Angular isomers

In another case, when an aromatic ring was joined to the pyridine ring, the cyclization took place at position 4 of the pyridine moiety to give the angular isomer (vide infra). 

The linear cyclobuta[e]cyclopropa[b]naphthalene (114) was synthesized very early by the carbene addition approach, while the synthesis of the angular isomer failed. The most strained of all isolable cycloproparenes, 1,4-dihydrocyclo-propa[b,g]naphthalene (115), was obtained originally by the same approach from the tetrabromide 116. The synthesis was recently improved. An alternative access to 115 via 117 as precursors is also available. The strain energy of 116 is extremely high, and it explodes upon melting (132 °C). ... 

Among type C azapentalenes it has been shown339 that, of two dibenzotetraazapentalenes, the centrosymmetric isomer 255 has zero dipole moment, whereas the angular isomer 254b has a moment of 4.36 D (benzene). 

A mixture of linear and angular chromenes (70) and (71) resulted from the reaction of 1,3 -dihydroxyacridone with the hydroxyacetal. Methylation of the angular isomer gave acronycine, which shows broad-spectrum antitumour activity. 

The constitution of the angular syn-bisdienophile 74 (see Scheme 20) maintains the bicyclic nature of the two dienophilic units. However, the [a,c]-fusion present in the phenanthrene constitution reduces the end-to-end symmetry of the rc-systems, rendering the two olefinic carbon atoms in each dienophilic unit constitutionally-heterotopic. The effect of this reduction in the symmetry of the 7t-systems in 74 upon the diastereoselectivities exhibited in its cycloadditions was of considerable interest to us. Furthermore, the incorporation of two phenanthrene units into a macropolycyclic belt could ultimately lead to the preparation of an angular isomer 70 of [12]cyclacene 50 (Fig. 20). Compared with [12]cyclacene 50, the angular isomer 70 has some appealing electronic features associated with it. 

The angular isomer 18e was obtained, in low yield, as a 1 1 mixture of the syn and anti diastereoisomers, from the tetraamine 311 with paraformaldehyde in TFA. The corresponding linear isomer was obtained instead, also as a 1 1 mixture of the syn and anti diastereoisomers, under the same reaction condition, from the tetraamine 312. A lower yield was observed when, instead of the methoxyaniline moiety, in the starting tetraamine there was a 2-amino-naphthalene moiety (Scheme 57) <2006OL4867>. 

The synthesis of quinacridone 8-1 was first claimed in a series of papers beginning in 1896 (Niementowski 1896, 1906 Ullman and Maag 1906 Baczynski and Niementowski 1919) all of which later proved to be an angular isomer of the linear compound first prepared in 1935 (Lieberman 1935 Liebermanefa/. 1935). Little was done with the compound until 1955 when Du Pont chemists recognized its polymorphic behaviour and associated favourable photochemical stability (Jaffe 1992). Jaffe has also noted that many acridone derivatives are polymorphic. Additional relevant references may be found in the recent review by Lincke (2000). 

Reaction of the cyclic amidines 247 with cyclic yJ-oxo esters yielded both possible linear and angular tricyclic derivatives 248 and 249 but generally only the linearly annelated major products 248 were isolated. - " The yield of the angular isomer 249 decreased " with increasing ring size. 

While the reaction of 2-aminobenzylamine with 3-hydroxy-y-butyrolac-tone (265) at 200 C under a nitrogen atmosphere gave 3-hydroxytetrahy-dropyrrolo[2,l-6]quinazoline (184) directly, the reaction with y-butyrolac-tone yielded 2-(3-hydroxypropyl)-3,4-dihydroquinazoline (122). Compound 122 was cyclized with phosphoryl chloride to a 1 2 mixture of the linear isomer 124 and the angular isomer 123 (see Section IV,A, 1). 

Isomerizations. A synthetically useful transformation is the rearrangement of linear triquinanes to the angular isomers. For the conversion of propargyl ethers to allenyl ethers microwave promotion is advantageous. ... 

However, this result has not been reproduced. The only successful application of the cyclization procedure is the synthesis of the linear 3,4-dihydro-l//-cyclobuta[3,4]benzocyclopropene (4), which was isolated in 5% yield from 4-bromo-5-(methoxymethyl)-l,2-dihydrocyclobutaben-zene (3). The angular isomer, cydobuta[2,3]benzocyclopropene 5, was not formed in the analogous cyclization. ... 

In general, cycloproparenes are thermally labile compounds, and they tend to decompose slowly at or above room temperature. When benzocyclopropene was heated to 100°C in a sealed tube, 9,10-dihydrophenanthrene was formed in 40% yield.The same product formed upon exposure of benzocyclopropene to copper(II) acetate (40%). In the presence of aluminum trichloride, however, benzocyclopropene dimerized to 9,10-dihydroanthracene and anthracene in 25% combined yield.Dihydroanthracene (45%) was also formed with silver(I) tetrafluoro-borate. Cyclopropa[A]naphthalene dimerized at 20°C in the presence of 1,3-diphenylisoben-zofuran or in its absence to give the linear 6,13-dihydropentacene (1) in ca. 85% yield.The angular isomer was not formed. The fact that 1,3-diphenylisobenzofuran did not react with cyclopropa[6]naphthalcne suggests that the dimerization does not proceed via biradicals. On the other hand, biradicals have been trapped with buta-1,3-diene under the conditions of the thermal dimerization of benzocyclopropene (80"C). ... 

The angular isomer on treatment with CHjMgl gives flindersine 99, a pyranoquinoline alkaloid... 

Three of the four possible angular isomers of the naphthoisothiazole system (92, 93, 94) have been prepared by methods similar to those used in... 

When the -amino-hydrazine (50.1) is treated with an orthoester, two products are possible triediyl orthoacetate yields only the linear molecule whereas the orthoformate gives a mixture of linear and angular isomers. 

The angular isomers, 2 and 3, have received much attention since they display good photochromic properties in solution under ambient conditions. Further structural diversity has been achieved by the fusion of aromatic and heterocyclic moieties onto 2Ff-[l]benzopyran and the isomeric naphthopyrans. This review discusses the consequences of the reversible opening of the pyran ring in such compounds under the influence of UV-irradiation and draws together the information reported in the scientific and patent literature concerning the synthesis and photochromic properties of these molecules. Particular attention is paid to the... 

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