The assembly of complex polycyclic skeletons of chemical and biomedical interest has become an important,challenging and active area of research in modern organic chemistry ^[1]. Among these skeletons,isocoumarin based fused ring system present in many natural products (Fig. 1) shows a broad range of biological activities ^[2]. In recent years,a variety of methods have been developed to prepare these structurally complex fused skeletons ^[3]. However,synthetic chemists are continuously searching for the development of new,cleaner and efficient chemical transformation methodologies,or modifications in the established synthetic pathways to ensure eco-friendly and cost effective synthesis with minimal or no use of toxic chemicals.

Fig. 1

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Fig. 1. Some representative natural products.

T_i ll date,excellent region-,chemo-,diastereo- and enantioselectivities are obtained for the preparation of complex molecules by developing several highly selective procedures ^[4]. The procedure usually used for the construction of such organic compounds involves a step-wise formation of individual bonds in the target molecule. However,it is much more desirable,if one could form several bonds in one go without isolating the intermediates and changing the reaction conditions. The waste produced in such synthetic procedures is very small as compared to step-wise pathways. Therefore,from the synthetic point of view, one-pot synthesis of fused-ring systems is an attractive procedure for searching bioactive compounds.

Coumarin derivatives ^[5] are important synthetic targets because of possessing diverse biological applications ^[6]. They are also known to have vasodilatory ^[7],anticoagulant ^[8],anti-HIV ^[9],antitumor ^[10] and anti-inflammatory ^[11] properties. The fluorescent properties of some isocoumarin derivatives are also reported ^[12]. Herein,we report one-pot synthesis of novel fused isocoumarin framework through highly selective domino multicyclizations under catalyst and solvent free reaction conditions. The resulting fused isocoumarin framework is an interesting scaffold for drug design and discovery and can play an important role in pharmaceutical research. 2. Experimental 2.1. Materials and methods

All reagents and solvents were used as obtained from the supplier or recrystallized/redistilled as required. Thin layer chromatography (TLC) was performed using aluminum sheets (Merck) coated with silica gel 60 F254. The melting points of compounds were determined using capillary tubes and an electrothermal melting point apparatus,model MP-D Mitamura Riken Kogyo,Japan. IR spectra of compounds were recorded on a Bio-Rad FTS 3000 MX spectrophotometer (400-4000 cm^-1 ). NMR spectra were recorded using a Bruker AM-300 spectrometer and chemical shifts are reported in ppmversustetramethylsilane with either tetramethylsilane or the residual solvent resonance used as an internal standard. Mass spectra were acquired on a Bruker Omniflex MALDI-TOF instrument and elemental analyses were carried out with a LECO-183 CHNS model. 2.2. Procedure for the synthesis of hexacyclic fused isocoumarin (6)

A mixture of 2,3-diphenylacrylic acid (2.07 g,9 mmol) and thionyl chloride (1 mL) was heated in the presence of a few drops of DMF for 30 min at 70°C. Completion of reaction was indicated by the disappearance of gas evolution. Excess thionyl chloride was removed under reduced pressure to afford 2,3-diphenylacryloyl chloride. Homophthalic acid (0.54 g,3 mmol) was then added to it and the mixture was heated first for 3.5 h at 200°C and then cooled to room temperature. Addition of aqueous solution of sodium carbonate (5%,200 mL),followed by filtration and washing thoroughly with water furnished the crude product,which was recrystallized in toluene to give compound 6 in pure form (68%). Mp: 135-136°C; IR (KBr,cm^-1):v 2918 (C-H),1704 (C=O),1569 (C=C); ¹H NMR (300 MHz,CDCl₃):δ 8.11 (dd,1H,J= 1.2,7.5 Hz), 7.91 (d,1H,J= 7.5 Hz),7.88-7.82 (m,1H),7.59-7.19 (m,12H),6.98 (dd,2H,J= 1.2,7.5 Hz),4.96 (s,1H); ¹³C NMR (75 MHz,CDCl₃):δ 204.6,161.4,151.1,142.7,137.5,135.8,135.7,135.3,133.1,130.6, 128.8,128.7,128.6,128.4,128.2,127.4,124.9,124.4,123.6,122.7, 107.8,62.7,48.2,31.0; MS [MALDI-TOF] (m/z): 427.13 [M+H⁺ ] (100),428.13 (33),429.13 (5). Anal. Calcd. for C₃₀H₁₈O₃: C,84.49; H,4.25; Found: C,84.53; H,4.23. 3. Results and discussion

We commenced our studies by reacting equimolar quantities of homophthalic acid1and 2,3-diphenylacryloyl chloride 2 at 200°C with an aim of getting isocoumarin derivative,3-(1,2-diphenylvinyl)-1H-isochromen-1-one 3 ^[13] (Scheme 1 and Fig. S1 in Supporting information). However,instead of obtaining the expected product 3,compound 4,i.e. 3-phenyl-1H-isochromen-1-one ^[14] and the novel fused hexacyclic isocoumarin framework 6,formed by the domino multicylization reaction,were isolated in 75% and 1% yields,respectively (Table 1,entry 1). Structure of the novel fused ring system6was established through spectral (IR, ¹H NMR, ¹³C NMR,COSY,NOESY,MS) and single crystal X-ray analyses (Fig. 2,Table 2). This unprecedented observation prompted us to pursue further the domino cyclization reaction from both the mechanistic as well as synthetic viewpoints. Accordingly,we retroanalyzed the fused isocoumarin skeleton 6 (Fig. S2 in Supporting information); it was hypothesized that some of the 2,3-diphenylacryloyl chloride 2 might be cyclized under the conditions to give 2-phenyl-1H-inden-1-one 5 ^[15],which reacted with the already observed 3-phenyl-1H-isochromen-1-one 4 through Michael addition reaction. To check our hypothesis,the concentration of 2 was gradually increased under the same reaction conditions. To our delight,the yield of 6significantly increased (1%→20%),while that of isocoumarin derivative 4 diminished (75%→45%) (Table 1,entry 2). Interestingly,when the concentration of2was enhanced to 3 equivalents,the yield of6 further increased (20%→30%),while that of 4 decreased (45%→28%) (Table 1,entry 3). At this stage,it was assumed that the formation of 2-phenyl-1H-inden-1-one 5compared with 3-phenyl-1H-isochromen-1-one4was slow and it needed more time to cyclize. Thus,the reaction time was increased from 2 to 3 h. It was found that the yield of4pronouncedly decreased (28%→7%), while that of 6 increased (30%→46%) (Table 1,entry 4). Surprisingly,further increase in the time duration (3→4 h) led to the formation of compounds 5 and 6 in 3% and 67% yields,respectively (Table 1,entry 5). It is pertinent to mention that compound4was not isolated under the conditions,indicating that compound 5reacts with it very quickly as soon as it forms. Interestingly,a slight decrease in the time duration (4→3.5 h) provided maximum yield (68%) of the fused compound6along with a trace amount of compound 5 as an impurity (Table 1,entry 6).

Table 1

Table 1 Optimization of reaction conditions for the synthesis of 6.

Table 1 Optimization of reaction conditions for the synthesis of 6.

Table 2

Table 2 X-ray crystallographic data of 6.

Table 2 X-ray crystallographic data of 6.

Scheme 1

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Scheme 1. Novel domino multicyclization reaction with all possible byproducts.

The attractive feature of this domino reaction is demonstrated by the fact that four new chemical bonds and three new rings were readily formed in domino fashion. In addition,work-up of the reaction is very simple. Water and phenylacetylene are the only byproducts,which may be evaporated under the reaction conditions/during the concentration of the reaction mixture, making the work-up very convenient simply by adding water/ filtration/washing/recrystallization. Finally,it is important to address here that only a single diastereomer of 6 was detected first by spectroscopic and then by X-ray diffraction analysis (Fig. 2 and Table 2).

Fig. 2

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Fig. 2. ORTEP diagram of 6.

The mechanism for this domino multicyclization reaction is proposed and shown in Scheme 2. It can be divided into three steps. The first step involves ring closure cascade reaction,which consists of regioselective condensation of acyl chloride 2 with homophthalic acid1leading to A and HCl (2 to A),intramolecular cyclization (A to B),removal of CO₂,phenyl acetylene and phenyl migration in a concerted fashion (BtoC),and finally dehydration (C to 4). The second step includes intramolecular cyclization of 2 to give intermediate 5. In the third step,double Michael addition reaction between 4 and 5 leads to intermediateEthroughD,which after aerobic oxidation,provides thermodynamically stable hexacyclic fused isocoumatin framework6. This mechanism has been partially supported by an experiment in which the isolated intermediates 4 and 5 were reacted at 200°C under solvent and catalyst free conditions; the hexacyclic product 6 was again generated in 66% yield (Scheme S1 in Supporting information). To the best of our knowledge,the synthetic strategy and mechanistic sequences described herein have not been reported so far.

Scheme 2

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Scheme 2. Proposed mechanism for the synthesis of 6.

Conclusively,a novel three component domino reaction was used for the construction of unprecedented hexacyclic fused isocoumarin framework. It is noteworthy for its cheap and readily available starting materials,eco-friendly procedure,easy work-up and potential biological applications of the resulting product. Our future efforts will be focused on using various computational and experimental methods for exploring the biological applications of this novel fused ring system. The facile one pot synthetic procedure may also be used to construct more useful and potential bioactive derivatives of this fused isocoumarin skeleton. Acknowledgment

We are highly grateful to the Higher Education Commission (HEC),Govt. of Pakistan for financial support. Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2014.03.022.