CHIRAL SYNTHONS FROM CARVONE .23. SYNTHESIS OF CHIRAL TRICYCLO[3.2.1.0(2,7)]OCTANES BY AN EFFICIENT 3-EXO-TRIG RADICAL CYCLIZATION REACTION

Synthesis of chiral tricyclo[3.2.1.0(2,7)]octanes by efficient and exc lusive 3-exo-trig radical cyclisation of either a bicyclo[3.2.1]oct-6- en-2-yl radical or a bicyclo[2.2.2]oct-5-en-2-yl radical is described. Reaction of the methoxy enones 7a-c with boron tribromide at low temp erature furnishes the bicyclo[3.2.1]octenyl bromides 9a-c and 10a-c al ong with varying amounts of 8a-c, whereas the methoxy enone 7d provide s the unrearranged bicyclo[2.2.2]octenyl bromide 8d. Treatment of the homoallyl bromides 9a-c and 10a-c and 8d with tributyltin hydride and AIBN generates, exclusively, the tricyclic ketones 15a-d.