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Indole-containing natural products have long been an inspiration to synthetic chemists.  However, many chemists are unaware that one of the most visible natural products, melanin pigments in human skin, hair, and eyes, are indole-based materials.  Eumelanin, the brown to black pigment in humans, is known to be composed of dihydroxyindole monomers; these are thought to form relatively short covalent oligomers that self-assemble into nanoparticles. The oligomers are composed of four to eight monomers and are heterogeneous in nature.  Such oligomers are within the range of modern organic synthesis, similar to oligoarene foldamers.  The ability to produce well-defined synthetic oligomers will advance our knowledge of this common but poorly understood biomaterial.  This grant focuses on developing the synthetic methods necessary to generate indole and dihydroxyindole oligomers of relevance to eumelanin.

Palladium catalyzed cross-coupling of aryl boronic acids with aryl halides––the Suzuki-Miyaura reaction––is a central method for constructing aryl-aryl carbon bonds, and is increasingly being adapted for oligoarene synthesis.  However, indole-indole Suzuki couplings are rare; thus, we began by examining indole dimerization.  Prior to the initiation of PRF funding, we had investigated Zhang and co-workers’ method¹ for ligand-free Suzuki reactions. In our hands, this method was inefficient for the desire coupling, providing 4-4’ biindolyl in 21% yield from 4-bromoindole with 4-boronic acid indole.  However, when 4-bromoindole was serendipitously left out of the reaction mixture we observed palladium black precipitation, a hallmark of reactivity in such reactions.  In fact, repeating the serendipitous reaction with an increased catalyst loading of 5 mol % Pd(OAc)₂ provided the desired  4-4’-biindolyl 1 in 69% yieldas a consequence of palladium-catalyzed homocoupling.

During the initial period of the grant, we focused on optimization of this reaction.  We are about to submit the results of this study to Tetrahedron (manuscript in preparation, three undergraduate co-authors).  In particular, following the work of Kabalka,² we found that including tosyl chloride (0.5 eq. relative to boron) in the reaction of 4-boronic acid indole yielded 4-4’ biindolyl in 81% yield.  Similarly, 5-5’ biindolyl and 6-6’ biindolyl were obtained in quantitative yield from 5- and 6-boronic acid indole, respectively, under the same conditions.  The homocoupling reactions are fast and operationally simple, employing air, water, and room temperature.  

While we optimized the homocoupling reaction, there was a significant advance in the Suzuki cross-coupling of indoles.  Using the XPhos ligand developed by Buchwald,³ in pioneering work Huleatt, Chai, and co-workers developed palladium-catalyzed borylation and cross-coupling conditions for biindolyls, generating both symmetric and non-symmetrical 5,6-dimethoxyindole dimers.⁴   We began studying cross-coupling reactions using the XPhos ligand this January, and quickly were able to attain 90% yields.  We are now using iridum-catalyzed C-H activation/borylation⁵ to install one or two boronate groups per indole.  These can be further transformed to halogens using and copper-mediated boron-to-bromide functional group exchange.⁶  A two-step, one-pot procedure provides 4,7-dihalogenated indoles useful for iterative synthesis in 87% yield.  In our hands this is much superior to the Bartoli reaction, a current route to 4,7-subsituted indoles.⁷  We are currently generating indole trimers using the diborylated and dibrominated indoles;  exploring the (random) oligomerization of diborylated and dibrominated indoles; and optimized the synthesis of 4-boronate-7-bromo indoles, which will serve as monomers for well-controlled iterative synthesis.

In addition to our synthetic efforts, we embarked on another melanin project that has quickly yielded an initial publication.⁸  Working with both melanin and boronic acids, we wondered if boronic acids could be used to inhibit melanin formation.  In fact, boronophenylalanine, originally considered by Mishima and co-workers⁹ as a mimic of the tyrosinase (the key enzyme involved in melanogenesis in vivo) substrates tyrosine and L-dopa, is a known melanogenesis inhibition.  Interestingly, it doesn’t act as a direct tyrosinase inhibitor, instead it inhibits melaninogenesis by binding to dihydroxyindole intermediates via boronic acid-diol covalent adduct formation.  I developed a simple spectroscopic assay to study this interaction with using commercially available aryl boronic acids.⁸  While I performed the experiments for the first paper myself, undergraduates can easily perform these kinetics measurements.  We started to investigate a series of heteroaryl boronic acids this summer, with benzothiophene 2-boronic acid so far the most potent inhibitor.  Potent compounds will be tested for their ability to inhibit melanogenesis in cell culture.

References:

 1.  Lui, L.; Zhang, Y.; Xin, B. “Synthesis of Biaryls and Polyaryls by Ligand-Free Suzuki Reaction in Aqueous Phase”  J. Org. Chem. 2006, 71, 3994-3997.

2.  Kabalka, G. W.; Wang, L.  “Ligandless palladium chloride-catalyzes homo-coupling of arylboronic acids in aqueous media” Tetrahedron Lett. 2002, 43, 3067-3068.

3.  Billingsley, K.; Buchwald, S. L. “Highly Efficient Monophosphine-Based Catalyst for the Palladium-Catalyzed Suzuki-Miyaura Reaction of Heteroaryl Halides and Heteroaryl Boronic Acids and Esters” J. Am. Chem. Soc. 2007, 129, 3358-3366. 

4.  Duong, H. A.; Chua, S.; Huleatt, P. B.; Chai, C. L. L.  “Synthesis of Biindolyls via Palladium-Catalyzed Reactions” J. Org. Chem. 2008, 73, 9177-9180.

5.   Lo, W. F.; Kaiser, H. M.; Spannenberg, A.; Beller, M.; Tse, M. K. “A highly selective Ir-catalyzed borylation of 2-substituted indoles: a new access to 2,7- and 2,4,7-substituted indoles” Tetrahedron Lett. 2007, 48, 371-375.

6. Thompson, A. L. S., Kabalka, G. W.; Akula, M. R.; Huffman, J. W. “The Conversion of Phenols to the Corresponding Aryl Halides Under Mild Conditions” Synthesis 2004, 547-550.

7.  Dobbs, A. P.; Voyle, M.; Whittall, N.; “Synthesis of Novel Indole Derivatives: Variations in the Bartoli Reaction” Synlett 1999, 1594-1596.

8.  Belitsky, J. M. “Aryl boronic acid inhibition of synthetic melanin polymerization” Bioorg. Med. Chem. Lett. 2010, 20, 4475-78.

9.  Mishima, Y.; Kondoh, H.  “Dual control of melanogenesis andd melanoma growth: overview molecular to clinical level and the reverse” Pigment Cell Res. 2000, Suppl. 8, 10-22.