Acta Chim. Slov. 2005, 52, 455–459
455
Scientific Paper
Sensitized Photooxygenation of Tinosponone, a Clerodane Diterpene from Tinospora Cordifolia
Jawaid Iqbal,* Adil Husain, and Anamika Gupta
Department of Chemistry, Organic Chemistry Section, Aligarh Muslim University, Aligarh-202002 (U.P. ) India.
E-mail: jawaid.iqbal0@lycos.com
Received 02-04-2005
Abstract
The reaction of tinosponone (1) with singlet oxygen was studied by using different combinations of photosensitizers (i.e. rose bengal, methylene blue, riboflavin and benzophenone), solvents (i.e. benzene, chloroform, acetone, acetonitrile and methanol) and singlet oxygen scavengers (i.e. DABCO and sodium azide). Two major products (3S,4aS,4bS,8R,8aR,10aR)-8-Hydroxy-3-(5'-hydroxy-2'-oxo-2',5'-dihydrofuran-3'-yl)-4a,8a-dimethyl-3,4,8,8a,9,10-hexahydro-10aH-benzo[f]isochromene-1,5(4aH,4bH)-dione (2) and (3S,4aS,4bS,8R,8aR,10aR)-8-Hydroxy-4a,8a-dimethyl-3-((1'R)-3'-oxo-4',6'-dioxa-bicyclo[3.1.0]hexan-1'-yl)-3,4,8,8a,9,10-hexahydro-10aH-benzo[f]isochromene-1,5(4aH,4bH)-dione (3) were isolated in all the solvents except methanol. In methanol a single product (3S,4aS,4bS,8R,8aR,10aR)-8-Hydroxy-3-(5'-hydroperoxy-2'-methoxy-2',5'-dihydrofuran-3'-yl)-4a,8a-dimethyl-3,4,8,8a,9,10-hexahydro-10aH-benzo[f]isochromene-1,5(4aH,4bH)-dione (4) was obtained. All products were characterized on the basis of IR, 1H NMR, 13C NMR and elemental analysis studies. The formation of products was explained by photooxidation of tinosponone. Effects of different solvents with the variation of added singlet oxygen sensitizers and singlet oxygen scavangers were observed on the yield of photooxidation products and were correlated to the rate of singlet oxygen formation.
Key words: photooxygenation, tinosponone, clerodane diterpene, Tinospora cordifolia, benzo[f]isochromene

Introduction
The photoreactivity of synthetic drugs have been intensively studied in the recent past for their phototoxicity and phototherapeutic value.1,2 In contrast photochemical studies on biologically active natural products are very limited. Plant materials are well known for their biological activity and medicinal values.3–5 Hence it is of importance to study the photoreactivity of biologically active plant metabolites for a correlation to their possible in vivo photoreactions and phototoxicity. The photochemical study is expected to through light on improving the stability of these compounds in medicinal formulations derived from natural plant extracts.
Within the context we have investigated the photooxidation of tinosponone (1), a clerodane diterpene isolated from Tinospora cordifolia. T. cordifolia is a plant of recognized medicinal values and is widely used as anti-bacterial, analgesic, antipyretic and also for the treatment of jaundice, skin diseases, diabetes, anemia etc.6–10 Several compounds containing 3-substituted furan moiety have been isolated from this plant species.11–13 In spite of immense medicinal use of this plant extract the photochemical sensitivity of their bioactive constituents has not been described in the literature. The 3-substituted furan moiety is quite
susceptible to attack by biological oxygens; we therefore, have investigated photooxygenation of tinosponone under different combinations of sensitizer dyes and solvents.
Results and discussion
Irradiation of air-saturated benzene solution of tinosponone with methylene blue as sensitizer in a water-cooled immersion well type photoreactor equipped with medium pressure mercury vapour lamp and purification of the crude product by silica gel column chromatography afforded two compound 2 and 3. When tinosponone was irradiated with methylene blue in methanol, the chromatographic analysis (TLC) of irradiated mixture did not show the presence of any of the previously identified products (2 and 3), rather a new product 4 was observed (Scheme 1). When these photoreactions were carried out in the absence of sensitizer same products were obtained but the reaction was observed to be slow.
The effect of nature of solvent on photooxidation was studied by using different solvents. The amount of substrate could not be kept same, as the solubility of substrate was different in different solvents. Therefore, relative yield of products was determined in these
Iqbal et al.     Sensitized Photooxygenation of Tinosponone
456
Acta Chim. Slov. 2005, 52, 455–459
cases. For this purpose, different reaction mixtures were irradiated under standard conditions for the same time period. Then 15 ml of each solution was taken out, concentrated and subjected to preparative TLC for the isolation of the products, and correlation of their yields. Yields of products in different solvents were found to vary with the polarity of the solvent. The yield was higher in polar solvents in comparison to non-polar solvents (Table 1). This observation may be attributed to longer lifetime of 1O2 in polar solvents.14,15 Owing to the solubility problem, the concentration of 1 was not same in all the solutions, as it was of methylene blue therefore, the possibility of energy transfer for different yields of products cannot be discarded. To confirm whether energy transfer or longer lifetime of 1O2 is responsible for different yields of products, we conducted experiments by varying the concentration of sensitizer (5Χ10-3-2Χ10-2 mol L-1) to the concentration of tinosponone in different solvents. Similar product patterns were obtained in these cases also, which supports the fact that lifetime of 1O2 and in turn polarity of solvent is responsible for the observed difference in the yields.
HOO
4' Hi,
o    v
6     4bf I7
1    \
v     A
V'   "OCH3
^<   2 H<fc
10a 10
102, hv/ Sens Methanol
8;
OH
9
OH
HO
102, hv/ Sens
Benzene
OH
OH
1o2
ox0
OH
2, 3, and 4
1a Scheme 1
The dependence of percentage yield of products on triplet energies of various sensitizers has also been studied. It was observed that rose bengal and methylene blue was much more efficient than riboflavin and benzophenone in the photosensitized decomposition of 1 (Table 2). This may be due to the fact that rose bengal and methylene blue, with lower triplet energies, produce singlet oxygen in large amount16,17 by type II
mechanism.18
Table 1. Yields of products, by methylene blue photosensitized reaction of Tinosponone with different solvents.
Solvent	Lifetime of 1O2a (΅s)	Yields of products (%)b (2+3)
Benzene	24	32.3 (19.6+12.7)
Acetone	26	31.6 (18.9+12.7)
Acetonitrile	30	35.2 (22.8+12.4)
Chloroform	60	40.4 (27.1+13.3)
Concentration of tinosponone =100 mg/200mL, 1.5 mM. Concentration of Methylene blue = 10% wt/wt of tinosponone. Time of irradiation = 4 hours. a See refs. 14, and 15. b Yields of the products were determined after isolation according to experimental part.
Table 2. Effect of Triplet energies of different sensitizers on the yields of products.
Sensitizer	Triplet energya (Kcal/mole)	Yields of products (%)b (2+3)
Methylene blue	33.5-34.0	31.7 (19.1+12.6)
Rose bengal	39.2-42.2	30.3 (17.1+13.2)
Riboflavin	57.8	21.2 (11.3+9.9)
Benzophenone	68.6-69.1	19.9 (9.9+10.0)
Concentration of tinosponone =100 mg/200mL, 1.5 mM. Concentration of Dye = 10% wt/wt of tinosponone. Time of irradiation = 4 hours. a See ref. 25. b Yields of the products were determined after isolation according to experimental part. Benzene was used as solvent.
On other hand riboflavin and benzophenone (higher triplet energies) act mainly by type I photosensitized photooxidation, do not produce significant amount of 1O2.19 The participation of 1O2 in the reaction was confirmed by studying the effect of some scavengers on the yield of this photooxidation reaction. The drastic lowering of the yield of products in presence of scavengers (DABCO-17%; sodium azide-14%) confirms that 1O2 is active oxidizing species in this photoreaction. Also no reaction was observed on conducting experiments under nitrogen atmosphere, which further support the involvement of 1O2 in this photoreaction. When irradiations were carried out by using silica bound rose bengal,20 same products were obtained but the reaction was observed to be slow.
The structure of the photoproducts was assigned on the basis of IR, 1H NMR, 13C NMR and elemental analysis studies. The spectral data of photoproducts 2, 3 and 4 were found to be similar with that of 1 except for the furan signals. The furan ring had been site of
r
r
O
4
1
O
O
2
3
1
O
Iqbal et al.     Sensitized Photooxygenation of Tinosponone
Acta Chim. Slov. 2005, 52, 455–459
457
attack is evident from absence of carbon/hydrogen signals due to furan moiety in the spectral data of all the identified photoproducts. The spectral study suggested that product 2 now posses S-hydroxy butenolide moiety instead of furan moiety. The additional IR bands at 3390 cm–1 (hydroxy group), 1670 (a,/3-unsaturated ketone) and extra carbonyl resonance at S 175.7 suggested an extra lactone carbonyl compared to that of parent compound. The 13C NMR signal at S 97.5 indicate that carbon must be attached to two oxygen atoms. This proton is in split with the proton of a,/3-unsaturated carbon at S 6.94 suggested that carbon attached to two oxygen atoms must be adjacent to a,/3-unsaturated carbon. The change of furan ring to 5-hydroxy butenolide moiety is also evident from the two carbon signals at S 136.5 and 175.7 instead of the olefinic signals of furan at S 121.5 and 139.7. The appearance of double signals for C-5’ indicated that compound 2 to be an epimeric mixture at C-5’.
The spectral data for compound 3 was also found to be similar with that of 1 except for the furan signals. On the basis of following spectral data we conclude the presence of epoxy lactone in 3. The 13C NMR spectrum indicated an additional lactone carbonyl resonance at S 171.1. A signal at S 120.4 was assigned to a dioxygenated carbon of the epoxylactone ring, with additional support from its 1H NMR signal at 55.23. Of the other carbons of the lactone ring, a carbon resonance value at S 36.2 along with a proton resonance at S 2.2 was assigned to the methylene carbon and at S 59.2 assigned to the quaternary carbon. The formation of lactone ring gets additional support from the IR spectrum of 3, which shows characteristic absorption for two lactone carbonyl at 1750 and 1710 cm–1 and for the epoxide ring at 3150, 1210, 950 and 745 cm–1. The appearance of double signals for C-5’ indicated compound 3 to be an epimeric mixture at C-5’.
The compound 4 was having a comparably similar spectral data to 1, with a basic difference in furan ring values. It was shown to contain a 2,5-dihydrofuran ring with an allylic hydroperoxy and a methoxy group. The 1H NMR spectrum recorded a highly deshielded signal at S 8.1 (brs, exch., 1H) and a three proton singlet at S 3.32, which were assigned to the allylic -OOH group (at C-5’) and -OMe group (at C-2’) respectively. This regiostructure gets support from the 1H NMR signals as a singlet at a low value of S 5.84 for C-2’ proton and a doublet at a high value of S 6.21 for C-5’ proton. Of olefinic carbons a carbon resonance at S 114.7 was assigned to C-4’ and at S 141.4 was assigned to C-3’. The signals for the protons at C-4’ and OCH3 were appropriately observed at S 5.72 and 3.32 respectively. Additional structural information for the compound 4 was inferred from its following chemical properties: 1) with Pb(OAc)4, gas was evolved, as it is characteristic
of compounds containing -OOH group; 2) With potassium iodide-acetic acid solution it liberate iodine (suggested presence of O-O bond).21
The formation of photoproducts 2, 3 and 4 can be envisaged to occur from unstable cyclic peroxide 1a, which initially results by the [4?+ 2?] cycloaddition of 1O2 to furan ring (Scheme 1). This unstable cyclic peroxide (1a) undergoes homolytic cleavage of O-O bond and afforded product 2 and 3 by following two competing processes:22,23 elimination of proton from bridgehead position and a subsequent rearrangement gives product 2; and 1,2 hydrogen shift gives product 3, as mechanistically rationalized in Scheme 2. In the presence of polar methanol solvent, the solvolysis induced transformation of intermediate 1a leads to the formation of product 4.
O O O
1a
O
I O
RX HJ   ^
I O
I
o
OH
OH O
b
R^^V    O.
V__/
I
H
-O
R       wO
CH3OH
I OOH
,0
R'     ^OCH3 4
R       O
‘   O II
R       O
I
O
O
3
R =
OH
Scheme 2
R
O
O
O
O
O
R
R
O
O
O
O
O
R
O
O
R
O
R
R
2
O
Iqbal et al.     Sensitized Photooxygenation of Tinosponone
458
Acta Chim. Slov. 2005, 52, 455–459
Experimental
Apparatus and chemicals
Irradiations were carried out in a photoreactor equipped with medium pressure mercury vapour lamp inserted in a water-cooled immersion well with continuous supply of water. IR spectra were recorded as KBr discs on a Perkin Elmer model spectrum RX1. 1H NMR and 13C NMR spectra were recorded on a Bruker Avance DRX-300 spectrometer using SiMe4 as internal standard and CDCl3 as solvent. Elemental analyses were carried on a Carlo Erba model 1108 Elemental analyzer. High-resolution mass spectra were determined with a VG-ZAB-BEQ9 spectrometer. All solvents and chemicals used were of HPLC and pharmaceutical grade. Tinosponone was isolated from stem of Tinospora cordifolia according to literature procedure.24 The purity of 1 was determined by comparison of its melting point and 1H NMR with that of literature value. Merck silica gel 60 F254 plates were used for analytical TLC; column chromatography was performed on Merck silica gel 60 (70-230 mesh).
Irradiation procedure
Irradiation of air-saturated solution of tinosponone (1) (100mg/200mL, 1.5 mM) in benzene with methylene blue (0.01 gm, 10% wt/wt of tinosponone) as sensitizer was carried out with medium pressure mercury vapour lamp (125 W) for 8 hours. Complete decomposition of
1 was monitored by thin layer chromatography (ethyl acetate: hexane; 3 : 7). Removal of solvent under reduced pressure and column chromatography of the resulting photoproduct on silica gel yielded compound
2 and 3. This photoreaction was also carried out under nitrogen atmosphere. The solutions to be irradiated were saturated with air/nitrogen prior to irradiation and were continuously bubbled during irradiation.
Similar experiments were carried out by using different combinations of solvents and sensitizers (Table I and II). Two different sets of reactions were also carried out in similar way by using D ABCO/sodium azide (10% wt/wt of tinosponone) with methylene blue as sensitizer.
Characterization of products
(3S,4aS,4bS,8R,8aR,10aR)-8-Hydroxy-3-(5'-hydroxy-2'-oxo-2',5'-dihydrofuran-3'-yl)-4a,8a-dimethyl-3,4,8,8a,9,10-hexahydro-10a//-benzo[f]isochromene-l,5(4a//,4b/7)-dione (2): mp 177 °C. Rf 0.83. [a]23D -93.7 (c 0.92, CHCl3). IR (KBr) v 3500, 3390, 1745, 1715, 1678, 1450 cm–1. 1H NMR (CDCl3) 8 6.94 (d, 1H, J 7.1 Hz), 6.67 (dd, 1H, J 10.8, 5.2 Hz), 6.15 (d, 1H, J 7.0 Hz), 5.91 (d, 1H, J 10.8 Hz), 4.91 (dd, 1H, J 12.2, 3.6 Hz), 4.82 (brs, exch., -OH), 4.28 (d, 1H, J 5.1 Hz), 2.42 (dd, 1H, J 14.8, 3.4 Hz), 2.38 (brs, 1H), 2.21 (m, 1H), 2.20 (brs, 1H), 2.18 (m, 1H), 1.65 (dd, 1H, J 15.4, 12.2
Hz), 1.61 (m, 1H), 1.42 (s, -CH3), 1.1 (dt, 1H, J 13.8, 4.5 Hz), 0.80 (s, -CH3). 13C NMR (CDCl3) 8 204.5, 176.2, 175.7, 141.5, 143.1, 136.5, 127.5, 97.1, 97.5, 74.5, 60.5,
52.6, 49.2, 42.9, 40.1, 34.8, 30.9, 29.9, 26.6, 19.2. HRMS-FAB (m/z): [M+] calcd for C19H22O7, 362.3738; found, 362.3731; m/z (relative intensity): 345 (C19H21O6+, 14), 318 (C18H22O5+, 100), 303 (C18H23O4+, 15), 274 (C17H22O3+ 10), 263 (C15H19O4+, 17), 261 (C17H25O2+, 9). Anal. Calcd for C19H22O7: C 62.97, H 6.12, O 30.91. Found: C 62.84, H 6.08, O 30.98.
(3S,4aS,4bS,8R,8aR,10aR)-8-Hydroxy-4a,8a-dimethyl-3-((l'R)-3'-oxo-4',6'-dioxa-bicyclo [3.1.0] hexan-l'-yl)-3,4,8,8a,9,10-hexahydro-10aff-benzo[f]isochromene-l,5(4a//,4b//)-dione (3): mp 164 °C. Rf 0.62. [a]23D -84.6 (c 0.47, CHCl3). IR (KBr) v 3430, 1750, 1710, 1660, 1210, 950, 745 cm–1. 1H NMR (CDCl3) 8 6.52 (dd, 1H, J 10.6, 4.8 Hz), 5.75 (d, 1H, J 10.4 Hz), 5.23 (s, 1H), 4.87 (dd, 1H, J 12.4, 3.8 Hz), 4.30 (d, 1H, J 5.1 Hz), 4.12 (brs, exch., -OH), 2.28 (dd, 1H, J 14.2, 2.9 Hz), 2.22 (brs, 1H), 2.36 (m, 1H), 2.2 (s, 1H), 2.16 (brs, 1H), 1.98 (m, 1H), 1.71 (m, 1H), 1.58 (dd, 1H, J 15.3, 11.8 Hz), 1.38 (s, CH3), 1.12 (dt, 1H, J 13.6, 4.4 Hz), 0.80 (s, CH3). 13C NMR (CDCl3) 8 202.6, 175.2, 171.1, 145.2, 130.7, 120.4, 75.2, 59.2, 56.8, 52.5, 48.1, 44.2, 41.7, 36.6, 36.2, 31.3, 18.6, 25.4, 18.8. HRMS-FAB (m/z): [M+] calcd for C19H22O7, 362.3738; found, 362.3751; m/z (relative intensity): 318 (C18H22O5+, 20), 290 (C17H22O4+, 100), 278 (C15H18O5+, 11), 276 (C17H24O3+, 11), 263 (C15H19O4+, 9), 221 (C14H21O2+, 16), 84 (C4H4O2+, 19). Anal. Calcd for C19H22O7: C 62.97, H 6.12, O 30.91. Found: C 62.74, H 6.34, O 31.05.
(3S,4aS,4bS,8R,8aR,10aR)-8-Hydroxy-3-(5'-hydroperoxy-2'-methoxy-2',5'-dihydrofuran-3'-yl)-4a,8a-dimethyl-3,4,8,8a,9,10-hexahydro-10a//-benzo[f]isochromene-l,5(4a//,4b/7)-dione (4): mp 169 °C. [a]23D -77.8 (c 0.34, CHCl3). IR (KBr) v 3520, 3500, 1670, 1250 cm–1. 1H NMR (CDCl3) 8 8.12 (brs, exch.), 6.61 (dd, 1H, J 10.7, 5.2 Hz), 6.21 (d, 1H, J 7.4 Hz), 5.84 (s, 1H), 5.72 (d, 1H, J 7.2 Hz), 5.62 (d, 1H, J 10.8 Hz), 4.94 (dd, 1H, J 12.1, 3.2 Hz), 4.64 (brs, exch., -OH) 4.26 (d, 1H, J 4.8 Hz), 3.22 (s, 3H), 2.48 (dd, 1H, J 13.8, 3.2 Hz), 2.30 (brs, 1H), 2.22 (m, 1H), 2.20 (brs, 1H), 2.12 (m, 1H), 1.68 (dd, 1H, J 15.2, 12.8 Hz), 1.65 (m, 1H), 1.48 (s, 3H), 1.1 (dt, 1H, J 13.8, 4.0 Hz), 0.82 (s, 3H). 13C NMR (CDCl3) 8 200.5, 173.4, 143.7, 141.4, 129.4, 114.7, 111.8, 112.2, 98.4, 73.8, 58.1, 53.8, 51.3, 47.6, 43.1, 42.5, 39.2,
33.7,  26.1, 25.2, 18.4. HRMS-FAB (m/z): [M+] calcd for C20H26O8 394.4156; found, 394.4152; m/z (relative intensity): 301 (C18H21O4+, 100), 363 (C19H23O7+, 18), 361 (C20H25O6+, 21), 331 (C19H23O5+, 13), 288 (C17H20O4+, 11), 206 (C14H22O+, 15), 84 (C4H4O2+, 21). Anal. Calcd for C20H26O8: C 60.90, H 6.64, O 32.45. Found: C 60.98, H 6.45, O 32.52.
Iqbal et al.     Sensitized Photooxygenation of Tinosponone
Acta Chim. Slov. 2005, 52, 455–459
459
References
1.  G. M. J. Beijersbergen Van Henegouwen, in: B. Testa U. Meyer (Ed.): Advances in Drug Research, Academic, New York and London, 1997, 29, pp.79–170.
2.  M. A. Miranda, in: J. V. Castell (Ed.): In Vitro Methods in Pharmaceutical Research, Academic: London, 1997, pp.289–315.
3.  N. R. Farnsworth, O. Akerele, A. S. Bingel, D. D. Soejarto, Z. Guo, Bull. WHO. 1985, 63, 965–981.
4.  G. M. Cragg, D. J. Newman, K. M. Snader, J. Nat. Prod.
1997, 60, 52–60.
5.  A. Kuruvilla, Indian J. Exp. Biol. 2002, 40, 7–11.
6.  Y. R. Chadha, The Wealth of India, Publication and information directorate, CSIR, New Delhi, 1976, 10, pp.251.
7.  B. Bishayi, S. Roychowdherry, S. Ghosh, M. Sengupta, J. Toxicol. Sci. 2002, 27, 139–146.
8.  P. S. Prince, V. P. Menon, J. Ethnopharmacol. 1999, 65, 277–281.
9.  G. C. Jagetia, K. Nayak, M. S. Vidyasagar, Cancer Lett.
1998, 127, 71–82.
10.   N. N. Rege, S. A. Dahanukar, J. Postgrad. Med. 1993, 31, 22–25.
11.   R. K. Bhatt, J. B. Hanuman, B. K. Sabata, Phytochemistry 1988, 27, 1212–1216.
12.   V. Wazir, R. Maurya, R. S. Kapil, Phytochemistry 1995, 38, 447–449.
13.   J. B. Hanuman, R. K. Bhatt, B. K. Sabata, Phytochemistry 1986, 25, 1677–1680.
14.   D. R. Kearns, in: H.H. Wasserman, R.W. Murray (Ed.): Singlet Oxygen, Academic, New York, 1979, pp.120.
15.   K. Gollnick, H. J. Kuhn, in: H. H. Wasserman, R. W. Murray (Ed.): Singlet Oxygen, Academic, New York,
1979, pp.290.
16.   E. M. Tuite, J. M. Kelly, J. Photochem. Photobiol. B: Biol. 1993, 21, 103–124.
17.   S. Roul, J. Cadet, J. Am. Chem. Soc. 1996, 118, 1892– 1898.
18.   C. S. Foote, Photochem. Photobiol. 1991, 54, 659.
19.   J. Cadet, C. Decarroz, S. Y. Wang, W. R. Midden, Isr. J. Chem. 1983, 23, 420–429.
20.   S. Tamagaki, C. E. Liesner, D. C. Neckers, J. Org. Chem.
1980, 45, 1573–1576.
21.   C. S. Foote, M. T. Wuesthoff, S. Wexler, I. G. Binstain, R. Denny, Tetrahedron 1967, 23, 2583–2599.
22.   M. Balci, Chem. Rev. 1981, 81, 91–108.
23.   K. Gollnick, A. Griesbeck, Tetrahedron 1985, 41, 2057–2068.
24.   R. Maurya, V. Wazir, A. Tyagi, R. S. Kapil, Phytochemistry 1995, 38, 659–661.
25.   J. D. Coyle, Photochemistry in Organic Synthesis, Burlington house, London, 1986, pp. 191.
Povzetek

Raziskovana je bila reakcija tinosponona (1) s singletnim kisikom v prisotnosti razliθnih kombinacij vzbujevalcev, topil in ponorov singletnega kisika. V vseh toipilih, razen v metanolu, smo izolirali dva glavna produkta: (3S,4aS,4bS,8R,8aR,10aR)-8-Hidroksi-3-(5'-hidroksi-2'-okso-2',5'-dihidrofuran-3'-il)-4a,8a-dimetil-3,4,8,8a,9,10-heksahidro-10aH-benzo[f]isokromen-1,5(4aH,4bH)-dion (2) in (3S,4aS,4bS,8R,8aR,10aR)-8-hydroksi-4a,8a-dimetil-3-((1'R)-3'-okso-4',6'-dioksa-biciklo[3.1.0]heksan-1'-il)-3,4,8,8a,9,10-heksahidro-10aH-benzo[f]isokromen-1,5(4aH,4bH)-dion (3). V metanolu smo dobili le (3S,4aS,4bS,8R,8aR,10aR)-8-hidroksi-3-(5'-hidroperoksi-2'-metoksi-2',5'-dihidrofuran-3'-il)-4a,8a-dimetil-3,4,8,8a,9,10-heksahidro-10aH-benzo[f]isokromen-1,5-(4aH,4bH)-dion (4). Strukture spojin smo potrdili na osnovi njihovih IR, 1H NMR, 13C NMR spektrov in elementne analize. Nastanek produktov razlagamo s fotooksidacijo tionosponona. Ugotovili smo, da izbrana kombinacija reagentov vpliva na izkoristek fotooksidacijskih produktov in da sledi hitrosti tvorbe singletnega kisika.
Iqbal et al.     Sensitized Photooxygenation of Tinosponone