Zur Lenkung der intramolekularen Diels-Alder-Reaktion von Cyclopentadienen mit olefinischen Substituenten

On the Course of the Intramolecular Diels-Alder-Reaction of Cyclopentadienes with Olefinic Substituents The 1:3 mixture of 4-bromobicyclo 3.2.0]hept-2-en-6-one and -7-one ( 1/2 ), available by N-bromosuccinimide bromination of bicyclo 3.2.0]hept-2-en-6-one, reacted rapidly with the organo-magnesium and -zinc reagents 3, 10a, 10b and 10d by cyclobutanone ring opening and bromide ion expulsion to give the 5-substituted cyclopentadienes 5, 12a, 12b/12c , and 12d as non-isolated intermediates. Further transformation occured in situ either by a direct intramolecular Diels-Alder reaction (path a) or by a 1,5]-H-migration prior to the intramolecular Diels-Alder reaction (path b). The intermediate 5 followed only path a to give the bridged norbornene derivative 7 , the intermediates 12a, 12b and 12c followed only path b to give the annellated norbornene derivatives 15a, 15b and 15c , respectively, and the intermediate 12d followed both paths to give the bridged 14d and the annellated norbornene derivative 15d (in the ration of about 1.4:1). These observations are discussed in terms of the relative velocities of 1,5]-H-migrations and intramolecular Diels-Alder reactions. The major conclusions are: (1) bridged norbornene derivatives with a six-membered ring C (such as 14d ) can be prepared by an intramolecular Diels-Alder reaction from 5-alkenyl-cyclopentadienes 12 , as long as the dienophilic double bond is activated by an appropriate substituent (as in 12d ); (2) such 5-alkenyl-cyclopentadienes 12 are available from the reaction of the bromo-bicyclo-heptenones 1/2 with suitable C-nucleophiles 10 .