Umwandlung von Bicyclo [3.2.0]hept‐2‐en‐6‐on in Cyclopentadienessigsäure‐Derivate

Conversion of Bicyclo [3.2.0]hept‐2‐en‐6‐one into Cyclopentadienylacetic Acid Derivatives The reaction of a mixture of 4 exo ‐bromobicyclo [3.2.0]hept‐2‐en‐6‐one ( 2 ) and ‐7‐one ( 3 ) with O‐ or N‐nucleophiles yielded cyclopentadien‐5′‐yl‐acetates 4a–f or‐acetamides 4g–h . Due to their rapid isomerization, the products 4 were not isolated, but some of them were demonstrated spectroscopically or captured in situ with maleimide as 10′‐substituted norbornene derivatives 7 . The formation of 4 from 2/3 involves a fragmentation of the bond between the carbonyl and the bridge‐head C‐atom, induced by the attacking nucleophile and the leaving Br‐ion and aided by the relief of the four‐membered ring strain. Some of the isomerization products of 4 , i.e. the cyclopentadiene‐1′‐yl‐ and 2′‐yl‐acetyl derivatives were captured with maleimide as the 1′‐ and 8′‐substituted norbornene‐derivatives 8 and 9 . Two C‐nucleophiles did not induce the fragmentation: sodium acetylacetonate substituted the Br‐atom and sodium (diethoxyphosphoryl)ethoxycarbonylmethide condensed with the carbonyl group of 2/3 , yielding 11/12 and 13/14 , respectively.