7-Oxanorbornadienes

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... 

Oligomers Oppolzer s camphor sultam (see also camphor derivatives) Organometallics Organosilicon Organosulfur Organozirconocenes Orthoquinodimethanes Oxabutadienes Oxanorbornadiene(s) Oxazinolactams Oxazinones Oxazoles... 

Scheme 6. Photochemical [2 + 2] cycloreversion of the dodecadehydro[18]annulene in furan as the solvent, followed by [4 + 2] Diels-Alder cycloaddition, gives the oxanorbornadiene adducts 38-40 [66]...

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]. 

Neither dimethyl oxanorbornadiene-2,3-dicarboxylate (128a) nor the related analogues 128b-d reacted with cyclooctyne [40] at 100 °C in a sealed... 

Scheme 35 Homo-Diels-Alder reactions of dimethyl oxanorbornadiene-2,3-dicarboxylate and related compounds 128 with cyclooctyne [81]...

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. 

Like the cyclization of the parent system, the photocyclization of hetero analogues of norbornadiene was reported many years ago. Recent interest in this area will serve to illustrate the potential of the conversions. Thus the irradiation of the oxanorbornadiene system 259 brings about conversion to the oxepine 260. The formation of 260 presumably arises by quadricyclane 261 formation, followed by secondary photolytic ring-opening. Other research has shown that irradiation of the oxanorbornadiene 262 follows the same reaction mode and it undergoes (2 -+ 2)-cycloaddition to the quadricyclane derivative 263. Apparently, in this instance, cycloreversion affords the ylide 264 that can be trapped by suitable addends, giving the adducts illustrated in Scheme 3. ... 

Both the acid induced and the photochemical rearrangement of the norbornaidiene derivative (146) affords the hydroxynaphthalene (147). The photo-process is not affected by triethylamine and it is thought to involve benzy lie-ally lie C-O bond cleavage. No evidence was put forward for the intermediacy of a quadricyclane. Prinzbach smd his coworkers have reported the synthesis of the oxanorbornadienes (148) and their photoconversion into the corresponding oxaquadricyclane derivatives (149) on irradiation. The azaquadricyclanes (150) have been prepared by the irradiation of the corresponding norbornadiene derivatives (151). ... 

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. 

---