A new synthesis of chlorins

Novel Foscan®-derived ring-fused chlorins for photodynamic therapy of cancer

Photodynamic therapy (PDT) is an established anticancer treatment that combines the use of a photosensitiser (PS) and a light source of a specific wavelength for the generation of reactive oxygen species (ROS) that are toxic to the tumour cells. Foscan® (mTHPC) is a clinically-approved chlorin used for the PDT treatment of advanced head and neck, prostate and pancreatic cancers but is characterized by being photochemically unstable and associated with prolonged skin photosensitivity. Herein, we report the synthesis of new 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine-fused chlorins, having the meso-tetra(3-hydroxyphenyl)macrocycle core of mTHPC, by exploring the [8π + 2π] cycloaddition of a meso-tetra(3-hydroxyphenyl)porphyrin derivative with diazafulvenium methides. These chlorins have photochemical properties similar to Foscan® but are much more photostable. Among the novel compounds, two chlorins with a hydroxymethyl group and its azide derivative present in the 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine-fused system, are promising photodynamic agents with activity in the 100 nM range against triple-negative breast cancer cells and, in the case of azidomethyl chlorin, a safer phototherapeutic index compared to Foscan®.

Synthesis of a BC-Dihydrodipyrrin Building Block of Bacteriochlorophyll a

A strategy for the synthesis of bacteriochlorophyll a relies on joining AD and BC halves that contain the requisite stereochemical configurations of the target macrocycle. The BC half (1) is a dihydrodipyrrin bearing a dimethoxymethyl group at the 1-position, a β-ketoester at the 8-position, and (R)-2-methyl and (R)-3-ethyl substituents in the pyrroline ring. An established route to AD-dihydrodipyrrins (Pd-mediated coupling of a 2-halopyrrole with a chiral 4-pentynoic acid followed by Petasis methenylation, acidic hydrolysis, Paal-Knorr ring closure, and Riley oxidation) proved to be unviable for BC-dihydrodipyrrins given the presence of the β-ketoester unit. A route presented here entails Pd-mediated coupling of a 2-halopyrrole (2) with (3R,4R)-4-ethyl-1,1-dimethoxy-3-methylhex-5-yn-2-one (3), anti-Markovnikov hydration of the alkyne to give the 1,4-diketone, and Paal-Knorr ring closure. Compound 3 was prepared by Schreiber-modified Nicholas reaction beginning with (S)-4-isopropyl-3-propionyloxazolidin-2-one and the hexacarbonyldicobalt complex of (±) 3-methoxy-1-(trimethylsilyl)pentyne followed by transformation of the aldehyde derived therefrom to the 1,1-dimethoxymethylcarbonyl motif. The absolute stereochemical configuration of the Schreiber-Nicholas alkylation product was confirmed by single-crystal X-ray diffraction, whereas the BC half (1) by 1H NMR spectroscopy showed a J value of 2.9 Hz consistent with the trans-configuration. Taken together, the route provides a key chiral building block for the synthesis of photosynthetic tetrapyrroles and analogues.

Shallow- and Deep-Water Ophiura Species Produce a Panel of Chlorin Compounds with Potent Photodynamic Anticancer Activities

A Pacific brittle star Ophiura sarsii has previously been shown to produce a chlorin (3S,4S)-14-Ethyl-9-(hydroxymethyl)-4,8,13,18-tetramethyl-20-oxo-3-phorbinepropanoic acid (ETPA) (1) with potent phototoxic activities, making it applicable to photodynamic therapy. Using extensive LC-MS metabolite profiling, molecular network analysis, and targeted isolation with de novo NMR structure elucidation, we herein identify five additional chlorin compounds from O. sarsii and its deep-sea relative O. ooplax: 10S-Hydroxypheophorbide a (2), Pheophorbide a (3), Pyropheophorbide a (4), (3S,4S,21R)-14-Ethyl-9-(hydroxymethyl)-21-(methoxycarbonyl)-4,8,13,18-tetramethyl-20-oxo-3-phorbinepropanoic acid (5), and (3S,4S,21R)-14-Ethyl-21-hydroxy-9-(hydroxymethyl)-4,8,13,18-tetramethyl-20-oxo-3-phorbinepropanoic acid (6). Chlorins 5 and 6 have not been previously reported in natural sources. Interestingly, low amounts of chlorins 1-4 and 6 could also be identified in a distant species, the basket star Gorgonocephalus cf. eucnemis, demonstrating that chlorins are produced by a wide spectrum of marine invertebrates of the class Ophiuroidea. Following the purification of these major Ophiura chlorin metabolites, we discovered the significant singlet oxygen quantum yield upon their photoinduction and the resulting phototoxicity against triple-negative breast cancer BT-20 cells. These studies identify an arsenal of brittle star chlorins as natural photosensitizers with potential photodynamic therapy applications.

Palladium-Catalyzed Chemo- and Regiocontrolled Tandem Cyclization/Cross-Coupling of 2-Benzyl-3-alkynyl Chromones with Aryl Iodides for the Synthesis of 4H-Furo[3,2-c]chromenes and Xanthones

A novel Pd-catalyzed chemo- and regiocontrolled tandem cyclization/cross-coupling reaction of 3-alkynyl chromone with aryl iodide was developed for the synthesis of 4H-furo[3,2-c]chromenes and xanthones. The difunctionalization of alkynes through O-attack/5-exo-dig and C-attack/6-endo-dig cyclization was reported by this rare approach, which was selectively controlled by the addition of KF or a bidentate phosphine ligand. A one-pot tandem process was demonstrated directly from γ-alkynyl-1,3-diketone for this method.

Synthesis of bacteriochlorins bearing diverse β-substituents

Eleven bacteriochlorins have been prepared for surface attachment, bioconjugation, water-solubilization, vibrational studies, and elaboration into multichromophore arrays.

Synthesis of AD-Dihydrodipyrrins Equipped with Latent Substituents of Native Chlorophylls and Bacteriochlorophylls

Native chlorophylls and bacteriochlorophylls share a common trans-substituted pyrroline ring D (17-propionic acid, 18-methyl), whereas diversity occurs in ring A particularly at the 3-position. Two dihydrodipyrrins equipped with native-like D-ring substituents and tailorable A-ring substituents have been synthesized. The synthesis relies on a Schreiber-modified Nicholas reaction to construct the stereochemically defined precursor to ring D, a dialkyl-substituted pent-4-ynoic acid. The carboxylic acid group of the intact propionic acid proved unworkable, whereupon protected propionate (-CO₂^tBu) and several latent propyl ethers were examined. The tert-butyldiphenylsilyl-protected propanol substituent proved satisfactory for reaction of the chiral N-acylated oxazolidinone, affording (2S,3S)-2-(3-((tert-butyldiphenylsilyl)oxy)propyl)-3-methylpent-4-ynoic acid in ∼30% yield over 8 steps. Two variants for ring A, 2-tert-butoxycarbonyl-3-Br/H-5-iodo-4-methylpyrrole, were prepared via the Barton-Zard route. Dihydrodipyrrin formation from the pyrrole and pentynoic acid entailed Jacobi Pd-mediated lactone formation, Petasis methenylation, and Paal-Knorr-type pyrroline formation. The two AD-dihydrodipyrrins bear the D-ring methyl and protected propanol groups with a stereochemical configuration identical to that of native (bacterio)chlorophylls, and a bromine or no substitution in ring A corresponding to the 3-position of (bacterio)chlorophylls. The analogous β-position of a lactone-pyrrole intermediate on the path to the dihydrodipyrrin also was successfully brominated, opening opportunities for late-stage diversification in the synthesis of (bacterio)chlorophylls.

Demonstration of the utility of DOS-derived fragment libraries for rapid hit derivatisation in a multidirectional fashion

Organic synthesis underpins the evolution of weak fragment hits into potent lead compounds. Deficiencies within current screening collections often result in the requirement of significant synthetic investment to enable multidirectional fragment growth, limiting the efficiency of the hit evolution process. Diversity-oriented synthesis (DOS)-derived fragment libraries are constructed in an efficient and modular fashion and thus are well-suited to address this challenge. To demonstrate the effective nature of such libraries within fragment-based drug discovery, we herein describe the screening of a 40-member DOS library against three functionally distinct biological targets using X-Ray crystallography. Firstly, we demonstrate the importance for diversity in aiding hit identification with four fragment binders resulting from these efforts. Moreover, we also exemplify the ability to readily access a library of analogues from cheap commercially available materials, which ultimately enabled the exploration of a minimum of four synthetic vectors from each molecule. In total, 10-14 analogues of each hit were rapidly accessed in three to six synthetic steps. Thus, we showcase how DOS-derived fragment libraries enable efficient hit derivatisation and can be utilised to remove the synthetic limitations encountered in early stage fragment-based drug discovery.

Riley Oxidation of Heterocyclic Intermediates on Paths to Hydroporphyrins-A Review

Riley oxidation of advanced heterocyclic intermediates (dihydrodipyrrins and tetrahydrodipyrrins) is pivotal in routes to synthetic hydroporphyrins including chlorins, bacteriochlorins, and model (bacterio)chlorophylls. Such macrocycles find wide use in studies ranging from energy sciences to photomedicine. The key transformation (–CH₃ → –CHO) is often inefficient, however, thereby crimping the synthesis of hydroporphyrins. The first part of the review summarizes 12 representative conditions for Riley oxidation across diverse (non-hydrodipyrrin) substrates. An interlude summarizes the proposed mechanisms and provides context concerning the nature of various selenium species other than SeO₂. The second part of the review comprehensively reports the conditions and results upon Riley oxidation of 45 1-methyltetrahydrodipyrrins and 1-methyldihydrodipyrrins. A comparison of the results provides insights into the tolerable structural features for Riley oxidation of hydrodipyrrins. In general, Riley oxidation of dihydrodipyrrins has a broad scope toward substituents, but proceeds in only modest yield. Too few tetrahydrodipyrrins have been examined to draw conclusions concerning scope. New reaction conditions or approaches will be required to achieve high yields for this critical transformation in the synthesis of hydroporphyrins.

Azulichlorins and Benzocarbachlorins Derived Therefrom

Acid-catalyzed condensation of azulidipyrrane aldehydes with a dihydrodipyrrin carbaldehyde afforded the first examples of azulichlorins. These macrocyclic products were isolated in a monoprotonated form, and the free bases proved to be somewhat unstable. The monocations were strongly diatropic, and proton NMR spectroscopy showed the internal C-H at ca. -2 ppm. Addition of TFA gave the related dications, but these exhibited significantly reduced aromatic ring currents. Reaction of an azulichlorin with tert-butyl hydroperoxide and KOH in dichloromethane/methanol gave a benzocarbachlorin and two related aldehydes. The UV-vis spectrum for the benzocarbachlorin showed a split Soret band at 414 and 430 nm, together with a strong chlorin-like absorption at 684 nm. The proton NMR spectrum indicated that the carbachlorin is strongly aromatic and the internal C-H was observed at -4.64 ppm. Addition of TFA afforded a C-protonated dication with a significantly increased diatropic ring current. The proton NMR spectrum, NICS calculations, and AICD plots indicated that the system favors a 22π electron delocalization pathway that runs through the fused benzo unit. Addition of TFA to the benzocarbachlorin aldehydes primarily led to the formation of monocations, and the generation of C-protonated dications was no longer favored due to the presence of electron-withdrawing formyl moieties.

Furan- and Thiophene-Based Auxochromes Red-shift Chlorin Absorptions and Enable Oxidative Chlorin Polymerizations

The de novo syntheses of chemically stable chlorins with five-membered heterocyclic (furane, thiophene, formylfurane and formylthiophene) substituents in selected meso- and β-positions are reported. Heterocycle incorporation in the 3- and 13-positions shifted the chlorin absorption and emission to the red (up to λem =680 nm), thus these readily incorporated substituents function analogously to auxochromes present in chlorophylls, for example, formyl and vinyl groups. Photophysical, theoretical and X-ray crystallographic experiments revealed small but significant differences between the behavior of the furan- and the thiophene-based auxochromes. Four regioisomeric bis-thienylchlorins (3,10; 3,13, 3,15 and 10,15) were oxidatively electropolymerized; the chlorin monomer geometry had a profound impact on the polymerization efficiency and the electrochemical properties of the resulting material. Chemical co-polymerization of 3,13-bis-thienylchlorin with 3-hexylthiophene yielded an organic-soluble red-emitting polymer.

Synthesis and photophysical characterization of bacteriochlorins equipped with integral swallowtail substituents

The two pyrroline units of bacteriochlorins can now bear gem-dialkyl or diaryl groups (L), which project above and below the macrocycle plane, whereas dimethyl groups generally have been accessible previously.

Synthesis of tailored hydrodipyrrins and their examination in directed routes to bacteriochlorins and tetradehydrocorrins

18 gem-dimethyl stabilized hydrodipyrrins with diverse α-substituents have been prepared and examined in directed syntheses of unsymmetrically substituted hydroporphyrins.

Synthesis and photophysical characteristics of 2,3,12,13-tetraalkylbacteriochlorins

Tetraalkylbacteriochlorins, available upon acid-mediated self-condensation of α-ester stabilized dihydrodipyrrin-carboxaldehydes, provide valuable models of the naturally occurring bacteriochlorophylls.

Progress Towards Synthetic Chlorins with Graded Polarity, Conjugatable Substituents, and Wavelength Tunability

Advances in chlorin synthetic chemistry now enable the de novo preparation of diverse chlorin-containing molecular architectures. Five distinct molecular designs have been explored here, including hydrophobic bioconjugatable (oxo)chlorins; a hydrophilic bioconjugatable chlorin; a trans-ethynyl/iodochlorin building block; a set of chlorins bearing electron-rich (methoxy, dimethylamino, methylthio) groups at the 3-position; and a set of ten 3,13-disubstituted chlorins chiefly bearing groups with extended π-moieties. Altogether 23 new chlorins (17 targets, 6 intermediates) have been prepared. The challenge associated with molecular designs that encompass the combination of "hydrophilic, bioconjugatable and wavelength-tunable" chiefly resides in the nature of the hydrophilic unit.

Isoxazolidine-fused meso-tetraarylchlorins as key tools for the synthesis of mono- and bis-annulated chlorins

An efficient synthetic approach to prepare novel annulated chlorin derivatives, a meso-tetraarylchlorin bearing an azepine ring and another bearing simultaneously an azepine and a 2H-pyran ring, is described.

How green is green chemistry? Chlorophylls as a bioresource from biorefineries and their commercial potential in medicine and photovoltaics

Chlorophylls are the natural green pigments par excellence and offer potential as therapeutics and in energy generation. This perspective outlines the state-of-the-art, their possible applications and indicates future directions in the context of green chemistry and their production from biorefineries.

The binding of analogs of porphyrins and chlorins with elongated side chains to albumin

In previous studies, we demonstrated that elongation of side chains of several sensitizers endowed them with higher affinity for artificial and natural membranes and caused their deeper localization in membranes. In the present study, we employed eight hematoporphyrin and protoporphyrin analogs and four groups containing three chlorin analogs each, all synthesized with variable numbers of methylenes in their alkyl carboxylic chains. We show that these tetrapyrroles' affinity for bovine serum albumin (BSA) and their localization in the binding site are also modulated by chain lengths. The binding constants of the hematoporphyrins and protoporphyrins to BSA increased as the number of methylenes was increased. The binding of the chlorins depended on the substitution at the meso position opposite to the chains. The quenching of the sensitizers' florescence by external iodide ions decreased as the side chains became longer, indicating to deeper insertion of the molecules into the BSA binding pocket. To corroborate this conclusion, we studied the efficiency of photodamage caused to tryptophan in BSA upon illumination of the bound sensitizers. The efficiency was found to depend on the side-chain lengths of the photosensitizer. We conclude that the protein site that hosts these sensitizers accommodates different analogs at positions that differ slightly from each other. These differences are manifested in the ease of access of iodide from the external aqueous phase, and in the proximity of the photosensitizers to the tryptophan. In the course of this study, we developed the kinetic equations that have to be employed when the sensitizer itself is being destroyed.

Toward a general synthesis of chlorins

Recently, we described a new synthesis of C,D-ring symmetric chlorins 11, involving 2 + 2 condensation of bis-formyl-dihydrodipyrrins 9 with symmetrically substituted dipyrromethane diacids 10 (Method I). However, while versatile in many aspects, Method I was unsuited to the broader goal of synthesizing fully non-symmetric chlorins of general structure 15, which requires regioselective control over the reacting centers in the A,B- and C,D-ring components. In this paper, we describe four new 2 + 2 strategies that accomplish this differentiation (Methods II-V). Of these, Method V, which combines operational simplicity with moderate to high product yields, proved to be the most effective route, exploiting reactivity differences between the two formyl groups of A,B-rings 9 to impart excellent regioselectivity. Methods II-IV are also useful alternatives to Method V, although in some cases, the appropriately functionalized precursors are less readily available. All four approaches generate single regioisomers of diversely substituted chlorins, and in every case, the 2 + 2 condensation is accomplished in a simple, one-flask procedure without need for additives such as oxidizing agents or metals. Taken together, these methodologies provide expanded access to an array of chlorins for SAR studies that may advance the effectiveness of PDT and other applications.

Synthesis of Hydrodipyrrins Tailored for Reactivity at the 1- and 9-Positions

A collection of 33 hydrodipyrrins (9 targets, 21 intermediates, and 3 byproducts) has been prepared. The hydrodipyrrins (dihydrodipyrrins, tetrahydrodipyrrins, and hexahydrodipyrrins) contain a pyrrole ring and a geminal-dimethyl substituted 1-pyrroline (or pyrrolidine) ring. The alpha-substituents on the pyrrole ring (H, Br, CHO) and pyrroline ring (H, CH(3), CH(OR)(2), OMe, SMe) provide different reactivity combinations (Nu(-), E(+)) and 0, 1, or 2 carbon atoms (which can give rise to the bridging meso-carbons in hydroporphyrins). Straightforward access to various hydrodipyrrins should facilitate development of syntheses of diverse hydroporphyrins.

Synthesis of Locked meso-β-Substituted Chlorins via 1,3-Dipolar Cycloaddition

A novel approach toward "locked" chlorins with increased stability has been studied in detail. The chlorin skeleton is assembled in a convergent fashion from two fragments via a porphyrin forming reaction, followed by 1,3-dipolar cycloaddition of azomethine ylides, which are formed in situ. Central to the success of the process is the presence of two electron-withdrawing groups in vicinal positions at the perimeter of the porphyrin. As a result, the 1,3-dipolar cycloaddition took place regioselectively, on the bond activated by two electron-withdrawing groups. Moreover, the chlorins formed are locked and hence more stable because of the presence of two quaternary carbon atoms. Overall, in just six steps locked chlorins were constructed from easily available materials. The large array of functionalities tolerated in this approach validates it for a broad use in more advanced studies. The correlation between the results of the 1,3-dipolar cycloaddition and dipolarophile (porphyrin) LUMO energy was extensively studied. There was a definite correlation between the reaction time and the LUMO energy level, and a partial correlation between the reaction yield and the distribution of the LUMO. Additionally, various approaches toward crucial building blocks, namely 3,4-disubstituted-2,5-diformylpyrroles, were investigated.

Studies in chlorin chemistry. 3. A practical synthesis of c,d-ring symmetric chlorins of potential utility in photodynamic therapy

[reaction: see text] C,D-ring symmetric chlorins 8 were prepared in 47-85% yield, on scales up to several hundred milligrams, by condensation of appropriately substituted bis-formyldihydrodipyrrins 6 and dipyrromethane bis-carboxylic acids 7 in 5% TFA/CH2Cl2 (25 examples). Target chlorins were chosen to systematically probe the effect of lipophilic and hydrophilic substituents on tissue partitioning and cellular membrane penetration in photodynamic therapy (PDT).

Studies in chlorin chemistry. II. A versatile synthesis of dihydrodipyrrins

[reaction: see text] Dihydrodipyrrins are key building blocks for the synthesis of hydroporphyrins, many of which have important biological activity. The title compounds were prepared in stereo- and regioselective fashion by a three-step sequence consisting of (1) Pd(0)-catalyzed coupling-cyclization of 2-iodopyrroles with gamma-alkynoic acids to afford enelactones of the desired substitution pattern, (2) methylenation at the lactone carbonyl group employing the Petasis reagent, and (3) in situ enol-ether hydrolysis and amination of the resultant 1,4-diketone to close the pyrroline ring (nine examples). Yields for each step were generally high, although in substrates not blocked by geminal substitution aromatization to a dipyrromethane is a competing side reaction.

Lewis acid-promoted oxidative addition of thioimidates to Pd0

The isomeric S-methyldihydropyrrins 9-Z and 9-E exhibit markedly different behavior in Pd(0)-catalyzed cross-coupling reactions. Thioimidates 9-Z are readily converted to imines 10-Z employing Pd(0)/AlkZnI. Under identical conditions 9-E are inert. Oxidative addition to Pd(0) requires activation by Zn or other Lewis acids, which is sterically unfavorable with 9-E. Analogous results were obtained with the related thioimidates 11-E,Z as well as with methylthiopyridines 19alpha-gamma. In the case of both 11 and 19 oxidative addition to Pd(0) was greatly facilitated in the presence of BF(3) x Et(2)O. The importance of Lewis acid activation to Pd(0) oxidative addition in such substrates appears to be a general phenomenon not previously recognized.

The rational synthesis of chlorins via rearrangement of porphodimethenes: influence of beta-substituents on the regioselectivity and stereoselectivity of pyrroline ring formation

The porphodimethene rearrangement methodology reported in this paper provides for a rational, step-by-step synthesis of chlorins from readily available pyrrole precursors. The intermediate porphodimethenes are furnished directly via the '2 + 2' MacDonald condensation, or by the less symmetry-constrained '3 + 1' condensation of a tripyrrane and bis-formyl pyrrole. The synthetic route is short and highly convergent, especially in the case of the '3 + 1' approach, and furnishes chlorins in good to moderate yields. The synthesis is highly regioselective and appears to be based on the ability of the beta-substituent to stabilize excess electron density, with an electron-neutral hydrogen or an electron-withdrawing carbonyl beta-substituent demonstrating the greatest influence on the formation of the pyrroline ring. The synthesis is highly stereoselective when epimerization of the pyrroline ring beta-carbons is possible, furnishing only the trans-reduced sterioisomer. Finally, there is substantial evidence that a fifth, axial ligand is involved in the transposition of peripheral hydrogens during the rearrangement of the pi-system from metalloporphodimethene to metallochlorin.

Synthesis of meso-substituted chlorins via tetrahydrobilene-a intermediates

Chlorin building blocks incorporating a geminal dimethyl group in the reduced ring and synthetic handles in specific patterns at the perimeter of the macrocycle are expected to have utility in biomimetic and materials chemistry. A prior route employed condensation of a dihydrodipyrrin (Western half) and a bromodipyrromethane-monocarbinol (Eastern half), followed by oxidative cyclization of the putative dihydrobilene-a to form the meso-substituted zinc chlorin in yields of approximately 10%. The limited stability of the dihydrodipyrrin precluded study of the chlorin-forming process. We now have refined this methodology. A tetrahydrodipyrrin Western half (2,3,4,5-tetrahydro-1,3,3-trimethyldipyrrin) has been synthesized and found to be quite stable. The condensation of the Western half and an Eastern half (100 mM each) proceeded smoothly in CH(3)CN containing 100 mM TFA at room temperature for 30 min. The resulting linear tetrapyrrole, a 2,3,4,5-tetrahydrobilene-a, also is quite stable, enabling study of the conversion to chlorin. Refined conditions for the oxidative cyclization were found to include the following: the tetrahydrobilene-a (10 mM), AgTf (3-5 molar equiv), Zn(OAc)(2) (15 molar equiv), and 2,2,6,6-tetramethylpiperidine (15 molar equiv) in CH(3)CN at reflux exposed to air for 4-6 h, affording the zinc chlorin. The chlorin-forming process could be implemented in either a two-flask process or a one-flask process. The two-flask process was applied to form six zinc chlorins bearing substituents such as pentafluorophenyl, 3,5-di-tert-butylphenyl, TMS-ethyl benzoate, iodophenyl, or ethynylphenyl (deprotection of the TMS-ethynyl group occurred during the oxidative cyclization process). The stepwise yields (isolated) for the condensation and oxidative cyclization processes forming the tetrahydrobilene and zinc chlorin were 32-72% and 27-62%, respectively, giving overall yields of zinc chlorin from the Eastern and Western halves of 12-45%. Taken together, the refinements introduced enable 100-mg quantities of chlorin building blocks to be prepared in a facile and rational manner.