorus calamus Linn commonly called sweet flag, grows wild in the hilly tracts of the country and is also cultivated as a commercial crop in Mysore. The rhizomes of this plant are known for their insecticidal properties. The phenylpropanoid, beta-cis-asarone (1), present to the extent of 15-20% in the rhizome oil, was shown to be the active insecticidal factor by Baxter and coworkers [1]. Recently, Saxena and coworkers [2] have shown that (1) has chemosterilant property and it acts neither as JH nor like precocines (anti-JH) but has a new type of antigonadal property. In our work with insecticides of natural origin, it was observed that a pure sample of (1) on storage got contaminated with a large number of oxidation products, presumably aided by light. This prompted us to investigate the photochemical behaviour of beta-cis-asarone (1).
A sample of cis-asarone (1, 3g, GLC pure) deposited in the form of this films in four covered petridishes (50 mg/cm
2) was exposed to direct sunlight for one month during the month of March with occassional TLC monitoring when about 70% of (1) disappeared. The mixture was subjected to column chromatography over silica gel (200g), and the column was successively eluted with 200 mL each of (a) hexane (Fr-1), (b) hexane/benzene (9:1) (Fr-2), (c) hexane/benzene (8:2) (Fr-3), (d) hexane/benzene (1:1) (Fr-4), (e) benzene (Fr-5) and (f) benzene/chloroform (8:2) (Fr-6).
Fr-1 gave unchanged (1) (0.8g). Fr-2 gave trans-asarone (2) [3] (0.26g, mp and mmp 43°). Its spectral data (PMR and mass) fully agreed with its structure. Fr-3, similarly gave a mixture of two compounds (TLC), one major and the other minor. Repeated crystallisation from ether-pet ether yielded (3) as the major compound as a pale yellow amorphous solid (0.19g), mp 138°; C12H14O4 (M
+ 222). It had a carobnyl (
sic) function (positive 2,4-DNP test), the presence of which was supported by a fragment at m/z 195 [100%, (M+1)-28] in its mass spectrum. Further, the absorption bands at 1650(s) and 2830(w) cm
-1 in its IR spectrum (KBr) showed it to be a conjugated aldehyde. The PMR spectrum (CDCl3) of (3) displayed two doubles at d7.17 (J = 16.0 Hz) and 8.13 (J = 16.0 Hz) for one proton each and an aldehydic proton as a doublet at 9.67 (J = 7.5 Hz). These data indicate trans-configuration of two protons on the propenal side-chain. Based on the presence of other groups as indicated by its PMR spectrum [d3.86(s, 3H, Ar-OCH3), 3.90(s, 3H, Ar-OCH3), 3.93(s, 3H, Ar-OCH3), 6.50(s, 1H, Ar-H on C3), 7.33(s, 1H, Ar-H on C6)], (3) has been characterised as (E)-3-(2,4,5-trimethoxyphenyl)propen-2-al.
Preparative TLC (acetone-benzene, 98:2) of the residue from above crystallisation gave in addition to (3), the minor compound (4) as a colourless solid (46mg), which crystallised from ether-pet ether in fine needles, mp 85°, C12H14O4 (Found: C 64.8, H 6.2; C12H14O4 requires C64.9, H 6.3%). It gave positive test with 2,4-DNP. This compound was characterised as the Z-isomer (4) of (3) from its spectral data; IR (KBr): 1645 cm
-1; PMR (CDCl3): d3.87, 3.92, 3.97 (each singlet 3H each, 3 x Ar-OCH3), 6.59(bs, 2H, Ar-H on C3 and Ar-CH=CH-), 6.80(d, J = 6.0 Hz, 1H, -CH=CH-CHO), 7.33(s, 1H, Ar-H on C6), 10.20(s, 1H, -CHO).
Fr-4 and Fr-5 were combined and gave asaraldehyde (5) [4] as the major product (1.18g). Fr-6 gave a glistening white solid from ether-pet ether (0.28g), mp 108° (lit mp 101°); M
+, m/z 416. Its IR, PMR and
13C NMR data and mass fragmentation pattern, fitted very well with the reported spectral data of acoradin (6), a symmetrical dimeric compound, reported [4] earlier as a constituent of this oil.
The formation of (3), (4) and (5) under these conditions without any sensitiser is significant. However, the formation of 3 and 4 in poor yields shows that this is not a favoured route of oxidation, although similar compounds have been postulated as intermediates during the photosensitised oxidation of isoeugenol to veratrol [5].
The dimer (6) isolated from the reaction product appears to be the first report of formation of such products from photoreaction, although dimers of other types have been reported [5] from eugenol under sensitesed conditions. Presumably asaraldehyde (5) formed in the initial stages acts as a sensitiser for this conversion.
The easy formation of (5) and (6) leads one to think that their reported natural occurrence in calamus oil is possibly due to their formation as artifacts during long storage without exclusion of air and light.
The formation of the different photoproducts from asarone (1) can be rationalised as shown in
Scheme 1.
References:[1] RM Baxter, PC Dandiya, SL Kandal. Nature 185 (1960) 466.
[2] BP Saxena, O Koul, K Tikku, CK Atal. Nature 270 (1977) 512.
[3] A Patra, AK Mitra. J Nat Prod 44 (1981) 668.
[4] A Patra, AK Mitra. Indian J Chem 17B (1979) 412.
[5] S Pandita, SS Chibber. Indian J Chem 21B (1982) 1043.
Scheme 1:Ar = 2,4,5-trimethoxybenzene
H H
| |
Ar---C===C---CH3 <--> Ar---CH2---CH===CH <--> Ar---C===C---CHO
| |
H H
(2) || (3)
||
||
||
|| H H
|| | |
|+------------> Ar---C===C---CHO
| (4)
|
|
V
H H H
| O-O | |
Ar---C---CH===CH2 <--- Ar---C===C---CH3
| hv
O---OH (1)
| |
| hv | SENSITISER
| | (aldehyde)
| |
| V
V
CH3---C---C---CH3
Ar-CHO | |
C---C
(5) | |
Ar Ar
(6)
