Synthesis of Porphyrins with ABAB Symmetry from Dipyrromethanes as Potential Phototherapeutic Agents

: Asymmetrically meso -substituted porphyrins were synthesized with ABAB symmetry pat-terns. The approach required the formation of dipyrromethanes, which were obtained from the condensation of an aldehyde (pentafluorobenzaldehyde, 4-nitrobenzaldehyde or N , N -diphenylamino-benzaldehyde) with a large excess of pyrrole (1:47 aldehyde/pyrrole mol ratio), catalyzed by trifluoroacetic acid in 70–94% yields. Then, acid-catalyzed condensation of these dipyrromethanes with an aldehyde ( N , N -dimethylaminobenzaldehyde, 4-carboxymethyl benzaldehyde or N -ethyl-3-carbazolecarbaldehyde) (1:1 mol ratio) in dichloromethane, followed by oxidation with 2,3-di-chloro-5,6-dicyano-1,4-benzoquinone affords the diseased porphyrins in 10–42% yields. These ABAB-porphyrins are interesting starting materials to obtain photoactive molecular structures as potential phototherapeutic agents.


Introduction
Porphyrin derivatives have been proposed as photosensitizers in the photodynamic inactivation of microorganisms [1,2]. However, depending on the substituents on the periphery of the tetrapyrrolic macrocycle, these molecules tend to aggregate, producing a loss of photodynamic activity. Thus, to achieve effective photoinactivation, these compounds can bind to different supports forming photoactive materials [3,4]. In this sense, it is interesting to develop porphyrins asymmetrically substituted in the meso positions by two different structures (A and B). In these compounds, structure A has a functional group that allows covalent attachment to other molecules, while B is substituted by groups, allowing changes in the properties of the tetrapyrrolic macrocycle [5,6].
A major limitation of available methods to synthesize porphyrins tetrapyrrole macrocycles is related to the possibility of attaching different groups at the four meso-positions of tetrapyrrole macrocycle. Porphyrins containing two types of meso-substituents can be synthesized by condensation of binary mixed aldehyde catalyzed by acid. However, this procedure is statistical in nature and frequently multiple porphyrins are formed [7]. In this case, six porphyrins can be obtained and the workup is complex due to the presence of subproducts. In general, the purification involves slowly chromatographic separation and no pure porphyrin is always possible, resulting in low yields of the desired product. More direct approaches to obtain trans-substituted porphyrins (ABAB-porphyrins) are provided by condensation of dipyrromethanes with aldehydes. This approach needs to obtain meso-substituted dipyrromethane that can be formed by the reaction of an aldehyde with pyrrole [8].
In the present work, ABAB-porphyrins were synthesized from the condensation of meso-(substituted)dipyrromethanes with benzaldehyde derivatives catalyzed by acid. These porphyrins are interesting starting materials to obtain photoactive molecular structures as potential phototherapeutic agents.

Equipment and Chemical Substances
Proton nuclear magnetic resonance ( 1 HNMR) spectra were carried out on an FT-NMR Bruker Avance DPX400 spectrometer (Bruker BioSpin, Rheinstetten, Germany). Mass spectra were performed on a Bruker micrO-TOF-QII (Bruker Daltonics, MA, USA), using an ESI source (ESI-MS). Absorption and fluorescence spectra were achieved on a Shimadzu UV-2401PC spectrometer (Shimadzu Corporation, Tokyo, Japan) and a Spex FluoroMax spectrofluorometer (Horiba Jobin Yvon Inc, Edison, NJ, USA), respectively. Chemical substances were purchased from Sigma-Aldrich (Milwaukee, WI, USA). These compounds were used without further purification. Silica gel thin-layer chromatography (TLC, 250 microns) plates were obtained from Analtech (Newark, DE, USA), while silica gel 60 (0.040-0.063 mm, 230-400 mesh) was acquired from Merck (Darmstadt, Germany). (1). Pentafluorobenzaldehyde (0.55 mL, 4.45 mmol) and pyrrole (14.5 mL, 209 mmol) were purged with argon for 15 min. Then, trifluoroacetic acid (TFA) (70 L, 0.90 mmol) was added. The mixture was stirred for 45 min at room temperature. After that, the sample was diluted with 25 mL of dichloromethane (DCM) and three washes of 15 mL each were carried out with NaOH (0.1 M). The solvent and excess pyrrole were removed under reduced pressure using a rotary evaporator. meso-(4-Nitrophenyl)dipyrromethane (2). A solution of 4-nitrobenzaldehyde (2.50 g, 16.6 mmol) and pyrrole (55 mL, 780 mmol) was degassed by bubbling with argon for 15 min, and then TFA (321 L, 4.17 mmol) was slowly added. The solution was stirred for 20 min at room temperature. The crude product was diluted with DCM. The organic phase was washed with aqueous 0.1 M NaOH. After that, the sample was washed with water. The solvent was removed under reduced pressure. The unreacted pyrrole was removed by vacuum distillation at room temperature. The product was purified by flash chromatography (silica gel, cyclohexane/ethyl acetate/triethylamine; 80:20:1) yielded 3.19 g (72%) of

Spectroscopic Studies
Absorption and fluorescence spectra were performed in a quartz cell of 1 cm path length using N,N-dimethylformamide (DMF) at 25.0 ± 0.5 °C. Absorbances (<0.05) were matched at the excitation wavelength (550 nm) and the areas of the emission spectra were integrated in the range 600-800 nm. The fluorescence quantum yield (ФF) of the porphyrins was calculated by comparison of the area below the corrected emission spectrum of 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin (TMP) as a reference [5].

Synthesis of Dipyrromethanes
Aldehydes and pyrrole undergo acid-catalyzed condensation at room temperature. Therefore, the condensation of substituted benzaldehydes with a large excess of pyrrole (1:47 aldehyde/pyrrole mol ratio) catalyzed by TFA affords meso-(substituted)dipyrromethane (1-3). After 25 min of stirring at room temperature, the reaction mixture showed complete consumption of the starting aldehyde. In this procedure, pyrrole was used as a reactant in excess and as the solvent of the condensation, which yielded the corresponding dipyrromethane (Scheme 1).
The dipyrromethanes were purified by flash chromatography on silica gel in a mildly basic medium, using n-hexane/ethyl acetate/TEA (80/20/1). The presence of 1% TEA was necessary to avoid the acidolysis of the dipyrromethane on silica gel column. Dipyrromethanes 1, 2 and 3 were obtained in 94, 72 and 70%, respectively. These compounds are stable upon storage at 0 °C in nitrogen atmosphere and absence of light. Therefore, dipyrromethanes can be easily synthesized with good yield and high purity, which is crucial for the formation of asymmetric meso-substituted porphyrins.

Synthesis of ABAB-Porphyrins
ABAB-porphyrins 4-7 were synthesized by the acid-catalyzed condensation of dipyrromethane 1-3 and the correspondent substituted benzaldehyde (Scheme 2). Mixedbenzaldehyde dipyrromethane condensations were performed using about [1:1] molar relation of dipyrromethane and substituted benzaldehyde. The reaction was performed using catalytic among of TFA or BF3 . OEt2 and DCM as solvent at room temperature. The reaction mixture was subject to oxidation with DDQ. Thus, this mixed condensation affords the corresponding ABAB-porphyrin.
These porphyrins were easily separated by flash chromatography with high purity using DCM/methanol gradient. In all these cases, the first purple band corresponds to the ABAB-porphyrin. Under these conditions, ABAB-porphyrins 4, 5, 6 and 7 were obtained in 24, 10, 21 and 42% yields, respectively.
This procedure was previously used to obtain ABAB-porphyrins from the condensation of a dipyrromethane bearing a sterically hindered substituent with an aldehyde [9].
The pentafluorophenyl group linked to the meso position of the compounds 4 and 5 can be used to obtain porphyrin derivatives by nucleophilic aromatic substitution reaction of the para-fluorine atom [3,10]. Thus, this substituent was used to covalently link the porphyrin ring to several functionalized structures [4]. In porphyrin 4, N,N-dimethylaminophenyl substituent can be used to obtain cationic intrinsic charge by methylation [5]. In addition, aminophenyl substituents can be obtained in porphyrin 6 by reduction of nitrophenyl group [11]. Moreover, porphyrin 5 and 6 can be hydrolyzed to form carboxylic acid groups, which can be linked to several structures [11]. Finally, both electroactive substituents of porphyrin 7 can form different polymers [12].
The steady-state fluorescence emission spectra of these porphyrins were obtained in DMF ( Table 1). The two bands are characteristic for similar meso-substituted porphyrin, which were assigned to Qx(0-0) and Qx(0-1) transitions [5,6]. Free-base porphyrins with D2h symmetry displayed vibronic structure unchanged upon excitation. Moreover, Stokes shifts of ~10 nm were obtained for the tetrapyrrolic macrocycles from the intersection of the absorption and fluorescence Qx(0-0) band. Therefore, the spectroscopic energies of these porphyrins were similar to the relaxed energies of the lowest singlet excited state S1. This behavior was in accordance with a rigid planar structure of tetrapyrrolic macrocycles. Fluorescence quantum yields (ФF) of these photosensitizers were calculated by comparison with TMP as a reference. The values of ФF for these porphyrins agree with values previously reported by similar porphyrin derivatives [5,15].

Conclusions
The following two basic steps were used sequentially in the synthesis of ABAB-porphyrin: (1) meso-(4-substituted) dipyrromethane was formed from correspondent benzaldehyde derivative and pyrrole catalyzed by acid, (2) condensation of dipyrromethane with appropriate benzaldehydes yields the ABAB-porphyrin, which was easily purified by flash chromatography. Thus, the desired ABAB-porphyrins 4-7, bearing different substituents were obtained with appreciable yields of 10-42%. Thus, the dipyrromethanes react with an aldehyde under the conditions of the two-step one-flask porphyrin synthesis, affording direct access to ABAB-porphyrins. Moreover, it has a relatively simple reaction workup and high yields. These trans-substituted porphyrins contain precursor groups of positive charges, which can be used to obtain cationic photosensitizers. Moreover, these tetrapyrrolic macrocycles can be covalently attached to molecular structures and be used to form polymeric materials. Therefore, these ABAB-porphyrins are interesting starting materials to obtain photoactive molecular structures as potential phototherapeutic agents.