Synthesis and NMR Spectral Studies of the 7-C60-Adduct of N,N-(Tetrachlorophthaloyl) Dehydroabietylamine

The 7-C60-adduct of N,N-(tetrachlorophthaloyl)dehydroabietylamine was synthesized for the first time and characterized by IR, UV-vis, mass and NMR spectral studies. The 1H-NMR and 13C-NMR resonance signals of the new compound are unambiguously assigned by using homo- and heteronuclear 2D NMR spectroscopic techniques such as COSY, ROESY, HSQC and HMBC. The C1 symmetric structure with 6,6-junction of compound was determined.


Introduction
Dehydroabietylamine, which possesses an aromatic diterpene structure with three rings and a reactive amino group and is the main component of disproportionated rosin amine, can be easily isolated from the latter. Dehydroabietylamine and its derivatives have attracted considerable interest due to their wide range of uses such as chiral resolution agents [1,2], antibacterial substances [3,4] and chiral surfactants [5,6].
Since the discovery of C 60 , its peculiar cage structure has attracted great attention. Cycloaddition reactions of C 60 , especially 1,3-dipolar addition reactions, have been the subject of a large variety of studies and shown to be a useful methods for the synthesis of functionalized fullerene derivatives [7][8][9][10][11]. These results prompted us to study new cycloadducts of dehydroabietylamine with C 60 as a part of an OPEN ACCESS on-going program for the development of new rosin amine derivatives with potential biological or material properties. So far, the majority of the studies on the chemical transformations of dehydroabietylamine have focused on the amine group and benzene ring, but the chemical transformation in other skeleton has seldom been reported. In this paper, we report the synthesis for the first time of the 7-C60-adduct of N,N-(tetrachlorophthaloyl)dehydroabietylamine (5, Figure 1) and describe the structure determination of the new compound, along with its detailed 1 H-and 13 C-NMR assignments.

Synthesis Procedures
The general procedure for the synthesis of compound 5 is shown in Scheme 1. Dehydroabietylamine (1) was prepared as described in the literature [12]. Dehydroabietylamine reacted with tetrachlorophthalic anhydride (TCPA, an amino protecting group) to give compound 2, which then was transformed into 3 by C-7 benzylic oxidation. Subsequently, the reaction of 3 with p-tosylhydrazide yielded p-tosylhydrazone 4. Compound 5 was prepared according to the reported method [13]. According to the literature [13], heating a solution of the p-tosylhydrazone with C 60 at 70 °C afford the [5,6]-open isomer. However, in our experiment, no traces of this expected [5,6]-open isomer were found and the only isolated product was the [6,6]-closed isomer (methanofullerene), which is likely due to a rearrangement of the 5,6-open isomer into the thermodynamically more stable 6,6-closed isomer at 70 °C. The IR spectrum of compound 5 showed bands indicating the presence of the different expected functional groups: Characteristic asymmetric and symmetric stretching bands of the imide carbonyl (C=O) at 1,776 and 1,721 cm −1 and at 526 cm −1 for the C 60 skeleton. In the UV-vis spectrum, the characteristic absorptions for the methanofullerenes was also observed at 436.50, 697.00 nm [13]. Meanwhile, the structure of compound 5 as a monoadduct was supported by the matrix-assisted laser desorption/ionization time of flight mass spectrum (MALDI-TOF MS) which display the expected peak at m/z 1,271. The 1 H-, 13 C-NMR and 2D NMR spectra for compound 5 are summarized in Table 1.

Analysis of Compound
First, the assignment of some protons is easily accomplished by analysis of the 1 H-NMR chemical shifts, signal multiplicity and coupling constants. The signals of the aromatic protons (7.26-8.40 ppm) can be readily identified by their chemical environment ( Figure 2).

13 C-NMR Spectrum Analysis of Compound 5
The proton noise decoupled 13 C-NMR spectrum (Figure 3) displayed 24 and 43 resonance signals respectively for (tetrachlorophthaloyl)dehydroabietylamine moiety and C 60 moiety of compound 5.
In the 13 C-NMR spectrum, the signal at δ C 164.51 ppm could be assigned to C-7', 8', which was supported by the cross peaks of δ C 164.51 ppm/H-18α, 18β in the HMBC spectrum.

General
1D ( 1 H and 13 C) and 2D NMR experiments were performed on a Bruker AVANCE AV-500 NMR spectrometer (500.13 MHz for 1 H and 125.77 MHz for 13 C). The samples were dissolved in 0.5 mL CDCl 3 , 1 H-NMR and 13 C-NMR spectra were recorded using TMS as an internal reference. Chemical shifts were reported in parts per million (ppm). FT infrared (IR) spectrum were recorded as KBr pellets on a Nicolet 360 FT-IR spectrometer and UV-vis spectra on a Shimadzu UV-2550 UV-VIS spectrometer. ESI mass spectrometric were obtained on Agilent 1100 Capillary LC/Micromass Q-Tof Micro mass spectrometer. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra was performed on a Bruker Daltonics Autoflex III mass spectrometer using α-cyano-4hydroxycinnamic acid (CHCA) as a matrix in a negative-ion reflector mode. All chemicals and solvents were obtained from commercial sources and used as received or dried according to standard procedures. Column chromatography was performed on silica gel (ZCXⅡ, 100-200 mesh). Chemical reactions were monitored by thin layer chromatography using precoated silica gel GF254 plates.

Preparation of 7-C 60 -Adduct of N,N-(Tetrachlorophthaloyl)dehydroabietylamine (5)
Compound 4 (408.9 mg, 0.556 mmol) was dissolved in dry pyridine (7 mL) in a dried three-necked flask under N 2 . Then, NaOMe (31.2 mg, 0.578 mmol) was added, and the mixture was stirred for 15 min at room temperature. A solution of C 60 (200 mg, 0.278 mmol) in chlorobenzene (55 mL) was added and the mixture was stirred at 70 °C for 24 h. After cooling to room temperature the solvent was evaporated in vacuo, the residue was column chromatographed on silica gel, pre-eluted with CS 2 to remove unreacted C 60 (67.7 mg) and then with CS 2 -CHCl 3 (10:1) to give 5 as a dark brown solid (145 mg, 41%

Conclusions
In conclusion, the new compound 5 was synthesized from dehydroabietylamine, the assignments of the proton and carbon signals for 7-C 60 -adduct of N,N-(tetrachlorophthaloyl)dehydroabietylamine were made possible by using 1D and 2D NMR techniques including 1 H-, 13 C-NMR, COSY, ROESY, HSQC and HMBC experiments. The two peaks at δ C 84.80, 77.66 ppm in the 13 C-NMR spectra correspond to the sp 3 -hybridized bridgehead carbons on the cyclopropyl moiety. This pattern is unambiguously diagnostic for C 1 symmetric structure with 6,6-junction. All the spectral data support and confirm the proposed structure of the target compound.