Cycloheptatriene ring

Attempts to synthesize l,2-diazacycl[3.2.2]azines (l,2,4-triazolo)[3,4,5-ti/]indolizines, for example, 337, have met with very limited success. To date the ring system is known only when fused on its ef-faces to a cycloheptatriene ring (see Section 12.16.6.4). It may be that the 1,2-diazacyclazine system is more strained than that of the parent cyclazines, since the N-N bond of the triazole ring is expected to be shorter than a C-C or C-N bond. Similarly, all attempts to form the N-N bond as the final step in a synthesis of the 1,2,4-triazacyclazine ring system, 359, have been unsuccessful <1987J(P1)1159>. 

Azulene can be written as fused cyclopentadiene and cycloheptatriene rings, neither of which alone is aromatic. However, some of its resonance structures have a fused cyclopentadienyl anion and cycloheptatrienyl cation, which accounts for its aromaticity and its dipole moment of 1.0 D. 

Deprotection of a dimethyl acetal at a late stage in a synthesis of the unusual diterpenoid tropone Hainanolido was not a problem.123 What was a problem was the fate of the aldehyde released because under most conditions, the aide-hyde underwent a very easy Prins-like reaction with the cycloheptatriene ring to give the tetracyclic product 573 [Scheme 2.57], This reaction could not be suppressed completely, but it was minimised by using zinc bromide to deprotect the dimethyl acetal 57.1. The desired aldehyde 573 was obtained in 61 % yield together with 16% of the unwanted 573. 

Cycloheptatriene 1 and substituted derivatives thereof do not generally contain detectable amounts of their norcaradiene valence tautomers at equilibrium1-2. However, the bicyclic (tropy-lidene-type) adduct is generally not obtained with diazenes, probably due to the decreased reactivity of the boat conformation of cycloheptatriene. Instead, the tricyclic (norcaradiene-type) adduct 2 is obtained with the most reactive diazenes, i.e., 4-phenyl- and 4-methyl-3W-1,2,4-tri-azolc-3,5(4W)-diones. Ene reaction can also compete. The reaction course is affected by the nature of the diazene, the reaction conditions, the substitution pattern and the nature of the substituents on the cycloheptatriene ring. 

The cycloheptatriene ring system is of great interest as a subject for excited state reactions induced by both thermal and photochemical excitation. Major contributions to the photochemistry of the troponoid system and subsequently, to the photochemistry of the cycloheptatriene system have been made by Chapman b whose pioneering work in this field has stimulated much interest in photochemistry in general. The photochemistry of the troponoid system has been reviewed quite thoroughly 1>, and mention will be made only of recent developments in this area. Dimerization reactions will not be discussed. 

Cu(TpMs)] catalyzes the addition of the iCHCC Et unit (derived from N2HCHCC>2Et) to benzene which yields a cycloheptatriene ring, in analogy with the Buchner reaction. When alkyl groups are linked to the aromatic rings, the selectivity of the reaction can be oriented toward insertion into an alkyl C-H bond by using [Cu(TpMs)].48 On the other hand, [Cu(Tp )] efficiently catalyzes the insertion of... 

An experimental distinction among these three alternatives proved to be difficult, particularly as norcaradienes and cycloheptatrienes usually interconvert easily via valence bond isomerization (norcaradiene - cycloheptatriene EA = 6.5 kcal/mol log A = 11.8) (64) and cycloheptatriene ring inversion (1,3,5-cyclo-heptatriene EA - 6.3 kcal/mol) (Figure 4 ref. 65). Therefore it was necessary to resort to such devices as introducing chirality into the molecules by way of an additional stereochemical label. 

Figure 4. Exo-endo equilibration via facile norcaradiene-cycloheptatriene valence bond isomerization (64) and cycloheptatriene ring inversion (65).

A secondary aspect of the solid-state interactions in tropolone compounds is the ability to form rr-stacking units. This effect is due largely to the delocalisation of the cycloheptatriene ring system over the total molecule. Typical tt-tt interactions in benzene molecules range from 3.5. 0 A (mean 3.81 A) and with a nearly parallel orientation of 0 ° (mean 0.67°) between individual benzene molecules. In tropolone compounds the interaction is of order 3.72(3) A with an inter-planar angle between tropolone molecules 0.7(8)°. We present in Table 15-3 a selection of tropolone compounds [in total 21 entries]. 

Thus, only dehydrogenation of cyclohexane is possible to any extent at these temperatures, this in point of fact being due to the far greater stabilization of the benzene ring by resonance compared with that of the cyolopentadiene and cycloheptatriene rings. 

Protonations of (83) and (84) give agostic complexes. Exo addition to the cycloheptatriene ring of (85) was established from experiments in D2O using H and NMR for product identification. Similar exo addition of (D ) occurs... 

Other compounds containing cycloheptatriene rings such as azulene (XXI), heptalene (XXII) and sesquifulvalene (XXIII) are considered in the final chapter dealing with bicyclic and polycyclic compounds. 

Tricyclic dibenzo derivatives of the seven-membered oxepine, azepine and cycloheptatriene rings are the parent systems of several clinically-active psychotropic drugs [l,2]- In six- ajid seven-membered-ring derivatives with central-nervous-system psychopharmacological activity ability to induce such action may be associated with... 

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