7-Oxanorbomadiene derivatives

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). 

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. 

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