The enolized p-0x0 ester 9 is reduced by NaB(CN)H, in weakly acidic methanol solution to afford an almost quantitative yield of a mixture of diastereomeric P-hydroxy esters 10. On treatment with phosphorus oxychloride in pyridine, 10 is converted into a mixture of the P-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 P-chloro esters producing the a$-unsaturated ester ?a in 84% yield based on 9. -Acid-catalysed equilibration of the known vinyl sulphides C2-and Cs-15 is carried out on large scale furnishing a 7 3~2 7 ratio of C2-and Cs-15. This mixture is oxidized to the sulphones C2-and Cs-16 which are allowed to react with sodium cyanide supported on aluminium oxide affording the u,punsaturated dinitrile 17 which is isolated by chromatography. Thus, 17 is now available on a 30-g scale. Methanolysis of l?by a prolonged treatment with hydrogen chloride in methanol followed by hydrolysis of the Pinner salt leads to ?b. -Both a$-unsaturated esters 7 a and b are readily brominated at the allylic positions by N-bromosuccinimide to yield the y,y'-dibromo esters 19a and b. Zinc-copper couple in tetrahydrofuran converts 19a and b into the semibullvalenedicarboxylates 2 a and b. While 2 a 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 2 a and b undergo a very rapid degenerate Cope rearrangement in solution. In addition, 2 b 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-l2,19a and bare established by X-ray diffraction analyses.The predicted accelerating effect of n-acceptor substituents on the degenerate Cope rearrangement of bridged homotropilidenes, e.g. bullvalenes, barbaralanes, and semibullvalenes[21, was demonstrated for the first time in the barbaralane series, with 2,6-dicyanobarbaralane as the andi id ate^^,^]. Shortly afterwards, the 2,6-dicyanosemibullvalene 1 b has been which not only corroborated this resultr6] but also stimulated new interest by exhibiting exceptional properties, such as being yellow in the absence of a chromophor, showing reversible thermochromism ['], and, in the solid state, a rapid non-degenerate Cope rearrangement between valence tautomers non-equivalent only by virtue of their environment in the crystal lattice"'. The interesting colour and thermochromism of 1 b are shared by the 1,5-bridged semibullvalene-2,6-dicarbonitrile 1 c[lo,lll but not by the 3,7-dicyanosemibullvalene 4a['2-141 nor the corresponding dimethyl 3,7-dicarboxylate 4 b"2,151. Grohmann's series of tetramethyl semibullvalenetetracarboxylates 3 seems to exhibit properties similar to those of the dinitriles 1. Unfortunately, an investigation of the thermochromism of 3 is s...
