Rules of Macrocycle Topology: A [13]‐Macrodilactone Case Study

Role of Macrocyclic Conformational Steering in a Kinetic Route toward Bielschowskysin

Macrocyclic furanobutenolide-derived cembranoids (FBCs) are the biosynthetic precursors to a wide variety of highly congested and oxygenated polycyclic (nor)diterpenes (e.g. plumarellide, verrillin, and bielschowskysin). These architecturally complex metabolites are thought to originate from site-selective oxidation of the macrocycle backbone and a series of intricate transannular reactions. Yet the development of a common biomimetic route has been hampered by a lack of synthetic methods for the pivotal furan dearomatization in a regio- and stereoselective manner. To address these shortcomings, a concise strategy of epoxidation followed by a kinetically controlled furan dearomatization is reported. The surprising switch of facial α:β-discrimination observed in the epoxidation of the most strained E-acerosolide versus E-deoxypukalide and E-bipinnatin J derived macrocycles has been rationalized by the variation of the 3D conformational landscape between macrocyclic scaffolds. A careful conformational analysis of these macrocycles by VT-NMR and NOESY experiments at low temperature was supported by DFT calculations to characterize these equilibrating macrocyclic conformers. The shift in conformational topology associated with a swing of the butenolide ring in E-deoxypukalide is in general agreement with the reversal of β-selectivity observed in the epoxidation. We also describe the downstream functionalization of FBC-macrocycles and how the C-7 epoxide configuration is retentively translated to the C-3 stereogenicity in dearomatized products under kinetic control to secure the requisite 3S,7S,8S configurations for the bielschowskysin synthesis. Unlike previously speculated, our results suggest that the most strained FBC-macrocycles bearing a E-(Δ^7,8)-alkene moiety may stand as the true biosynthetic precursors to bielschowskysin and several other polycyclic natural products of this class.

Exploring the physicochemical and antiproliferative properties of biaryl-linked [13]-macrodilactones

A macrocyclic motif fosters productive protein-small molecule interactions. There are numerous examples of both natural product and designed, synthetic macrocycles that modulate the immune system, slow microbial infection, or kill eukaryotic cells. Reported here are the synthesis, physicochemical characterization, and antiproliferative activity of a group of [13]-macrodilactones decorated with a pendant biaryl moiety. Biaryl analogs were prepared by Suzuki reactions conducted on a common intermediate that contained a bromophenyl unit alpha to one of the carbonyls of the [13]-macrodilactone. Principal component analysis placed the new compounds in physicochemical context relative to a variety of pharmaceuticals and natural products. Modest inhibition of proliferation was observed in ASZ cells, a murine basal cell carcinoma line. This work underscores the value of an approach toward the identification of bioactive compounds that places the evaluation of physicochemical parameters early in the search process.

Structures of three disubstituted [13]-macrodilactones reveal effects of substitution on macrocycle conformation

The synthesis and crystal structures of three new disubstituted [13]-macrodilactones, trans-4,8-dimethyl-1,10-dioxa­cyclo­tridec-5-ene-2,9-dione, cis-4-(4-bromo­phen­yl)-13-methyl-1,10-dioxa­cyclo­tridec-5-ene-2,9-dione, and trans-11-methyl-4-phenyl-1,10-dioxa­cyclo­tridec-5-ene-2,9-dione are reported and their conformations are put in the context of other [13]-macrodilactone structures reported previously, showing that the number, location, and relative disposition of groups attached at the termini of planar units of the [13]-macrodilactones subtly influence their aspect ratios.

The synthesis and crystal structures of three new disubstituted [13]-macro­di­lactones, namely, trans-4,8-dimethyl-1,10-dioxa­cyclo­tridec-5-ene-2,9-dione, C₁₃H₂₀O₄, I, cis-4-(4-bromo­phen­yl)-13-methyl-1,10-dioxa­cyclo­tridec-5-ene-2,9-dione C₁₈H₂₁BrO₄, II, and trans-11-methyl-4-phenyl-1,10-dioxa­cyclo­tridec-5-ene-2,9-dione, C₁₈H₂₂O₄, III, are reported and their conformations are put in the context of other [13]-macrodilactone structures reported previously. Together, they show that the number, location, and relative disposition of groups attached at the termini of planar units of the [13]-macrodilactones subtly influence their aspect ratios.

Synthesis and structure of a carbohydrate-fused [15]-macrodilactone

The design, synthesis and structural characterization of a new α-d-glucose fused [15]-macrodilactone is reported. The macrolide was synthesized by a route involving sequential acylations of glucose at the C4' and C6' hydroxyl groups followed by an intramolecular Stille reaction previously established for other [15]-macrodilactones. Analysis of the X-ray crystallographic structure of the macrolide revealed a unique conformation of this macrocycle that differs from earlier models for [13]- and [15]-macrodilactones. Organizing the three planar units and the pyranose moiety into a macrocyclic ring resulted in a cup-shaped structure with planar chirality. Further, the gt conformation of the exocyclic hydroxymethyl group in the glucose unit was found to be crucial for controlling the planar chirality and, hence, governing the molecular shape and overall topology of the compound.