Cis" and "trans" doubly bridged S-heterocyclic ethylenes result by stereospecific [2,3] sigmatropic rearrangement of, respectively, cis and trans isomers of spirocyclic sulfonium ylides (eq 1 and 2). This approach has been applied to ylides derived from 7-methylene-1 -thiaspiro[5,11 ]heptadecane (2), leading to derivatives of "cí's"and "trons"-15-thiabicyclo[10.7.0]nonadec-l(12)-enes 5 and 6. The latter is a "thiabetweenanene",2a whose short (seven atom) bridge was further shortened by sulfur extrusion via a Ramberg-Backlund variant.16 Hydrogenation of the Ramberg-Backlund product diene, 9, gave a [10.6]betweenanene derivative, 10. The approach appears to be uniquely suitable for the synthesis of short-bridged betweenanenes.Bis(trans-poIymethylene)ethylenes (I), or betweenanenes,2 first recognized as a structural class by Cahn, Ingold, and Prelog,3 have been recently synthesized by Marshall's2 and by Nakazaki's4 groups. Marshall's approach involves
The reactivity in methanol of methoxide ion with each of 4-fluoro-. 4-chloro-, 4-bromo-, 5-chloro-, 4-chloro-5-nitro-, 5-chloro-4-nitro-, and 5-bromo-4-nitro-benzofurazan, and 5-chloro-and 5-chloro-4-nitro-benzothiadiazole has been studied. The results indicate that the reaction proceeds through a two-step mechanism of &Ar type.In benzofurazans a nitro-group, ortho to the leaving group, increases the substitution rate by a factor of 1 Os-1 O8 except in the case of 5-halogeno-6-nitrobenzofurazans which are practically inert. The increase in velocity is less in the case of the corresponding benzothiadiazole derivatives and is explained by the different situation in the two cases of the fixed double-bond between C(4) and C(5). Some of the results are compared with the other recent work.WHILE studying benzo-X-diazoles of types ( 1)-( 111) we established that the presence of an annelated furazan ring activated the substitution of a chloro-in a similar action gave the normal nucleophilic substitution product according to Scheme 1 with yields above 90% in clean ~~~n d -o r d e r reactions (Table 1).Reactions between 4-Halogeno-5-nitrobenzofurazans or 5-Halogeno-4-nitrobenzofurazans (Cl, Br) and Methoxide Ion in Methanol.-By carrying out the reaction of 4-halogpo-5-nitrobenzofurazan with a large excess of methoxide we obtain the final Meisenheimer-like product as in Scheme 2.way to that Observed for a nitro-@oup in chloronitrobenzenes. Introduction Of a nitro-group in the ring carrying halogen in benzofurazan derivatives greatly increases the reactivity.2 Besides the reaction between
Anionic desilylation of -silylated (E)-4,5-epoxythiacyclononane 1,1-dioxides results in formation of transannular cyclization products whose distribution is independent of the stereochemistry of the starting material. Unlike the species formed by direct BuLi metalation,1 the intermediate arising from the 9-silyl regioisomer rapidly and indiscriminately attacks either epoxidic carbon forming both cis-and trans-joined bicyclic products. This lack of selectivity suggests the reactive intermediate is not a hypervalent silicon species, but is more likely a carbanion which, being associated to a large cation (K+ or Bu4N+) is less discriminating than that arising from BuLi metalation. The intermediate formed by desilylation of the 2-silyl regioisomer appears to cyclize relatively slowly, such that transfer of the carbanionic functionality from C-2 to C-9 becomes competitive with cyclization. With this substrate an unexpected phenomenon has emerged, namely that t-BuOK may act as a base or a desilylating agent depending
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