Zur Stereochemie der aromatischen CLAISEN‐Umlagerung

Abstract

AbstractThe possible transition state conformations (chair (S), boat (W), and twist (T), respectively cross (K) forms) and methods for their determination in the thermal ortho‐CLAISEN rearrangement of allyl aryl ethers are discussed.Crotyl 3,5‐dimethylphenyl ether (11) gives a mixture of 2‐(α‐methylallyl)‐3,5‐dimethyl‐phenol (12) and 4‐crotyl‐3,5‐dimethyl‐phenol (13) on heating in N, N‐diethylaniline. Values of 3 and 31 were obtained for the ratio of 12/13 for trans‐11 and cis‐11, respectively. It therefore follows that both ethers rearrange steroselectively ( > 90%) by the S or W forms of the activated complex.αMethylallyl 6‐alkylphenyl ethers rearrange on heating in various solvents to a mixture of trans‐and cis‐2‐crotyl‐6‐alkyl‐phenols. The amount of the cis‐phenols in the rearrangement products decreases with the increasing bulk of the 6‐alkyl substituent. This result is only obvious if the chair form of the transition state during the rearrangement of these ethers is highly favoured.trans‐Crotyl 2,6‐dimethylphenyl ether (trans‐33) rearranges highly steroselectively (94%) on heating to trans‐4‐crotyl‐2,6‐dimethyl‐phenol (trans‐34). In the case of the corresponding cis ether 33, the rapid cis → trans isomerisation of this ether and the cis/trans ratio of the phenol 34 indicate that the reverse rearrangement of the intermediate ortho‐dienone to the ether 33 and the further rearrangement to 4‐crotyl‐2,6‐dimethyl‐phenol (34) has little stereoselective character.