Photochromism of novel chromenes constrained to be part of [2.2]paracyclophane: remarkable ‘phane’ effects on the colored o-quinonoid intermediates

Abstract

The photochemistry of rationally designed chromenes that are constrained to be part of [2.2]paracyclophane, i.e., CP-H and CP-OMe, was investigated to examine the effect of through-space delocalization in the cyclophane core (phane effect) on the photochromic behavior. In contrast to the parent chromene, i.e., 2,2-diphenylbenzopyran CH, for which the photoinduced coloration is not observable at room temperature, the cyclophanochromenes CP-H and CP-OMe lend themselves to readily observable photochromism. The photogenerated o-quinonoid intermediates that are responsible for the observed color revert slowly to permit kinetic monitoring of their decays. The difference in the kinetic rate constants for reversion of the photogenerated o-quinonoid intermediates derived from CP-H and CP-OMe attests to remarkable through-space delocalization. To the best of our knowledge, the observed results constitute the first examples of the influence of phane effects on the stabilization or otherwise of the reactive intermediates.

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Introduction

Development of functional materials lies at the center of focus of research in organic chemistry at present. One witnesses utilization of a number of organic materials in diverse electronic devices in today's modern society.¹ Organic photochromic materials are a unique class of functional materials, which are assuming increasing importance in ophthalmic lenses, optical data storage devices, optoelectronic materials, molecular machines and switches, etc.;² photochromism is defined as reversible interconversion of a chemical species between two or more structural forms with distinct optical properties brought about at least in one direction by light.² A large number of photochromic systems based on various reaction types and classes of compounds have been heretofore reported.^2,3

We have been interested in the photochromic phenomenon exhibited by the class of 2,2-diarylbenzopyrans, referred to generally as chromenes (CHs, Chart 1). The latter undergo photoinduced C–O bond cleavage leading to the so-called o-quinonoid intermediates that are colored. In general, CH (Chart 1) does not lend itself to readily observable photochromism at room temperature; the photolysis leads to a mixture of colored quinonoid intermediates (vide infra), which revert too rapidly to preclude their detection at room temperature.⁴ One of the chief strategies to modulate the phenomenon of photochromism in chromenes has been annulation as, for example, in naphthopyrans.⁵ We have shown that simple arylation of the chromene nucleus permits control of the photochromic behavior via a mesomeric effect.⁶ Further, we have inquired into how sterics,⁷ toroidal conjugation in hexaarylbenzenes⁸ and polar–π interactions in cofacially oriented aryl rings with respect to the photogenerated o-quinonoid chromophore⁹ influence persistence as well as spectral properties. In continuation with these studies, our attention was drawn toward exploitation of the [2.2]paracyclophane scaffold in a wide array of areas that span optoelectronic materials,¹⁰ asymmetric synthesis,¹¹ polymers,¹² and dye-sensitized solar cells,¹³etc. [2.2]Paracyclophane represents a unique molecular entity that permits creation of different chromophores at a common junction and exploration of through-space chromophore interactions.¹⁴ Extensive investigations on paracyclophane-based bichromophores have shown that π–π interactions between the aromatic planes manifest in complete delocalization (phane effect) leading to absorption and emission properties that are not readily described by each of the aromatic chromophores.¹⁵ In the backdrop of noted remarkable phane effects, we wondered as to how the latter might influence spectrokinetic properties of colored o-quinonoid intermediates generated by the photolysis of the precursor chromenes. Investigation of the effects of through-space delocalization on the properties of the reactive intermediates is of significant importance from the standpoints of both fundamental understanding and practical applications. We thus rationally designed cyclophane-based chromenes (cyclophanochromenes) CP-H and CP-OMe as shown in Chart 1 to examine the extent to which the covalently bound and cofacially oriented arenes influence thermal reversion kinetics or persistence of photogenerated o-quinonoid intermediates. The methoxy derivative of CP-H, i.e., CP-OMe, was specifically designed based on our previous investigations on naphthalenes functionalized with cofacially aligned chromenes and aryl rings.⁹ Herein, we report remarkable ‘phane’ effects that permit the phenomenon of photochromism to be observed at room temperature for simple chromenes.

Chart 1

Results and discussion

Synthesis of cyclophanochromenes CP-H and CP-OMe

The [2.2]paracyclophane-based chromenes CP-H and CP-OMe were synthesized starting from suitably substituted hydroxy- and pseudo para-dihydroxycyclophanes (Scheme 1). The required monohydroxycyclophane was prepared by following the literature-reported procedure.¹⁶ The dihydroxycyclophane was synthesized starting from mono-formylcyclophane. Further formylation of the latter with dichloromethyl methyl ether in the presence of TiCl₄ yielded the required diformyl compound,¹⁷ which upon performing the Dakin reaction yielded the required dihydroxycyclophane.¹⁸ This product was monomethylated with MeI in DMSO–KOH. The hydroxycyclophanes thus derived were subjected to cyclocondensation with 1,1-diphenylpropargyl alcohol in the presence of pyridinium-p-toluenesulfonate (PPTS) as the catalyst to afford CP-H and CP-OMe. Both cyclophanochromenes were thoroughly characterized by IR, ¹H NMR, ¹³C NMR and ESI-MS spectroscopic analyses.

Scheme 1

X-ray crystal structure determination of CP-H

In order to establish if 2,2-diarylpyrannulation of the parent [2.2]paracyclophane modifies structural characteristics, we sought to determine the X-ray crystal structures of both cyclophanochromenes CP-H and CP-OMe. While the crystals of CP-H could be readily grown by slow evaporation of its solution in chloroform at room temperature, we were unsuccessful in obtaining the crystals of CP-OMe amenable for X-ray studies. In Fig. 1 are shown two perspective drawings of the molecular structure, which reveal that both benzene rings that are bonded by the ethylene bridges undergo considerable distortion. The calculated distance between the centers of the two rings that make up the cyclophane is 2.97 Å. Similarly, the distance between the carbons that are connected by the ethylene linkage is 2.77 Å. These geometrical parameters compare very well with those observed for the parent [2.2]paracyclophane and its derivatives,¹⁹ suggesting thereby that the 2,2-diarylpyrannulation of the paracyclophane does not manifest in unexpected structural features.

Fig. 1 The X-ray determined molecular structures of cyclophanochromene CP-H. The cofacial orientation of the benzene nucleus with respect to the benzopyran moiety is clearly evident in both. The structure on the right side reveals distortion of the aryl rings brought about by the ethylene linkages.

Photochemistry of chromenes CP-H and CP-OMe

Photolysis of the solutions of chromenes CP-H and CP-OMe in toluene (4–5 × 10⁻³ M) contained in quartz cuvettes at 298 K for 1–2 min in a Luzchem photoreactor (λ ≃ 350 nm, 14 lamps of 8 W power) led to reddish-brown (rust) coloration. In Fig. 2 are shown the absorption spectra of the solutions before and after irradiation; all the spectra for the chromenes were recorded at room temperature, i.e., 298 K. The spectra in Fig. 2 show that the photogenerated colored intermediates of both cyclophanochromenes exhibit absorption properties in the visible region that are virtually indistinguishable. The absorptions of the intermediates are characterized by a sharp band at ca. 417–420 nm followed by a broad band in the range of 450–700 nm. The rust color of the photolysates was found to disappear in 2–3 min upon standing in the dark. Thus, reversion of the colored species to colorless forms was kinetically followed by monitoring the disappearance of absorption at 420 and 524 nm for both of the cyclophanochromenes. The decay profiles of the colored species are also shown in Fig. 2. Fitting of the absorption decays to a single exponential function yielded kinetic rate constants for thermal reversion of the colored species. The decay rate constants thus extracted are provided along with their decay profiles.

Fig. 2 The absorption spectra of chromenes CP-H (a) and CP-OMe (b) before (black) and after (red) irradiation. Also given are the decay profiles for thermal reversion of the colored species for CP-H (c) and CP-OMe (d) at 298 K; the decay in each case monitored by following the absorption at 420 nm.

As mentioned at the outset, the parent chromene CH (Chart 1) does not exhibit visible coloration at room temperature upon photolysis.⁴ In contrast, the cyclophanochromenes CP-H and CP-OMe exhibit ready coloration and significantly longer-lived intermediates that permit decays to be followed spectrally at room temperature. Of the two cyclophanochromenes, the colored species of CP-OMe decays rather slowly signifying the fact that the substituents in the aryl ring aligned cofacially with respect to the chromene ring contribute to stabilization and otherwise of the intermediates responsible for color. It is pertinent to compare the rate constants for the decays of the colored intermediates of CP-H (k = 0.023 s⁻¹) and CP-OMe (k = 0.012 s⁻¹) with those of the parent CH and a benzo-annulated chromene such as naphthochromene. The very absence of observation of any coloration at room temperature for the intermediate of CH allows one to set k ≥ 1.0 s⁻¹ as the lower limit,⁴ while the reported value of k for reversion of the colored intermediate of naphthochromene is 0.09 s⁻¹ at 298 K.²⁰

Influence of phane effects on the photochromic behavior

The mechanism of photochromism of chromenes has been well established.²¹ In short, the C–O bond cleavage following photoexcitation may in principle lead to four possible geometrical isomers of o-quinonoid intermediates; these isomers for the cyclophanochromenes are shown in Scheme 2. The CC isomer is expected to revert back quite rapidly or undergo bond rotation to the TC isomer, while the CT isomer is believed to be not populated due to steric constraints. The color observed under steady-state photolysis is generally attributed to TC and TT isomers; the TT isomer is formed as a consequence of the 2-photon process and becomes important only under the conditions of long durations of steady-state photolysis. Thus, it is a mixture of TC (major) and TT (minor) isomers that accounts for the observed color in steady-state photolysis, while only the TC isomer becomes accessible upon laser flash photolysis with low power. Indeed, at very short durations of irradiation, the formation of the TT isomer can be minimized even under steady-state photolysis conditions such that only the TC isomer is entirely responsible for the color. In our experiments, we found that the decays for the colored species of both CP-H and CP-OMe could be nicely fitted to a single exponential function when the duration of irradiation was limited to 1–2 min. Based on the foregoing discussion, we attribute the photogenerated colored species in both CP-H and CP-OMe to the TC isomer (Scheme 2).

Scheme 2

The colored o-quinonoid intermediate in the case of the parent chromene, i.e., CH, has been reported to be red in color; the absorption spectrum recorded at low temperatures shows a sharp band at ca. 410 nm and a broad band in the range of 450–600 nm.⁴ The photogenerated intermediates of cyclophanochromenes CP-H and CP-OMe also exhibit similar absorption features with the only difference that the broad absorption is more pronounced and red shifted. This perhaps contributes to the color of the irradiated solutions being reddish-brown, see Fig. 2. Given that one cannot observe the color changes upon photolysis of CH at room temperature, the origin of the photochromic behavior of the chromenes constrained to be part of cyclophanes, i.e., CP-H and CP-OMe, at room temperature should be traceable to the manifestation of geometric attributes inherent to the cyclophane scaffold. Insofar as the [2.2]paracyclophanes substituted in the two rings with different chromophores are concerned, the optical characteristics are better described by two states, namely ‘chromophore state’ and the ‘phane state’;¹⁵ while the former describes through-bond delocalization in the chromophore, the latter characterizes delocalization across the cyclophane core. It has emerged from incisive investigations of Bazan and co-workers^14,15a,c that the through-space (phane) effect decreases with increasing conjugation length. Otherwise, the phane effect could be considerable for suitably substituted chromophores. The X-ray determined crystal structure for CP-H shows that the linkage of the two benzene nuclei via ethylene bridges renders the carbon atoms of the two rings at distances less than the sum of their van der Waals' radii (3.40 Å). The calculated center-to-center distance between the two benzene nuclei and the distance between the carbons that are linked by ethylene bridges are 2.97 and 2.77 Å, respectively. As mentioned earlier, these values compare closely with those observed for [2.2]paracyclophane and a number of its derivatives.¹⁹ Clearly, a close overlap of the orbitals must facilitate chromophore delocalization to account for the observation of photochromism in cyclophanochromenes. Indeed, the ‘phane’ effects must be significant in the photochemically-generated o-quinonoid intermediates than in the precursor cyclophanochromenes, since the chromophore in the former could be considered as e-deficient. The fact that the through-space delocalization does afford significant stability is underscored from the behavior of the methoxy-substituted derivative, i.e., CP-OMe, for which the colored intermediate is significantly longer lived.

A perusal of the absorption spectra for the photogenerated intermediates of both CP-H and CP-OMe in Fig. 2 shows that the absorptions of the colored intermediates in the visible region are similar with no perceptible differences. This signifies the fact that there is no signature for a charge-transfer kind of complex formation between the o-quinonoid e-deficient chromophore and the e-rich arene. The observed phenomenon in cyclophanochromenes is thus a consequence of stabilization of the photogenerated o-quinonoid intermediate via chromophore delocalization across the cyclophane core. Indeed, these results constitute first examples of exploitation of the [2.2]paracyclophane scaffold to modulate the behavior of reactive intermediates, which in the present instance are o-quinonoids derived from chromenes.

Conclusions

Photochromism of two rationally designed chromenes that are constrained to be part of [2.2]paracyclophane has been investigated to examine the extent to which the ‘phane’ effects influence the spectrokinetic properties of the photogenerated colored o-quinonoid intermediates. Given that the two chromophores of the chromenes constrained to be part of the cyclophane are not highly conjugated, the phane effects involving through-space delocalization between the aryl and the photogenerated o-quinonoid chromophores manifest in remarkable stabilization of the latter. While the o-quinonoid intermediate in the case of the parent chromene CH is highly labile to preclude its detection at room temperature, the colored intermediates in the case of cyclophanochromenes, i.e., CP-H and CP-OMe, are remarkably longer lived to permit their thermal reversion to be followed at room temperature. The significance of phane effects is strikingly revealed from the photochromic behavior of the methoxy derivative CP-OMe, for which the colored o-quinonoid intermediate is ca. 2-fold longer lived than that of CP-H.

Experimental section

General aspects

1,2-Dichloroethane was distilled over calcium hydride under nitrogen prior to use. Anhyd. tetrahydrofuran (THF) and toluene were freshly distilled over sodium under a nitrogen gas atmosphere. All other solvents were distilled before use. Column chromatography was conducted with silica gel of 60–120 μ mesh.

Preparation of cyclophanochromene, CP-H

An oven dry two-necked round bottom flask was charged with [2.2]paracyclophane (0.25 g, 1.20 mmol) and dry DCM (5 mL) under a nitrogen atmosphere. The container was cooled to 0 °C and TiCl₄ (0.25 mL, 2.40 mmol) followed by dichloromethyl methyl ether (0.11 mL, 1.26 mmol) was introduced dropwise. The reaction mixture was brought to room temperature and stirred for 6 h. To this was added crushed ice and stirred for an additional 2 h. The organic matter was subsequently extracted with CHCl₃ (30 mL × 3). The combined extract was dried over anhyd. Na₂SO₄, filtered and evaporated to yield crude racemic 4-formyl[2.2]paracyclophane,¹⁶ which was further purified by silica-gel column chromatography, yield 90%.

rac-4-Formyl[2.2]paracyclophane¹⁶

Colorless solid, yield 90%; mp 140–142 °C (lit. mp 142–145 °C); ¹H NMR (CDCl₃, 500 MHz) δ 9.95 (s, 1H), 7.01 (d, J = 1.8 Hz, 1H), 6.73 (dd, J₁ = 7.7 Hz, J₂ = 1.7 Hz, 1H), 6.59 (d, J = 7.8 Hz, 1H), 6.56 (dd, J₁ = 7.8 Hz, J₂ = 1.7 Hz, 1H), 6.50 (dd, J₁ = 7.9 Hz, J₂ = 1.6 Hz, 1H), 6.43 (dd, J₁ = 8.0 Hz, J₂ = 1.7 Hz, 1H), 6.37 (dd, J₁ = 7.7 Hz, J₂ = 2.0 Hz, 1H), 4.10 (ddd, J₁ = 12.7, 10.3, 1.4 Hz, 1H), 3.17–3.29 (m, 3H), 3.01–3.12 (m, 3H), 2.95 (ddd, J₁ = 13.1, 10.1, 6.7 Hz, 1H).

A solution of rac-4-formyl[2.2]paracyclophane (0.50 g, 2.12 mmol) in DCM–MeOH (1 : 1) was treated with H₂O₂ (0.30 mL, 30% in water) and conc. H₂SO₄ (0.016 mL), and the contents were stirred at room temperature for 24 h. After completion of the reaction as judged from TLC analysis, the solvent was removed and the organic matter was extracted with CHCl₃. The combined organic extract was dried over anhyd. Na₂SO₄, filtered and evaporated to furnish crude rac-4-hydroxy[2.2]paracyclophane,¹⁶ which was further purified by silica-gel column chromatography, yield 70%.

rac-4-Hydroxy[2.2]paracyclophane¹⁶

Colorless solid, yield 70%; mp 223–225 °C (lit. mp 225 °C); ¹H NMR (CDCl₃, 500 MHz) δ 7.00 (dd, J₁ = 7.9 Hz, J₂ = 1.8 Hz, 1H), 6.55 (dd, J₁ = 7.9 Hz, J₂ = 1.8 Hz, 1H), 6.45 (dd, J₁ = 7.9 Hz, J₂ = 1.8 Hz, 1H), 6.38–6.41 (m, 2H), 6.26 (dd, J₁ = 7.5 Hz, J₂ = 1.5 Hz, 1H), 5.54 (s, 1H), 3.31–3.36 (m, 1H), 2.88–3.13 (m, 6H), 2.63–2.69 (m, 1H).

To a round bottom flask containing 15 mL of dry 1,2-dichloroethane were added rac-4-hydroxy[2.2]paracyclophane (0.25 g, 1.12 mmol), 1,1-diphenylprop-2-yn-1-ol (0.47 g, 2.24 mmol) and a catalytic amount of pyridinium-p-toluenesulfonate (PPTS, 5 mol%). The contents were heated at reflux under a nitrogen gas atmosphere for 36 h. After this period, the reaction mixture was cooled, washed with saturated Na₂CO₃ solution, and the organic matter was extracted with chloroform (20 mL × 3). The combined organic extract was dried over anhyd. Na₂SO₄, filtered and evaporated to yield the crude cyclophanochromene CP-H, which was further purified by silica-gel column chromatography, yield 60%.

Cyclophanochromene, CP-H

Colorless solid, yield 60%; mp 115–117 °C; IR (KBr) cm⁻¹ 2924, 2853, 1625, 1490, 1446; ¹H NMR (CDCl₃, 500 MHz) δ 7.59 (d, J = 7.8 Hz, 2H), 7.44 (t, J = 7.6 Hz, 2H), 7.33–7.35 (m, 3H), 7.17 (t, J = 7.2 Hz, 2H), 7.09–7.12 (m, 1H), 6.84 (dd, J₁ = 7.6 Hz, J₂ = 1.5 Hz, 1H), 6.57–6.61 (m, 2H), 6.55 (d, J = 9.6 Hz, 1H), 6.48 (dd, J₁ = 7.6 Hz, J₂ = 1.6 Hz, 1H), 6.35 (d, J = 7.6 Hz, 1H), 6.23 (d, J = 10.0 Hz, 1H), 6.13 (d, J = 7.6 Hz, 1H), 3.45–3.51 (m, 1H, –CH₂–), 3.27–3.32 (m, 1H, –CH₂–), 3.03–3.15 (m, 3H, –CH₂–), 2.89–2.95 (m, 1H, –CH₂–), 2.71–2.77 (m, 1H, –CH₂–), 2.58–2.64 (m, 1H, –CH₂–); ¹³C NMR (125 MHz, CDCl₃) δ 31.4, 31.9, 34.3, 34.7, 81.3, 122.2, 122.4, 125.8, 126.4, 126.6, 127.1, 127.2, 127.3, 127.80, 127.84, 128.3, 129.6, 132.4, 132.7, 137.4, 137.9, 139.5, 144.9, 146.3, 151.1; ESI-MS⁺m/z calcd for C₃₁H₂₇O 415.2062 [M+H], found 415.2065.

Preparation of methoxy-cyclophanochromene, CP-OMe

Racemic 4-formyl[2.2]paracyclophane (0.50 g, 2.12 mmol) was treated with TiCl₄ (0.47 mL, 4.24 mmol) and 1,1-dichloromethyl methyl ether (0.20 mL, 2.23 mmol) in dry DCM at 0 °C, and the reaction mixture was subsequently allowed to stir at rt under a nitrogen atmosphere for 12 h. At the end of this period, the reaction mixture was quenched by adding crushed ice and stirring the mixture at rt for 3 h. Later, the organic matter was extracted with CHCl₃ and dried over anhyd. Na₂SO₄, filtered and evaporated to afford the crude 4,16-diformyl[2.2]paracyclophane,¹⁷ which was further purified by silica-gel column chromatography, yield 54%.

4,16-Diformyl[2.2]paracyclophane¹⁷

Colorless solid, yield 54%; mp 218–221 °C; ¹H NMR (CDCl₃, 500 MHz) δ 9.94 (s, 2H), 7.05 (d, J = 2.1 Hz, 2H), 6.64 (dd, J₁ = 7.7 Hz, J₂ = 2.1 Hz, 2H), 6.53 (d, J = 7.9 Hz, 2H), 4.11–4.15 (m, 2H), 3.23–3.31 (m, 2H), 3.10–3.18 (m, 2H), 2.99–3.04 (m, 2H).

A similar Dakin oxidation protocol as described above was adopted to prepare 4,16-dihydroxy[2.2]paracyclophane from 4,16-diformyl[2.2]paracyclophane (0.50 g, 1.89 mmol), H₂O₂ (0.50 mL, 30% in water) and conc. H₂SO₄ (0.03 mL) in DCM–MeOH solution (1 : 1 mixture).

4,16-Dihydroxy[2.2]paracyclophane¹⁸

Colorless solid, yield 68%; mp >300 °C; ¹H NMR (CDCl₃, 500 MHz) δ 6.90 (d, J = 7.9 Hz, 2H), 6.19 (d, J = 7.6 Hz, 2H), 5.58 (d, J = 1.2 Hz, 2H), 3.34–3.37 (m, 2H), 2.81–3.06 (m, 4H), 2.61–2.64 (m, 2H).

To the solution of the above compound (0.10 g, 0.42 mmol) in dry DMSO was added KOH (0.02 g, 0.42 mmol). The resultant solution was cooled to ca. 15 °C and MeI (0.03 mL, 0.42 mmol) was introduced. After stirring the reaction mixture at this temperature for 3–4 h, it was quenched with little ice. The organic matter was extracted with CHCl₃ and the combined extract was dried over anhyd. Na₂SO₄, filtered and evaporated to yield crude 4-hydroxy-16-methoxy[2.2]paracyclophane. The latter was further purified by silica-gel column chromatography, yield 58%.

4-Hydroxy-16-methoxy[2.2]paracyclophane

Colorless solid, yield 58%; mp 241–243 °C; IR (KBr) cm⁻¹ 3387, 2923, 2851, 1609, 1510, 1244; ¹H NMR (CDCl₃, 500 MHz) δ 6.98 (d, J = 1.8 Hz, 1H), 6.63 (d, J = 7.6 Hz, 1H), 6.50 (d, J = 7.6 Hz, 1H), 6.38 (dd, J₁ = 7.6 Hz, J₂ = 1.5 Hz, 1H), 6.31 (dd, J₁ = 7.6 Hz, J₂ = 1.5 Hz, 1H), 5.83 (d, J = 1.2 Hz, 1H), 3.87 (s, 3H), 3.39–3.44 (m, 1H), 3.29–3.34 (m, 1H), 3.12–3.17 (m, 1H), 3.03–3.09 (m, 1H), 2.84–2.91 (m, 1H), 2.74–2.81 (m, 2H), 2.48–2.54 (m, 1H); ¹³C NMR (125 MHz, CDCl₃) δ 30.8, 33.1, 33.8, 34.0, 55.3, 118.5, 124.7, 125.4, 128.3, 131.9, 134.9, 135.1, 136.2, 139.1, 142.6, 153.4, 158.6; ESI-MS⁺m/z calcd for C₁₇H₁₉O₂ 255.1385 [M+H], found 255.1388.

Cyclocondensation of 4-hydroxy-16-methoxy[2.2]paracyclophane with 1,1-diphenylprop-2-yn-1-ol according to the procedure described above led to the required chromene CP-OMe as a colorless solid, yield 65%.

Methoxy-cyclophanochromene, CP-OMe

Colorless solid, yield 65%; mp 155–158 °C; IR (KBr) cm⁻¹ 3027, 2924, 2856, 1453, 1416; ¹H NMR (CDCl₃, 500 MHz) δ 7.56 (d, J = 7.2 Hz, 2H), 7.42 (t, J = 7.5 Hz, 2H), 7.23–7.28 (m, 3H), 6.99–7.11 (m, 4H), 6.74 (d, J = 7.8 Hz, 1H), 6.69 (d, J = 9.5 Hz, 1H), 6.59 (d, J = 7.5 Hz, 1H), 6.44 (d, J = 9.5 Hz, 1H), 6.40 (d, J = 7.7 Hz, 1H), 6.13 (d, J = 7.7 Hz, 1H), 3.85 (s, 3H, –OCH₃), 3.65–3.71 (m, 1H, –CH₂–), 3.38–3.42 (m, 1H, –CH₂–), 3.22 (t, J = 12.3 Hz, 1H, –CH₂–), 3.07–3.19 (m, 1H, –CH₂–), 2.87–2.94 (m, 2H, –CH₂–), 2.71–2.79 (m, 2H, –CH₂–); ¹³C NMR (125 MHz, CDCl₃) δ 27.3, 28.4, 34.1, 35.1, 51.4, 80.7, 123.1, 123.2, 125.1, 126.0, 126.4, 126.8, 127.1, 127.7, 127.8, 128.2, 128.3, 129.1, 130.0, 134.3, 135.7, 136.2, 136.3, 137.8, 142.9, 144.6, 144.8, 151.5; ESI-MS⁺m/z calcd for C₃₂H₂₉O₂ 445.2168 [M+H], found 445.2162.

Acknowledgements

JNM thankfully acknowledges the financial support from CSIR (Council of Scientific and Industrial Research), India. SM is grateful to CSIR for a senior research fellowship. AK gratefully acknowledges the fellowship from Indian Academy of Sciences, which allowed his summer internship away from his host institution, namely IISER, Pune.

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Footnotes

† This article is included in the All Aboard 2013 themed issue.

‡ Electronic supplementary information (ESI) available. CCDC reference number 891747. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2nj40575j

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