Photomediated Transformation of Salannin, a Tetranortriterpenoid from Azadirachta indica A. Juss

Abstract

Photolysis of Salannin (1), an important bioactive compound from Azadirachta indica A.Juss (Neem), affords Δ¹⁷-isosalanninolide (2), a hitherto unknown compound, along with isosalanninolide (3) and salanninolide (4). A probable mechanism has been suggested. While the mechanism of formation of 2 involves both a [4+2] cycloaddition and a [1,3] sigmatropic shift in the furan and D-rings, respectively, formation of 3 and 4 involves the decomposition of an ozonoide-like peroxide intermediate.

Introduction

Neem, Azadirachta indica A. Juss (Meliaceae) seeds contain a number of complex triterpenoids which are of great interest as they exhibit a variety of biological properties [1,2,3,4]. Several of these contain UV chromophores such as α,β - unsaturated ketone, furan, α,β -unsaturated ester and tigloyl groups. The bio-efficacy of .neem formulations is altered due to degradation of the triterpenoids under light exposure [5,6]. Hence, it is relevant to study the formation, isolation and bio-activity of the products formed when pure samples of these limonoids are exposed to sunlight. Herein the photolysis of salannin, one of the major triterpenoids present in the neem seed is reported.

Irradiation of salannin (1) using a sunlamp with a wider spectrum of irradiation equivalent to exposure to sunlight yielded three compounds (Figure 1). The present paper describes the sequence of events leading to the formation of Δ¹⁷-isosalanninolide (2) and its complete characterization. It may be noted that the compound 2 exhibited antifeedancy against Spodoptera litura comparable to Azadirachtin A, the most potent antifeedant in neem extract. Compounds isosalanninolide (3) and salanninolide (4) [7] have been previously reported to be formed in the photolysis of salannin (1) using UV irradiation in presence of oxygen.

Figure 1.

Figure 1.

Results and Discussion

Salannin (1) isolated from seed kernel of A. indica [8,9] was irradiated using a sunlamp in benzene for eight hours. Purification of the crude product by HPLC yielded compounds 2, 3 and 4. The mechanism for the formation of 2 is envisaged as given below (Figure 2) while the mechanism for 3 and 4 is assumed to be similar to that of the the reported photooxygenation of cedrelone[10].

Figure 2.

Figure 2.

The formation of the intermediate 5 can be explained by: a) the addition of ¹O₂ to furan generating an unstable endoperoxide similar to that of Diels-Alder reaction.[11]; b) [1,3] suprafacial sigmatropic hydrogen shift resulting in the 1,2-shift of the double bond in the D-ring. Further the unstable endoperoxide functionality in compound 5 rearranges to yield hydroxy butenolide in ring E of compound 2 [12].

The structure of the compound 2 was established on the basis of the following spectral evidence. Compound 2 exhibited a λ max at 298, 227 nm (log ε 3.782, 4.395) indicating an extra conjugation, compared to salannin (1) (λ max at 225 nm, log ε 4.356). The infrared spectrum of 2 showed additional absorptions at 1708 cm^-1 (α,β- unsaturated ketone) and 3376 cm^-1 (hydroxyl group). The ¹H- and ¹³C-NMR spectrum of 2 was similar to that of 1 except for changes observed in the D and furan rings. The ¹³C-NMR exhibited signals due to a lactone (169.2 ppm), a deoxygenated methine at 88.1 ppm (proton of which resonated at 5.52 ppm as a triplet), an allylic methylene at 31.1 ppm (attached to protons at 2.31 and 2.37 ppm as multiplets) and 4 olefinic carbons (154.0, 119.5, 130.9, 98.1) among which only the signal at 98.1 showed connectivity with a hydrogen at 5.92 ppm (as a singlet). The changes observed in the olefinic region indicated that the double bond between C-13 and C-14 has now been reorganized to C-13 and C-17 and is in conjugation with C-20, C-22 which lie in the same plane. The D₂O exchanged ¹H-NMR spectrum revealed the presence of a -OH at 5.93 ppm, The structure assigned was further supported by the mass spectra of 2 which showed a M⁺ at m/z 628.

It must be noted that though salannin 1, the major constituent in neem formulations, did not exhibit significant antifeedant activity against Spodoptera litura, its photoproduct 2 exhibits remarkable activity comparable to that of Azadirachtin A, the results of which are published elsewhere[13].