1,6-Oxido annulene

C. fi-Oxido ) annulene. A 2-1., three-necked, round-bottomed flask equipped with a sealed mechanical stirrer and a reflux condenser protected by a calcium chloride drying tube is charged with 81 g. (1.5 moles) of sodium methoxide (Note 9) and 600 ml. of anhydrous ether. To this slurry is added, with stirring, 117 g, (0.25 mole) of flnely powdered 3,4,8,9-tetrabromo-ll-oxatricyclo[4.4.1.0 - ]undecane. The reaction mixture is refluxed with stirring for 10 hours and is then allowed to stand overnight. Following this, 500 ml. of water is added slowly to dissolve the solids. The ether layer is separated and the aqueous layer is extracted with two 100-ml. portions of ether. The combined ethereal solution is washed with 250 ml. of water and dried over anhydrous potassium carbonate. Removal of the ether under reduced pressure on a rotary evaporator affords a brown oil that is distilled to give 34.4-35.1 g. of yellow l,0-oxido[10]annulene, b.p. 77° (0.02 mm.) (Note 10). This material readily solidifies at room temperature. After two recrystallizations at —40° from 225-ml. portions of pentane-ether (5 1), 18.3-18.5 g. (51%) of l,6-oxido[10]annulene is obtained as pale yellow needles, m.p. 51-52° (Notes 11 and 12). 

Epoxide opening with nucleophiles occurs at the less substituted carbon atom of the oxlrane ting. Cataiytic hydrogenolysis yields the more substituted alcohol. The scheme below contains also an example for trons-dibromination of a C—C double bond followed by dehy-drobromination with strong base for overall conversion into a conjugated diene. The bicycKc tetraene then isomerizes spontaneously to the aromatic l,6-oxido[l0]annulene (E. Vogel, 1964). 

As reported by Shani and Sondheimer,1 the dehydrohalogenation of the tetrabromide by means of potassium hydroxide in ethanol at 50-55° affords a mixture, which is readily separated by chromatography on alumina, of l,6-oxido[10]annulene and the isomeric 1-benzo-xepin. The latter compound is also formed during chromatography of l,6-oxido[10]annulene on silica gel.7... 

In order to avoid acid-catalyzed rearrangements of 1,6-oxido-[10]annulene it is recommended that the distillation flask be treated with a base before use. 

The preparation of l,6-oxido[10]annulene, described simultaneously by Sondheimer and Shani4 0 and by Vogel, Biskup, Pretzer, and Boll, is illustrative of the rather general synthesis of aromatic 1,6-bridged [10]annulenes starting from 1,4,5,8-tetrahydronaphthalene. Apart from the present compound, the following bridged [10]annulenes have thus far been obtained by this approach l,6-methano[10J-annulene,2-8-9 the ll,ll-dihalo-l,6-methano[10]annulenes,9 10 and 1,6-imino [ 1 OJannulene.11... 

The first step in this preparation, the epoxidation of 1,4,5,8-tetra-hydronaphthalene, exemplifies the well-known selectivity exerted by peracids in their reaction with alkenes possessing double bonds that differ in the degree of alkyl substitution.12 As regards the method of aromatization employed in the conversion of ll-oxatricyclo[4.4.1.01-6]-undeca-3,8-diene to l,6-oxido[10]annulene, the two-step bromination-dehydrobromination sequence is given preference to the one-step DDQ-dehydrogenation, which was advantageously applied in the synthesis of l,6-metliano[10]annulene,2,9 since it affords the product in higher yield and purity. 

Oxido[10]annulene closely resembles l,6-methano[lOJannulene in many of its spectral properties, particularly in its proton magnetic resonance, ultraviolet, infrared, and electron spin resonance spectra,1 but is chemically less versatile than the hydrocarbon analog due to its relatively high sensitivity toward proton and Lewis acids. 

The recent upsurge of interest in systems of theoretical interest demands practical syntheses of several important compounds. These are BICYCLO[2.l.0]PENT-2-ENE, BENZOCYCLOPROPENE, 1,6-OXIDO[10]ANNULENE, and others. ewdo-TRICYCLO[4.4.0.02 5]-DECA-3,8-DIENE-7,10-DIONE is utilized as a model for the use of CYCLOBUTADIENE IN SYNTHESIS, and a stable monomeric ketene, icri-BUTYLCYANOKETENE offers opportunities for further studies of this interesting species. 

Experimental measurements on the enthalpy of formation are also lacking for the saturated parent bicyclic hydrocarbon, as well as for any other substituted and/or hetero derivative, save the equally conceptually complicated l,6-oxido[10]annulene. 

The H NMR spectrum of 7.80 was also recorded by Elix. While it was of insufficient quality to allow an extract assignment of structure vide supra), it did allow Elix to conclude that, as expected given its 4n n-electron formulation, compound 7.80 is not aromatic as judged by the absence of any substantial macrocyclic ring-current effects. This conclusion reached, it should be noted, that the formally 4n + 2 pentaoxa[30]pentaphyrin-(2.2.2.2.2) isomers also failed to display any kind of substantial ring-current effects. This led Elix to conclude that there is no fundamental difference in electronic character between large 4n and 4n + 2 7r-electron oxido-bridged annulenes. ... 

A final sub-group within the bridged 10 7r-electron systems includes those compounds bridged by heteroatoms. The synthesis and n.m.r. investigation of l,6-oxido-[10]annulene 137-139,142) and many of its derivatives 17,125,138,140) have accordingly been executed. The n.m.r. spectrum 17 of l,6-oxido-[10]annulene itself (30, see Table 6) consists of an AA BB system centered at t 2.65 (in CCI4 also quoted at t 2.52... 

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