7-Oxanorbornadiene

Three decades ago the preparation of oxepin represented a considerable synthetic challenge. The theoretical impetus for these efforts was the consideration that oxepin can be regarded as an analog of cyclooctatetraene in the same sense that furan is an analog of benzene. The possibility of such an electronic relationship was supported by molecular orbital calculations suggesting that oxepin might possess a certain amount of aromatic character, despite the fact that it appears to violate the [4n + 2] requirement for aromaticity. By analogy with the closely related cycloheptatriene/norcaradiene system, it was also postulated that oxepin represents a valence tautomer of benzene oxide. Other isomers of oxepin are 7-oxanorbornadiene and 3-oxaquadricyclane.1 Both have been shown to isomerize to oxepin and benzene oxide, respectively (see Section 1.1.2.1.). 

As discussed, 3-oxaquadricyclanes are generally obtained from 7-oxanorbornadienes by a photochemical intramolecular [2+2] cycloaddition. When the irradiation is carried out at ambient temperature, the 3-oxaquadricyclanes cannot be isolated but react further to oxepins... 

Takeshita H., Mori A., Tian G. R. Carbon-Carbon Double Bond Formation by Means of High-Pressure Cycloaddition-Retro-Diels-Alder Reaction Between 2,3-Bis(Methoxycarbonyl)-7-Oxanorbornadiene and Dienes Yuki Gosei Kagaku Kyo-kaishi 1990 48 132-143... 

Confirmation that the polymerizations proceed via metallacyclic intermediates was obtained by studying the ROMP of functionalized 7-oxanorbornadienes. These polymerize slower than their norbornene analogs, allowing NMR identification of the metallacyclobutane resonances and subsequent monitoring of ring opening to the first insertion product. In addition, the X-ray crystallographic structure of complex (212) has been reported.533... 

Alkenes have also been prepared by retro-Diels-Alder reaction of resin-bound cyclohexenes. Figure 3.40 depicts an interesting example of this strategy, in which the retro-Diels-Alder reaction is induced by conversion of a thermally stable, resin-bound 7-oxanorbornadiene into a thermally unstable 7-oxanorbornene [795]. 

Arene oxides 42 and 49 were obtained as minor products by an unusual thermal isomerization of the photooxide of 3-diphenylisobenzofuran. Although the thermal isomerization reaction of 3-oxaquadricyclane has been used in the synthesis of benzene oxide 1, this route to benzene oxides was initially limited to specifically substituted derivatives owing to the unavailability of the unsubstituted 7-oxanorbornadiene. Thus most of the substituted arene oxides prepared by this route contained electron-withdrawing substituents X [X = —CF3 (77), —COaMe (76, 86, 88-92, 94), and —C02Et (93)] which increased their thermal stability. 

The unsaturated lactones (341) behave in a similar fashion and yield the isomers (342). The formation of the oxygen-bridged quadricyclane (343) is a consequence of intramolecular addition in 7-oxa-norbornadiene (344) many analogous transformations have been reported in substituted 7-oxanorbornadienes and in l,4-epoxy-l,4-dihydronaphthalenes, which are further converted to benzoxepins. 

Oxanorbornadiene (2). This hydrocarbon has been obtained from (1) by the modified Corey-Winter synthesis (5, 661). Some interesting reactions of... 

More recently, a Pd(II) salt was shown to catalyze the 1,2-insertion polymerization of a 7-oxanorbornadiene derivative (Fig. 10-16) [50]. The resulting saturated polymer, when heated, gives polyacetylene via a retro-Diels-Alder reaction. (This reaction is reminiscent of the Durham route to polyacetylene discussed below). One advantage of this technique over other routes is that it employs a late transition metal polymerization catalyst. Catalysts using later transition metals tend to be less oxophilic than the d° early transition metal complexes typically used for alkene and alkyne polymerizations [109,110]. Whereas tungsten alkylidene catalysts must be handled under dry anaerobic conditions, the Pd(II)-catalyzed reaction of water-insoluble monomers may be run as an aqueous emulson polymerization. 

The most efficient syntheses of oxepines from five-membered 0-heterocycles are based on Diels-Alder reaction of furans with alkynes, photolysis of 7-oxanorbornadienes obtained to respective 3-... 

Additional well-defined side-chain liquid crystalline polymers should be synthesized by controlled polymerizations of mesogen-ic acrylates (anionic or free radical polymerizations), styrenes (anionic, cationic or free radical), vinyl pyridines (anionic), various heterocyclic monomers (anionic, cationic and metalloporphyrin-initiated), cyclobutenes (ROMP), and 7-oxanorbornenes and 7-oxanorbornadienes (ROMP). Ideally, the kinetics of these living polymerizations will be determined by measuring the individual rate constants for termination and... 

Two intramolecular [2 + 2] photocycloadditions of note have been reported. The quantitative conversion of 7-oxanorbornadiene to the corresponding oxaquadri-cyclane has now been achieved despite many earlier efforts. Few syntheses of prismane derivatives can match the efficiency displayed in the cyclization to afford (45) which results from cycloaddition across the 1,4-diene system of the bicyclo-hexadiene, being best effected with light of wavelength 350 nm, which the product... 

Reaction of 7-oxanorbornadienes (e.g., LXVII) with [Rh(CO)2Cl]2 in CHCI3 affords 6-hydroxyfulvenes (e.g., LXVIII) in good yields (Hogeveen... 

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