Helicenes Do the Twist

Significance

Wang and co-workers report the syntheses and X-ray structures for sterically congested helical molecules. Although the cyclization reflects a formal [4+2] Diels-Alder cycloaddition, the authors postulate a step-wise mechanism involving a biradical intermediate (see also: W. Dai, J. L. Petersen, K. K. Wang Org. Lett. 2004, 6, 4355-4357). A protropic rearrangement driven by aromatization leads to the final product. The X-ray structure indicates that the dihedral twist angle in the phenanthrene core is a remarkable 57.8°. The phenyl substituents are roughly parallel to the nearest fused-ring benzene and with a C-C distance of 3.00 Å at the closest points.