Deborah A. Smithen

Use of F-BODIPYs as a Protection Strategy for Dipyrrins: Optimization of BF2 Removal

We recently reported the first general method for the deprotection of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes (F-BODIPYs) involving a microwave-assisted procedure for the removal of the BF(2) moiety, and liberation of the corresponding free-base dipyrrin. Further optimization of the reaction has resulted in a more convenient and accessible protocol. The availability of this new methodology enables BF(2)-complexation to be used as a dipyrrin protection strategy. Herein lies a detailed examination of the deprotection reaction, with a view to optimization and gaining mechanistic insight, and its application in facilitating a multistep synthesis of pyrrolyldipyrrins.

Eight-membered ring-containing jadomycins: implications for non-enzymatic natural products biosynthesis.

Jadomycin Oct (1) was isolated from Streptomyces venezuelae ISP5230 and characterized as a structurally unique eight-membered l-ornithine ring-containing jadomycin. The structure was elucidated through the semisynthetic derivatization of starting material via chemoselective acylation of the l-ornithine α-amino group using activated succinimidyl esters. Incorporation of 5-aminovaleric acid led to jadomycin AVA, a second eight-membered ring-containing jadomycin. These natural products illustrate the structural diversity permissible from a non-enzymatic step within a biosynthetic pathway and exemplifies the potential for discovery of novel scaffolds.

Antimicrobial activity of non-natural prodigiosenes

Tripyrrolic prodigiosenes, derivatives of the natural product prodigiosin, have been produced via multi-step synthesis beginning with 2-formyl pyrroles bearing various functionalities at the 4-position. Two tin complexes are also reported, and these feature a prodigiosene ligand bearing a conjugated benzyl-ester. Antimicrobial activities of prodigiosenes are evaluated against Gram positive and Gram negative bacterial strains, as well as a yeast.

An improved method for the synthesis of F-BODIPYs from dipyrrins and bis(dipyrrin)s.

An improved methodology for the synthesis of F-BODIPYs from dipyrrins and bis(dipyrrin)s is reported. This strategy employs lithium salts of dipyrrins as intermediates that are then treated with only 1 equiv of boron trifluoride diethyletherate to obtain the corresponding F-BODIPYs. This scalable route to F-BODIPYs renders high yields with a facile purification process involving merely filtration of the reaction mixture through Celite in many cases.

One-Pot Synthesis of Asymmetric Annulated Bis(pyrrole)s

Condensation of activated functionalized pyrroles with acetone results in asymmetric bis(pyrrole)s, formed via ring annulation. The methodology is somewhat general and can be applied to a variety of ketones, as well as to a range of pyrrolic substrates that do not bear electron-withdrawing groups directly adjacent to the pyrrole ring.

Synthesis and Photobiological Activity of Ru(II) Dyads Derived from Pyrrole-2-Carboxylate Thionoesters

The synthesis and characterization of a series of heteroleptic ruthenium(II) dyads derived from pyrrole-2-carboxylate thionoesters are reported. Ligands bearing a conjugated thiocarbonyl group were found to be more reactive toward Ru(II) complexation compared to analogous all-oxygen pyrrole-2-carboxylate esters, and salient features of the resulting complexes were determined using X-ray crystallography, electronic absorption, and NMR spectroscopy. Selected complexes were evaluated for their potential in photobiological applications, whereupon all compounds demonstrated in vitro photodynamic therapy effects in HL-60 and SK-MEL-28 cells, with low nanomolar activities observed, and exhibited some of the largest photocytotoxicity indices to date (>2000). Importantly, the Ru(II) dyads could be activated by relatively soft doses of visible (100 J cm-2, 29 mW cm-2) or red light (100 J cm-2, 34 mW cm-2), which is compatible with therapeutic applications. Some compounds even demonstrated up to five-fold selectivity for malignant cells over noncancerous cells. These complexes were also shown to photocleave, and in some cases unwind, DNA in cell-free experiments. Thus, this new class of Ru(II) dyads has the capacity to interact with and damage biological macromolecules in the cell, making them attractive agents for photodynamic therapy.

Probing the hydrolytic reactivity of 2-difluoromethyl pyrroles

α-Difluoromethyl pyrroles were found to be stable while N-protected with an electron-withdrawing group. Due to the propensity of pyrroles to access azafulvenium-like intermediates, the C-F bonds of an α-difluoromethyl substituent are labile under hydrolytic conditions. The presence of certain electron-withdrawing substituents about the pyrrolic ring can accelerate this process, as determined through a kinetic comparison of the deprotection and subsequent hydrolysis reactions of N-protected β-aryl α-difluoromethyl pyrroles.

Thionation reactions of 2-pyrrole carboxylates

Reaction of 2-pyrrole carboxylates with Lawessons reagent at elevated temperatures results in the corresponding thionoesters, concurrent with the production of a new class of pyrrole annulated with the (1,3,2)-thiazaphospholidine unit. X-ray crystallography was used to identify the pyrrolic thiazaphospholidine, which was found to have unique structural features compared to literature analogues. Addition of BF3·OEt2 to the thionation procedure was found to produce the corresponding F-BODIPY, constituting a four-step reaction in one-pot. The scope and limitations of these reactions involving the promiscuous Lawessons reagent are discussed herein.

Abstract 4258: In vivo anti-leukemia activity of novel C-ring modified prodigiosenes in a zebrafish xenograft model

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL Acute myeloid leukemia (AML) remains fatal for 40% of patients, both due to refractory disease and toxicity from traditional therapeutic agents. Zebrafish provide an efficient and robust leukemia model with the capacity for embryonic screens to test new drugs in vivo. Naturally-occurring Prodigiosin has demonstrated potential as an anti-cancer agent, but toxicity prevents its use as a chemotherapeutic. Here, we present four novel derivatives, prodigiosenes designated DSB-8, -31, -39, -50, synthesized using a multi-step sequence beginning with simple pyrroles. These synthetic compounds demonstrated anti-leukemia activity in vitro against cells in the NIH/NCI Developmental Therapeutics Program, including K562, CCRF-CEM, MOLT-4, RPMI-8226 and SR cell lines. We have previously xenografted human leukemia cells into 48 hour zebrafish embryos and developed a cell quantification assay to evaluate drug responses (Corkery et al BJH, 2011). To assess the anti-leukemia activity and toxicity of our novel prodigiosenes in vivo, we first conducted toxicity curves using 48-hour zebrafish embryos treated for 72 hours to determine an optimal dose (50% of maximum tolerated dose [MTD]). Prodigiosenes DSB-39 (1.0 uM) and DSB-50 (1.5 uM) were tolerated better than DSB-8 (0.2 uM) and DSB-31 (0.2 uM). Subsequently, 40-50 CM-DiI-labeled K562 chronic myelogenous leukemia (CML) cells were injected into the yolk sac of 48-hour zebrafish embryos, which were treated with prodigiosenes at 50% MTD. Engrafted embryos were followed using live cell microscopy. Embryos treated with 0.03% DMSO served as negative control, showing abundant K562 cell proliferation and entry of cells into circulation. Embryos treated with 20 µM imatinib mesylate (IM), a targeted inhibitor of the BCR-ABL1 oncoprotein in K562 cells, served as a positive control, demonstrating no cell proliferation or migration. By comparison, prodigiosenes inhibited K562 cell activity to a similar or greater degree than IM. Qualitatively, the effects of prodigiosenes were graduated: DSB-8 (n=6/7) was cytostatic, DSB-31 (n=5/8) and DSB-50 (n=6/9) were moderately cytotoxic, and DSB-39 (n=7/7) was strongly cytotoxic. To quantify this effect, we are applying a modified ex vivo proliferation assay. Embryos are dissociated to a single cell suspension at 24 and 72 hours post-injection followed by immunohistochemistry directed to human CD34. Our work extends the use of zebrafish xenografts to determine the in vivo sensitivity of human cancer cells to novel drugs and suggests that tailored prodigiosenes may represent novel therapeutic agents for leukemia with improved anti-cancer potency and reduced toxicity. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4258. doi:1538-7445.AM2012-4258