A "methyl extension" strategy for polyketide natural product linker site validation and its application to dictyostatin

Total synthesis of the actinoallolides and a designed photoaffinity probe for target identification

The actinoallolides are a family of polyketide natural products isolated from the bacterium Actinoallomurus fulvus. They show potent biological activity against trypanosomes, the causative agents of the neglected tropical diseases human African trypanosomiasis (sleeping sickness) and Chagas disease, while exhibiting no cytotoxicity against human cell lines. Herein, we give a full account of our strategy evolution towards the synthesis of this structurally unique class of 12-membered macrolides, which culminated in the first total synthesis of (+)-actinoallolide A in 20 steps and 8% overall yield. Subsequent late-stage diversification then provided ready access to the congeneric (+)-actinoallolides B-E. Enabled by this flexible and efficient endgame sequence, we also describe the design and synthesis of a photoaffinity probe based on actinoallolide A to investigate its biological mode of action. This will allow ongoing labelling studies to identify their protein binding target(s).

Development of α,α-Disubstituted Crotylboronate Reagents and Stereoselective Crotylation via Brønsted or Lewis Acid Catalysis

The development of α,α-disubstituted crotylboronate reagents is reported. Chiral Brønsted acid-catalyzed asymmetric aldehyde addition with the developed E-crotylboron reagent gave (E)-anti-1,2-oxaborinan-3-enes with excellent enantioselectivities and E-selectivities. With BF₃·OEt₂ catalysis, the stereoselectivity is reversed, and (Z)-δ-boryl-anti-homoallylic alcohols are obtained with excellent Z-selectivities from the same E-crotylboron reagent. The Z-crotylboron reagent also participates in BF₃·OEt₂-catalyzed crotylation to furnish (Z)-δ-boryl-syn-homoallylic alcohols with good Z-selectivities. DFT computations establish the origins of observed enantio- and stereoselectivities of chiral Brønsted acid-catalyzed asymmetric allylation. Stereochemical models for BF₃·OEt₂-catalyzed reactions are proposed to rationalize the Z-selective allyl additions. These reactions generate highly valuable homoallylic alcohol products with a stereodefined trisubstituted alkene unit. The synthetic utility is further demonstrated by the total syntheses of salinipyrones A and B.

Catalytic Allylation of Aldehydes Using Unactivated Alkenes

Simple feedstock organic molecules, especially alkenes, are attractive starting materials in organic synthesis because of their wide availability. Direct utilization of such bulk, inert organic molecules for practical and selective chemical reactions, however, remains limited. Herein, we developed a ternary hybrid catalyst system comprising a photoredox catalyst, a hydrogen-atom-transfer catalyst, and a chromium complex catalyst, enabling catalytic allylation of aldehydes with simple alkenes, including feedstock lower alkenes. The reaction proceeded under visible-light irradiation at room temperature and with high functional group tolerance. The reaction was extended to an asymmetric variant by employing a chiral chromium complex catalyst.

Design and 22-step synthesis of highly potent D-ring modified and linker-equipped analogs of spongistatin 1

Spongistatin 1 is among the most potent anti-proliferative agents ever discovered rendering it an attractive candidate for development as a payload for antibody-drug conjugates and other targeted delivery approaches. Unfortunately, it is unavailable from natural sources and its size and complex stereostructure render chemical synthesis highly time- and resource-intensive. As a result, the design and synthesis of more acid-stable and linker functional group-equipped analogs that retain the low picomolar potency of the parent natural product requires more efficient and step-economical synthetic access. Using uniquely enabling direct complex fragment coupling crotyl- and alkallylsilylation reactions, we report a 22-step synthesis of a rationally designed D-ring modified analog of spongistatin 1 that is characterized by GI50 values in the low picomolar range, and a proof-of-concept result that the C(15) acetate may be replaced with linker functional group-bearing esters with only minimal reductions in potency.

Direct, Mild, and General n-Bu4NBr-Catalyzed Aldehyde Allylsilylation with Allyl Chlorides

A direct, mild, and general method for the enantioselective allylsilylation of aldehydes with allyl chlorides is reported. The reactions are effectively catalyzed by 5 mol % of n-Bu₄NBr, and this rate acceleration allows the use of complex allyl donors in fragment-coupling reactions and of electron-deficient allyl donors. The results are (1) significant progress toward a "universal" asymmetric aldehyde allylation reaction that can reliably and highly stereoselectively couple any allyl chloride_aldehyde combination and (2) the discovery of a novel mode of nucleophilic catalysis for aldehyde allylsilylation reactions.

Importance of a 4-Alkyl Substituent for Activity in the Englerin Series

The ring closing metathesis/transannular etherification approach to the englerin nucleus was adapted to provide two key intermediates for analogue synthesis: the 4-desmethyl Δ^5,6 tricycle and the 4-oxo Δ^5,6 tricycle. The former was elaborated to 4-desmethyl englerin A and the latter served as a common precursor for englerin A, 4-ethyl englerin A, and 4-isopropyl englerin A. 4-Desmethyl englerin A was less active than the natural product by an order of magnitude, but the 4-ethyl and 4-isopropyl analogues were comparable in activity to englerin A. These results are consistent with the premise that the 4-alkyl group enforces the binding conformation of the cinnamoyl ester substituent. Furthermore, they suggest that 4-alkyl englerin structures may prove to be useful tool compounds.

Synthesis and Evaluation of a Linkable Functional Group-Equipped Analogue of the Epothilones

An approach to the validation of a linker strategy for the epothilone family of microtubule-stabilizing agents is reported. An analogue of epothilone B in which the C(6) methyl group has been replaced with a 4-azidobutyl group has been prepared by total chemical synthesis, and amides derived from the azido group have been shown to retain the activity of the parent compound. These results set the stage for an evaluation of the potential of the epothilones to serve as the drug component of antibody-drug conjugates and other selective tumor cell-targeting conjugates.

Strained alkynes derived from 2,2′-dihydroxy-1,1′-biaryls; synthesis and copper-free cycloaddition with azides

A series of strained alkynes were prepared from 2,2′-dihydroxy-biaryls, and were demonstrated to react with azides without a copper catalyst.

Structural Determinants of the Dictyostatin Chemotype for Tubulin Binding Affinity and Antitumor Activity Against Taxane- and Epothilone-Resistant Cancer Cells

A combined biochemical, structural, and cell biology characterization of dictyostatin is described, which enables an improved understanding of the structural determinants responsible for the high-affinity binding of this anticancer agent to the taxane site in microtubules (MTs). The study reveals that this macrolide is highly optimized for MT binding and that only a few of the structural modifications featured in a library of synthetic analogues resulted in small gains in binding affinity. The high efficiency of the dictyostatin chemotype in overcoming various kinds of clinically relevant resistance mechanisms highlights its potential for therapeutic development for the treatment of drug-resistant tumors. A structural explanation is advanced to account for the synergy observed between dictyostatin and taxanes on the basis of their differential effects on the MT lattice. The X-ray crystal structure of a tubulin-dictyostatin complex and additional molecular modeling have allowed the rationalization of the structure-activity relationships for a set of synthetic dictyostatin analogues, including the highly active hybrid 12 with discodermolide. Altogether, the work reported here is anticipated to facilitate the improved design and synthesis of more efficacious dictyostatin analogues and hybrids with other MT-stabilizing agents.