BRIDGED MEDIUM RINGS WITH HYPERSTABLE DOUBLE-BONDS - SYNTHESES AND REACTIONS

Bridged medium rings contain considerable steric strain that is reliev ed to some extent by the introduction of unsaturation. Nevertheless, s train makes the respective polyenes prone to undergo transannular rear rangements and hinders their ring inversion. Double bonds positioned a t a bridgehead are especially effective in the release of strain and a re appropriately called hyperstable. In this account four bicyclo[6.2. 2]- and bicyclo[7.2.2]alkenes with a bridgehead double bond are descri bed. A key step in their synthesis is the oxy-Cope rearrangement of pr ecursors with a smaller bicyclic skeleton. The double bonds in these c ompounds belong to the most hyperstable in bicyclic hydrocarbons. Thei r interconversion as well as their reaction with protons and with bora ne are described. In a sense the chemistry of hyperstable double bonds miners that of their aromatic counterparts: they undergo electrophili c substitution and refrain from addition, but they cannot reverse the keto-enol equilibrium. In contrast to aromatic double bonds hyperstabl e double bonds readily take part in transannular reactions.