7-Oxanorbomadiene

As a bridging function the silyl ether of 5,6-bis(hydroxymethyl)-7-oxanorbomadiene can be used giving products 15.143... 

All Diels-Alder reactions of tropones 51 as dienes with different types of dienophiles shown in Scheme 11 are accelerated by pressure, so that in some cases the desired cycloadducts are only formed at high pressure. An interesting synthetic equivalent of the unreactive acetylene in Diels-Alder syntheses is the oxanorbomadiene derivative 52 (Scheme 11 entry 2). 52 reacts with tropones forming the adducts 53, 54 and 55, which undergo a retro-Diels-Alder reaction leading to 56 and 57, the formal [4+2] cycloadducts of tropones to acetylene. 

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 oxanorbomadiene system 259 brings about conversion to the oxepine 260138. The formation of 260 presumably arises by quadricyclane 261 formation, followed by secondary photolytic ring-opening. Other research has shown that irradiation of the oxanorbomadiene 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 3139. 

Cycloaddition reactions with furans remain a source of new compounds. On photochemical excitation of benzofuro-annulated oxanorbomadiene 42 in the presence of dipolarophiles Ill-addition compounds are obtained. The formation of 43 constitutes one of the rare examples of the cycloaddition of a nitrile as a dipolarophile <95LA1503>. 

Cycloaddition of 7-oxanorbomadiene derivatives has been reported (Scheme 247). 5-Bromo-7-azanorbomenes undergo regioselective Pauson-Khand reaction to give (168). The bromine is lost during the reaction (Scheme 248). A-Protected alkynylamides undergo intramolecular reactions with alkenes to give 2-amidocyclopentenones (Scheme 249). 

Qxanorbornene and 7-oxanorbomadiene-derivatives were found to form observable and even isolable molybdametallacycles. Thus, 7-oxa-2,3-(bistrifluoro-mefhyl)norbornadiene reacts wifh Mo(N-2,6-i-Pr2-C,H),)(CHCMe),)(OCMe),)2 to form a remarkably stable metaUacycle [106]. A similar tungstametallacycle was observed in fhe reaction of 2,3-bis(trifluoromethyl)norbornadiene with W(N-2,6-i-Pr2-CgHsKCHCMesKOCMeslz [98]. 

Competition experiments have shown that 203 reacts more than 50 times faster with Mo(=CHCMe2Ph)(=NAr)(OCMe3)2 than does its norbomadiene analogue (139). The oxygen in the 7-position enhances the activity, presumably by assisting the initial coordination of the monomer to the molybdenum centre, although it is released once the metallacycle has formed. The comparative stability of the metallacycles formed from the 7-oxanorbomadiene derivatives is not the result of any direct interaction between the oxygen and the metal, and must therefore be due to inductive effects. Further competition experiments have shown that 203 also reacts about 30 times faster than norbomene with this initiator, but that when... 

Polymers of 7-oxanorbomene and 7-oxanorbomadiene derivatives can be degraded to telechelic oligomers using (Me3SiCH2CH=)2 with Mo(=CHCMe2Ph) (=NAr)[OCMe2CF3]2 as initiator (Viswanathan 1994). 

Fig. 12 Solid state mechanoactivation of oxanorbomadiene, producing an alkyne in the polymer backbone and releasing a small molecule furan. Reprinted with permission from [58] (Copyright American Chemical Society 2013)...

Kislukhin, A. A., Higginson, C. J., Hong, V. R, Finn, M. G. (2012). Degradable Conjugates from Oxanorbomadiene Reagents. Journal of the American Chemical Society, 134, 6491-6497. 

The Diels-Alder reaction of furans has been studied in detail [11]. Synthetically important are cycloadditions with acetylenic dienophiles, for example, acetylenedicarboxylates primary cycloadducts are oxanorbomadienes (e.g., 23), which isomerize to phenols on treatment with acids (e.g., 24). The selective hydrogenation of 23 to 25 followed by a (4 + 2)-cycloreversion yields the furan-3,4-dicarboxylate 26 ... 

Diels-Alder reaction/retro-Diels-Alder sequence can be applied to the synthesis of furans with unusual substitution pattern. For example, the reaction of furan 69 and 2-methylfuran 83 with hexafluorobut-2-yne 84 afforded 7-oxanorbomadienes 85, which were further transformed into 3,4-bis(trifluoromethyl)furans 87 through the intermediate 86 [72-75], Temperature of retro-Diels-Alder reaction should be relatively high in this case. However, this drawback can be circumvented by the addition of tetrazine 88. It was shown that reaction of 85a with this reagent proceeds at room temperature giving rise to 3,4-bis(trifluoromethyl)furan 87a through the Diels-Alder cycloaddition and double retro-Diels-Alder reaction [76]. 

Hydrogenation of the intermediate 7-oxanorbomadiene is not always a necessary step. Thus, reaction of ethyl trifluorobutynoate with 2,5-dunethytfiiran 94 at 150 °C for 24 h afforded a mixture of cycloadduct 95 and tetra-substituted furan 96 in a ratio of 1 9 [80]. 

Acetone-sensitized photoreaction of 137 with butyne-2 gave products of mono-248 and bis [2h-2] cycloaddition 249 [149], Oppositely, 3,4-bis(trifluoromethyl) furans 250 participate in the Diels-Alder reaction. They reacted with hexafluorobut-2-yne 84 giving 7-oxanorbomadiene 251 [72, 73, 77, 150], Treatment of 251 with TiCl4/LiAlH4/Et3N system led to the formation of tetrakis(trifluoromethyl)benzene 252 [77], Irradiation of 251 produced mixture of cyclopentadienes 253 and 254 as well as oxepin 255 in variable ratios depending on the solvent [150]. 

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