2,6‐Dimethyl Semibullvalene‐2,6‐dicarboxylates

The enolized β-oxo ester 9 is reduced by NaB(CN)H3 in weakly acidic methanol solution to afford an almost quantitative yield of a mixture of diastereomeric β-hydroxy esters 10. On treatment with phosphorus oxychloride in pyridine, 10 is converted into a mixture of the β-chloro esters exo-11, endo-11, exo-12, endo-12, and 13 which is separated in part by chromatography. Sodium methoxide in methanol eliminates hydrogen chloride from the crude mixture of β-chloro esters producing the α,β-unsaturated ester 7a in 84% yield based on 9. – Acid-catalysed equilibration of the known vinyl sulphides C2- and Cs-15 is carried out on large scale furnishing a 73:27 ratio of C2- and Cs-15. This mixture is oxidized to the sulphones C2- and Cs-16 which are allowed to react with sodium cyanide supported on aluminium oxide affording the α,β-unsaturated dinitrile 17 which is isolated by chromatography. Thus, 17 is now available on a 30-g scale. Methanolysis of 17 by a prolonged treatment with hydrogen chloride in methanol followed by hydrolysis of the Pinner salt leads to 7b. – Both α,β-unsaturated esters 7a and b are readily brominated at the allylic positions by N-bromosuccinimide to yield the γ,γ′-dibromo esters 19a and b. Zinc-copper couple in tetrahydrofuran converts 19a and b into the semibullvalenedicarboxylates 2a and b. While 2a is persistent only in dilute solutions, thus frustrating all attempts at its isolation, 2b exhibits considerable stability in solution and forms lemon-coloured crystals. The semibullvalenedicarboxylates 2a and b undergo a very rapid degenerate Cope rearrangement in solution. In addition, 2b exhibits reversible thermochromism in both solution and solid state. The structures of the new compounds are based on spectroscopic evidence, including mass, IR, and NMR spectra. The configurations of exo-11, exo-12, 19a and b are established by X-ray diffraction analyses.