Synthesis of homo- and heteromultivalent carbohydrate-functionalized oligo(amidoamines) using novel glyco-building blocks

• Felix Wojcik,
• Sinaida Lel,
• Alexander G. O’Brien,
• Peter H. Seeberger and
• Laura Hartmann

Beilstein J. Org. Chem. 2013, 9, 2395–2403, doi:10.3762/bjoc.9.276

• -CONH2 (14): Compound 14 (30 mg, 16.2 μmol) was obtained as white hygroscopic powder after cleavage from the resin, precipitation into diethyl ether and deactylation with a yield of 81%. MALDI–TOF–MS: [M + Na]+ calcd for C75H135N13O33S3Na, 1864.83 (monoisotopic); found, 1864.63; RP-HPLC analysis 5% to 95

• deactylation compound 15 (11 mg, 5.6 μmol) was obtained as white hygroscopic powder with 28% yield. MALDI–TOF–MS: [M + H]+ calcd for C79H140N11O40S3, 1978.84 (monoisotopic); found, 1979.06; [M + Na]+ calcd for C79H139N11O40S3Na, 2000.82 (monoisotopic); found, 2000.88; [M + K]+ calcd for C79H139N11O40S3K

• deactylation in solution, the crude material was purified via preparative RP-HPLC (5 to 50% MeCN in 30 min) and freezed-dried. Compound 16 (12.6 mg, 4.2 μmol) was obtained as white hygroscopic powder with 21% yield. MALDI–TOF–MS: [M + Na]+ calcd for C124H217N23O49S6Na, 3027.34 (monoisotopic); found, 3027.71

Linkage of α-cyclodextrin-terminated poly(dimethylsiloxanes) by inclusion of quasi bifunctional ferrocene

• Helmut Ritter,
• Berit Knudsen and
• Valerij Durnev

Beilstein J. Org. Chem. 2013, 9, 1278–1284, doi:10.3762/bjoc.9.144

• ), 4.78 (m, 12H), 5.06 (m, 12H), 5.55 (m, 12H); MALDI–TOF–MS m/z: 3962.58 [M + Na]+ for n = 12. Complexation reactions Complexation of α-CD-disiloxane (4) with ferrocene: α-CD-disiloxane (100 mg, 0.015 mmol) is dissolved in a solution of ferrocene (2.8 mg, 0.015 mmol) and 2.0 mL of chloroform and stirred

Influence of cyclodextrin on the solubility and the polymerization of (meth)acrylated Triton^® X-100

• Melanie Kemnitz and
• Helmut Ritter

Beilstein J. Org. Chem. 2012, 8, 2176–2183, doi:10.3762/bjoc.8.245

• , matrix-assisted laser desorption ionization mass spectrometry (MALDI-TOF MS), dynamic light scattering (DLS), gel-permeation chromatography (GPC) and turbidity measurements. Additionally, the viscosity change of the methacrylic homopolymer with RAMEB-CD was evaluated. Keywords: (meth)acrylated Triton

S-Fluorenylmethyl protection of the cysteine side chain upon N^α-Fmoc deprotection

• Johannes W. Wehner and
• Thisbe K. Lindhorst

Beilstein J. Org. Chem. 2012, 8, 2149–2155, doi:10.3762/bjoc.8.242

• “Fmoc” refers to fluorenylmethoxycarbonyl and “Fm” to fluorenylmethyl. ESI mass spectra were recorded on a Mariner ESI-TOF 5280 (Applied Biosystems) instrument and MALDI-MS measurements on a MALDI-TOF-MS-Biflex III (Bruker) instrument by using 2,5-dihydroxybenzoic acid (DHB) or α-cyano-4-hydroxycinnamic

• -OCH2CH2), 66.1 (C-4), 62.4 (C-6), 54.0 (C-α), 46.9 (CH2-Fm), 38.7 (C-β), 38.6 (man-OCH2CH2), 36.4 (CH2-CHFm), 21.0, 20.9, 20.7, 20.7 (4 C(O)CH3) ppm; HRMS–ESI (m/z): calcd for C33H41N2O11S, 673.24; found, 673.21; MALDI–TOF–MS (CCA) m/z 673.73. S-(Fluoren-9-yl)-L-cysteine-[2-(α-D-mannopyranosyloxy)ethyl

Influence of cyclodextrin on the solubility of a classically prepared 2-vinylcyclopropane macromonomer in aqueous solution

• Helmut Ritter,
• Jia Cheng and
• Monir Tabatabai

Beilstein J. Org. Chem. 2012, 8, 1528–1535, doi:10.3762/bjoc.8.173

• amino group at δ 2.85 ppm and the methylene protons adjacent to the thio group at δ 2.68 ppm which also confirms the structure of 3. The mass differences between the signals of the main series in MALDI-TOF MS correspond to the molar mass of the monomer NiPAAm units equal to 113 g/mol and the mass of end

• at 1 °C/min−1 in a magnetically stirred cell; values of the cloud points were defined as the temperature at which the deepest point of the derivative curve was achieved. MALDI-TOF MS was performed on a Bruker Ultraflex TDF mass spectrometer using a 337 nm nitrogen laser. GPC analyses were performed

• is flushed with nitrogen until it is colorless. Then, 4 mL of dimethyl sulfide (DMS) are added to the solution. The reaction mixture is stirred overnight at rt. Methanol is removed under reduced pressure. Mn = 1.8 × 104 g/mol, DI = 2.2; dn = 5.2 nm. MALDI-TOF MS of amino-terminated poly(NiPAAm) 3

Formation of carbohydrate-functionalised polystyrene and glass slides and their analysis by MALDI-TOF MS

• Martin J. Weissenborn,
• Johannes W. Wehner,
• Christopher J. Gray,
• Robert Šardzík,
• Claire E. Eyers,
• Thisbe K. Lindhorst and
• Sabine L. Flitsch

Beilstein J. Org. Chem. 2012, 8, 753–762, doi:10.3762/bjoc.8.86

• groups can be used as simple hydrophobic, noncovalently linked anchors for ligands on surfaces and at the same time facilitate the in situ mass spectrometric analysis of such ligands. Keywords: carbohydrate array; conductive tape; MALDI-TOF MS; nonconductive surface; trityl-mediated adhesion

• , and have investigated the use of matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) mass spectrometry (MS) analysis (Figure 1B), which has been highly successful on ligands immobilised on gold plates [10]. MALDI-TOF MS requires an electrically conducting surface and a matrix for

• target for MALDI-TOF MS analysis. Based on previous work one predicts that photoelectrons generated by UV laser irradiation are not dissipated by the polymeric surface. These photoelectrons distort the local electric field causing a significant loss in resolution of the analyte ions and a nonlinear shift

Acceptor-influenced and donor-tuned base-promoted glycosylation

• Stephan Boettcher,
• Martin Matwiejuk and
• Joachim Thiem

Beilstein J. Org. Chem. 2012, 8, 413–420, doi:10.3762/bjoc.8.46

• , 127.6, 127.6, 127.5 (Car), 104.2 (C-1’), 97.9 (C-1), 82.3 (C-3’), 79.4 (C-2’), 79.1 (C-2), 79.1 (C-5), 76.3 (C-4’), 75.0, 74.7, 73.2, 73.0 (-O-CH2-Ph), 72.6 (C-3), 70.6 (C-5’), 70.1 (C-4), 68.8 (C-6), 55.2 (-OCH3), 16.7 (C-6’) ppm; MALDI–TOF–MS (DHB, positive mode; m/z): [M + Na]+ calcd for C41H48O10Na

• ), 79.1 (C-2), 75.2, 74.7, 74.5, 73.5, 72.9 (-O-CH2-Ph), 73.5 (C-4), 74.6 (C-5), 74.2 (C-2’), 73.4 (C-4), 71.4 (C-5’), 69.3 (C-6), 68.7 (C-6’), 57.1 (-OCH3) ppm; MALDI–TOF–MS (DHB, positive mode; m/z): [M + Na]+ calcd for C48H54O11Na, 829.36; found, 830.1. Monobenzylated methyl α- and β-D-gluco- and

Synthesis of mesomeric betaine compounds with imidazolium-enolate structure

• Nina Gonsior,
• Fabian Mohr and
• Helmut Ritter

Beilstein J. Org. Chem. 2012, 8, 390–397, doi:10.3762/bjoc.8.42

• -time-of-flight mass spectrometry (MALDI-TOF-MS) was performed on a Bruker Ultraflex TOF mass spectrometer. Ions formed with a pulsed nitrogen laser (25 Hz, 337 nm) were accelerated to 25 kV, with the molecular masses being recorded in the linear mode. 2,5-Dihydroxybenzoic acid (DBH) in acetonitrile

Highly efficient cyclosarin degradation mediated by a β-cyclodextrin derivative containing an oxime-derived substituent

• Michael Zengerle,
• Florian Brandhuber,
• Christian Schneider,
• Franz Worek,
• Georg Reiter and
• Stefan Kubik

Beilstein J. Org. Chem. 2011, 7, 1543–1554, doi:10.3762/bjoc.7.182

• 600, Bruker DPX 400; MALDI-TOF-MS, Bruker Ultraflex TOF/TOF; ESI–MS, Bruker Esquire 3000; IR, FT-IR System Spectrum BX, Perkin-Elmer; elemental analysis, Elementar vario Micro cube. All chemicals, unless other stated, are commercially available and were used without further purification. Cyclosarin

Redox-active tetrathiafulvalene and dithiolene compounds derived from allylic 1,4-diol rearrangement products of disubstituted 1,3-dithiole derivatives

• Filipe Vilela,
• Peter J. Skabara,
• Christopher R. Mason,
• Thomas D. J. Westgate,
• Asun Luquin,
• Simon J. Coles and
• Michael B. Hursthouse

Beilstein J. Org. Chem. 2010, 6, 1002–1014, doi:10.3762/bjoc.6.113

• .); MALDI-TOF MS: 580; Accurate Mass calculated for C26H12S8 579.8699 found 579.8707; 1H NMR (major isomer, 400 MHz, CDCl3) δH (ppm): 7.71 (1H, d, J = 0.7 Hz), 7.49 (1H, dd, J = 5.1 and 1.1 Hz), 7.34 (1H, d, J = 5.5 Hz), 7.26 (1H, dd, J = 3.5 and 1.1 Hz), 7.23 (1H, dd, J = 5.5 and 0.7 Hz), 7.19 (1H, dd, J

Donor-acceptor substituted phenylethynyltriphenylenes – excited state intramolecular charge transfer, solvatochromic absorption and fluorescence emission

• Ritesh Nandy and
• Sethuraman Sankararaman

Beilstein J. Org. Chem. 2010, 6, 992–1001, doi:10.3762/bjoc.6.112

• , 129.9, 129.1, 128.9, 127.7, 127.6, 127.5, 127.4, 127.3, 123.5, 123.4, 123.3, 120.6, 84.1, 77.9 ppm; MALDI-TOF MS m/z (%) 252 (72) [M+], 253 (100) [M+ + 1], 254 (22) [M+ + 2]; Anal. calcd. for C20H12 C, 95.23; H, 4.75. Found C, 95.04, H 4.60. General procedure for the synthesis of 1a–e. 1a–e were

• , 128.5 (q, 3JC-F = 3.75 Hz), 127.7, 127.6, 127.41, 127.39, 127.0, 124.87 (q, 3JC-F = 3.75 Hz), 124.3, 123.5, 123.4, 123.3, 122.7, 121.1, 91.4, 88.6 ppm; MALDI-TOF MS C27H15F3 m/z (%) 396 (100) [M+], 397 (84) [M+ + 1], 398 (22) [M+ + 2]. 4-(2-Triphenylenylethynyl)benzonitrile (1c). The crude product was

• , 115.4, 113.5, 113.5, 90.6, 88.2, 69.4, 69.2, 31.9, 29.7, 29.6, 29.4, 29.36, 29.30, 29.26, 26.1, 22.7, 14.1 ppm; MALDI-TOF MS: m/z (%) 640 (100) [M+], 641 (49) [M+ + 1], 642 (9) [M+ + 2]. N,N-Dimethyl-4-(2-triphenylenylethynyl)aniline (1g). The crude product was purified by column chromatography with a

Towards racemizable chiral organogelators

• Jian Bin Lin,
• Debarshi Dasgupta,
• Seda Cantekin and
• Albertus P. H. J. Schenning

Beilstein J. Org. Chem. 2010, 6, 960–965, doi:10.3762/bjoc.6.107

• Varian Mercury NMR Spectrometer. IR spectra were measured on a Perkin Elmer 1600 FT-IR. MALDI-TOF MS spectra were measured on a Perseptive DE Voyager Mass Spectrometer with α-cyano-4-hydroxycinnamic acid as the matrix. Synthesis Phenylglycinamide R-2, S-2: The ester salt R-1, S-1 (5.04 g, 25 mmol) was

• ): 7.43 (m, 2H, ArH); 7.31 (m, 3H, ArH); 4.84 (br, 4H, NH2); 4.44 (s, 1H, COCH). MALDI-TOF MS (calc MW = 150.08, C8H10N2O): 150.97 [M + H]+. IR ν (cm−1): 3339; 3073; 1660; 1454; 1405; 1271; 869; 699. R-3, S-3: To a solution of 3,4-didodecyloxybenzaldehyde (237 mg, 0.5 mmol) in toluene, was added compound

• ) ; 1.83 (m, 32H, CH2) ; 1.83 (m, 6H, CH3). 13C NMR (CDCl3, 100 MHz): 199.20; 174.34; 162.90; 152.33; 149.32; 144.98; 139.45; 128.65; 128.43; 127.83; 127.25; 123.52; 112.46; 111.78; 69.35; 69.07; 31.90; 29.62; 29.42; 29.34; 29.25; 29.08; 26.03; 25.96; 22.66; 14.09. MALDI-TOF MS (calc MW = 606.48

Formation of epoxide-amine oligo-adducts as OH-functionalized initiators for the ring-opening polymerization of ε-caprolactone

• Julia Theis and
• Helmut Ritter

Beilstein J. Org. Chem. 2010, 6, 938–944, doi:10.3762/bjoc.6.105

• two very weak signals, which were difficult to discern, at 2.81 and 2.66 ppm, corresponding to the CH2 of the epoxide group. MALDI-TOF MS measurements (Figure 2) definitely indicated the formation of an oligomer homologous series of epoxide-amine addition products of 1 and 3. The amino alcohol

• the midpoint method. The melting point (Tm) values are reported as the average peak maxima of the second and the third heating cycle. Matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS) was performed on a Bruker Ultraflex TOF mass spectrometer. Ions formed with

• obtained. IR (diamond): 2944, 2895, 2865 ν(CH)CH3, CH2, 1721 ν(CO), 1512 ν(C=C)aromatic, 1471 δ(CH), 1365 δs(CH), 1293 δ(OH), 1239 ν(CO)ArOCH2, 1176 ν(CO) cm−1. 13C NMR spectra of 4, measured in CDCl3. MALDI-TOF MS spectra (linear mode) of epoxide-amine product 4. GPC curve of epoxide-amine product 4

Synthesis and crystal structures of multifunctional tosylates as basis for star-shaped poly(2-ethyl-2-oxazoline)s

• Richard Hoogenboom,
• Martin W. M. Fijten,
• Guido Kickelbick and
• Ulrich S. Schubert

Beilstein J. Org. Chem. 2010, 6, 773–783, doi:10.3762/bjoc.6.96

• . 1H and 13C NMR spectroscopy and elemental analysis, the chemical structures of the TetraTos and HexaTos were verified by MALDI-TOF MS, revealing only the desired mass peak corresponding to full tosylation (Figure 2). The absence of residual hydroxyl groups is of major importance for the use of these

• compared to the previously discussed multi-tosylates, partly counteracting the advantages of multi-tosylate initiators compared to multi-halides and multi-triflates. Figure 8 depicts the MALDI-TOF MS spectrum of the porphyrin initiator TetraTos-B. The formation of TetraTos-B (mass 1582) and a minor

• pump, a 2414 refractive index detector and a Waters Styragel HT4 column utilizing DMF containing 5 mM NH4PF6 at a flow rate of 0.5 mL/min as eluent and the column oven set to 50 °C (PEG calibration). MALDI-TOF-MS was performed on a Voyager-DE™ PRO Biospectrometry™ Workstation (Applied Biosystems) time

En route to photoaffinity labeling of the bacterial lectin FimH

• Thisbe K. Lindhorst,
• Michaela Märten,
• Andreas Fuchs and
• Stefan D. Knight

Beilstein J. Org. Chem. 2010, 6, 810–822, doi:10.3762/bjoc.6.91

• spectrometry For mass spectrometric analyses of the photo-crosslinked products, a Bruker Biflex III instrument (MALDI-TOF-MS, Prof. Th. K. Lindhorst, CAU), or a Bruker Biflex II instrument (ESI-FT-ICR-MS/MS, group of PD Dr. B. Lindner at the Research Center Borstel), or the 4700 Proteomics Analyzer mass

• ), 72.53 (C-3man), 72.05 (C-2man), 68.64 (C-4man), 68.12 (Fmoc-CH2), 67.17 (manOCH2CH2), 66.71 (manOCH2CH2), 51.63 (C-α), 48.30 (CH-Fmoc), 40.35 (C-6), 38.55 (C-β), 28.21 (OC(CH3)3) ppm. MALDI-TOF-MS: m/z = 639.2 [M + Na]+, 656.2 [M + K]+; ESI-MS: m/z = 639.26 [M + Na]+ (616.66 calcd. for C31H40N2O11). N

• ), 40.42 (C-6man), 38.74 (C-β), 28.33 (C(CH3)3) ppm. MALDI-TOF-MS: m/z = 639.2 [M + Na]+; ESI-MS: m/z = 639.26 [M + Na]+ (616.66 calcd. for C31H40N2O11). N-(Fluoren-9-ylmethoxycarbonyl)-4-[2-(α-D-mannopyranosyloxy)ethyl]-1-[(+)-biotinylamidopropyl]-L-aspartic acid diamide (9) The glycoamino acid 7 (220 mg

A bivalent glycopeptide to target two putative carbohydrate binding sites on FimH

• Thisbe K. Lindhorst,
• Kathrin Bruegge,
• Andreas Fuchs and
• Oliver Sperling

Beilstein J. Org. Chem. 2010, 6, 801–809, doi:10.3762/bjoc.6.90

• , length of cell: 1 dm). ESI-MS measurements were recorded on a Mariner ESI-TOF 5280 (Applied Biosystems) instrument and MALDI-MS measurements on a MALDI-TOF-MS-Biflex III (Bruker) instrument. 2-[N-(Nω-tert-Butyloxycarbonyl-pentaglycyl)]-amidoethyl α-D-mannopyranoside (7) Glyc5Boc (300 mg, 0.74 mmol) was

• = 175.96 (NH-C(O)O), 174.60, 174.36, 173.74 (5 NH-CH2-CO), 101.98 (C-1), 84.15 (C(CH3)3), 75.11 (C-5), 72.80 (C-3), 72.32 (C-2), 69.10 (C-4), 68.02 (manOCH2CH2N), 63.25 (C-6), 45.82 (manOCH2CH2N), 44.81 (4 NH-CH2-CO), 41.27 (NH-CH2-CO), 29.89 (C(CH3)3) ppm; MALDI-TOF-MS: m/z 631.1, [M + Na]+ (631.3 calcd

• , H-5, H-6a, H-6b, manOCH2CH2N, NH-CH2-CO) ppm; MALDI-TOF-MS: m/z 531.1 [M + Na]+ (531.2 calcd. for the free amine C18H32N6O11 + Na). 2-Azidoethyl 2,4-di-O-benzoyl-3,6-di-O-(2,3,4,6-tetra-O-benzoyl-α-D-mannopyranosyl)-α-D-mannopyranoside (13) The 2,4-di-O-benzoyl-protected mannoside 11 [28] (250 mg

Novel multi-responsive P2VP-block-PNIPAAm block copolymers via nitroxide-mediated radical polymerization

• Cathrin Corten,
• Katja Kretschmer and
• Dirk Kuckling

Beilstein J. Org. Chem. 2010, 6, 756–765, doi:10.3762/bjoc.6.89

• nitroxide terminated polymers were characterized by nuclear magnetic resonance (NMR) spectroscopy, size exclusion chromatography (SEC) and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). Thermal properties were investigated by the differential scanning

• having higher molecular weights. By performing NMRP on 2VP, the resulting polymer should possess defined end groups (Scheme 1). In order to analyze the polymer structure MALDI-TOF MS was employed. The spectra of samples obtained from the polymerization at 110 °C with a [initiator/monomer] ratio of 1:140

• weight determined by MALDI-TOF MS only increased from Mn = 1530 g/mol to Mn = 2800 g/mol. One reason might be the laser energy used to desorb the polymer led to P2VP chain degradation or fragmentation. Since SEC calibration has been done with P2VP standards, one can assume that the different results can

Synthesis, spectral characterization, electron microscopic study and thermogravimetric analysis of a phosphorus containing dendrimer with diphenylsilanediol as core unit

• E. Dadapeer,
• B. Hari Babu,
• C. Suresh Reddy and
• Naga Raju Charmarthi

Beilstein J. Org. Chem. 2010, 6, 726–731, doi:10.3762/bjoc.6.85

• groups. The structures of the intermediate compounds were confirmed by IR, GCMS and 31P NMR. The final compound was characterized by 1H, 13C, 31P NMR, MALDI-TOF MS and CHN analysis. Scanning electron microscopic and thermogravimetric analysis/differential scanning calorimetric studies were also performed

Calix[4]arene-click-cyclodextrin and supramolecular structures with watersoluble NIPAAM-copolymers bearing adamantyl units: “Rings on ring on chain”

• Bernd Garska,
• Monir Tabatabai and
• Helmut Ritter

Beilstein J. Org. Chem. 2010, 6, 784–788, doi:10.3762/bjoc.6.83

• NMR spectrum of the successful cycloaddition of 2 and 3 indicates that the di-substituted calix[4]arene 4 was the major product along with a little amount of the mono-substituted compound. In addition, the MALDI-TOF-MS clearly confirmed the existence of the covalently combined rings (4) with a

• desorption ionization time of flight mass spectrometry (MALDI-TOF-MS) was performed on a Bruker Ultraflex TOF mass spectrometer. Ions formed with a pulsed nitrogen laser (25 Hz, 337 nm) were accelerated to 25 kV, the molecular masses being recorded in linear mode. 2-(4-Hydroxyphenylazo)benzoic eacid (HABA

Synthesis of 6-PEtN-α-D-GalpNAc-(1–>6)-β-D-Galp-(1–>4)-β-D-GlcpNAc-(1–>3)-β-D-Galp-(1–>4)-β-D-Glcp, a Haemophilus influenzae lipopolysacharide structure, and biotin and protein conjugates thereof

• Andreas Sundgren,
• Martina Lahmann and
• Stefan Oscarson

Beilstein J. Org. Chem. 2010, 6, 704–708, doi:10.3762/bjoc.6.80

• of 23, from which the Boc-group was removed by acid hydrolysis to afford derivative 24. Protein conjugations were performed by reaction of 24 (20 equiv) with human serum albumin (HSA). Compound 20 was similarly activated with dimethyl squarate and conjugated to HSA. MALDI-TOF MS of the HSA-conjugates

New diarylmethanofullerene derivatives and their properties for organic thin- film solar cells

• Daisuke Sukeguchi,
• Surya Prakash Singh,
• Mamidi Ramesh Reddy,
• Hideyuki Yoshiyama,
• Rakesh A. Afre,
• Yasuhiko Hayashi,
• Hiroki Inukai,
• Tetsuo Soga,
• Shuichi Nakamura,
• Norio Shibata and
• Takeshi Toru

Beilstein J. Org. Chem. 2009, 5, No. 7, doi:10.3762/bjoc.5.7

• ; FT-IR (KBr, cm−1): 2923, 2853, 1725, 1608, 1511, 1464, 1430, 1275, 1186, 1136, 1107; MALDI-TOF MS: m/z 1200; Anal. calcd. for C91H44O4: C; 90.98, H; 3.69. Found: C; 89.41, H; 3.45. Photovoltaic cells After rubbing with cloth to remove the protrusions, the ITO-glass (FINE brand, Furuuchi Co. Ltd., 15

Dimerization of propargyl and homopropargyl 6-azido- 6-deoxy- glycosides upon 1,3-dipolar cycloaddition

• Nikolas Pietrzik,
• Daniel Schmollinger and
• Thomas Ziegler

Beilstein J. Org. Chem. 2008, 4, No. 30, doi:10.3762/bjoc.4.30

• product of dimerization though. Only oligomers 8 were observed in this case (for details see Supporting Information File 1). At first, it was unclear whether 7a was formed by an intramolecular cyclization or a dimerization of 4a since its concentration-dependent ESI-MS and MALDI-TOF-MS spectra both showed