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 .