Efficient synthesis of monoacyl dipyrromethanes and their use in the preparation of sterically unhindered trans-porphyrins

The condensation of an aldehyde with a dipyrromethane bearing a sterically unhindered aryl substituent at the 5-position typically results in low yield and a mixture of porphyrin products derived from acidolytic scrambling. We have developed a concise nonscrambling synthesis of such trans-porphyrins that takes advantage of the availability of multigram quantities of dipyrromethanes. This route involves the selective monoacylation of the dipyrromethanes with a pyridyl thioester, reduction of the monoacyl dipyrromethane to the corresponding carbinol, and self-condensation of the carbinol to form the porphyrin. The monoacylation procedure has wide scope as demonstrated by the preparation of a set of 15 diverse monoacyl dipyrromethanes in good yield at the multigram scale. The dipyrromethanecarbinol self-condensation reaction is extremely rapid (<3 min) under mild room-temperature conditions and affords the trans-porphyrin in 16-28% yield. Analysis by laser-desorption mass spectrometry (LD-MS) of samples from the crude reaction mixture revealed no scrambling within the limit of detection (1 part in 100). The self-condensation is compatible with a range of electron-withdrawing or -releasing substituents as well as substituents for building block applications (TMS-ethyne, ethyne, iodo, ester). The absence of any detectable scrambling in the self-condensation enables a simple purification. The synthesis readily affords gram quantities of pure, sterically unhindered trans-porphyrins in a process involving minimal chromatography.     

Rational or statistical routes from 1-acyldipyrromethanes to meso-substituted porphyrins. Distinct patterns, multiple pyridyl substituents, and amphipathic architectures

New methodology is described for the synthesis of porphyrins bearing four (A 4, cis-A 2B 2, cis-ABC 2, trans-A 2B 2) or fewer (A, cis-AB, cis-A 2, trans-A 2) meso substituents. The method entails condensation of two 1-acyldipyrromethanes in the presence of a metal salt (MgBr 2, 3 mol equiv) and a noncoordinating base (DBU, 10 mol equiv) in a noncoordinating solvent (toluene) with heating (conventional or microwave irradiation) and exposure to air. The rational synthesis of trans-A 2B 2- or trans-A 2-porphyrins was achieved via condensation of two identical 1-acyldipyrromethanes. The statistical synthesis of various meso-substituted porphyrins was achieved via condensation of two nonidentical 1-acyldipyrromethanes. Both routes possess attractive features including (1) no scrambling, (2) good yield (up to 60%) at high concentration (100 mM) for the macrocycle-forming step, (3) reasonable scope (aryl, heteroaryl, alkyl, or no substituent), (4) short reaction time ( approximately 2 h) via microwave irradiation, (5) magnesium porphyrins as the products, which easily undergo demetalation, and (6) facile chromatographic purification. A key advantage of the statistical route is to obtain a cis-substituted porphyrin without the corresponding trans isomer. For example, reaction of an A/B-substituted 1-acyldipyrromethane and the fully unsubstituted 1-formyldipyrromethane gave the magnesium chelates of three porphyrins: the trans-A 2B 2-porphyrin, the "hybrid" cis-AB-porphyrin, and porphine (no trans-AB-porphyrin can form), which were readily demetalated and separated as the free base species. Altogether 26 1-acyldipyrromethanes and 26 target porphyrins have been prepared, including many with two different pyridyl substituents. One set of amphipathic porphyrins includes cis-A 2B 2- or cis-A 2BC-porphyrins wherein A = pentyl and B/C = pyridyl ( o-, m-, p-). Taken together, the rational and statistical routes enable facile conversion of readily available 1-acyldipyrromethanes to diverse porphyrins bearing 1-4 meso substituents for which access is limited via other methods.     

A tin-complexation strategy for use with diverse acylation methods in the preparation of 1,9-diacyldipyrromethanes

The acylation of dipyrromethanes to form 1,9-diacyldipyrromethanes is an essential step in the rational synthesis of porphyrins. Although several methods for acylation are available, purification is difficult because 1,9-diacyldipyrromethanes typically streak extensively upon chromatography and give amorphous powders upon attempted crystallization. A solution to this problem has been achieved by reacting the 1,9-diacyldipyrromethane with Bu(2)SnCl(2) to give the corresponding dibutyl(5,10-dihydrodipyrrinato)tin(IV) complex. The reaction is selective for dipyrromethanes that bear acyl groups at both the 1- and 9-positions but otherwise is quite tolerant of diverse substituents. The diacyldipyrromethane-tin complexes are stable to air and water, are highly soluble in common organic solvents, crystallize readily, and chromatograph without streaking. Four methods (Friedel-Crafts, Grignard, Vilsmeier, benzoxathiolium salt) were examined for the direct 1,9-diacylation of a dipyrromethane or the 9-acylation of a 1-acyldipyrromethane. In each case, treatment of the crude reaction mixture with Bu(2)SnCl(2) and TEA at room temperature enabled facile isolation of multigram quantities of the 1,9-diacyldipyrromethane-tin complex. The diacyldipyrromethane-tin complexes could be decomplexed with TFA in nearly quantitative yield. Alternatively, use of a diacyldipyrromethane-tin complex in a porphyrin-forming reaction (reduction with NaBH(4), acid-catalyzed condensation with a dipyrromethane, DDQ oxidation) afforded the desired free base porphyrin in yield comparable to that obtained from the uncomplexed diacyldipyrromethane. The acylation/tin-complexation strategy has been applied to a bis(dipyrromethane) and a porphyrin-dipyrromethane. In summary, the tin-complexation strategy has broad scope, is compatible with diverse acylation methods, and greatly facilitates access to 1,9-diacyldipyrromethanes.     

Facile synthesis of meso-substituted dipyrromethanes and porphyrins using cation exchange resins

The macroporosity and acidity of cation exchange resins play a crucial role in the synthesis of dipyrromethanes and porphyrins; for the first time, cation exchange resins have been used as heterogeneous solid acid catalysts to produce dipyrromethanes and porphyrins in good yields. The reaction, at room temperature, of substituted aromatic aldehydes with pyrrole catalysed by cation exchange resin afforded the corresponding meso-substituted dipyrromethane in yields ranging from 70 to 80%, indicating the broad scope of the reaction. Further, the condensation of meso-substituted dipyrromethane with similar or different substituted aromatic aldehydes, using cation exchange resins furnished meso-tetrakis-symmetrical and mixed porphyrins, respectively. One-pot synthesis of porphyrins can also be carried out directly from the aldehydes and pyrrole under the above conditions. Acidolysis of the dipyrromethane is negligible under the conditions of the porphyrin-forming reaction.     

An Efficient Synthesis of Porphyrins with Different meso Substituents that Avoids Scrambling in Aqueous Media

We have developed new conditions that afford regioisomerically pure trans‐A₂B₂‐, A₃B‐, and trans‐AB₂C‐porphyrins bearing aryl and arylethynyl substituents. The porphyrins were prepared by the acid‐catalyzed condensation of dipyrromethanes with aldehydes followed by oxidation with p‐chloranil or 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ). Optimal conditions for the condensation were identified after examining various reaction parameters such as solvent composition, acid concentration, and reaction time. The conditions identified (for aromatic aldehydes: EtOH/H₂O 4:1, [DPM]=4 mM , [aldehyde]=4 mM , [HCl]=38 mM , 16 h; for arylethynyl aldehydes: THF/H₂O 2:1, [DPM]=13 mM , [aldehyde]=13 mM , [HCl]=150 mM , 3 h) resulted in the formation of porphyrins in yields of 9–38 % without detectable scrambling. This synthesis is compatible with diverse functionalities such as ester or nitrile. In total, 20 new trans‐A₂B₂‐, A₃B‐, and trans‐AB₂C‐porphyrins were prepared. The scope and limitations of the two sets of reaction conditions have been explored. The methodological advantage of this approach is its straightforward access to building blocks and the formation of the porphyrin core in higher yields than by any other methodology and by using environmentally benign and nonhazardous chemicals.     

Synthesis of meso-tetraphenyl porphyrins via condensation of dipyrromethanes with N-tosyl imines

A new synthetic route for the synthesis of 5,10,15,20-tetraphenyl porphyrins has been developed based on the reaction of 5-substituted dipyrromethanes with N-tosyl imines in the presence of a metal triflate catalyst. meso-Substituted tetraphenyl porphyrins were synthesized in a two-step process. The first step of the method is the metal triflate-catalyzed condensation of 5-substituted dipyrromethanes with N-tosyl imines to form a porphyrinogen intermediate and the second step is the oxidation of the porphyrinogen to porphyrin. The method was applied to the synthesis of trans-A₂B₂-tetraarylporphyrins and the products were obtained with only a trace amount of one scrambling product. The synthesis of two important building blocks for porphyrin synthesis, mono and di-sulfonamide alkylated 5-substituted dipyrromethanes, was achieved by the addition of 5-substituted dipyrromethane to N-tosyl imine. The application of mono and di-sulfonamide alkylated 5-substituted dipyrromethanes in ‘2+2’ porphyrin formation reactions is presented.     

Formation of porphyrins in the presence of acid-labile metalloporphyrins: a new route to mixed-metal multiporphyrin arrays

The ability to incorporate distinct metalloporphyrins at designated sites in multiporphyrin arrays is essential for diverse applications in materials and biomimetic chemistry. The synthesis of such mixed-metal arrays via acid catalyzed reactions has largely been restricted to metalloporphyrins of stability class II (e.g., Cu, Co, Ni) or I. We describe routes for the rational synthesis of mixed-metal arrays via acid-catalyzed condensations that are compatible with metalloporphyrins of stability class III (e.g., Zn) and IV (e.g., Mg). The routes are demonstrated for p-phenylene-linked arrays. The key finding is that several mild Lewis acids [InCl(3), Sc(OTf)(3), Yb(OTf)(3), and Dy(OTf)(3)], which are known to catalyze the dipyrromethane + dipyrromethane-dicarbinol condensation in CH(2)Cl(2) at room temperature without acidolysis, do not demetalate zinc or magnesium porphyrins under the same conditions. Rational routes to porphyrin dyads and triads employ reaction of a (porphyrin)-dipyrromethane and a (porphyrin)-dipyrromethane-dicarbinol. The porphyrin-forming reactions (six examples) proceed in yields of 18-28%. The metalation states of the arrays prepared in this manner include Zn-free base (ZnFb), MgFb, ZnFbMg, ZnFbZn, and ZnFbFb. Studies of the catalysis process indicate that the dipyrromethane + dipyrromethane-dicarbinol condensation is catalyzed by both the Lewis acid and a Br?nsted acid derived in situ from the Lewis acid. Taken together, the ability to employ otherwise "acid-labile" metalloporphyrins as precursors in condensation procedures should broaden the scope of accessible mixed-metal multiporphyrin arrays and motivate further studies of the application of mild Lewis acid catalysts in porphyrin chemistry.     

Masked imidazolyl-dipyrromethanes in the synthesis of imidazole-substituted porphyrins

Imidazole-substituted metalloporphyrins are valuable for studies of self-assembly and for applications where water solubility is required. Rational syntheses of porphyrins bearing one or two imidazol-2-yl or imidazol-4-yl groups at the meso positions have been developed. The syntheses employ dipyrromethanes, 1-acyldipyrromethanes, and 1,9-diacyldipyrromethanes bearing an imidazole group at the 5-position. The polar, reactive imidazole unit was successfully masked by use of (1) the 2-(trimethylsilyl)ethoxymethyl (SEM) group at the imidazole pyrrolic nitrogen, and (2) a dialkylboron motif bound to the pyrrole of the dipyrromethane and coordinated to the imidazole imino nitrogen. The nonpolar nature of such doubly masked imidazolyl-dipyrromethanes facilitated handling. Selected masked dipyrromethanes were characterized by 11B and 15N NMR spectroscopy. Five distinct methods were examined to obtain trans-A2B2-, trans-AB2C-, and trans-AB-porphyrins. Each porphyrin contained one or two SEM-protected imidazole units. The SEM group could be removed with TBAF or HCl. Two zinc(II) porphyrins and a palladium(II) porphyrin bearing a single imidazole moiety were prepared and subjected to alkylation (with ethyl iodide, 1,3-propane sultone, or 1,4-butane sultone) to give water-soluble imidazolium- porphyrins. This work establishes the foundation for the rational synthesis of a variety of porphyrins containing imidazole units.     

Synthesis of mono- and disubstituted porphyrins: A- and 5,10-A2-type systems

General syntheses have been developed for meso-substituted porphyrins with one or two substituents in the 5,10-positions and no beta substituents. 5-Substituted porphyrins with only one meso substituent are easily prepared by an acid-catalyzed condensation of dipyrromethane, pyrrole-2-carbaldehyde, and an appropriate aldehyde using a "[2+1+1]" approach. Similarly, 5,10-disubstituted porphyrins are accessible by simple condensation of unsubstituted tripyrrane with pyrrole and various aldehydes using a "[3+1]" approach. The yields for these reactions are low to moderate and additional formation of either di- or monosubstituted porphyrins due to scrambling of the intermediates is observed. However, the reactions can be performed quite easily and the desired target compounds are easily removed due to large differences in solubility. A complementary and more selective synthesis involves the use of organolithium reagents for S(N)Ar reactions. Reaction of in situ generated porphyrin (porphine) with 1.1-8 equivalents of RLi gave the monosubstituted porphyrins, while reaction with 3-6 equivalents of RLi gave the 5,10-disubstituted porphyrins in yields ranging from 43 to 90 %. These hitherto almost inaccessible compounds complete the series of different homologues of A-, 5,15-A(2)-, 5,10-A(2)-, A(3)-, and A(4)-type porphyrins and allow an investigation of the gradual influence of type, number, and regiochemical arrangement of substituents on the properties of meso-substituted porphyrins. They also present important starting materials for the synthesis of ABCD porphyrins and are potential synthons for supramolecular materials requiring specific substituent orientations.     

Effects of aldehyde or dipyrromethane substituents on the reaction course leading to meso-substituted porphyrins

To better understand the effects of diverse substituents on reactions leading to porphyrins, pyrrole+aldehyde condensations and related reactions of dipyrromethanes were examined. The course of pyrrole+aldehyde condensations was investigated by monitoring the yield of porphyrin (by UV-Vis spectroscopy), reaction of aldehyde (by TLC), and changes in the composition of oligomers (by laser desorption mass spectrometry). Reaction reversibility was examined via exchange experiments. Reversibility of reactions leading to porphyrin was further probed with studies of dipyrromethanes. The reaction course was found to depend on the nature of the substituent and the acid catalyst. Alkyl or electron-donating substituents displayed levels of reversibility (exchange/scrambling) on par or greater than that of the phenyl substituent, whereas electron-withdrawing or sterically bulky substituents exhibited little to no reversibility. The results obtained provide insight into the electronic and steric effects of different substituents and should facilitate the design of synthetic plans for preparing porphyrinic macrocycles.     

1,9-Bis(N,N-dimethylaminomethyl)dipyrromethanes in the synthesis of porphyrins bearing one or two meso substituents

A synthesis of 5,15-disubstituted zinc-porphyrins has been developed that employs condensation of a 1,9-bis(N,N-dimethylaminomethyl)dipyrromethane+a dipyrromethane in refluxing ethanol containing zinc acetate followed by oxidation with DDQ. The N,N-dimethylaminomethylation of the dipyrromethane was achieved via Eschenmoser's reagent (N,N-dimethylmethyleneammonium iodide) in CH₂Cl₂ at room temperature. The synthesis is compatible with diverse substituents (e.g., alkyl, aryl, ester, acetal) and enables rapid synthesis of trans-AB-, A₂-, and A-porphyrins. The synthesis of >40 zinc porphyrins has been surveyed; 13 zinc porphyrins were isolated in yields of 5-20% without detectable scrambling.     

Remarkable isolation, structural characterisation and electrochemistry of unexpected scrambling analogues of 5-ferrocenyl-10,20-diphenylporphyrin

Selective condensation of 5-ferrocenyldipyrromethane, 1, and dipyrromethane, 2, with benzaldehyde, 3, led to 5-ferrocenyl-10,20-diphenylporphyrin, 5. During the condensation, an unusually large amount of scrambling was observed which led to the isolation of two further ferrocenylated porphyrin analogues 6 and 7. The structure of 6 was confirmed by a single-crystal X-ray study. A mechanism is proposed for this atypical scrambling which is likely to involve acid-catalysed reversion of the dipyrromethane synthesis. (1)H NMR further elucidated the structures of each complex and showed the existence of atropisomerism. An electrochemical study (cyclic voltammetry, Osteryoung square wave and linear sweep voltammetry) showed that there exists a linear relationship between the sum of the group electronegativities of meso substituents of the obtained porphyrins and the formal reduction potentials of the two observed ring-centred reduction processes, the meso substituent ferrocenyl-based oxidation process and the first ring-centred oxidation wave. These four relationships could be mathematically quantified. Due to the strong electron-withdrawing properties of the oxidised ferrocenium group, the second ring centred oxidation wave fell outside the potential window of dichloromethane as solvent.     

Porphyrin architectures bearing functionalized xanthene spacers

A modular synthetic strategy for the construction of cofacial porphyrin architectures bearing hydrogen-bond synthons on a xanthene platform is presented. The convergent approach is based on a xanthene aldehyde-ester building block that is easily obtainable on a multigram scale with minimal purification. Treatment of this xanthene derivative with a variety of aryl aldehydes and pyrrole under standard Lindsey conditions affords a family of meso-substituted porphyrins bearing a single functionalized xanthene spacer. Direct modification of the hydrogen-bond synthon after macrocyclization proceeds smoothly to furnish porphyrin systems with a variety of cofacial functionalities (e.g., carboxylic acid, ester, amide). Porphyrins bearing two trans-functionalized xanthene spacers are prepared by the MacDonald [2 + 2] condensation of the xanthene aldehyde-ester with readily available 5-aryl-substituted dipyrromethanes such as 5-mesityldipyrromethane to afford the pure alpha,alpha- and alpha,beta-porphyrin atropisomers after chromatographic separation. The versatility of this synthetic method offers intriguing opportunities for the use of these and related templates for the study of proton-coupled activation of small molecules.     

A simple and versatile one-pot synthesis of meso-substituted trans-A2B-corroles

We have developed a new methodology that affords regioisomerically pure trans-A2B-corroles. The corrole formation reaction involves the acid-catalyzed condensation of a dipyrromethane and an aldehyde followed by oxidation with DDQ. Optimal conditions for the condensation were identified after examining various reaction parameters (solvent, acid, concentration, time). The conditions identified (CH2Cl2, [DPM] = 33 mM, [aldehyde] = 17 mM, [TFA] = 1.3 mM (for sterically hindered DPMs) or [TFA] = 0.26 mM (for sterically unhindered DPMs), 5 h, room temperature) resulted in the formation of corroles in 3-25% yield without detectable scrambling. The synthesis is compatible with diverse functionalities: ester, nitrile, ether, fluoro, hydroxy, iodo, nitro, thioacetate, methylsulfoxy. In total 21 corroles of type trans-A2B were prepared. Three exemplary corrole syntheses were successfully carried out at 8 mmol scale. Corroles 23, 30, and 41 (160-600 mg) were obtained in essentially the same yield as in small scale experiments.     

Use of orthoesters in the synthesis of meso-substituted porphyrins

The use of two orthoesters, trimethyl orthoacetate and trimethyl orthobenzoate, in the synthesis of porphyrins from 5-phenyldipyrromethanes is described. Previously unreported 5,15-diphenyl-10,20-dimethyl porphyrins can be accessed conveniently by this route. A relationship between the steric bulk of the orthoester and the amount of scrambling of the porphyrin products has been found. Strong electron-withdrawing substituents on the phenyl ring of the dipyrromethane also enhance scrambling.     

Synthesis of 5,15-diaryltetrabenzoporphyrins

A general method of synthesis of 5,15-diaryltetrabenzoporphyrins (Ar 2TBPs) has been developed, based on 2 + 2 condensation of dipyrromethanes followed by oxidative aromatization. Two pathways to Ar 2TBPs were investigated: the tetrahydroisoindole pathway and the dihydroisoindole pathway. In the tetrahydroisoindole pathway, precursor 5,15-diaryltetracyclohexenoporphyrins (5,15-Ar 2TCHPs) were assembled from cyclohexeno-fused meso-unsubstituted dipyrromethanes and aromatic aldehydes or, alternatively, by way of the classical MacDonald synthesis. In the first case, scrambling was observed. Aromatization by tetracyclone was more effective than aromatization by DDQ but failed in the cases of porphyrins with electron-withdrawing substituents in the meso-aryl rings. The dihydroisoindole pathway was found to be much superior to the tetrahydroisoindole pathway, and it was developed into a general preparative method, consisting of (1) the synthesis of 4,7-dihydroisoindole and its transformation into meso-unsubstituted dipyrromethanes, (2) the synthesis of 5,15-diaryloctahydrotetrabenzoporphyrins (5,15-Ar 2OHTBPs), and (3) their subsequent aromatization by DDQ. Ar 2TBP free bases exhibit optical absorption spectra similar to those of meso-unsubstituted tetrabenzoporphyrins and fluoresce with high quantum yields. Pd complex of Ph 2TBP was found to be highly phosphorescent at room temperature.     

Simple route to meso-substituted trans-A2B2-porphyrins bearing pyridyl units

We have developed a methodology that affords regioisomerically pure trans-A₂B₂-porphyrins bearing pyridyl substituents. The optimal conditions for their synthesis were identified by the modification of known conditions for the reaction of dipyrromethanes with aromatic aldehydes. A total of five new porphyrins were synthesized.     

Base-catalyzed direct conversion of dipyrromethanes to 1,9-dicarbinols: a [2 + 2] approach for porphyrins

A variant of the MacDonald approach was devised for the synthesis of porphyrins. A new base-catalyzed one-step synthesis of 1,9-dipyrromethane-dicarbinols was achieved by Friedel-Crafts alkylation of dipyrromethanes using commercially available ethyl glyoxylate solution in toluene. This method avoids the use of acid chlorides, Grignard reagents, borohydride reductions, and acidic conditions. The [2 + 2] condensation of dipyrromethanedicarbinols and dipyrromethanes yielded 5,15-di(ethoxycarbonyl)porphyrins.     

Synthesis of meso-substituted porphyrins carrying carboranes and oligo(ethylene glycol) units for potential applications in boron neutron capture therapy

Selective delivery of 10B to tumours is one of the major remaining problems in boron neutron capture therapy (BNCT) of cancer. Porphyrins are selectively accumulated in tumours. Thus two series of carborane-carrying porphyrins were constructed, with additional functionality for attachment of uncharged potentially water-solubilising polyethers. 3-(1,2-Dicarbaclosododecaboran(12)-1-ylmethoxy)benzaldehyde was prepared by protection of the aldehyde of 3-(prop-2-ynyloxy)benzaldehyde as a dithioacetal, treatment with decaborane(14) and deprotection. Condensation with a 3-nitrophenyldipyrromethane gave a separable mixture of meso-(3-nitrophenyl)-meso-(3-carboranylmethoxyphenyl)porphyrins, resulting from extensive scrambling at the porphyrinogen stage. Similarly, condensation of 3-(1,2-dicarbaclosododecaboran(12)-1-yl)benzaldehyde with this dipyrromethane gave an analogous mixture of meso-(3-nitrophenyl)-meso-(3-carboranylphenyl)porphyrins. In this second series, the two regioisomeric bis(nitrophenyl)bis(carboranylphenyl)porphyrins could only be distinguished by X-ray crystallography, their NMR spectra being identical. The nitro groups of the mono(nitrophenyl)porphyrins and the bis(nitrophenyl)-porphyrins were reduced to the corresponding amines with tin(II) chloride and the monoamines were coupled with a omega-methoxy poly(ethylene glycol) chloroformate of mean MW 600 to give the MeOPEGylated tricarboranyl porphyrins.     

Efficient synthesis of meso-substituted corroles in a H2O-MeOH mixture

New and efficient conditions for the synthesis of meso-substituted corroles were developed. The first step, involving the reaction of aldehydes with pyrrole, was carried out in a water-methanol mixture in the presence of HCl. A relatively narrow distribution of aldehyde-pyrrole oligocondensates was obtained through careful control of their solubility in the reaction medium. After thorough optimization of various reaction parameters (cosolvent, reagent, and acid concentration), high yields of bilanes were obtained. Additionally, the bilane derived from 4-cyanobenzaldehyde was isolated, and the oxidative macrocyclization reaction was performed under various reaction conditions (different solvents, different concentrations, and various oxidants). As a result, triphenylcorrole was obtained in the highest yield (32%) reported to date. The scope and limitations studied showed that this method was particularly efficient for moderately reactive aldehydes and those bearing electron-donating groups (yields 14-27%). Using these conditions, corroles bearing strongly electron-donating groups were obtained for the first time. In addition, it was found that the reaction of unhindered dipyrromethanes with aldehydes under analogous conditions afforded trans-A2B-corroles in very high yields (45-56%; 8-fold higher than previously reported) without scrambling. The fact that scrambling was not observed in this reaction despite a very high HCl concentration (0.3 M) is unprecedented. Detailed studies on the oxidation of bilane, derived from sterically hindered dipyrromethane, allowed us to unequivocally establish that the yield of macrocyclization is insensitive to the concentration. It was found the 1H NMR spectra of corroles in deuterated TFA gave very sharp signals.