1.6- Methano annulene structure

Prior to the work of Cheng et al. Rubin et al. reported a similar [4 + 4] pho-tocyclization reaction [299]. Rubin has shown that a cyclohexadiene derivative underwent a very facile photochemically promoted rearrangement to the stable bridged bisfulleroid 198 (Scheme 76). This process occurs via the initial [4 + 4] photoadduct (not observed), which undergoes a thermally allowed [2 + 2 + 2] cycloreversion to afford a bis-methano[12]annulene structure 198. Alternatively, compound 198 can be obtained by photolysis of the allylic alcohol 197 under reflux and acidic conditions in good yields. The allylic alcohol is obtained by acidic... 

Heteroannulenes electrophilic reactions, 7, 726 tr-excessive synthesis, 7, 727 nucleophilic reactions, 7, 727 Hetero[l IJannulenes structure, 7, 715 Hetero[ 12]annulenes pyridine-like methano-bridged, 7, 715 Hetero[ 13]annulenes unrestricted structure, 7, 716... 

Thus, l,6-methano[10]annulene (77) and its oxygen and nitrogen analogs 78 and 79 have been prepared and are stable compounds that undergo aromatic substitution and are diatropic. For example, the perimeter protons of 77 are found at 6.9-7.3 5, while the bridge protons are at —0.5 5. The crystal structure of 77 shows that the perimeter is nonplanar, but the bond distances are in the range 1.37-1.42A. It has therefore been amply demonstrated that a closed loop of 10 electrons is an aromatic system, although some molecules that could conceivably have such a system are too distorted from planarity to be aromatic. A small distortion from planarity (as in 77) does not prevent aromaticity, at least in part because the s orbitals so distort themselves as to maximize the favorable (parallel) overlap of p... 

Dimers with a 1,2-bismethylenecyclobutane structure were obtained from 585 [240], 590 [238], 591 [241], 592 [242], 593 [243] and from the pinene derivative 597 [244]. The interception of 592 by 1,3-diphenylisobenzofuran (DPIBF) afforded two diastereomeric [4+ 2]-cycloadducts [245], Bicyclo[5.1.0]octa-3,4-diene (594) was generated by /3-elimination and trapped by sodium pyrrolidide because of the question of the extent to which the corresponding bicyclooctyne is formed in addition to 594 [184], Liberated by /3-elimination from ll,ll-dichloro-l,6-methano[10]annulene in... 

While the synthesis of the requisite methano[10]annulenes is not always easy, the [ 2 + 2 + 2] cycloreversion has provided the essential structural variations of cyclopropa fusion into an aromatic moiety that complement other methodologies (Section II.E). Moreover, the sequence lends itself to further exploitation. Thus Vogel s group50 has obtained the dicyanoacetylene adduct 40 which provides phenylacetylene on fvp (equation 9). It is likely that methylenecyclopropabenzene (9) is initially formed as a molecule too... 

For the first time in our work we were able to isolate in the case of ll-cyano-1,6-methano[10]annulene a roughly 1 1 mixture of the anti-endo-Sidduci 48 and anti-exo-adduct 49 whose structures and configurations were determined by X-ray crystallography. ... 

In the reaction of l,6-methano[10]annulene with bis-triazolinediones the 2 1 adduct (lid) has the following structure ... 

Finally, if the perturbation due to the substituent as well as its position in the molecular framework is known, MCD spectroscopy can be used to investigate the nature and the ordering of the frontier orbitals. In this way, the relative importance of transannular interactions compared to substituent effects and geometrical distortions for the electronic structure of bridged an-nulenes such as l,6-methano[10]annulene (14) may be assessed by determining the energy ordering of the frontier orbitals of substituted derivatives of 14 from their MCD spectra. This is shown in detail in Example 3.12. 

When the parent compound of the bridged [lOjannulene series, l,6-methano-[10]annulene 11>120) 28 (see Table 6), was first prepared, it was necessary to distinguish between three possible structures a) the "double norcaradiene structure 20, b) a rapidly fluctuating system (c.f. cyclooctatetraene), possibly proceeding through low equilibrium concentrations of the double norcaradiene, 21, or c) a delocalized 10 -electron system 22. 

The cation possesses a more favourable geometry than 1,6-methano-[10]annulene, being able to incorporate the 10 -system into a more planar structure. It is evidently more stable than the tropylium ion, as it can be formed from its neutral hydrocarbon precursor 57 by a hydride-transfer reaction with tropylium tetrafluoroborate 136>. The anion, on the other hand, involves substantial steric strain and is not as planar as either the cation or the neutral bridged [lOJannulene. Whereas formation of the cation from 57 was seen to be most favourable, the anion precursor 58 is much less acidic than cyclopentadiene, although in the presence of... 

A number of structures have been prepared that avoid the steric problems associated with the 1,3,5,7,9-cyclodecapentaenes. 1,6-Methano[10]annulene (1) can be prepared from naphthalene in multigram quantities in four steps. ... 

The principle developed by Vogel of methano- or oxy-bridging of medium-sized annulenes [8] to strengthen the perimeter has proved advantageous in the synthesis of larger-membered heteroannulenes of various structures. 

The molecular structure of oxy-bridged annulenes has been studied using a range of semiempirical methods including HF/STO-3G, HF/4-31G, MP2/STO-3G, MNDO, and correlation corrected MNDO <86JA4768>. The barrier to inversion, at the ether oxygen, of l,6-methano-8,13-oxido[14]-annulene (11) was predicted to be 27 kcal mol . ... 

The dihydro-derivative (58) of the unique sesquiterpenoid spiniferin-1 (57), which incorporates the novel l,6-methano[10]annulene skeleton, has been synthesized (Scheme 4) and this confirms beyond doubt its precise structure. ... 

CNDO/2 calculations of spin densities. ) Tentative assignment of structure. McLachlan calculation of spin densities. From [79Gerl]. Replaced by a(D) = 0.046 mT upon deuteration at positions 6 and 8. Determination of dissociation enthalpy of dimer. ) McLachlan calculation of spin densities. Radical generated by thermolysis of l,6-methano[10]annulene-ll-t-butylperoxy ester. ... 

Such a structure may be described as a homonaphthalenc. In the case of 1,5-methano[10]annulene, discussed a little later in this section, there is in particular spectroscopic evidence that it resembles a homoazulene. 

An interesting property of l,6-methano[10]annulene is the influence of bridge-substituents on the stability of the aromatic system. For example, there is chemical and spectroscopic evidence in favour of the existence of a direct 1—6 bond in the 11,11-dimethyl derivative, thus producing the bisnorcaradiene structure (73). In contrast, the 11,11-difluoro-compound... 

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