Utility of Chlorosulfonyl Isocyanate (CSI) in the Synthesis of Fused Ring Systems and Biological Evaluation of Tetracyclic Analogues
Introduction
Chlorosulfonyl isocyanate (CSI) is a reagent with a variety of synthetic applications. While numerous annulation reactions have been observed, the N/S heteroatoms are largely maintained within the ring in these examples, and formation of cyclic ketones has not been previously observed. Here we describe the discovery that 6/7/5/6 and 6/6/5/6 tetracyclic fused ring compounds can be formed by treatment with CSI of benzosuberene and tetrahydronaphthalene analogues bearing a pendant aryl ring. While numerous synthetic approaches have been utilized to access fused ring systems, to the best of our knowledge an annulation of this type has not been reported through the use of CSI. These analogues serving as substrates for this reaction are potent inhibitors of tubulin polymerization that bind in the colchicine site.
Methods
Drawing structural inspiration from colchicine and combretastatin A-4 (CA4), a variety of small-molecule inhibitors of tubulin polymerization have been designed and synthesized by the Pinney Research Group. A subset of these molecules demonstrates dual mechanism of action, functioning as potent antiproliferative agents (cytotoxins) and vascular disrupting agents (VDAs). To further investigate SAR correlations and expand structural diversity around a benzosuberene-based molecule (KGP18), we discovered that treatment of KGP18 with CSI produced a unique tetracyclic ketone fused ring system, confirmed by various analytical techniques. To explore the scope of this reaction, various benzosuberene analogues containing structural modifications to the pendant aryl ring (through substituents of varied electronics) were synthesized and subjected to reaction with CSI.
Results
Six tetra-fused analogues have been synthesized maintaining the trimethoxy-aryl ring. One analogue, KGP521, accessed through reaction of TBS-protected KGP18 with CSI, generated the corresponding tetracyclic ring system in 84% yield. A separate TBS-protected dihydronaphthalene-based molecule, KGP03, upon treatment with CSI, also yielded a tetracyclic fused ring system. The potential scope of this reaction was explored by variation of the substituents on the pendant trimethoxy-aryl ring. It was postulated that, without an electron rich aromatic ring, the reaction might not proceed, but this was not the case because the reaction proceeded in the absence of appreciable electron donating groups on the pendant aryl ring. Cyclization to form a 6/7/5/6 tetra-fused ring system was observed when the pendant aryl ring contained a single methoxy group in the para position, two methoxy groups in the meta positions, a methyl group in the para position, and with an analogue bearing an unsubstituted phenyl ring. The initial tetracyclic analogue, derived from KGP18, demonstrated potent inhibition of tubulin polymerization (IC50 = 1.8 µM). Prepared as its corresponding water-soluble phosphate prodrug salt (KGP556), this molecule significantly decreased signal (compared to vehicle control) in a preliminary in vivo bioluminescence imaging (BLI) study in BALB/c mice bearing orthotopic RENCA kidney tumors. Additional tetracyclic compounds bearing the trimethoxy ring were evaluated for inhibition of tubulin polymerization, and KGP669, containing an additional benzylic olefin in the seven-membered ring, demonstrated strong activity (IC50 = 0.87 µM).
Conclusion
Reaction of benzosuberene KGP18 with CSI resulted in the formation of a tetracyclic fused ring system (6,7,5,6), and this reaction was expanded to include various analogues. To the best of our knowledge, this synthetic application of CSI has not been previously reported. Successful application of this CSI-mediated tetracyclic fused ring formation reaction has been demonstrated with several substrates bearing functional group variations on the pendant aryl ring. Several analogues demonstrated inhibition of tubulin polymerization, and one analogue demonstrated preliminary in vivo efficacy as a VDA.