Total Synthesis of (-)-Disorazole C1

Cyclization by Intramolecular Suzuki-Miyaura Cross-Coupling-A Review

Ring systems of all sizes are frequent core or substructures in natural products and they are important elements of many drug molecules, as they often confer high binding affinity to and selectivity for disease-relevant biological targets. A uniform key transformation in the synthesis of such structures is the cyclization step. Among the various approaches that have been developed for ring closure, the intramolecular Suzuki-Miyaura reaction has emerged as a powerful option for the construction of normal- and medium-sized rings as well as macrocycles, due to its stereospecificity, the mild reaction conditions, and the non-toxic nature of the boron by-products. In this review, we summarize the state-of-the-art of the application of intramolecular Suzuki-Miyaura cross-coupling reactions in the construction of (macro)cyclic frameworks of natural products and bioactive molecules of synthetic origin, covering (mostly) examples that have been reported since 2015. Target molecules prepared via intramolecular Suzuki-Miyaura cross-coupling as a key step range from natural products/natural product analogs to synthetic drug candidates, featuring ring sizes from 4 to ≫12. We highlight the utility, scope, and limitations of the reaction for different ring sizes and arrays of functional groups. Where possible, comparisons with other methods of cyclization are provided.

Yamaguchi esterification: a key step toward the synthesis of natural products and their analogs-a review

The Yamaguchi reagent, based on 2,4,6-trichlorobenzoyl chloride (TCBC) and 4-dimethylaminopyridine (DMAP), is an efficient tool for conducting the intermolecular (esterification) reaction between an acid and an alcohol in the presence of a suitable base (Et3N or i Pr2NEt) and solvent (THF, DCM, or toluene). The Yamaguchi protocol is renowned for its ability to efficiently produce a diverse array of functionalized esters, promoting high yields, regioselectivity, and easy handling under mild conditions with short reaction times. Here, the recent utilization of the Yamaguchi reagent was reviewed in the synthesis of various natural products such as macrolides, terpenoids, polyketides, peptides, and metabolites.

Synthesis of a Non-Symmetrical Disorazole C1-Analogue and Its Biological Activity

The synthesis of a novel disorazole C1 analogue is described, and its biological activity as a cytotoxic compound is reported. Based on our convergent and flexible route to the disorazole core, we wish to report a robust strategy to synthesize a non-symmetrical disorazole in which we couple one half of the molecule containing the naturally occurring oxazole heterocycle and the second half of the disorazole macrocycle containing a thiazole heterocycle. This resulted in a very unusual non-symmetrical disorazole C1 analogue containing two different heterocycles, and its biological activity was studied. This provided exciting information about SAR (structure-activity-relationship) for this highly potent class of antitumor compounds.

Effect of Extra-Framework Anion Substitution on the Properties of a Chiral Crystalline Sponge

Chiral metal-organic materials, CMOMs, are of interest as they can offer selective binding sites for chiral guests. Such binding sites can enable CMOMs to serve as chiral crystalline sponges (CCSs) to determine molecular structure and/or purify enantiomers. We recently reported on the chiral recognition properties of a homochiral cationic diamondoid, dia, network {[Ni(S-IDEC)(bipy)(H2O)][NO3]}n (S-IDEC = S-indoline-2-carboxylicate, bipy = 4,4'-bipyridine), CMOM-5[NO3]. The modularity of CMOM-5[NO3] means there are five feasible approaches to fine-tune structures and properties via substitution of one or more of the following components: metal cation (Ni2+); bridging ligand (S-IDEC); linker (bipy); extra-framework anion (NO3-); and terminal ligand (H2O). Herein, we report the effect of anion substitution on the CCS properties of CMOM-5[NO3] by preparing and characterizing {[Ni(S-IDEC)(bipy)(H2O)][BF4]}n, CMOM-5[BF4]. The chiral channels in CMOM-5[BF4] enabled it to function as a CCS for determination of the absolute crystal structures of both enantiomers of three chiral compounds: 1-phenyl-1-butanol (1P1B); methyl mandelate (MM); ethyl mandelate (EM). Chiral resolution experiments revealed CMOM-5[BF4] to be highly selective toward the S-isomers of MM and EM with enantiomeric excess, ee, values of 82.6 and 78.4%, respectively. The ee measured for S-EM surpasses the 64.3% exhibited by [DyNaL(H2O)4] 6H2O and far exceeds that of CMOM-5[NO3] (6.0%). Structural studies of the binding sites in CMOM-5[BF4] provide insight into their high enantioselectivity.

Crystal Engineering of a Chiral Crystalline Sponge That Enables Absolute Structure Determination and Enantiomeric Separation

Chiral metal-organic materials (CMOMs), can offer molecular binding sites that mimic the enantioselectivity exhibited by biomolecules and are amenable to systematic fine-tuning of structure and properties. Herein, we report that the reaction of Ni(NO₃)₂, S-indoline-2-carboxylic acid (S-IDECH), and 4,4'-bipyridine (bipy) afforded a homochiral cationic diamondoid, dia, network, [Ni(S-IDEC)(bipy)(H₂O)][NO₃], CMOM-5. Composed of rod building blocks (RBBs) cross-linked by bipy linkers, the activated form of CMOM-5 adapted its pore structure to bind four guest molecules, 1-phenyl-1-butanol (1P1B), 4-phenyl-2-butanol (4P2B), 1-(4-methoxyphenyl)ethanol (MPE), and methyl mandelate (MM), making it an example of a chiral crystalline sponge (CCS). Chiral resolution experiments revealed enantiomeric excess, ee, values of 36.2-93.5%. The structural adaptability of CMOM-5 enabled eight enantiomer@CMOM-5 crystal structures to be determined. The five ordered crystal structures revealed that host-guest hydrogen-bonding interactions are behind the observed enantioselectivity, three of which represent the first crystal structures determined of the ambient liquids R-4P2B, S-4P2B, and R-MPE.

Synthesis and Biological Evaluation of C(13)/C(13')-Bis(desmethyl)disorazole Z

We describe the total synthesis of the macrodiolide C(13)/C(13')-bis(desmethyl)disorazole Z through double inter-/intramolecular Stille cross-coupling of a monomeric vinyl stannane/vinyl iodide precursor to form the macrocycle. The key step in the synthesis of this precursor was a stereoselective aldol reaction of a formal Evans acetate aldol product with crotonaldehyde. As demonstrated by X-ray crystallography, the binding mode of C(13)/C(13')-bis(desmethyl)disorazole Z to tubulin is virtually identical with that of the natural product disorazole Z. Likewise, C(13)/C(13')-bis(desmethyl)disorazole Z inhibits tubulin assembly with at least the same potency as disorazole Z and it appears to be a more potent cell growth inhibitor.

Extending the Structure-Activity Relationship of Disorazole C1 : Exchanging the Oxazole Ring by Thiazole and Influence of Chiral Centers within the Disorazole Core on Cytotoxicity

The synthesis of novel disorazole C1 analogues is described and their biological activity as cytotoxic compounds is reported. Based on our convergent entry to the disorazole core we present a flexible and robust strategy to construct a variety of interesting new analogues. In particular, two regions of the molecules were examined for structural modification: 1. Replacement of the heterocyclic moiety by an exchange of the oxazole ring by a thiazole; and 2. Evaluation of the influence of the absolute configuration of the chiral centers of the molecule. Predicated on our flexible strategy we were able to construct all analogues in an efficient way and could perform an exciting SAR (structure-activity-relationship) study to obtain insight in the cytotoxic activity influenced by the chiral centers of the disorazole core.