Two Steps to Bicyclo[4.2.0]octadienes from Cyclooctatetraene: Total Synthesis of Kingianic Acid A.

Synthetic approaches to bicyclo[4.2.0]octadiene natural products frequently employ the synthesis of linear tetraenes to initiate a biosynthetic 8π/6π-electrocyclization cascade. This work forges a functionalized bicyclo[4.2.0]octadiene in two steps from cyclooctatetraene. The versatility of this method is demonstrated through natural product synthesis, including the first total synthesis of kingianic acid A and formal syntheses of kingianins A, D, and F and cryptobeilic acid D ethyl ester. The unexpected formation of an E,E,Z,E-tetraene byproduct is rationalized through density functional theory modeling.

Guest-Dependent Isomer Convergence of a Permanently Fluxional Coordination Cage.

A fluxional bis-monodentate ligand, based on the archetypal shape-shifting molecule bullvalene, self-assembles with M2+ (M=Pd2+ or Pt2+ ) to produce a highly complex ensemble of permanently fluxional coordination cages. Metal-mediated self-assembly selects for an M2 L4 architecture while maintaining shape-shifting ligand complexity. A second level of simplification is achieved with guest-exchange; the binding of halides within the M2 L4 cage mixture results in a convergence to a cage species with all four ligands present as the "B isomer". Within this confine, the reaction graph of the bullvalene is greatly restricted, but gives rise to a mixture of 38 possible diastereoisomers in rapid exchange. X-ray crystallography reveals a preference for an achiral form consisting of both ligand enantiomers. Through a combination of NMR spectroscopy and DFT calculations, we elucidate the restricted isomerisation pathway of the permanently fluxional M2 L4 assembly.

Ambimodal Pericyclic Rearrangements of Dialkenyl-Bullvalenes Give Tetrahydro-1,8-ethenoheptalenes.

The fluxional structure of bullvalene is expanded by the discovery of a [5,5]-sigmatropic rearrangement of dialkenyl substituted derivatives. This gives rise to tetrahydro-1,8-ethenoheptalenes (THEH), representing the first examples of this tricyclic scaffold. Variation of the substitution pattern alters the product distribution, including one thermodynamically balanced between THEH and bullvalene isomers. DFT calculations are used to explore the thermodynamic landscape and reaction mechanism revealing a pretransition state bifurcation leading to a concerted ambimodal rearrangement pathway.

Stereomutation of Substituted Bullvalenes.

The stereomutation of substituted bullvalenes is an inevitable consequence of the valence isomerism that automerizes this unique fluxional hydrocarbon. The introduction of external stereogenicity in the substituents expands the reaction graphs and leads to a wealth of complex diastereochemical relationships. In this communication, we explore these possibilities and prepare a range of stereochemically rich substituted bullvalenes. This includes a series of disubstituted bullvalenes with two external stereocenters as a platform for fluxional, shape-diverse compound libraries. We also prepare a tethered bisbullvalene with central stereogenicity in the tether as an ensemble of 900 unique isomers that are completely stereomutable.

Boronate Ester Bullvalenes.

Boronate ester bullvalenes are now accessible in two to four operationally simple steps. This unlocks late-stage diversification through Suzuki cross-coupling reactions to give mono-, di-, and trisubstituted bullvalenes. Moreover, a linchpin strategy enables preprogrammed installation of two different substituents. Analysis of solution phase isomer distributions and single-crystal X-ray structures reveals that isomer preference in the crystal lattice is due to general shape selectivity.