Abstract: | The stereochemistry of the transition state in the CLAISEN rearrangement of crotyl propenyl ethers ( 2 ) has been established. By heating trans, cis-, cis, cis- and trans, trans-crotyl propenyl ether at 142,5°, erythro and threo 2, 3-dimethylpent-4-en-1-al ( 3 and 4 ) were obtained. From the ratio 3/4 it was shown that the rearrangement of the three ethers largely involves (97–98%) a chair-like transition state. |