Applications synthétiques de la cyclisation d'alcools tertiaires γ‐éthyléniques en α‐bromotétrahydrofurannes sous l'action du N‐bromosuccinimide. II. Cyclisation du (±)‐nérolidol en diméthyl‐2,5‐(méthyl‐4‐pentène‐3‐yl)‐2‐cycloheptène‐4‐one, tétraméthyl‐3, 3, 7, 10‐oxa‐2‐tricyclo[5.5.0.01,4]‐dodécène‐9, β‐acoratriène, cédradiène‐2,8, épi‐2‐α‐cédrène et α‐cédrène

The ionic reaction of (±)‐nerolidol (cis/trans mixture) with N‐bromosuccinimide in CCI4 at room temperature afforded 2‐methyl‐2‐vinyl‐5‐(2‐bromo‐6‐methyl‐hept‐5‐en‐2‐yl)‐tetrahydrofuran (4) in high yield. This compound was readily dehydrobrominated by refluxing collidine to the intermediate allyl vinyl ether 8, which immediately undergoes [3,3]‐sigmatropic rearrangement to 2, 5‐dimethyl‐2‐(4‐methylpent‐3‐enyl)‐cyclohept‐4‐enone (11). By treatment with SnCI4 in nitromethane at room temperature 11 was in turn cyclised to cis‐3, 3, 7, 10‐tetramethyl‐2‐oxa‐tricyclo[5.5.0.01,4]dodec‐9‐ene (12), an oxetane closely related to the sesquiterpene carotol. This oxetane (12) underwent a stereospecific ring contraction when treated by Lewis acids such as H2AICI or HAlCI2, to form the β‐acoratriene (13).Finally, the BF3‐catalysed cycli‐ sation of the latter afforded 2,8‐cedradiene (19) from which 2‐epi‐α‐cedrene (20) was easily obtained by partial, regio‐selective hydrogenation. α‐Cedrene (22) itself, together with its epimer 20, resulted from the Wolff‐Kishner reduction of the 2‐epi‐α‐cedren‐3‐one (21), prepared by selective hydroboration/oxidation of 19. A transformation of nerolidol to α‐cedrene was thus achieved by a unique stepwise cyclisation process.