GILBERTSTORK AND ,4. W. BURGSTAHLER
,5068
Vol. 77
[CONTRIRUTIOS I)ROM THE CHEMICAL LABORATORIES O F HARVARD UNIVERSITY A S D THE CHAYD1,ER 1 , A n O P A l O R Y O F COLUMBIA UNIVERSITY]
The Stereochemistry of Polyene Cyclization BY GILBERTSTORK AND 4 . W. BURGSTAHLER RECEIVED MARCH17, I%% l'he steric course of the cyclizatioii of polyciie. i\ iiesylacetic acid : i i i d relalecl iiio:iocyclic isonier;.
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The formation of six-membered rings by the acidcatalyzed cyclization of 1,j-dienes is a well-known reaction which has been observed frequently in the terpene series. The structural requirements which
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must be met before a diene will undergo this type of cyclization have been discussed by Hibbit and Linstead.2 It is mechanistically significant that although sixmembered rings are the usual products from the cyclization of l,S-dienes, five-membered rings also have obtained (a) in the cyclization of 2,i-dimethyl2,B-octadiene (I) and (b) in cyclizations involving attack of an aromatic ring which lead to the formation of variable quantities of spirans as with I1 (R = H).4 These results are rationalized easily in the case of I which simply involves a combination of steric and electrical factors favorable to fivemembered ring formation and in the case of I1 the low nucleophilicity of the aromatic system allows the reaction to proceed non-concertedly with the formation of the better initiating carbonium ion. The preference for the formation of six-membered rings in this reaction, ceteris paribus, is well illustrated by the suppression of spiran formation when R in formula I1 is a methyl group.6
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