Synthesis, conformational preferences, and biological activity of conformational analogues of the microtubule-stabilizing agents, (-)-zampanolide and (-)-dactylolide

Linear (-)-Zampanolide: Flexibility in Conformation-Activity Relationships

Through an understanding of the conformational preferences of the polyketide natural product (-)-zampanolide, and the structural motifs that control these preferences, we developed a linear zampanolide analogue that exhibits potent cytotoxicity against cancer cell lines. This discovery provides a set of three structural handles for further structure-activity relationship (SAR) studies of this potent microtubule-stabilizing agent. Moreover, it provides additional evidence of the complex relationship between ligand preorganization, conformational flexibility, and biological potency. In contrast to medicinal chemistry dogma, these results demonstrate that increased overall conformational flexibility is not necessarily detrimental to protein binding affinity and biological activity.

In Vivo Evaluation of (-)-Zampanolide Demonstrates Potent and Persistent Antitumor Efficacy When Targeted to the Tumor Site

Microtubule-stabilizing agents (MSAs) are a class of compounds used in the treatment of triple-negative breast cancer (TNBC), a subtype of breast cancer where chemotherapy remains the standard-of-care for patients. Taxanes like paclitaxel and docetaxel have demonstrated efficacy against TNBC in the clinic, however new classes of MSAs need to be identified due to the rise of taxane resistance in patients. (−)-Zampanolide is a covalent microtubule stabilizer that can circumvent taxane resistance in vitro but has not been evaluated for in vivo antitumor efficacy. Here, we determine that (−)-zampanolide has similar potency and efficacy to paclitaxel in TNBC cell lines, but is significantly more persistent due to its covalent binding. We also provide the first reported in vivo antitumor evaluation of (−)-zampanolide where we determine that it has potent and persistent antitumor efficacy when delivered intratumorally. Future work on zampanolide to further evaluate its pharmacophore and determine ways to improve its systemic therapeutic window would make this compound a potential candidate for clinical development through its ability to circumvent taxane-resistance mechanisms.

Synthesis of Morpholine-Based Analogues of (-)-Zampanolide and Their Biological Activity

We describe the convergent synthesis of three prototypical examples of a new class of analogues of the complex, cytotoxic marine macrolide (-)-zampanolide that incorporate an embedded N-substituted morpholine moiety in place of the natural tetrahydropyran ring. The final construction of the macrolactone core was based on a high-yielding intramolecular HWE olefination, while the hemiaminal-linked side chain was elaborated through a stereoselective, BINAL-H-mediated addition of (Z,E)-sorbamide to a macrocyclic aldehyde precursor. The synthesis of the common functionalized morpholine building block involved two consecutive epoxide openings with tosylamide and the product of the first opening reaction, respectively, as nucleophiles. Of the three morpholino-zampanolides investigated, the N-acetyl and the N-benzoyl derivatives both exhibited nanomolar antiproliferative activity, thus being essentially equipotent with the natural product. In contrast, the activity of the N-tosyl derivative was significantly reduced.

Studies toward the Synthesis of an Oxazole-Based Analog of (-)-Zampanolide

Studies are described toward the synthesis of an oxazole-based analog of (-)-zampanolide (2). Construction of (-)-dactylolide analog 22 was achieved via alcohol 5 and acid 4 through esterification and Horner-Wadsworth-Emmons (HWE)-based macrocyclization; however, attempts to attach (Z,E)-sorbamide to 22 proved unsuccessful. The C(8)-C(9) double bond of the macrocycle was prone to migration into conjugation with the oxazole ring, which may generally limit the usefulness of zampanolide analogs with aromatic moieties as tetrahydropyran replacements.

A Method for the Stereoselective Construction of the Hemiaminal Center in Zampanolides

We have developed a new method for the stereoselective establishment of the N-acyl hemiaminal moiety in zampanolide-type structures that involves the reaction of (Z,E)-sorbamide (3) with BINAL-H and subsequent amide transfer from a putative aluminum carboximidoate complex to the aldehyde moiety of a dactylolide precursor, such as 2 or 5. The method has enabled the efficient synthesis of 13-desmethylene-(-)-zampanolide (4), which was found to be an equipotent cell growth inhibitor as the natural product (-)-zampanolide (1).

A Comparative Synthetic Strategy Perspective on α-Amino Acid- and Non-Amino Acid-Derived Synthons towards Total Syntheses of Selected Natural Macrolides

Macrocyclic alkaloids (macrolides) and cyclopeptides have an immense range of applications in drug discovery research because of their natural abundance and potential biological and physicochemical properties. Presently, more than 100 approved drugs or clinical drug candidates contain macrocyclic scaffolds as the biologically active component. This review provides an interesting perspective about the use of amino acid-derived chiral pools versus other methods derived from miscellaneous synthons towards the total synthesis of non-peptidic macrolides. The synthetic routes and the key strategies involved in the total syntheses of ten natural macrolides have been discussed. Both the amino acid-derived and non-amino acid-derived synthetic routes have been illustrated to present a comparative study between the two approaches.

Biocatalytic dynamic kinetic reductive resolution with ketoreductase from Klebsiella pneumoniae: the asymmetric synthesis of functionalized tetrahydropyrans

Ketoreductase from growing cells of Klebsiella pneumoniae (NBRC 3319) acts as an efficient reagent for converting racemic α-benzyl/cinnamyl substituted-β-ketoesters to the corresponding β-hydroxy esters with excellent yields and stereoselectivities (ee and de >99 %). The reactions described herein followed a biocatalytic dynamic kinetic reductive resolution (DKRR) pathway, which is reported for the first time with such substrates. It was found that the enzyme system can accept substituted mono-aryl rings with different electronic natures. In addition, it also accepts a substituted naphthyl ring and heteroaryl ring in the α-position of the parent β-ketoester. The synthesized enantiopure β-hydroxy esters were then synthetically manipulated to valuable tetrahydropyran building blocks.