Annulation 18-annulene

The role of a-electrons in the stabilization of aromatics has been repeatedly emphasized by Shaik and Hiberty48 and has recently received experimental confirmation after a study of the effects of deuterium substitution of annulenes with internal hydrogens, such as the anti-aromatic [16] annul ene and the aromatic [18]annulene.49 50... 

Maleic anhydride cycloaddition to furanothia[ll]annulene (28) led to the formation of the triply annulated parent (29) (see p. 715). 

H-Chemical shifts of cyclododeceno[c]furan (an annulated [12]-annulene) and cyclotetradeceno[c]furan (an annulated [14]annulene) have been calculated. Remarkably stable compounds result when a carbonyl... 

The heavily annulated benzo- and furanothia[ll]annulenes depicted in 6182 and 6283 were synthesized by condensation of dialdehyde (47) with properly structured Wittig components. Further, treatment of 62 with maleic anhydride produced a mixture of endo and exo cycloadducts of general formula 63 whose hexatriene segment that was not directly bound to the heteroatom was found to be thermally labile, readily... 

Bisdehydrothia-annulene (94) and the annulated bisdehydrooxa- and thia-annulenes (99 and 100) were synthesized73 in two steps as shown in Scheme 4, the key initial step entailing cross-condensation of 45 with properly designed dialdehydes. Acyclic models (93, 97, and 98) were reported to be quite unstable, and this appears to have prevented their... 

The benzannulation technique is also applicable to the benzene homologation and functionalization of annulenes, as well to a quadruple arene modification of dendritic cores. The reaction of chromium carbene functionalized l,6-methano[10]annulene 82 with 3-hexyne under standard conditions afforded a fair yield of the benzannulation product 83 after protection and oxidative work-up (Scheme 32) [75]. The chromium complex 84 evidently partly survived the oxidation conditions using Feln a syn-stereochemistry with respect to the Cr(CO)3 fragment and the methano bridge was suggested on the basis of NMR data, which is in contrast to the preferred formation of anti-annulation products bearing cydophane skeletons [75b]. 

Annulene is a very unstable compound that undergoes cyclization to bicyclic isomers and can be kept only at very low temperature. The NMR spectrum has been studied at low temperature. " Besides indicating the double-bond geometry shown in the structure below, the spectrum reveals a paramagnetic ring current, the opposite to what is observed for aromatic systems. This feature is quite characteristic of the [4n]annulenes and has been useful in characterizing the aromaticity or lack of it in annules. ... 

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