Site-Selective Modification of a Porpholactone—Selective Synthesis of 12,13- and 17,18-Dihydroporpholactones

The reaction of meso-tetrakis(pentafluorophenyl)porpholactone with azomethine ylides and nitrones affords pyrrolidine-fused and isoxazolidine-fused dihydroporpholactones that display, respectively, isobacteriochlorin- and chlorin-type UV–Vis spectra. These reactions are site-selective, yielding, respectively, 17,18- or 12,13-dihydroporpholactones. The crystal and molecular features of pyrrolidine-fused and isoxazolidine-fused dihydroporpholactones were unveiled from single-crystal X-ray diffraction studies.


Introduction
Pyrrole-modified porphyrins have attracted considerable attention in recent years. This group of compounds comprises a large diversity of porphyrin analogues where one or more pyrrolic units of the porphyrin macrocycle are replaced by non-pyrrolic units. The synthesis, reactivity, structural features, optical properties, and applications of these compounds have been comprehensively reviewed [1][2][3][4][5][6]. In short, these compounds may be prepared by two complementary routes: by constructing the porphyrin macrocycle using adequate pyrrolic and non-pyrrolic building blocks or by chemical modification of already existing porphyrins. The latter route has been extensively used by the group of Brückner that, by using the "breaking and mending of porphyrins" approach [7], was able to produce a large diversity of pyrrole-modified porphyrins [2]. Focusing on their optical properties, these porphyrinoids may display porphyrin-, chlorin-, isobacteriochlorin-, or bacteriochlorin-like spectra, which is an important feature for a range of applications, including biological (photodynamic therapy (PDT) of cancer or multimodal imaging contrast agents) and technical ones (catalysis or optical sensors) [3,[8][9][10][11][12].
Porpholactones, a class of pyrrole-modified porphyrins in which a pyrrolic unit is formally replaced by an oxazolone unit, can be conveniently obtained from meso-tetraarylporphyrins in a two-step procedure (osmium tetroxide-mediated dihydroxylation of the porphyrin followed by KMnO 4 -induced cleavage of 2,3-dihydroxychlorin) [13] or simply by oxidation of a meso-tetraarylporphyrin with Oxone ® in the presence of a catalytic amount of RuCl 3 [14].
In the late 1990s, the group of Cavaleiro reported that the porphyrin macrocycle can participate as a dienophile in Diels-Alder reactions (D-A) [15] or as a dipolarophile in 1,3-dipolar cycloadditions (1,3-DC) [16]. The resulting monoadducts are heterocycle-fused chlorins, while bisadducts are bacteriochlorins or isobacteriochlorins. Concerning the 1,3-DC, the reaction of porphyrins with various types of 1,3-dipoles [17], namely, azomethine ylides [16,18,19] and nitrones [20,21], just to refer to the two types used in this work, has been reported. This is, effectively, an excellent and selective route to porphyrin derivatives with adequate optical properties for biological applications. Considering that 1,3-DC have also been successfully used for the peripheral functionalization of other porphyrin-type macrocycles, namely, tetraazaporphyrins [22], N-confused porphyrins [23], octaphyrins [24], and triply linked diporphyrins [25], the rarity of studies concerning the use of pyrrole-modified porphyrins as dipolarophiles is surprising [26]. In this article, we report the reaction of a porpholactone with two azomethine ylides and two nitrones. Since the porpholactone macrocycle is not symmetric, some site selectivity in these 1,3-DC reactions governed by the lactone unit was expected.
Porpholactones display porphyrin-like UV-Vis spectra, which indicates that the lactone moiety mimics very well the electronic properties of a β-β' bond [27]. However, the reactivity of the three remaining β-β' bonds is influenced differently by the lactone unit. This influence is observed, for instance, when porpholactone 1 reacts with Woollins' reagent and PhMe2SiH [28,29] to afford, selectively, a compound with a hydrogenated pyrrole unit adjacent to the lactone moiety (A, Figure  1) or when porpholactones react with hydrazine [30] (or tosylhydrazide) [31] to give compounds with a hydrogenated β-β' bond opposite to the lactone moiety (B). The selective formation of oppporphodilactones [32] and 12,13-dihydroxyporpholactone osmate esters (C) [11] from porpholactones are other examples of the orientation effect of the lactone unit. These observations prompted us to investigate the effect of the lactone unit on the reactivity of porpholactones with azomethine ylides and nitrones and, specifically, to find if it induces site selectivity and, eventually, regioselectivity. In these studies, we used meso-tetrakis(pentafluorophenyl)porpholactone (1) that was prepared by Zhang's method [14].

Results and Discussion
The reaction of porpholactone 1 with azomethine ylide 2a (generated in situ from Nmethylglycine and paraformaldehyde) was performed under the same reaction conditions applied to meso-tetrakis(pentafluorophenyl)porphyrin [16] (Scheme 1). Interestingly, a single monoadduct was formed, indicating clearly that the reaction was site-selective. A similar reaction with azomethine ylide 2b (generated in situ from N-benzylglycine and paraformaldehyde) also afforded a single monoadduct. Both cycloadducts displayed isobacteriochlorin-type UV-Vis spectra ( Figure 2 and Supplementary Materials) similar to that reported for dihydroporpholactone A [28], signifying that the cycloadditions occurred selectively at one of the pyrrolic units adjacent to the lactone moiety (i.e., they should have structures 3 or 4). Their emission spectra were also similar to that of an isobacteriochlorin [28], with a main band around 580 nm and a shoulder around 630 nm. The quantum yields were determined to be 0.59 and 0.69 for 3a and 3b, respectively. These values also correlate well with the quantum yields reported for isobacteriochlorins. Their 1 H NMR spectra

Results and Discussion
The reaction of porpholactone 1 with azomethine ylide 2a (generated in situ from N-methylglycine and paraformaldehyde) was performed under the same reaction conditions applied to meso-tetrakis(pentafluorophenyl)porphyrin [16] (Scheme 1). Interestingly, a single monoadduct was formed, indicating clearly that the reaction was site-selective. A similar reaction with azomethine ylide 2b (generated in situ from N-benzylglycine and paraformaldehyde) also afforded a single monoadduct. Both cycloadducts displayed isobacteriochlorin-type UV-Vis spectra ( Figure 2 and Supplementary Materials) similar to that reported for dihydroporpholactone A [28], signifying that the cycloadditions occurred selectively at one of the pyrrolic units adjacent to the lactone moiety (i.e., they should have structures 3 or 4). Their emission spectra were also similar to that of an isobacteriochlorin [28], with a main band around 580 nm and a shoulder around 630 nm. The quantum yields were determined to be 0.59 and 0.69 for 3a and 3b, respectively. These values also correlate well with the quantum yields reported for isobacteriochlorins. Their 1 H NMR spectra revealed the signals of the NH protons at 3.58 and 4.50 ppm (for the N-methyl derivative) and at 3.58 and 4.60 ppm (for the N-benzyl derivative).
These signals disappeared after shaking with D 2 O. 1 H NMR spectra of porphyrin derivatives with NH signals at such low fields are typical of isobacteriochlorins [15,18,21,33,34], and the same pattern is also observed in isobacteriochlorin-type structures such as compound A [28]. The reaction of porpholactone 1 with nitrone 5a (generated in situ from N-methylhydroxylamine hydrochloride and paraformaldehyde in the presence of potassium carbonate) was performed under the same reaction conditions applied to meso-tetrakis(pentafluorophenyl)porphyrin [21]. TLC analysis of the reaction mixture showed one spot corresponding to porpholactone 1 and two very close spots corresponding to new compounds. The unreacted porpholactone and the two products could be separated by preparative TLC (silica gel) using a 1:1 mixture of CH2Cl2/hexane as the eluent. Similarly, when nitrone 5b was used (generated in situ from N-benzylhydroxylamine hydrochloride and paraformaldehyde in the presence of potassium carbonate), two products with similar Rf were also obtained. The UV-Vis spectra of the new compounds were identical to that of 12,13dihydroxyporpholactone osmate ester C, 11 showing Q bands at λmax 682 nm ( Figure 2). These spectra clearly indicate that the cycloadditions occurred selectively at the pyrrolic unit opposite to the lactone moiety. Consequently, the cycloadducts are regioisomers 6 and 7 [35]. The mass spectra of these products indicated that they are all monoadducts, showing the protonated molecular peaks (M + H) + = 1052 for 6a and 7a and (M + H) + = 1128 for 6b and 7b. Each pair of regioisomers was obtained in a ratio of ca. 2.4:1, indicating that cycloaddition, besides being site-selective, was also regioselective. A slow crystallization of the main cycloadduct formed in the reaction with nitrone 5a allowed obtaining crystals adequate for single-crystal X-ray diffraction studies. These studies revealed that it has structure 6a ( Figure 4) and, therefore, the minor isomer has structure 7a.
The emission spectra of adducts 6 and 7 presented two bands around 650 nm and 690 nm and quantum yields between 0.14 and 0.23, which were much smaller than those for adducts 3 (Table S1, SM). The 1 H NMR spectra of compounds 6 and 7 were markedly different from those of compounds 3a/3b, particularly at the chemical shifts of the signals of the NH protons: −1.53 and −1.19 ppm for 6a, −1.67 and −1.36 ppm for 7a, −1.68 and −1.31 ppm for 6b, and −1.73 and −1.43 ppm for 7b. The 13 C and 19 F NMR spectra of the cycloadducts discussed above are also compatible with the proposed structures, displaying various sets of peaks as expected for porpholactone derivatives with reduced symmetry. In contrast to azomethine adducts 3, nitrone cycloadducts 6 and 7 revealed to be unstable in solution at room temperature, affording, slowly, porpholactone 1. This transformation, probably via retrocycloaddition, was easily observed by TLC and was also detected in the 13 C NMR spectra of the cycloadducts that showed a signal corresponding to the carbonyl group of the adduct and a small one due to the carbonyl group of porpholactone 1.  N-benzyl derivative). These signals disappeared after shaking with D2O. 1 H NMR spectra of porphyrin derivatives with NH signals at such low fields are typical of isobacteriochlorins [15,18,21,33,34], and the same pattern is also observed in isobacteriochlorin-type structures such as compound A [28].
The N-benzyl cycloadduct led to the isolation of poor crystals, which could be, nevertheless, investigated using single-crystal X-ray diffraction studies. The crystallographic studies performed revealed that the material crystallized in the centrosymmetric P-1 triclinic space group, with the asymmetric unit being composed of two disordered halves of two independent molecular units. A full disclosure of the structure of the cycloadduct was instead achieved by solving and refining the structure in the P1 triclinic space group (by discarding the center of inversion). With this procedure, it was possible to unequivocally confirm that the cycloadduct has structure 3b (and not 4b, see Scheme 1 and Figure 3). Though the X-ray diffraction reliability factors were high (due to the poor diffraction of the crystal), the disclosed structural features were in perfect agreement with those obtained from additional characterization techniques, namely, NMR and UV-Vis spectroscopy.   The N-benzyl cycloadduct led to the isolation of poor crystals, which could be, nevertheless, investigated using single-crystal X-ray diffraction studies. The crystallographic studies performed revealed that the material crystallized in the centrosymmetric P-1 triclinic space group, with the asymmetric unit being composed of two disordered halves of two independent molecular units. A full disclosure of the structure of the cycloadduct was instead achieved by solving and refining the structure in the P1 triclinic space group (by discarding the center of inversion). With this procedure, it was possible to unequivocally confirm that the cycloadduct has structure 3b (and not 4b, see Scheme 1 and Figure 3). Though the X-ray diffraction reliability factors were high (due to the poor diffraction of the crystal), the disclosed structural features were in perfect agreement with those obtained from additional characterization techniques, namely, NMR and UV-Vis spectroscopy.   The reaction of porpholactone 1 with nitrone 5a (generated in situ from N-methylhydroxylamine hydrochloride and paraformaldehyde in the presence of potassium carbonate) was performed under the same reaction conditions applied to meso-tetrakis(pentafluorophenyl)porphyrin [21]. TLC analysis of the reaction mixture showed one spot corresponding to porpholactone 1 and two very close spots corresponding to new compounds. The unreacted porpholactone and the two products could be separated by preparative TLC (silica gel) using a 1:1 mixture of CH 2 Cl 2 /hexane as the eluent. Similarly, when nitrone 5b was used (generated in situ from N-benzylhydroxylamine hydrochloride and paraformaldehyde in the presence of potassium carbonate), two products with similar R f were also obtained. The UV-Vis spectra of the new compounds were identical to that of 12,13-dihydroxyporpholactone osmate ester C, 11 showing Q bands at λ max 682 nm (Figure 2). These spectra clearly indicate that the cycloadditions occurred selectively at the pyrrolic unit opposite to the lactone moiety. Consequently, the cycloadducts are regioisomers 6 and 7 [35]. The mass spectra of these products indicated that they are all monoadducts, showing the protonated molecular peaks (M + H) + = 1052 for 6a and 7a and (M + H) + = 1128 for 6b and 7b. Each pair of regioisomers was obtained in a ratio of ca. 2.4:1, indicating that cycloaddition, besides being site-selective, was also regioselective. A slow crystallization of the main cycloadduct formed in the reaction with nitrone 5a allowed obtaining crystals adequate for single-crystal X-ray diffraction studies. These studies revealed that it has structure 6a ( Figure 4) and, therefore, the minor isomer has structure 7a. In a previous work [21], we have shown that the addition of an azomethine ylide to a chlorin results in the formation of an isobacteriochlorin, while the addition of a nitrone to a chlorin leads to the formation of a bacteriochlorin. Although being electronically related to porphyrins (and not to chlorins), porpholactones react with azomethine ylides and nitrones with site selectivity identical to The emission spectra of adducts 6 and 7 presented two bands around 650 nm and 690 nm and quantum yields between 0.14 and 0.23, which were much smaller than those for adducts 3 (Table S1, SM). The 1 H NMR spectra of compounds 6 and 7 were markedly different from those of compounds 3a/3b, particularly at the chemical shifts of the signals of the NH protons: −1.53 and −1.19 ppm for 6a, −1 .67 and −1.36 ppm for 7a, −1.68 and −1.31 ppm for 6b, and −1.73 and −1.43 ppm for 7b. The 13 C and 19 F NMR spectra of the cycloadducts discussed above are also compatible with the proposed structures, displaying various sets of peaks as expected for porpholactone derivatives with reduced symmetry. In contrast to azomethine adducts 3, nitrone cycloadducts 6 and 7 revealed to be unstable in solution at room temperature, affording, slowly, porpholactone 1. This transformation, probably via retrocycloaddition, was easily observed by TLC and was also detected in the 13 C NMR spectra of the cycloadducts that showed a signal corresponding to the carbonyl group of the adduct and a small one due to the carbonyl group of porpholactone 1.
In a previous work [21], we have shown that the addition of an azomethine ylide to a chlorin results in the formation of an isobacteriochlorin, while the addition of a nitrone to a chlorin leads to the formation of a bacteriochlorin. Although being electronically related to porphyrins (and not to chlorins), porpholactones react with azomethine ylides and nitrones with site selectivity identical to that expected for a chlorin, and this is a noteworthy observation. The reason for the site selectivity observed in the reaction of 1,3-DC with chlorins was elucidated by a DFT study [36]: while the addition of an azomethine ylide is irreversible and kinetically controlled, the addition of a nitrone is reversible, pointing to a thermodynamic control of the reaction. Experimental observations in this work, namely, the retrocycloaddition observed with nitrone cycloadducts, suggest that a similar reaction control also operates with porpholactones.
In conclusion, this study showed that the reactions of meso-tetrakis(pentafluorophenyl) porpholactone with 1,3-dipoles are site-selective and, depending on the type of 1,3-dipole used, the resulting cycloadducts are 12,13-or 17,18-dihydroporpholactones, which display, respectively, chlorinor isobacteriochlorin-type UV-Vis spectra. Considering that a large diversity of substituents may be linked to these cycloadducts by nucleophilic substitutions at the pentafluorophenyl groups [37,38], the reaction of porpholactone 1 with 1,3-dipoles may be an excellent route to a myriad of compounds for biological or technical applications.

Chemicals and Instrumentation
1 H, 13 C and 19 F NMR spectra were recorded on Bruker AVANCE 300 or Bruker AVANCE 500 spectrometers. CDCl 3 was used as solvent, and tetramethylsilane (TMS) as an internal reference. The chemical shifts are expressed in δ (ppm), and the coupling constants (J) in hertz (Hz). UV-Vis spectra were recorded on a Shimadzu UV-2501PC spectrophotometer using CHCl 3 as the solvent. Fluorescence emission spectra were recorded with a Jobin Yvon FluoroMax-3 spectrofluorometer using CHCl 3 as the solvent. Fluorescence quantum yields Φ F were determined using 5,10,15,20-tetraphenylporphyrin (TPP) in chloroform (Φ F = 0.11) as the fluorescence standard. Mass spectra were recorded using a Micromass Q-TOF-2 TM mass spectrometer and CHCl 3 as the solvent. Preparative thin-layer chromatography was carried out on 20 × 20 cm glass plates coated with silica gel (1 mm thick, Merck, Kenilworth, NJ, USA). Analytical TLC was carried out on sheets precoated with silica gel (Merck 60, 0.2 mm thick). Solvents were purified or dried according to the literature procedures. Compound 1 was prepared according to a published procedure [14].