Properties of Configurationally Stable Atropoenantiomers in Macrocyclic Natural Products and the Chrysophaentin Family

development of antibiotics, antineoplastics, and therapeutics for other diseases. Natural products are unique among all other small molecules in that they are produced by dedicated enzymatic assembly lines that are the protein products of biosynthetic gene clusters. As the products of chiral macromolecules, natural products have distinct three-dimensional shapes and stereochemistry is often encoded in their structures through the presence of stereocenters, or in the case of molecules that lack a stereocenter, the presence of an axis or plane of chirality. In the latter forms of chirality, if the barrier to rotation about the chiral axis or chiral plane is sufficiently high, stable conformers may exist allowing for isolation of discrete conformers, also known as atropisomers. Importantly, the diverse functions and biological activities of natural products are contingent upon their structures, stereochemistry and molecular shape. With continued innovation in methods for natural products discovery, synthetic chemistry, and analytical and computational tools, new insights into atropisomerism in natural products and related scaffolds are being made. As molecular complexity increases, more than one form of stereoisomerism may exist in a single compound (for example, point chirality, chiral axes, and chiral planes), sometimes creating atypical or noncanonical atropisomers, a term used to distinguish physically noninterconvertable atropisomers from typical atropisomers.Here we provide an account of the discovery and unusual structural and stereochemical features of the chrysophaentins, algal derived inhibitors of the bacterial cytoskeletal protein FtsZ and its associated protein partners. Eleven members of the chrysophaentin family have been discovered to date; seven of these are macrocyclic bis-bibenzyl ethers wherein the site of the ether linkage yields either a symmetrical or asymmetrical macrocyclic ring system. The asymmetrical ring system is highly strained and corresponds to the compounds having the most potent antimicrobial activity among the family. We review the structure elucidation and NMR properties that indicate restricted rotation between axes of two biaryl ethers, and the plane represented by the substituted 2-Z-butene bridge common to all of the macrocycles. Computational studies that corroborate high barriers to rotation about one representative plane, on the order of 20+ kcal/mol are presented. These barriers to rotation fix the conformation of the macrocycle into a bowl-like structure and suggest that an atropisomer should exist. Experimental evidence for atropisomerism is presented, consistent with computational predictions. These properties are discussed in the context of the total synthesis of 9-dechlorochrysophaenin A and its ring C isomers. Last, we discuss the implications for the presence of enantiomers in the biological activity and macrocyclization of the natural product.

Computational Insight into the Rope-Skipping Isomerization of Diarylether Cyclophanes

The restricted rotation of chemical bonds may lead to the formation of stable, conformationally chiral molecules. While the asymmetry in chiral molecules is generally observed in the presence of one or more stereocenters, asymmetry exhibited by conformational chirality in compounds lacking stereocenters, called atropisomerism, depends on structural and temperature factors that are still not fully understood. This atropisomerism is observed in natural diarylether heptanoids where the length of the intramolecular tether constrains the compounds to isolable enantiomers at room temperature. In this work, we examine the impact tether length has on the activation free energies to isomerization of a diarylether cyclophane substructure with a tether ranging from 6 to 14 carbons. Racemization activation energies are observed to decay from 48 kcal/mol for a 7-carbon tether to 9.2 kcal/mol for a 14-carbon tether. Synthetic efforts to experimentally test these constraints are also presented. This work will likely guide the design and synthesis of novel asymmetric cyclophanes that will be of interest in the catalysis community given the importance of atropisomeric ligands in the field of asymmetric catalysis.

Recent Studies on Cyclic 1,7-Diarylheptanoids: Their Isolation, Structures, Biological Activities, and Chemical Synthesis

Diarylheptanoids are a family of plant secondary metabolites with a 7 carbon skeleton possessing two phenyl rings at the 1- and 7-positions. They can be subdivided into acyclic and cyclic diarylheptanoids where the latter are further divided into meta,meta-bridged biphenyls ([7.0]metacyclophanes) and meta,para-bridged diphenyl ether heptanoids (oxa[7.1]metapara-cyclophanes). Since the isolation of curcumin from the rhizomes of turmeric (Curcuma longa) in 1815 which was named curcumin, a variety of diarylheptanoids have been isolated from a number of plant families such as Aceraceae, Actinidiaceae, Betulaceae, Burseraceae, Casuarinaceae, Juglandaceae, Leguminosae, Myricaceae, and Zingiberaceae. Earlier studies on these diarylheptanoids have been summarized on several occasions, of which the main themes only focus on isolation, structure elucidation, and the biological properties of linear types. Only a few have covered cyclic diarylheptanoids and their chemical synthesis has been covered lastly by Zhu et al. in 2000. The present paper has, therefore, covered recent progress in cyclic diarylheptanoids focusing on the isolation, structural and biological features, and chemical synthesis.

Total Synthesis of Tedarene A

Tedarene A is a macrocyclic diaryl ether heptanoid isolated from the marine sponge Tedania ignis showing an inhibitory effect against nitric oxide production. The first total synthesis of tedarene A was achieved starting from the commercially available 3-(4-methoxyphenyl)propan-1-ol in nine steps and 15.3% overall yield. The synthetic sequence featured an E,Z-dienic bond introduction and a macrocyclization under Ullman conditions. During the synthesis, the E,E-isomer of tedarene A was also obtained and fully characterized.

A chiral "Siamese-Twin" calix[4]pyrrole tetramer

We describe our results in the attempted template syntheses of oligomacrocycle calix[4]pyrrole dimer 4, using Hay coupling reaction conditions, tetraalkynyl calix[4]pyrrole 5 as starting material and two bipyridyl N-oxides of different length as templates. We found that the short bis-N-oxide 3 was not an efficient template for the macrocyclization reaction producing an insoluble crude reaction mixture containing exclusively oligomerization and polycondensation products. On the other hand, when we used the long bis-N-oxide 6 as template we obtained a soluble crude reaction mixture in which we did not detect the expected calix[4]pyrrole dimer 4. Instead, we isolated, in low yield, an encapsulation complex of the bis-N-oxide 6 in a partially reacted calix[4]pyrrole dimer. The major isolated species was an unprecedented calix[4]pyrrole tetramer encapsulating two molecules of 6. The complex adopted a chiral helical-like conformation in the solid state resembling the previously described so-called "Siamese-Twin porphyrins".

Molecular Motion and Conformational Interconversion of Ir(I)·COD Included in Rebek's Self-Folding Octaamide Cavitand

We report experimental and theoretical evidence of restrained axial rotation for heteroleptic L2·Ir(I)·1,5-cyclooctadiene (COD) complexes included in the aromatic cavity of Rebek's self-folding octaamide cavitand. At 298 K, the axial spinning motion of the included organometallic guests was slow on the (1)H NMR time scale and produced a proton spectrum for the bound host indicative of C2 symmetry. Signals corresponding to aromatic protons of the bound host coalesced at 323 K, indicating that the spinning process of the included guest became fast on the (1)H NMR time scale and that the complex approached C4 symmetry. Surprisingly, lowering the temperature of the solution to 193 K induced an additional splitting of the proton signals observed at room temperature for both the bound host and the included guest. We propose the emergence of a new element of chirality in the complexes, which was associated with a slow interconversion, on the (1)H NMR time scale between the two chiral twisted-boat conformers of the chelated COD included in the already chiral cavity of the container. This leads to the inclusion complexes existing in solution as pairs of two racemic diastereomers. We estimated that the racemization barrier for the two cyclochiral conformers of the Ir(I) chelated COD was 5 kcal mol(-1) higher as an included organometallic complex than as free in solution. Furthermore, we performed a van't Hoff plot and determined that the inclusion of the organometallic complex in the cavitand was endothermic and exclusively driven by entropy (ΔH = 5.9 kcal mol(-1) and ΔS = 33.9 cal mol(-1) K(-1)).