Background

1,8-dioxo-octahydroxanthenes are important class of oxygen heterocycle in which a phenyl substituted pyran ring is fused on either side with two cyclohexenone rings. Presence of conjugated bis-dienone functionality makes these compounds sensitive to attack by nucleophiles and light energy. In the recent past, there is a renewed interest in the synthesis of this class of compounds as number of its applications increased, both in the field of medicinal chemistry and material science. 1,8-dioxo-octahydroxanthenes shows useful biological activities such as anti-inflammatory, antibacterial, antiviral activities [1], finds use in biodegradable agrochemicals [2, 3], cosmetics and pigments [4], fluorescent materials [5], photodynamic therapy [6], luminescent sensors [7], and in laser technologies [8]. The formation of intermediate, 2,2'-arylmethylenebis(3-hydroxy-5,5-dimethyl-2-cyclohexene-1-one), in the synthesis of 1,8-dioxo-octahydroxanthenes can take place even without a catalyst [9]. However, there is always need for a catalyst in the cyclization step to get the cyclized compound [9–12]. A variety of reagents, mainly acids, have been employed to accomplish this transformation [11, 13–27]. However many of these reagents has its own disadvantages, such as use of expensive reagents, high catalyst loading and low yield.

On exposure to water, conventional Lewis acids such as AlCl3, ZnCl2, BCl3 etc., decomposes whereas lanthanide Lewis acids shows high hydrolytic stability. In fact, several organic reactions catalysed by lanthanide Lewis acids are carried out in water [28]. The catalysts can be recovered and reused without loss of much activity, thus considered to be green Lewis acids. SmCl3 is one of the readily available inexpensive Lewis acids which is ranked as "hard" according to the hard soft acids and bases (HSAB) concept [29]. SmCl3 is an excellent catalyst used in several important organic transformations such as selective removal of acid sensitive protecting groups like Boc, THP and TBDMS in the presence of one another [30], C-acylation of 1, 3-dicarbonyl compounds and malononitrile [31], and in the formation of ether from allylic alcohols [32]. SmCl3 is also used in electro reductive alkylation of ketones [33] and cleavage of allyl ethers [34]. Considering that there is a need for the use of more useful greener acid catalyst we examined SmCl3, a water resistant, reusable, lanthanide Lewis acid for the synthesis of 1,8-dioxo-octahydroxanthenes. Here in we report a highly efficient and clean synthesis of 1,8-dioxo-octahydroxanthenes using SmCl3.

Results and Discussion

Reaction between 4-NO2-benzaldehyde (1d, 1 mole) and dimedone (2, 2 mole) in presence of SmCl3 was identified as the test reaction and different reaction parameters were studied for the formation of corresponding 1,8-dioxo-octahydroxanthene. Considering the fact that SmCl3 is a water resistant Lewis acid the reaction was carried out in water at room temperature (RT). The reaction was complete in 15 min. However, it was only, the open chain, compound 2,2'-((4-nitrophenyl)methylene)bis(3-hydroxy-5,5-dimethylcyclohex-2-enone) (3d, Table 1, Scheme 1, entry 1) obtained as the sole product, in very good yield. Formation of the open chain compound was confirmed from 1H-NMR spectra of 3d which shows a peak corresponding to phenyl methylene proton at δ 5.54. The reaction mixture when left at RT for 24 h in water (entry 2) or heated at 100°C in water for 24 h (entry 3) gave only the open chain compound 3d. Heating a neat reaction mixture with SmCl3 (10 mol %, entry 4) at 100°C, for 24 h gave cyclised product 4d in 40% yield. However, heating the neat reaction mixture in presence of SmCl3 (20 mol%) at 120°C for 8 h, led to the formation of cyclised product 4d in excellent yield (entry 5, 98%).

Scheme 1
scheme 1

Optimisation of condition for reaction between dimedone and 4-NO 2 -benzaldehyde in the presence of SmCl 3 .

Full size image
Table 1 Optimisation of condition for reaction between dimedone and 4-NO2-benzaldehyde in the presence of SmCl3
Full size table

Based on these results it was concluded that use of SmCl3 (10 mol %) in water, at room temperature and SmCl3 (20 mol %) without solvent at 120°C would be the useful conditions for the formation of open chain compound 3d and the 9-phenyl1,8-dioxo-octahydroxanthane (4d) respectively. A series of examples illustrating versatility of this method was studied using aromatic, hetero aromatic and aliphatic aldehydes (Scheme 2) and the results are summarized in Table 2.

Scheme 2
scheme 2

SmCl 3 catalysed synthesis of 9-aryl-3,4,5,6,7,9-hexahydro-2H-xanthene-1,8-dione derivatives or its open chain form.

Full size image
Table 2 SmCl3 catalysed synthesis of 9-aryl-3,4,5,6,7,9-hexahydro-2H-xanthene-1,8-dione derivatives or its open chain form
Full size table

In case of formation of open chain compounds 3a to 3o, irrespective of whether it has electron donating or withdrawing substituents on the aromatic ring all the starting materials underwent reaction within 30 min. However, wide difference in reaction rate could be observed, in the formation of xanthenediones 4a to 4m. Among the different nitro benzaldehydes studied, 4-nitro-benzaldehyde (1d) underwent reaction faster (8 h) compared to 3-nitro-benzaldehyde (1c, 10 h, entry 3) and 2-nitro-benzaldehyde (1b, 13 h) to give the product 4b, 4c and 4d respectively. Steric hindrance of the 2-nitro group may the reason for slow reaction rate. In case of halo substituted benzaldehydes 1e-1g, the mesomerically electron donating character was predominant hence the reaction took place fast (entry 5, 6 and 7) compared to substrates with electron withdrawing substituents.

The effect of 4-CH3 substituent was so mild that the 4-CH3-benzaldehyde (1h) did not make any difference in reaction rate (entry 8, 5 h) compared to unsubstituted benzaldehyde (1a). In case of benzaldehydes 1i and 1j (entry 9, and 10), containing just one electron donating substituent, the rate of reaction was fast compared to compounds with electron withdrawing substituents. A significant increase in the rate of the reaction could be observed when more than one electron donating groups (entry 11, 12 and 13) are present. The reaction condition was tolerant to ether, phenolic -OH group.

In case of furfuraldehyde (1n) and cinnamaldhyde (1o) the open chain compounds 3n and 3o were obtained in excellent yield where as the corresponding cyclised products 4n and 4o was obtained only in minor quantity along with a mixture of side products. In order to improve the yield of cyclised products 4n and 4o, different reaction conditions were tried by varying the quantity of the catalyst from 20 mol % to 100 mol % and the reaction temperature from ambient to 85°C. None of the reaction conditions gave better result. Polymerisable nature of the furyl ring and cinnamoyl group may be the reason for low yield of the product. Spectral data and melting point observed for all the products were comparable with data reported in the literature (please check additional files 1 and 2).

In order to know the efficiency of SmCl3 the data reported in the literature for the synthesis of compound 4d, using different type of the catalysts was compared with SmCl3. The data on catalysts ranging from neutral slats, protic acids, and Lewis acids (Table 3, Scheme 3) were used for the comparison. This clearly shows that SmCl3 is a better catalyst for preparation of 1,8-dioxo-octahydroxanthanes in terms of high yield of the product, short reaction time and simple workup procedure.

Scheme 3
scheme 3

Comparison of the efficiency of catalysts reported for the synthesis of 14-(4-nitrophenyl)-14H-dibenzo [a, j] xanthenes and SmCl 3 .

Full size image
Table 3 Comparison of the efficiency of catalysts reported for the synthesis of 14-(4-nitrophenyl)-14H-dibenzo [a, j] xanthenes and SmCl3
Full size table

Considering the Lewis acidic nature of SmCl3 a possible mechanism is proposed (Scheme 4). Reaction of dimedone in its enol form is expected to react with aromatic aldehyde, under the influence of SmCl3, to give rise to ortho-quinone methides (o-QMs) III by Knoevenagel adduct formation. The intermediate III may further undergo Michael addition with another molecule of dimedone to form intermediate IV. The presence of water as solvent might not have facilitated dehydration reaction to give the cyclised product 4. However, when reaction was carried out in the absence of water a facile dehydration reaction took place to give rise to the cyclic compound 4.

Scheme 4
scheme 4

Proposed mechanism for the SmCl 3 catalysed synthesis of 3,3,6,6-tetramethyl-9-aryl-1,8-dioxo-2,3,4,5,6,7-hexahydroxanthene and its open chain form.

Full size image

Experimental

General Procedure for the Preparation of 2, 2'-aryl/alkylmethylene bis (3-hydroxy-5, 5- dimethyl-2-cyclohexene-1-one) (3)

To a mixture of an aldehyde (1 mole) and 5, 5-dimethyl-1,3-cyclohexanedione (2 mole) in water SmCl3, (10 mol%) was added and stirred at ambient temperature, till the reaction goes to completion. Progress of the reaction was monitored by TLC (Silica gel, 9:1, Hexane:EtOAc). After completion of the reaction, the solid separated was filtered, washed with water and dried to get the product.

General Procedure for the Preparation of 3,3,6,6-Tetramethyl-9-aryl-3,4,5,6,7,9-hexahydro-2H-xanthene-1,8-dione (4)

To a solution of an aldehyde (1 mole) and 5,5-dimethyl-1,3-cyclohexanedione (2 mole), SmCl3 (20 mol%) was added and stirred at 120°C. The progress of the reaction was monitored by TLC (Silica gel, 7:3, Hexane:EtOAc). After completion of the reaction it was cooled to room temperature and water (5 mL) was added, solid separated was filtered washed with water (5 mL) and dried under vacuum to get the desired xanthandione derivative. The combined aqueous layer was washed with EtOAc to remove any other organic impurities, concentrated and dried under vacuum at 120°C for 2 h to recover the catalyst in almost quantitative yield.

1. 2, 2'-(4-nitrophenylmethylene)bis(3-hydroxy-5,5- dimethyl-2-cyclohexene-1-one) (3d)

The reaction was carried out according to the general experimental procedure using 4-nitrobenzaldehyde (100 mg, 0.0006 mole), 5,5-dimethyl-1,3-cyclohexanedione (170 mg, 0.0013 mole) and SmCl3 (16.9 mg, 10 mol%) in water (1 mL). Conditions: room temperature, 30 minutes. The title compound 3d was obtained (251 mg, 91%) as a solid. The spectral data for the compound 3d was in agreement with the values already reported in the literature [35].

Mp: 194-196°C. 1H NMR (400 MHz, CDCl3): δ 1.04 (s, 6H, 2 × CH3), 1.16 (s, 6H, 2 × CH3), 2.24-2.44 (m, 8H, 4 × CH2), 5.57 (s, 1H, CH), 7.03-7.51 (m, 4H, Ar), 11.87 (s, 1H, OH).

2. 2,2'-(4-methoxyphenylmethylene)bis(3-hydroxy-5,5-dimethyl-2-cyclohexene-1-one) (3i)

The reaction was carried out according to the general experimental procedure using 4-methoxybenzaldehyde (100 mg, 0.0007 mole), 5,5-dimethyl-1,3-cyclohexanedione (188 mg, 0.0014 mole) and SmCl3 (18.8 mg, 10 mol%) in water (1 mL). Conditions: room temperature, 30 minutes. The title compound 3i was obtained (242 mg, 82%) as a solid. The spectral data for the compound 3i was in agreement with the values already reported in the literature [35].

Mp:146-148°C. 1H NMR (400 MHz, CDCl3): δ 1.02 (s, 6H, 2 × CH3), 1.15 (s, 6H, 2 × CH3), 2.21-2.40 (m, 8H, 4 × CH2), 3.70 (s, 3H, OCH3), 5.41 (s, 1H, CH), 6.75 (d, J = 8.8 Hz, 2H, Ar), 6.93 (d, J = 8.0 Hz, 2H, Ar), 11.84 (s, 1H, OH).

3. 3,3,6,6-tetramethyl-9-(4-nitrophenyl)-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (4d)

The reaction was carried out according to the general experimental procedure using 4-nitrobenzaldehyde (100 mg, 0.0006 mole), 5,5-dimethyl-1,3-cyclohexanedione (170 mg, 0.0013 mole) and SmCl3 (33.9 mg, 20 mol%). Conditions: 120°C, 8 h. The title compound 4d was obtained (257 mg, 97%) as a solid. The spectral data for the compound 4d was in agreement with the values already reported in the literature [34]. Mp: 225-226°C. The same reaction was repeated with the recovered catalyst for four times to get 97%, 96%, 96% and 96% of the product.

1H NMR (400 MHz, CDCl3): δ 0.99 (s, 6H, 2 × CH3), 1.11 (s, 6H, 2 × CH3), 2.18 (q, J = 19.6 Hz, 4H, 2 × CH2), 2.49 (s, 4H, 2 × CH2), 4.82 (s, 1H, CH), 7.47(d, J = 19.6 Hz, 2H, Ar), 7.47 (d, J = 5.2 Hz, 2H, Ar).

4. 9-(4-hydroxyphenyl)-3,3,6,6-tetramethyl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8(2H)-dione (4j)

The reaction was carried out according to the general experimental procedure using 4-hydroxybenzaldehyde (100 mg, 0.0008 mole), 5,5-dimethyl-1,3-cyclohexanedione (210 mg, 0.0016 mole) and SmCl3 (42 mg, 20 mol%). Conditions: 120°C, 9 h. The title compound 4j was obtained (298 mg, 98%) as a solid. The spectral data for the compound 4j was in agreement with the values already reported in the literature [36]. Mp: 244-246°C. 1H NMR (400 MHz, CDCl3): δ 0.99 (s, 6H, 2 × CH3), 1.09 (s, 6H, 2 × CH3), 2.21 (q, J = 8.8 Hz, 4H, 2 × CH2), 2.45 (s, 4H, 2 × CH2), 4.67 (s, 1H, CH), 6.61 (d, J = 8 Hz, 2H, Ar), 7.11 (d, J = 8 Hz, 2H, Ar).

Conclusion

In conclusion, SmCl3, a water resistant Lewis acid was used for the synthesis of 3,3,6,6-tetramethyl-9-aryl-3,4,5,6,7,9-hexahydro-2H-xanthene-1,8-dione derivatives or its open chain analogue depending upon the reaction condition. Water was used as the green solvent, and the catalyst could be recovered and reused. The reaction work up is very simple, solid separated from the reaction mixture was filtered washed with water to recover the catalyst and dried to get the desired xanthenedione derivatives. This procedure offers several advantages such as use of inexpensive green catalyst, operational simplicity, cleaner reaction and minimal environmental impact, high yield, thus making it one of the attractive and practical protocols for the synthesis of xanthenediones. Further studies on the use of xanthenediones as visible light sensitizers and as better excited state electron donors to TiO2 for application in dye sensitised solar cells are in progress.