HETEROCYCLES

Paper | Regular issue | Vol. 87, No. 1, 2013, pp. 65-78
Received, 9th October, 2012, Accepted, 31st October, 2012, Published online, 8th November, 2012.
DOI: 10.3987/COM-12-12602

■ Synthesis of Bicyclic Dioxetanes Bearing a 4-(Benzimidazol-2-yl)-3-hydroxyphenyl Group and Their Base-Induced Chemiluminescent Decomposition in an Aprotic Medium and in an Aqueous Medium

Hiromasa Hagiwara, Nobuko Watanabe, Hisako K. Ijuin, Masashi Yamada, and Masakatsu Matsumoto*

Department of Chemistry, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka 259-1293, Japan

Abstract

Bicyclic dioxetane, 5-tert-butyl-4,4-dimethyl-2,6,7-trioxabicyclo[3.2.0]heptane, bearing a 4-(benzimidazol-2-yl)-3-hydroxyphenyl group at the 1-position and its N-substituted benzimidazolyl-analogs were synthesized. N-Methylbenzimidazolyl-analog and N-phenylbenzimidazolyl-analog were found to undergo charge-transfer-induced decomposition (CTID) to effectively give light in both TBAF/MeCN and in NaOH/H2O. The CTID of N-(4-carboxybutyl)benzimidazolyl-analog gave also effectively light both in MeCN and in H2O. On the other hand, chemiluminescent CTID of the unsubstituted benzimidazolyl-analog changed depending on the base used: TBAF/MeCN induced weak emission of yellow light due to a dianion of the dioxetane, while TMG(tetramethylguanidine)/MeCN induced strong emission of blue light due to a monoanion of the dioxetane.

INTRODUCTION
Upon treatment with a base, a hydroxyphenyl-substituted dioxetane is deprotonated to give an unstable oxidophenyl-substituted dioxetane which rapidly decomposes with an accompanying emission of light by intramolecular charge-transfer-induced decomposition (CTID) mechanism. This phenomenon has received considerable attention from the viewpoints of application to clinical and biological analysis as well as of mechanistic interest related to bioluminescence and chemiluminescence.1−7 One of such CTID-active dioxetanes is bicyclic compound 1 bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl group, which effectively emits light in an aqueous system as well as in an aprotic polar solvent.8 Furthermore, dioxetane 1 has very recently been found to undergo solvent-promoted decomposition, which is an entropy-controlled reaction leading to effective chemiluminescence.9

These facts prompted us to realize bicyclic dioxetanes 2 bearing a 4-(benzimidazol-2-yl)-3-hydroxyphenyl group with expectation that the skeleton of 2 could be developed to novel chemiluminescence substrates bearing various auxiliaries, since a saturated nitrogen of benzimidazolyl group could be easily functionalized or tethered, differently from benzothiazolyl or benzoxazolyl group. Thus, we basically investigated here whether or not dioxetanes 2 showed effective chemiluminescence in an aqueous medium as well as in an aprotic medium. Dioxetanes investigated here were parent 2a bearing a 4-(benzimidazol-2-yl)-3-hydroxyphenyl group and its N-methylbenzimidazolyl- 2b, N-phenylbenzimidazolyl- 2c and N-(4-carboxybutyl)benzimidazolyl-analog 2d (Figure 1).

RESULTS AND DISCUSSION
Synthesis of bicyclic dioxetanes bearing a 4-(benzimidazol-2-yl)-3-hydroxyphenyl group
All of the dioxetanes 2a−d investigated here were prepared by singlet oxygenation of the corresponding 5-[4-(benzimidazol-2-yl)-3-hydroxyphenyl]-4-tert-butyl-3,3-dimethyl-2,3-dihydrofurans 3a−d. These precursors were synthesized through several steps starting from 4-tert-butyl-5-(4-carboxy-3-methoxyphenyl)-3,3-dimethyl-2,3-dihydrofuran (5), which was synthesized from 5-(4-bromo-3-methoxyphenyl)-4-tert-butyl-3,3-dimethyl-2,3-dihydrofuran (4),10 as illustrated in Scheme 1.
The initial step was condensation of carboxylic acid 5 with benzene-1,2-diamines 6a−c, which smoothly proceeded by using triphenylphosphonium anhydride trifluoromethanesulfonate (POP)11 in CH2Cl2 at room temperature to give the corresponding benzimidazoles 7a−c. Nucleophilic substitution of benzimidazole 7a with ethyl 5-bromopentanoate gave ester 8. These 4-benzimidazolyl-3-methoxyphenyl-substituted dihydrofurans 7a−c and 8 were demethylated effectively with sodium methylthiolate to give the desired precursors 3a−d: for the case of 8, saponification of the ester function also proceeded. All of dihydrofurans 3a−d were individually irradiated with Na-lamp in the presence of catalytic amount of tetraphenylporphin (TPP) in acetone or CH2Cl2 under O2 atmosphere at 0 °C to selectively give the corresponding dioxetanes 2a−d. The structures of dioxetanes 2a−d were determined by 1H NMR, 13C NMR, IR, MS and HRMass spectral analyses.

Base-induced chemiluminescent decomposition of dioxetanes bearing a 4-(benzimidazol-2-yl)-3-hydroxyphenyl group
When a solution of dioxetane 2a in MeCN was added to a solution of tetrabutylammonium fluoride (TBAF, large excess) in MeCN at 45 ºC, 2a decomposed according to the pseudo-first order kinetics independent of the TBAF concentration to emit yellow light, the spectrum of which is shown in Figure 2(A). The chemiluminescence properties of 2a were as follows: maximum wavelength λmaxCL = 510 nm, chemiluminescence efficiency ΦCL = 0.065,12,13 rate of CTID kCTID = 4.6 x 10−3 s-1, and half-life t1/2CTID = 150 s (Table 1). On similar treatment with TBAF, 2b and 2c showed bright chemiluminescence, the properties of which are shown in Table 1 (Figure 2(A)). We can see from Table 1 that 2b and 2c emitted light as effectively as benzothiazolyl-analog 1, while, in contrast, parent 2a gave light in poor yield, and the λmaxCL was considerably longer for 2a than those for 2b and 2c.
Next, we carried out CTID of dioxetanes 2a−c in an aqueous system. When dioxetanes 2a−c were individually treated with 0.1 M NaOH aqueous solution at 45 ºC, they decomposed with the accompanying chemiluminescence. Their chemiluminescence properties and spectra are summarized in Table 1 and Figure 2(B), respectively. Table 1 shows that all of three dioxetanes 2a−c emitted light in high yields, which were >1000 times higher than that for simple bicyclic dioxetane, 5-tert-butyl-1-(3-hydroxyphenyl)-4,4-dimethyl-2,6,7-trioxabicyclo[3.2.0]heptane (9),14 though they were somewhat lower than that for 1. If we compare here features of CTID for 2a−c in a NaOH/H2O system to those in a TBAF/MeCN, we can see that only parent dioxetane 2a showed the noticeable differences in λmaxCL and in ΦCL between these two systems. The λmaxCL for 2a was 33 nm shorter in an aqueous system than in an MeCN system, though those for 2b and 2c were not so much different between in these two systems. Chemiluminescence efficiency for 2a was unexpectedly higher in the aqueous system than in the MeCN system, while those for 2b and 2c somewhat decreased in an aqueous system. Notably, 2a was the first example that a CTID-active dioxetane emitted light more effectively in an aqueous system than in an aprotic polar solvent system.

The freshly spent reaction mixture for 2a−c in both base systems effectively gave the corresponding keto ester 10a−c after careful neutralization. Authentic oxido anions 11a−c generated by dissolving 10a−c in NaOH/H2O gave fluorescences, the spectra of which coincided with the corresponding chemiluminescence spectra of 2a−c (Figure 2(D)). The results suggest that 11a−c were undoubtedly emitters for CTID of the corresponding dioxetanes 2a−c in a NaOH/H2O system (Scheme 2). On the other hand, in a TBAF/MeCN system, fluorescence spectrum of 11a generated from 10a did not coincide with chemiluminescence spectrum from 2a, though fluorescence spectra of authentic 11b and 11c coincided with the corresponding chemiluminescence spectra of 2b and 2c (Figure 2(C)).

We carried out several experiments to understand the discrepancy between fluorescence spectrum of 11a and chemiluminescence spectrum of 2a in a TBAF/MeCN. Prominent difference in the structure between 2a and 2b or 2c was that only benzimidazolyl group in 2a possesses a weakly acidic NH. This structural difference suggested that 2a would decompose in different manner depending on a base system. Thus, we attempted to use tetramethylguanidine (TMG, pKa = 13.6) as a base far weaker than TBAF (pKa >> 15) though strong enough for CTID of 2a in MeCN. When a solution of 2a in MeCN was added to a solution of MeCN including a large excess of TMG instead of TBAF at 45 °C, 2a showed chemiluminescence with λmaxCL = 471 nm, ΦCL = 0.22, kCTID = 1.9 x 10−4 and t1/2 = 3700 s.
This λmaxCL was 39 nm shorter than that in a TBAF/MeCN, and coincided with λmaxfl of fluorescence from authentic 11a in TBAF/MeCN as well as in TMG/MeCN (Figure 2(C)). We can understand from Table 1 that ΦCL of 2a in a TMG/MeCN system increased more than 3 times from that in a TBAF/MeCN system, and was same as that for 2b in a TBAF/MeCN. These results suggested that a strong base TBAF produced dianion 14a of keto ester in the excited state through dianion 13a of dioxetane 2a, whereas TMG could abstract only a phenolic proton of 2a to produce monoanion of dioxetane 12a which decomposed into monoanion 11a in the excited state (Scheme 2).

As described above, dioxetane 2b bearing a 3-hydroxy-4-(N-methylbenzimidazol-2-yl)phenyl group underwent CTID to effectively give light in both TBAF/MeCN and NaOH/H2O systems. Thus, we investigated whether or not the substitution with ω-functionalized alkyl group instead of N-methyl in 2b could keep up chemiluminescence properties, especially high ΦCL, for base-induced decomposition. As a representative of ω-functionalized alkyl group, we selected 4-carboxybutyl group, which could tether various auxiliaries or pendants (Figure 1). Dioxetane bearing an 4-[N-(4-carboxybutyl)-benzimidazol-2-yl]-3-hydroxyphenyl group 2d decomposed to effectively emit light in both TBAF/MeCN and NaOH/H2O systems (Figure 2). The results summarized in Table 1 show that chemiluminescence properties for 2d were practically similar to those for 2b.

CONCLUSION
Bicyclic dioxetane bearing a 4-(benzimidazol-2-yl)-3-hydroxyphenyl group 2a and its N-substituted benzimidazolyl-analogs 2b−2d were synthesized. N-Methylbenzimidazolyl-analog 2b and N-phenylbenzimidazolyl-analog 2c were found to undergo CTID to effectively give light in both TBAF/MeCN and in NaOH/H2O. On the other hand, ΦCL, λmaxCL and kCTID for CTID of unsubstituted benzimidazolyl-analog 2a changed depending on the base used: especially ΦCL in TBAF/MeCN system was quite low and was only <1/3 of ΦCL in TMG/MeCN system. CTID of N-(4-carboxybutyl)-benzimidazolyl-analog 2d gave also effectively light both in MeCN and in H2O. The results presented here show that design of new CTID-dioxetanes tethering various auxiliaries through an N-spacer can become possible.

EXPERIMENTAL
General
Melting points were uncorrected. IR spectra were taken on a FT/IR infrared spectrometer. 1H and 13C NMR spectra were recorded on a 400 MHz and 500 MHz spectrometers. Mass spectra were obtained by using a double-focusing mass spectrometer and an ESI-TOF mass spectrometer. Column chromatography was carried out using SiO2.

Synthesis of 5-(4-carboxy-3-methoxyphenyl)-4-tert-butyl-3,3-dimethyl-2,3-dihydrofuran (5): BuLi (4.30 mL, 1.62 M in hexane, 6.97 mmol) was added to a solution of 5-(4-bromo-3-methoxyphenyl)-4-tert-butyl-3,3-dimethyl-2,3-dihydrofuran (4) (2.22 g, 6.56 mmol) in dry THF (20 mL) under a nitrogen atmosphere at -78 °C. After stirring for 30 min, dry ice was added to the solution and the reaction mixture was warmed slowly to room temperature. The reaction mixture was poured into 1 M HCl and extracted with AcOEt. The organic layer was washed twice with sat. aq. NaCl, dried over anhydrous MgSO4, and concentrated in vacuo. The residue was chromatographed on silica gel and eluted with AcOEt−hexane (1:9~1:1) to give 5 (1.73 g, 5.69 mmol, 87%). 5: colorless needles, mp 103.0−10.4.0 °C (from AcOEt−hexane). 1H NMR (400 MHz, CDCl3): δH 1.07 (s, 9H), 1.35 (s, 6H), 3.90 (s, 2H), 4.09 (s, 3H), 6.98 (d, J = 1.1 Hz, 1H), 7.11 (dd, J = 7.9 and 1.1 Hz, 1H), 8.15 (d, J = 7.9 Hz, 1H), 10.60 (br s, 1H) ppm. 13C NMR (125 MHz, CDCl3): δC 27.3, 32.5, 32.5, 47.4, 56.8, 83.4, 113.2, 117.1, 124.1, 127.2, 133.4, 143.4, 148.0, 157.5, 165.0 ppm. IR (KBr): 3448, 2956, 1687, 1604, 1561 cm−1. Mass (m/z, %): 304 (M+, 22), 289 (100), 245 (22), 215 (30), 179 (29), 52 (21). HRMS (ESI): 327.1554 calcd for C18H24O4Na [M+Na+] 327.1572.
Synthesis of 5-[4-(benzimidazol-2-yl)-3-methoxyphenyl]-4-tert-butyl-3,3-dimethyl-2,3-dihydrofuran (7a): Typical procedure. Triphenylphosphonium anhydride trifluoromethanesulfonate (POP) was prepared by adding trifluoromethanesulfonic anhydride (2.19 mL, 13.0 mmol) to a solution of triphenylphosphine oxide (7.26 g, 26.1 mmol) in dry CH2Cl2 (15 mL) under a nitrogen atmosphere at room temperature and stirring for 20 min. To the POP solution, 5-(4-carboxy-3-methoxyphenyl)-4-tert-butyl-3,3-dimethyl-2,3-dihydrofuran (5) (1.00 g, 3.29 mmol) and 1,2-phenylenediamine (353 mg, 3.26 mmol) in dry CH2Cl2 (10 mL) were added and stirred at room temperature over night. The reaction mixture was poured into sat. aq. NaHCO3 and extracted with AcOEt. The organic layer was washed twice with sat. aq. NaCl, dried over anhydrous MgSO4, and concentrated in vacuo. The residue was chromatographed on silica gel and eluted with AcOEt−hexane (1:2) to give 7a (753 mg, 2.00 mmol, 62%) as a pale yellow solid. 7a: colorless needles, mp 233.5−234.0 °C (from AcOEt). 1H NMR (400 MHz, CDCl3): δH 1.09 (s, 9H), 1.36 (s, 6H), 3.92 (s, 2H), 4.11 (s, 3H), 7.01 (s with fine coupling, 1H), 7.12 (d with fine coupling, J = 7.9 Hz, 1H), 7.23−7.30 (m, 2H), 7.42−7.58 (m, 1H), 7.72−7.90 (m, 1H), 8.56 (d, J = 7.9 Hz, 1H), 10.60 (br s, 1H) ppm. 13C NMR (125 MHz, DMSO-d6): δC 27.3, 32.3, 32.5, 47.0, 56.1, 82.5, 112.2, 113.4, 118.1, 118.7, 121.8, 122.4, 122.8, 125.8, 129.5, 134.9, 139.0, 142.9, 148.7, 149.1, 156.3 ppm. IR (KBr): 3435, 3056, 2962, 2870, 1646, 1611, 1570 cm−1. Mass (m/z, %): 377 (M++1, 13), 376 (M+, 43), 362 (28), 361 (100), 305 (30), 251 (13). HRMS (ESI): 377.2206 calcd for C24H29N2O2 [M+H+] 377.2229. Anal. Calcd for C24H28N2O2: C, 76.56; H, 7.50; N, 7.44. Found: C, 76.32; H, 7.69; N, 7.42.
4-tert-Butyl-5-[3-methoxy-4-(N-methylbenzimidazol-2-yl)phenyl]-3,3-dimethyl-2,3-dihydrofuran (7b): 79% yield. Colorless plates, mp 158.0–158.5 °C (from CH2Cl2–hexane). 1H NMR (400 MHz, CDCl3): δH 1.10 (s, 9H), 1.37 (s, 6H), 3.64 (s, 3H), 3.83 (s, 3H), 3.93 (s, 2H), 6.96 (d, J = 1.2 Hz, 1H), 7.07 (dd, J = 7.6 and 1.2 Hz, 1H), 7.26−7.35 (m, 2H), 7.38−7.41 (m, 1H), 7.55 (d, J = 7.6 Hz, 1H), 7.80−7.83 (m, 1H) ppm. 13C NMR (125 MHz, CDCl3): δC 27.3, 30.8, 32.4, 32.5, 47.3, 55.6, 83.2, 109.3, 112.5, 119.4, 119.7, 121.9, 122.4, 122.7, 126.4, 131.8, 136.0, 139.6, 143.1, 149.1, 151.7, 157.0 ppm. IR (KBr): 3049, 2958, 2871, 1655, 1609, 1563 cm−1. Mass (m/z, %): 391 (M++1, 16), 390 (M+, 53), 376 (29), 375 (100), 319 (35), 265 (8). HRMS (ESI) : 391.2401 calcd for C25H31N2O2 [M+H+] 391.2386. Anal. Calcd for C25H30N2O2: C, 76.89; H, 7.74; N, 7.17. Found: C, 76.98; H, 7.93; N, 7.19.
4-tert-Butyl-5-[3-methoxy-4-(N-phenylbenzimidazol-2-yl)phenyl]-3,3-dimethyl-2,3-dihydrofuran (7c): 56% yield. Colorless amorphous solid. 1H NMR (400 MHz, CDCl3): δH 1.04 (s, 9H), 1.33 (s, 6H), 3.33 (s, 3H), 3.89 (s, 2H), 6.66 (d, J = 1.2 Hz, 1H), 7.01 (dd, J = 7.6 and 1.2 Hz, 1H), 7.20−7.40 (m, 8H), 7.63 (d, J = 7.6 Hz, 1H), 7.88 (d, J = 7.6 Hz, 1H) ppm. 13C NMR (125 MHz, CDCl3): δC 27.3, 32.4, 32.4, 47.2, 54.7, 83.2, 110.2, 112.4, 119.7, 119.9, 122.5, 123.1, 125.8, 126.3, 127,5, 129.0, 131.6, 136.0, 137.1, 139.3, 143.1, 149.1, 150.9, 156.4 ppm. IR (KBr): 3057, 2956, 2865, 1652, 1604, 1565, 1499 cm−1. Mass (m/z, %): 453 (M++1, 24), 452 (M+, 70), 438 (34), 437 (100), 381 (29), 327 (13), 298 (12). HRMS (ESI): 453.2520 calcd for C30H33N2O2 [M+H+] 453.2542.
Synthesis of 4-tert-butyl-5-{4-[N-(4-ethoxycarbonylbutyl)benzimidazol-2-yl]-3-methoxyphenyl}-3,3-dimethyl-2,3-dihydrofuran (8): 4-tert-Butyl-5-[4-(benzimidazol-2-yl)-3-methoxyphenyl]-3,3-dimethyl-2,3-dihydrofuran (7a) (500 mg, 1.33 mmol) was added to a suspension of NaH (60% in oil, 70.2 mg, 1.76 mmol) in dry DMF (10 mL) under a nitrogen atmosphere at room temperature. After stirring for 30 min, ethyl 5-bromopentanoate (0.32 mL, 2.0 mmol) was added to the solution at room temperature and stirred for 2 days. The reaction mixture was poured into sat. aq. NH4Cl and extracted with AcOEt. The organic layer was washed twice with sat. aq. NaCl, dried over anhydrous MgSO4, and concentrated in vacuo. The residue was chromatographed on silica gel and eluted with AcOEt−hexane (1:1) to give 8 (666 mg, 1.32 mmol, 99%). 8: Yellow oil. 1H NMR (400 MHz, CDCl3): δH 1.09 (s, 9H), 1.21 (t, J = 7.1 Hz, 3H), 1.37 (s, 6H), 1.42−1.52 (m, 2H), 1.69−1.79 (m, 2H), 2.16 (t, J = 7.3 Hz, 2H), 3.81 (s, 3H), 3.93 (s, 2H), 4.01−4.11 (m, 4H), 6.96 (s, 1H), 7.06 (dd, J = 7.6 and 1.2 Hz, 1H), 7.23−7.33 (m, 2H), 7.38−7.43 (m, 1H), 7.47 (d, J = 7.6 Hz, 1H), 7.78−7.84 (m, 1H) ppm. 13C NMR (125 MHz, CDCl3): δC 14.1, 22.0, 27.3, 28.7, 32.4, 32.5, 33.6, 44.1, 47.3, 55.6, 60.3, 83.2, 109.8, 112.6, 119.7, 120.0, 121.9, 122.4, 122.7, 126.5, 131.7, 135.0, 139.5, 143.3, 149.0, 151.2, 156.9, 172.8 ppm. IR (liquid film): 3055, 2957, 2868, 1732, 1651, 1609, 1563 cm−1. Mass (m/z, %): 505 (M++1, 23), 504 (M+, 60), 490 (37), 489 (100), 459 (11). HRMS (ESI): 505.3039 calcd for C31H41N2O4 [M+H+] 505.3066.
Synthesis of 5-[4-(benzimidazol-2-yl)-4-tert-butyl-3-hydroxyphenyl]-3,3-dimethyl-2,3-dihydrofuran (3a): Typical procedure. MeSNa (95%, 120 mg, 1.63 mmol) was added to a solution of 7a (210 mg, 0.56 mmol) in dry DMF (5 mL) under a nitrogen atmosphere at room temperature and stirred for 30 min at 140 °C. The reaction mixture was poured into 1 M aq. HCl and sat. aq. NaCl, and extracted with AcOEt. The organic layer was washed twice with sat. aq. NaCl, dried over anhydrous MgSO4, and concentrated in vacuo. The residue was chromatographed on silica gel and eluted with AcOEt−hexane (1:4) to give 3a (200 mg, 0.552 mmol, 99%) as a pale yellow solid. 3a: Colorless needles, mp 284.5−285.0 °C (from AcOEt). 1H NMR (400 MHz, CDCl3): δH 1.09 (s, 9H), 1.35 (s, 6H), 3.90 (s, 2H), 6.91 (dd, J = 8.0 and 1.5 Hz, 1H), 7.08 (d, J = 1.5 Hz, 1H), 7.28−7.34 (m, 2H), 7.46−7.53 (m, 1H), 7.52 (d, J = 8.0 Hz, 1H), 7.71−7.78 (m, 1H), 9.45 (br s, 1H), 13.09 (br s, 1H) ppm. 13C NMR (125 MHz, DMSO-d6): δC 27.2, 32.3, 32.4, 47.0, 82.5, 111.7 (br), 112.6, 118.1 (br), 118.5, 121.0, 122.7 (br), 123.4 (br), 125.6, 126.0, 133.4 (br), 139.5, 141.1 (br), 149.0, 151.5, 157.5 ppm. IR (KBr): 3302, 2958, 2868, 2630, 1630, 1580 cm−1. Mass (m/z, %): 363 (M++1, 11), 362 (M+, 39), 348 (27), 347 (100), 291 (38). HRMS (ESI): 363.2043 calcd for C23H27N2O2 [M+H+] 363.2073. Anal. Calcd for C23H26N2O2: C, 76.21; H, 7.23; N, 7.73. Found: C, 76.47; H, 7.43; N, 7.72.
4-tert-Butyl-5-[3-hydroxy-4-(N-methylbenzimidazol-2-yl)phenyl]-3,3-dimethyl-2,3-dihydrofuran (3b): 97% yield. Colorless columns, mp 136.0−137.0 °C (from AcOEt). 1H NMR (400 MHz, CDCl3): δH 1.11 (s, 9H), 1.35 (s, 6H), 3.90 (s, 2H), 4.07 (s, 3H), 6.93 (dd, J = 8.1 and 1.7 Hz, 1H), 7.13 (d, J = 1.7, 1H), 7.31−7.39 (m, 2H), 7.41−7.44 (m, 1H), 7.70 (d, J = 8.1 Hz, 1H), 7.75−7.78 (m, 1H), 12.92 (br s, 1H) ppm. 13C NMR (125 MHz, CDCl3): δC 27.3, 32.5, 32.5, 33.0, 47.3, 83.2, 109.5, 112.7, 118.8, 119.6, 120.2, 123.0, 123.3, 126.1, 126.5, 135.6, 139.6, 140.3, 149.0, 151.4, 158.6 ppm. IR (KBr): 3417, 2956, 2867, 1624, 1566, 1466 cm-1. Mass (m/z, %): 377 (M++1, 14), 376 (M+, 48), 362 (28), 361 (100), 305 (40). HRMS (ESI): 377.2212 calcd for C24H29N2O2 [M+H+] 377.2229 Anal. Calcd for C24H28N2O2: C, 76.56; H, 7.50; N, 7.44. Found: C, 76.52; H, 7.68; N, 7.46.
4-tert-Butyl-5-[3-hydroxy-4-(N-phenylbenzimidazol-2-yl)phenyl]-3,3-dimethyl-2,3-dihydrofuran (3c): 82% yield. Pale yellow plates, mp 180.5-181.5 °C (from CH2Cl2). 1H NMR (400 MHz, CDCl3): δH 1.05 (s, 9H), 1.30 (s, 6H), 3.83 (s, 2H), 6.49 (dd, J = 8.3 and 1.7 Hz, 1H), 6.79 (d, J = 8.3 Hz, 1H), 7.06 (d, J = 1.7 Hz, 1H), 7.08 (d, J = 8.1 Hz, 1H), 7.23−7.29 (m, 1H), 7.32−7.42 (m, 3H), 7.56−7.63 (m, 3H), 7.80 (d, J = 8.1 Hz, 1H), 13.48 (br s, 1H) ppm. 13C NMR (125 MHz, CDCl3): δC 27.2, 32.4, 32.4, 47.2, 83.1, 110.3, 111.9, 118.6, 119.5, 119.7, 123.4, 123.8, 126.0, 126.7, 127.9, 129.5, 130.3, 136.5, 137.1, 139.4, 140.0, 149.0, 150.6, 159.1 ppm. IR (KBr): 3431, 3065, 2955, 2867, 1623, 1596, 1565 cm−1. Mass (m/z, %): 439 (M++1, 20), 438 (M+, 60), 424 (33), 423 (100), 381 (13), 368 (11), 367 (40), 285 (12). HRMS (ESI): 439.2363 calcd for C29H31N2O2 [M+H+] 439.2386.
4-tert-Butyl-5-{4-[N-(4-carboxybutyl)benzimidazol-2-yl]-3-hydroxyphenyl}-3,3-dimethyl-2,3-dihydrofuran (3d): 89% yield. Pale yellow columns, mp 174.0−175.0 °C (from CH2Cl2−hexane). 1H NMR (400 MHz, CDCl3): δH 1.11 (s, 9H), 1.35 (s, 6H), 1.73−1.82 (m, 2H), 1.99−2.09 (m, 2H), 2.44 (t, J = 7.1 Hz, 2H), 3.90 (s, 2H), 4.39−4.48 (m, 2H), 6.94 (dd, J = 8.1 and 1.7 Hz, 1H), 7.13 (d, J = 1.7 Hz, 1H), 7.30−7.38 (m, 2H), 7.40−7.45 (m, 1H), 7.58 (d, J = 8.1 Hz, 1H), 7.73−7.80 (m, 1H) ppm. 13C NMR (125 MHz, CDCl3): δC 21.8, 27.3, 29.1, 32.5, 32.5, 33.2, 45.2, 47.2, 83.2, 109.7, 112.8, 118.9, 119.9, 120.5, 123.1, 123.4, 125.9, 126.3, 134.9, 139.6, 140.3, 148.8, 150.7, 158.4, 178.4 ppm. IR (KBr): 3386, 3061, 2955, 2865, 1723, 1654, 1618, 1558 cm−1. Mass (m/z, %): 463 (M++1, 25), 462 (M+, 77), 448 (36), 447 (100), 445 (21), 403 (17), 391 (30), 389 (20), 347 (32), 291 (18), 57 (18). HRMS (ESI): 463.2591 calcd for C28H35N2O4 [M+H+] 463.2597.
Synthesis of 1-[4-(benzimidazol-2-yl)-3-hydroxyphenyl]-5-tert-butyl-4,4-dimethyl-2,6,7-trioxabicyclo[3.2.0]heptane (2a): Typical procedure. A solution of 3a (164 mg, 0.45 mmol) and tetraphenylporphine (TPP) (2.0 mg) in acetone (10 mL) was irradiated externally with 940W Na lamp under an oxygen atmosphere for 1.5 h at 0 °C. The reaction mixture was concentrated in vacuo. The photolysate was rinsed with CH2Cl2 to give dioxetane 2a (139 mg, 78%). 2a: Pale yellow granules, mp 284.0−285.0 °C (dec.) (from THF−hexane). 1H NMR (400 MHz, DMSO-d6): δH 0.99 (s, 9H), 1.10 (s, 3H), 1.38 (s, 3H), 3.92 (d, J = 8.1 Hz, 1H), 4.38 (d, J = 8.1 Hz, 1H), 7.18 (d, J = 1.5, 1H), 7.22 (dd, J = 8.3 and 1.5 Hz, 1H), 7.26−7.34 (m, 2H), 7.60−7.77 (m, 2H), 8.14 (d, J = 8.3 Hz, 1H), 13.24 (br s, 1H) ppm. 13C NMR (125 MHz, THF-d8): δC 18.7, 25.2, 27. 3, 37.5, 46.4, 80.9, 105.7, 111.7, 114.4, 117.0, 118.8, 119.3, 119.5, 123.4, 124.3, 125.5, 134.3, 140.6, 142.7, 152.5, 159.7 ppm. IR (KBr) : 3417, 3288, 2969, 2901, 1630, 1588, 1543 cm−1. Mass (m/z, %) : 395 (M++1, 19), 394 (M+, 67), 338 (20), 294 (15), 255 (11), 254 (27), 238 (19), 237 (100), 210 (27), 209 (28), 181 (19), 57 (22). HRMS (ESI) : 395.1947 calcd for C23H27N2O4 [M+H+] 395.1971. Anal. Calcd for C23H26N2O4: C, 70.03; H, 6.64; N, 7.10. Found: C, 69.98; H, 6.75; N, 7.02.
5-tert-Butyl-1-[3-hydroxy-4-(N-methylbenzimidazol-2-yl)phenyl]-4,4-dimethyl-2,6,7-trioxabicyclo[3.2.0]heptane (2b): 94% yield. Pale yellow needles, mp 162.5−163.0 °C (dec.) (from CH2Cl2−hexane). 1H NMR (400 MHz, CDCl3): δH 1.05 (s, 9H), 1.17 (s, 3H), 1.41 (s, 3H), 3,85 (d, J = 8.3 Hz, 1H), 4.09 (s, 3H), 4.61 (d, J = 8.3 Hz, 1H), 7.28 (dd, J = 8.5 and 1.8 Hz, 1H), 7.33−7.46 (m, 4H), 7.76−7.80 (m, 2H), 13.07 (br s, 1H) ppm. 13C NMR (125 MHz, CDCl3): δC 18.4, 25.0, 26.9, 33.1, 36.7, 45.6, 80.3, 105.2, 109.6, 113.9, 116.2, 118.2, 118.4, 118.9, 123.1, 123.5, 126.4, 135.6, 139.2, 140.2, 150.9, 158.7 ppm. IR (KBr): 3428, 2969, 2893, 1625, 1577 cm−1. Mass (m/z, %): 409 (M++1, 23), 408 (M+, 97), 352 (10), 308 (21), 278 (12), 268 (14), 267 (23), 252 (19), 251 (100), 224 (37), 223 (39), 195 (18), 57 (36). HRMS (ESI): 409.2114 calcd for C24H29N2O4 [M+H+] 409.2127. Anal. Calcd for C24H28N2O4: C, 70.57; H, 6.91; N, 6.86. Found: C, 70.26; H, 7.01; N, 6.85.
5-tert-Butyl-1-[3-hydroxy-4-(N-phenylbenzimidazol-2-yl)phenyl]-4,4-dimethyl-2,6,7-trioxabicyclo[3.2.0]heptane (2c): 96% yield. Colorless plates, mp 164.0−165.0 °C (dec.) (from CH2Cl2−hexane). 1H NMR (500 MHz, CDCl3): δH 0.99 (s, 9H), 1.13 (s, 3H), 1.34 (s, 3H), 3.78 (d, J = 8.4 Hz, 1H), 4.54 (d, J = 8.4 Hz, 1H), 6.81 (dd, J = 8.5 and 1.7 Hz, 1H), 6.87 (d, J = 8.5 Hz, 1H), 7.10 (d, J = 8.2 Hz, 1H), 7.25−7.30 (m, 5H), 7.58−7.64 (m, 3H), 7.81 (d, J = 7.9 Hz, 1H), 13.61 (br s, 1H) ppm. 13C NMR (125 MHz, CDCl3): δC 18.4, 25.0, 26.9, 36.7, 45.6, 80.3, 105.1, 110.4, 113.1, 116.1, 117.9, 118.1, 118.7, 123.6, 124.0, 126.6, 127.8, 129.7, 130.4, 136.5, 136.9, 139.1, 139.9, 150.1, 159.2 ppm. IR (KBr): 3433, 3065, 2993, 2969, 2898, 1631, 1595, 1574 cm−1. Mass (m/z, %): 471 (M++1, 36), 470 (M+, 100), 414 (13), 370 (17), 330 (15), 329 (16), 314 (19), 313 (77), 286 (40), 285 (47), 257 (12), 256 (22), 255 (14), 57 (23). HRMS (ESI): 471.2279 calcd for C29H31N2O4 [M+H+] 471.2284.
5-tert-Butyl-1-{4-[N-(4-carboxybutyl)benzimidazol-2-yl]-3-hydroxyphenyl}-4,4-dimethyl-2,6,7-trioxabicyclo[3.2.0]heptane (2d): 99% yield. Pale yellow amorphous solid. 1H NMR (500 MHz, CDCl3): δH 1.05 (s, 9H), 1.17 (s, 3H), 1.41 (s, 3H), 1.74−1.83 (m, 2H), 2.01−2.10 (m, 2H), 2.44 (t, J = 7.3 Hz, 2H), 3.84 (d, J = 8.2 Hz, 1H), 4.41−4.49 (m, 2H), 4.60 (d, J = 8.2 Hz, 1H), 7.28 (dd, J = 8.2 and 1.8 Hz, 1H), 7.32−7.39 (m, 2H), 7.41−7.46 (m, 2H), 7.66 (d, J = 8.2 Hz, 1H), 7.76−7.79 (m, 1H) ppm. 13C NMR (125 MHz, CDCl3): δC 18.4, 21.7, 25.0, 26.9, 29.1, 33.1, 36.7, 45.3, 45.6, 80.3, 105.3, 109.8, 114.0, 116.2, 118.5, 118.6, 119.0, 123.3, 123.7, 125.8, 135.0, 139.4, 140.2, 150.3, 158.6, 178.4 ppm. IR (KBr): 3448, 2965, 1716, 1625, 1542 cm−1. Mass (m/z, %): 495 (M++1, 31), 494 (M+, 100), 478 (17), 477 (49), 435 (36), 422 (24), 421 (67), 409 (23), 408 (30), 394 (27), 338 (29), 337 (78), 319 (40), 310 (46), 309 (61), 281 (45), 254 (27), 237 (56), 181 (47), 57 (88). HRMS (ESI): 495.2485 calcd for C28H35N2O6 [M+H+] 495.2495.
Thermal decomposition of 2a to 2,2,4,4-tetramethyl-3-oxopentyl 4-(benzimidazol-2-yl)-3-hydroxybenzoate (10a): Typical procedure. A solution of 2a (48.0 mg, 0.12 mmol) in p-xylene was stirred under a nitrogen atmosphere at 140 °C for 3 h. After cooling, the reaction mixture was concentrated in vacuo. The residue was chromatographed on silica gel and eluted with hexane−AcOEt to give 10a (47.4 mg, 99%). 10a: Colorless granules, mp 285.0−286.0 °C (dec.) (from THF−hexane). 1H NMR (400 MHz, DMSO-d6): δH 1.23 (s, 9H), 1.35 (s, 6H), 4.35 (s, 2H), 7.28−7.35 (m, 2H), 7.48 (d, J = 1.5 Hz, 1H), 7.53 (dd, J = 8.1 and 1.5 Hz, 1H), 7.66−7.74 (m, 2H), 8.21 (d, J = 8.1 Hz, 1H), 13.34 (br s, 1H) ppm. 13C NMR (125 MHz, DMSO-d6): δC 23.3, 28.0, 45.4, 48.8, 71.9, 112.3 (br), 117.1, 117.6, 118.2 (br), 119.6, 123.4 (br), 126.9, 131.9, 133.7 (br), 140.9 (br), 150.5, 157.8, 164.9, 215.5 ppm. IR (KBr) : 3347, 3318, 2969, 1709, 1681, 1612, 1582 cm−1. Mass (m/z, %) 395 (M++1, 19), 394 (M+, 70), 338 (18), 294 (14), 255 (11), 254 (27), 238 (18), 237 (100), 210 (25), 209 (25), 181 (19), 57 (24). HRMS (ESI) : 395.1980 calcd for C23H27N2O4 [M+H+] 395.1971. Anal. Calcd for C23H26N2O4: C, 70.03; H, 6.64; N, 7.10. Found: C, 70.02; H, 6.78; N, 7.08.
2,2,4,4-Tetramethyl-3-oxopentyl 3-hydroxy-4-(N-methylbenzimidazol-2-yl)benzoate (10b): 94% yield. Pale yellow columns, mp 174.0−175.0 °C (from CH2Cl2−hexane). 1H NMR (400 MHz, CDCl3): δH 1.31 (s, 9H), 1.41 (s, 6H), 4.09 (s, 3H), 4.43 (s, 2H), 7.34−7.47 (m, 3H), 7.59 (dd, J = 8.3 and 1.7 Hz, 1H), 7.74 (d, J = 1.7 Hz, 1H), 7.76−7.82 (m, 2H), 13.13 (br s, 1H) ppm. 13C NMR (125 MHz, CDCl3): δC 23.6, 28.1, 33.1, 45.9, 49.1, 72.2, 109.6, 116.7, 118.9, 119.0, 119.3, 123.2, 123.8, 126.7, 132.3, 135.6, 140.0, 150.4, 158.9, 165.5, 215.9 ppm. IR (KBr) : 3423, 3071, 2961, 1712, 1680, 1573 cm−1. Mass (m/z, %) : 409 (M ++1, 29), 408 (M+, 100), 352 (11), 308 (17), 268 (12), 267 (19), 252 (16), 251 (87), 224 (27), 223 (30), 195 (11), 57 (16). HRMS (ESI) : 409.2139 calcd for C24H29N2O4 [M+H+] 409.2127. Anal. Calcd for C24H28N2O4: C, 70.57; H, 6.91; N, 6.86. Found: C, 70.28; H, 6.98; N, 6.91.
2,2,4,4-Tetramethyl-3-oxopentyl 3-hydroxy-4-(N-phenylbenzimidazol-2-yl)benzoate (10c): 97% yield. Pale yellow columns, mp 180.5−181.0 °C (from CH2Cl2−hexane). 1H NMR (400 MHz, CDCl3): δH 1.27 (s, 9H), 1.37 (s, 6H), 4.37 (s, 2H), 6.89 (d, J = 8.4 Hz, 1H), 7.10−7.15 (m, 2H), 7.28−7.44 (m, 4H), 7.62−7.66 (m, 3H), 7.68 (d, J = 1.6 Hz, 1H), 7.84 (d, J = 7.9 Hz, 1H), 13.67 (br s, 1H) ppm. 13C NMR (125 MHz, CDCl3): δC 23.6, 28.2, 45.8, 49.1, 72.1, 110.5, 116.1, 118.8, 118.9, 119.0, 123.7, 124.3, 127.1, 127.8, 129.8, 130.5, 132.2, 136.5, 136.8, 139.8, 149.7, 159.4, 165.6, 215.9 ppm. IR (KBr) : 3431, 3060, 2971, 2874, 1724, 1685, 1579 cm−1. Mass (m/z, %): 471 (M++1, 36), 470 (M+, 100), 414 (14), 370 (24), 330 (18), 329 (22), 314 (21), 313 (86), 287 (11), 286 (53), 285 (64), 257 (20), 256 (34), 255 (20), 57 (30). HRMS (ESI): 471.2278 calcd for C29H31N2O4 [M+H+] 471.2284.
2,2,4,4-Tetramethyl-3-oxopentyl 4-[N-(4-carbonylbutyl)benimidazol-2-yl]-3-hydroxybenzoate (10d): 82% yield. Pale yellow amorphous solid. 1H NMR (500 MHz, CDCl3): δH 1.30 (s, 9H), 1.41 (s, 6H), 1.75−1.83 (m, 2H), 2.02−2.10 (m, 2H), 2.45 (t, J = 7.1 Hz, 2H), 4.42 (s, 2H), 4.42−4.49 (m, 2H), 7.33−7.40 (m, 2H), 7.43−7.47 (m, 1H), 7.59 (dd, J = 8.2 and 1.8 Hz, 1H), 7.69 (d, J = 8.2 Hz, 1H), 7.73 (d, J = 1.8 Hz, 1H), 7.77−7.80 (m, 1H) ppm. 13C NMR (125 MHz, CDCl3): δC 21.7, 23.6, 28.1, 29.1, 33.1, 45.3, 45.9, 49.1, 72.2, 109.9, 116.9, 119. 1, 119.2, 119.6, 123.4, 123.9, 126.4, 132.4, 135.0, 140.0, 149.8, 158.7, 165.5, 178.1, 216.1 ppm. IR (KBr) : 2970, 2932, 2877, 1719, 1704, 1684, 1525, cm−1. Mass (m/z, %) : 495 (M++1, 34), 494 (M+, 100), 493 (41), 477 (54), 435 (36), 421 (64), 408 (25), 337 (58), 319 (84), 309 (37), 292 (39), 237 (34), 57 (60). HRMS (ESI) : 495.2481 calcd for C28H35N2O6 [M+H+] 495.2495.

Measurement of chemiluminescence and time-course of the base-induced decomposition of dioxetanes; General Procedure: Chemiluminescence was measured using a JASCO FP-750 and/or FP-6500 spectrometer, and a Hamamatsu Photonics PMA-11 multi-channel detector.
TBAF/MeCN system. A freshly prepared solution (2.00 mL) of TBAF (1.0 x 10−2 mol/L) in MeCN was transferred to a quartz cell (10 x 10 x 50 mm), which was placed in a spectrometer that was thermostated with stirring at 45 °C. After 3−5 min, a solution of dioxetane 2 in MeCN (1.0 x 10−5 mol/L, 1.00 mL) was added by means of a syringe, and measurement was started immediately. The time-course of the intensity of light emission was recorded and processed according to first-order kinetics. The total light emission was estimated by comparing it with that of an adamantylidene dioxetane, the chemiluminescent efficiency ΦCL of which has been reported to be 0.29 and which was used here as a standard.12,13
NaOH/H2O system. A solution of NaOH (0.1 M, 2.90 mL) in H2O was transferred to a quartz cell (10 x 10 x 50 mm), which was placed in a spectrometer that was thermostated with stirring at 45 °C. After 3−5 min, a solution of dioxetane 2 in MeCN (1.0 x 10−4 mol/L, 0.10 mL) was added by means of a syringe, and measurement was started immediately. The time-course of the intensity of light emission was recorded and processed according to first-order kinetics.

ACKNOWLEDGEMENTS
The authors gratefully acknowledge financial assistance in the form of Grants-in-aid (No. 21550052 and No. 22550046) for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

References

1. (a) A. P. Schaap and S. D. Gagnon, J. Am. Chem. Soc., 1982, 104, 3504; CrossRef (b) A. P. Schaap, R. S. Handley, and B. P. Giri, Tetrahedron Lett., 1987, 28, 935; CrossRef (c) A. P. Schaap, T. S. Chen, R. S. Handley, R. DeSilva, and B. P. Giri, Tetrahedron Lett., 1987, 28, 1155. CrossRef
2. (a) J.-Y. Koo and G. B. Schuster, J. Am. Chem. Soc., 1977, 99, 6107; CrossRef (b) J.-Y. Koo and G. B. Schuster, J. Am. Chem. Soc., 1978, 100, 4496; CrossRef (c) W. Adam, I. Bronstein, T. Trofimov, and R. F. Vasil’ev, J. Am. Chem. Soc., 1999, 121, 958; CrossRef (d) W. Adam, M. Matsumoto, and T. Trofimov, J. Am. Chem. Soc., 2000, 122, 8631; CrossRef (e) A. L. P. Nery, D. Weiss, L. H. Catalani, and W. J. Baader, Tetrahedron, 2000, 56, 5317. CrossRef
3. (a) L. H. Catalani and T. Wilson, J. Am. Chem. Soc., 1989, 111, 2633; CrossRef (b) F. J. McCapra, Photochem. Photobiol. A. Chem., 1990, 51, 21; CrossRef (c) T. Wilson, Photochem. Photobiol., 1995, 62, 601. CrossRef
4. (a) Y. Takano, T. Tsunesada, H. Isobe, Y. Yoshioka, K. Yamaguchi, and I. Saito, Bull. Chem. Soc. Jpn., 1999, 72, 213; CrossRef (b) J. Tanaka, C. Tanaka, and M. Matsumoto, PCCP, 2011, 13, 16005; CrossRef (c) H. Isobe, Y. Takano, M. Okumura, S. Kuramitsu, and K. Yamaguchi, J. Am. Chem. Soc., 2005, 127, 8667. CrossRef
5. (a) S. Beck and H. Köster, Anal. Chem., 1990, 62, 2258; CrossRef (b) W. Adam, D. Reihardt, and C. R. Saha-Möller, Analyst, 1996, 121, 1527. CrossRef
6. (a) M. Matsumoto, J. Photochem. Photobiol. C: Photochem. Rev., 2004, 5, 27; CrossRef (b) M. Matsumoto and N. Watanabe, Bull. Chem. Soc. Jpn., 2005, 78, 1899. CrossRef
7. B. Edwards, A. Sparks, J. C. Voyta, and I. Bronstein, ‘Bioluminescence and Chemiluminescence, Fundamentals and Applied Aspects’, ed. by A. K. Campbell, L. J. Kricka, and P. E. Stanley, Wiley, Chichester, 1994, pp. 56–59.
8. (a) M. Matsumoto, T. Akimoto, Y. Matsumoto, and N. Watanabe, Tetrahedron Lett., 2005, 46, 6075; CrossRef (b) M. Tanimura, N. Watanabe, H. K. Ijuin, and M. Matsumoto, J. Org. Chem., 2010, 75, 3678. CrossRef
9. M. Tanimura, N. Watanabe, H. K. Ijuin, and M. Matsumoto, J. Org. Chem., 2011, 75, 902. CrossRef
10. M. Matsumoto, T. Sakuma, and N. Watanabe, Luminescence, 2001, 16, 275. CrossRef
11. (a) A. R. Katrizky, Tetrahedron, 1980, 36, 679; CrossRef (b) A. R. Katrizky and C. M. Marson, Angew. Chem. Int. Ed. Engl., 1984, 23, 420; CrossRef (c) A. R. Katrizky and G. Musumara, Chem. Soc. Rev., 1984, 13, 47; CrossRef (d) A. R. Katrizky, K. Sakizadeh, and G. Musumara, Heterocycles, 1985, 23, 1765. CrossRef
12. ΦCL was estimated based on the value 0.29 for the chemiluminescent decomposition of 3-(3-tert-butyldimethylsiloxyphenyl)-3-methoxy-4-(2’-spiroadamantane)-1,2-dioxetane in a TBAF/DMSO system13.
13. A. V. Trofimov, K. Mielke, R. F. Vasil’ev, and W. Adam, Photochem. Photobiol., 1996, 63, 463. CrossRef
14. M. Matsumoto, Y. Mizoguchi, T. Motoyama, and N. Watanabe, Tetrahedron Lett., 2001, 42, 8869. CrossRef