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[5.5.0.0 1,4 ]‐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 CCI 4 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 SnCI 4 in nitromethane at room temperature 11 was in turn cyclised to cis ‐3, 3, 7, 10‐tetramethyl‐2‐oxa‐tricyclo[5.5.0.0 1,4 ]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 H 2 AICI or HAlCI 2 , to form the β‐acoratriene ( 13 ).Finally, the BF 3 ‐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.