Tetra-/-butylbisdehydro annulene

The C chemical shifts of sp-hybridized carbon atoms in the paratropic tris-dehydro[16]- (310), -[20]- (311), and diatropic tetra-/-butylbisdehydro[14]- (26Sa) and -[18]- (282) annulenes are summarized in Table 12==. The bisdehydro[4 -l-2]-annulenes give signals ascribable to sp-hybridized carbons at an intermediate field... 

Acetyl-7,10,14-tri-/-butyl-l,8-bisdehydro[14]annulene (269) was obtained on treatment of tetra-t-butylbisdehydro[14]annuIene (265a) with acetic anhydride in the presence of zinc chloride at 140 °C for 30 s The structure of 269 has been confirmed by a stepwise synthesis . The acetyl derivative (269) showed a strong diatropicity. 

Treatment of the polyenyne ketones 286, 287 and 289 with potassium hydroxide in liquid ammonia, and subsequent reduction of the resulting cyclic glycols with tin(ii) chloride and hydrogen chloride, yielded tetra-r-butylbisdehydro[22]- (289) , -[26]- (290) and [30]- (291) annulenes as highly coloured crystals. Thus, a series of bisdehydro[4n-t-2]annulenes ranging from 14 to 30- electron systems (265a, 282, 289, 290 and 291) have been obtained. 

It has been predicted theoretically that planar [4/i-t-2]annulcnes will be aromatic up to [22]annulene, but that [26]annulene will no longer be aromatic . The findings that monodehydro[22]annulene , [22]annulene and tetra-r-butylbisdehydro-[22]annulene (289) are diatropic are consistent with the theoretical prediction. Trisdehydro[26]annulene (292) and monodehydro[26]annulene (293) prepared... 

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