Radical akcarboxylation of isorOngifolic acid & @ords in good yield the &o-ether 4 through a reaction involving a clean IJ-transannular hydrogen shift. The thio-ether 4 was fiuther converted in almost quantitative yield into the o&in Z through sequentkl oxidation-elimination. Longifolene 1 is a sesquiterpene discovered by Simonsen in Pinus bngifolia. and encountered in many other Pinus speciesl. Its structure was established by X-ray crystallography2 and extensive degradative studies3. Longifolene allies the rigidity of the camphene skeleton and a supplementary severe steric hindrance due to the presence of the cycloheptane ring bridging C-l and C-5. Compounds of this type can undergo numerous Wagner-Merwein rearrangements and cationic hydride migrations4.
Furthermore, when treated with tetrabromomethane (or bromoaichloromethane) in the presence of peroxides, longifolene 1 did not afford the normal product of addition of TX3 and X* moieties (X= Br, Cl) across the 7( 15) double bond, but yielded unexpected trihalogeno-bromo derivatives (10% and 30% yields respectively) to which structure 2 were assigned on the basis of nmr data and the study of the products of solvolysis5. A mechanistic interpretation was proposed in which the 1,5-shift of an H-atom was the key step. Although a few examples of such transannular hydrogen migrations were later reportedo, the possible operation of an ionic pathway ( viz. normal addition across the double bond followed by heterolysis of the C-Br bond and hydride migration from C-3 to C-7, possibly catalysed by traces of HBr) could also be put forward.