反应详情

Interestingly, 2,4,5-trimethoxyphenylpropane when treated with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) furnished α-asarone (compared with standard α-asarone) and an intense yellow coloured spot with some unreacted starting material (clearly visible on TLC plate). Increase in the amount of DDQ further favoured the formation of yellow colouring material rather than the α-asarone. All three products were separated on column chromatography in which yellow solid (mp 140° C.) showed IR absorption band at 1648 cm−1 (conjugated C═O) and also gave positive 2,4-DNP test, thus, confirming the presence of carbonyl group. UV spectra of yellow solid (λmax 244, 298, 366 nm) confirmed an increase in conjugation than the starting material 2,4,5-trimethoxyphenylpropane (288 nm) and β-asarone (269, 301 nm). 1H NMR (FIG. 1) of yellow solid showed the 14 number of protons (see Example I) in which two doublets and one doublet of doublet for three protons appeared at δ 9.65 (1H, d, J=7.8 Hz), 7.81 (1H, d, J=15.8 Hz) and δ 6.64 (1H, dd, J=15.8 Hz, J=7.8 Hz) respectively. Further, the position of two aromatic singlet protons and three singlet for nine protons for three trimethoxy groups were more or less at same δ value as compared to β-asarone (Patra, A. and Mitra, A. K., Phytochemistry, 44: 668-669 (1981)). IR and 1H NMR has supported the possibility of unsaturated aldehyde group (—CH═CH—CHO) attached with trimethoxy (nine protons) substituted phenyl ring (two protons). Similarly, the 13C NMR (FIG. 2) of the yellow solid that appeared at δ 194.1, 154.1, 153.2, 147.6, 143.3, 126.4, 114.5, 110.5, 96.5, 56.4, 56.2, 56.0 clearly indicated the presence of 12 carbons as similar to the 12 carbons of β-asarone except that the position of side propyl group which appeared at δ 194.1 (C-3′), 154.1 (C-1′) and 126.4 (C-2′) could be possible due to β-unsaturated aldehyde (—CH═CH—CHO) group. The EI mass spectrum (FIG. 3) of yellow solid showed a clear [M]+ peak at m/z 222. On the basis of above spectral data, the yellow solid was postulated to be 2,4,5-trimethoxycinnamaldehyde as a trans-isomer (Example I). The formation of this unexpected trans-2,4,5-trimethoxycinnamaldehyde was finally confirmed by its (i) oxidation with neutral KMnO4 in the cold acetone to well known 2,4,5-trimethoxybenzaldehyde (Example II) (Birch, A. J., Jackson, A. H., Shannon, P. V. R. and Steward, G. W., Journal of Chemical Society Perkin Trans I, 2492-2501, (1973) and Starkovsky, N. A., Journal of Organic Chemistry, 27, 3733-3734, (1962)) (ii) direct oxidation of β-asarone with selenium dioxide (Liu M C, Lin T S & Sartorelli A C, J Med Chem, 35, 3672 (1992)) into 2,4,5-trimethoxycinnamaldehyde (Example III) and its comparison with reported natural cinnamaldehyde. Treatment of β-asarone with selenium dioxide and few drop of base such as pyridine, triethylamine etc in dioxane gave two distinguished spots on TLC plate in which one yellow spot is expected for 2,4,5-trimethoxycinnamaldehyde while minor spot for corresponding cinnamyl alcohol derivative as clearly confirmed by the absorbance of peak at 1648 (carbonyl) and 3480 (hydroxyl group) in IR spectra. The latter was formed even when the amount of selenium dioxide was increased up to 1.3 equiv. Formation of side product alcohol are common with aldehyde during the allylic oxidation of several analogs of β-asarone with SeO2. However, we observed that without any separation, treatment of the mixture of cinnamaldehyde and cinnamyl alcohol with pyridinium chlorochromate (PCC) (Lin, S. J., Short, R. E., Ford, S. P., Grings, E. E. and Rasazza, P. N., J Nat Prod, 61, 51-56 (1998)) afforded 2,4,5-trimethoxycinnamaldehyde as a single spot since alcohol got oxidized into cinnamaldehyde. The 1H-NMR spectral data of cinnamaldehyde is similar to the reported natural (Kulkarni, M. M., Sohani, J., Rojatkar, S. R. and Nagasampagi, B. A., Indian J. Chem., 25B: 981-982 (1986)) and its 13C-NMR spectral data is reported here for the first time. Thus, isolation and characterization of above cinnamaldehyde has opened a new route to prepare several substituted cinnamaldehydes in a single step starting from phenylpropane derivatives.