Annulenes, dehydro reactions

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. 

Hetero[13]annulene, bis-dehydro-dimethylated structure, 7, 716 H etero[ 17]annulenes H NMR, 7, 719, 721 restricted structure, 7, 720 Heteroaromatic compounds reactions... 

Guss, J. M. Mason, R. (1973) Reactions of [14]annulene and dehydro[14]annulene with organotricarbonylchromium complexes Crystal and molecular structures of hexacarbonyl-trans-6a,12a-dihydrooctalenedichromium(0), tricarbonyl-1,4-dihydrophenanthrenechromium(O), and tricarbonylphen-anthrenechromium(O), J. Chem. Soc. Dalton Trans., 1834— 1840. 

Recently, further examples for dehydro Diels-Alder reactions were published, which are believed to proceed via intermediates of the type 205. To explain the formation of products besides those of the type 206, the authors [132] proposed a remarkable reaction cascade and supported it by quantum-chemical calculations. Accordingly, an isonaphthalene of the type 205 undergoes an electrocyclic ring expansion to give a l,2-dehydro[10]annulene derivative, in which a configurational isomerization occurs followed by an electrocyclic ring closure, yielding a further isonaphthalene of the type 205, and aromatization. 

In the fused compounds (241) and (242) the furan ring fails to react as a diene and Diels-Alder reaction with dienophiles occurs on the terminal carbocyclic rings. However, (243) and (244) afford monoadducts with dimethyl fumarate by addition to the furan rings (70JA972). The rates of reaction (Table 2) of a number of dehydroannuleno[c]furans with maleic anhydride, which yield fully conjugated dehydroannulenes of the exo type (247), have been correlated with the aromaticity or antiaromaticity of the products (76JA6052). It was assumed that the transition state for the reactions resembled products to some extent, and the relative rates therefore are a measure of the resonance energy of the products. The reaction of the open-chain compound (250), which yields the adduct (251), was taken as a model. Hence the dehydro[4 + 2]annulenes from (246) and (249) are stabilized compared to (251), and the dehydro[4 ]annulenes from (245) and (248) are destabilized (Scheme 84). 

Formation of substituted products by electrophilic reactions is one of the characteristics of benzenoid compounds. It has been shown that a few dehydro-[4/j + 2]annulenes afford substituted annulenes under strictly limited reaction conditions. Monodehydro[14]annulene, on treatment at room temperature with copper(ii) nitrate-acetic anhydride, oleum-dioxane and subsequent methylation and acetic anhydride-borontrifluoride etherate, yielded monosubstituted products, 169a, 169b and 169c, respectively. The electrophilic reactions must have resulted in substitution of one of the outer protons, but the exact point of attack has not been determined . 

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. 

Base-catalysed prototropic rearrangements of cyclic polyacetylenes have often been used to synthesize annulene and dehydroannulene systems. In such reactions, transannular carbon-carbon bond formation between proximate triple bonds occasionally takes place to give polycyclic compounds. Treatment with a base of cyclic polyacetylenes 57 and 59 gave benzenoid aromatic compounds 58 and 61, with the desired dehydro[16] and [I8]annulenes, respectively. ... 

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). 

The highly twisted Ti-conjugated macrocycle (184) with two allq nyl moieties underwent intramolecular [2 + 2] photocycloaddition to give the thiophene-fused bisdehydro[12]annulene (185) in 61% yield. Photolysis of the bis(dithienyl)ethynes (186) in the presence of iodine resulted in sequential electrophilic and photochemical cyclizations to yield the tetrathienonaphthalenes (183) in one pot reactions. The compounds (187) showed a significant potential as a cruciform scaffold for nanostructured Ji-electron materials. The alkenyl-substituted bis(dithienyl)ethynes (188) underwent p-benzoquinone photosensitized double 5-exo-cyclization to give the diarylated dithienofulvalenes (189). The atylpyridinylethynes (190) and (191) in aqueous HCl solution underwent photo-dehydro-Diels-Alder reaction to afford... 

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