Annulenes, dehydro preparation

By utilization of this methodology, dehydro[12]annulenes were prepared in two steps (Scheme 41). ... 

Annulene and dehydro[22]annulene are also diatropic. A dehydroben-zo[22]annulene has been prepared that has eight C=C units, is planar and possesses a weak induced ring current. In the latter compound there are 13 outer protons at 6.25-8.45 8 and 7 inner protons at 0.70-3.45 5. Some aromatic bridged [22]annu-lenes are also known. The [26]annulene has not yet been prepared, but several dehydro[26]annulenes are aromatic.Furthermore, the dianion of 1,3,7,9,13,15, 19,21-octadehydro[24]annulene is another 26-electron system that is aromatic. Ojima and co-workers prepared bridged dehydro derivatives of [26], [30], and [34]annulenes. All of these are diatropic. The same workers prepared a bridged tetradehydro[38]annulene, which showed no ring current. On the other hand, the dianion of the cyclophane 89 also has 38 perimeter electrons, and this species is diatropic. ... 

The [12]annulene (96) has been prepared. In solution this molecule exhibits rapid conformational mobility (as do many other annulenes), so that above a certain temperature, in this case — 150°C, all protons are magnetically equivalent. However, at — 170°C the mobility is greatly slowed and the three inner protons are found at 85 while the nine outer protons are at 68. Annulene 96 suffers from hydrogen interference and is certainly not planar. It is very unstable and above —50°C rearranges to 97. Several bridged and dehydro[12]annulenes are known. 

In order to get information on the effect of annelation of a 6tz ring onto macro-cyclic 4 starting material. The reference compound 337 was prepared by the reaction of 334 with maleic anhydride. It was found that the degree of paratropicity decreased in the sequence 337 > 336 > 335 > 333 ci 331. This is the same order as found for the reduction of diatropicity in the annelated bis-dehydro[14]annulenes, 324, 325, 327, 328 and 329. 

Related cyclization reactions are the formation of zethrene (134) [69], which was the product of all attempts to prepare the corresponding bisnaphthalenediyne system 132 (Scheme 8-16). Also, l,6-dehydro[10]annulene (40) [19], already an aromatic system, spontaneously cyclizes to give naphthalene via 1,5-dehydronaphthalene (135) at low temperature [19] (Scheme 8-17). 

Palmer et al. [184] prepared a stable dehydro[14]annulene by reaction of a l-sUyl-2-stannylethyne and a bromoaromatic functionality. Lukevics et al. [185] synthesized unsymmetrical diynes by reactions between alkynylstannanes and terminal bromoalkynes. 

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