Synthesis of C4-Modified Tetracyclines with Anticancer Activity

Abstract

Tetracyclines are a class of compounds that includes clinically employed antibacterial agents such as minocycline. Other tetracyclines, including the semisynthetic compound Col-3 and the natural product SF2575, exhibit anticancer activity. This dissertation focuses on the synthesis of tetracyclines as anticancer agents, specifically fully synthetic tetracyclines with structural modifications at the C4 position of the tetracycline scaffold.

An improved synthetic route to enone 56, a key intermediate for the synthesis of C4- modified tetracyclines, was developed. The new route provided 56 in higher overall yield and in fewer steps than a previously established route: 16% yield over 9 steps vs. 5% yield over 11 steps. In the new route, it was found that an alternative chiral ligand could be used successfully for an enantioselective vinylation reaction in place of the originally reported ligand, affording equally high enantioselectivity. The alternative ligand could be derived from a starting material that, unlike the precursor of the original ligand, was relatively inexpensive and not a controlled substance. 

In addition to enone 56, six variously substituted phenyl o-toluate esters were also prepared as tetracycline precursors. The four-ring core of the tetracycline structure was constructed by reaction of 56 with a phenyl o-toluate ester, and subsequent efforts were directed toward structural diversification at the C4 position on the tetracyclic core, especially via reactions of a selectively deprotected hydroxyl group at C4. Some attempts to activate a C4 alcohol as a leaving group resulted in intramolecular alkylation, forming a product containing a norbornyl ring system. Esterification of the C4 hydroxyl under certain Mitsunobu reaction conditions unexpectedly proceeded with stereochemical retention at C4, not inversion. Results of ¹⁸O-labeling experiments were consistent with an acylative reaction mechanism, which accounts for the observed configurational retention. 

The products of the stereoretentive esterification reactions were converted to C4-modified tetracyclines. These tetracyclines were cytotoxic to K-562 human chronic myelogenous leukemia cells and A549 human lung epithelial carcinoma cells with IC₅₀ values of 0.49–3.07 μM, and they were less toxic toward human primary small airway epithelial cells. The four C4-modified tetracyclines exhibited generally stronger cytotoxicity in the K-562 and A549 cell lines than their structurally related counterparts that differ in the type of substituent present at C4.