On the geometric requirements for concerted 1,2-carbonyl migration in α,β-epoxy ketones

Solid-Acid Catalysed Rearrangement of Cyclic α,β-Epoxy Ketones

“…The assignments cis and trans were based on measurement of the optical rotation of this mixture and known optical rotation values for the separate isomers from ref. [5] Piperitone Oxide (9): Piperitone oxide was prepared from piperitone according to the procedure described above. Yield of crude product 41%.…”

“…Acid-catalysed conversion of 2-hydroxy-2-isopropenyl-5-methylcyclohexanone (8) to 2,2,5-trimethylcycloheptane-1,3-dione (6) In striking contrast, homogeneous rearrangement of 5 with BF 3 ⋅Et 2 O in refluxing benzene gave 6 in an isolated yield of 92%. [5] In this case, it was shown [5] that the reaction proceeds via fluorohydrin intermediates (see Scheme 6). Since the flexible fluorohydrins could readily achieve the required conformation for concerted 1,2-acyl rearrangement, both diastereomers of pulegone oxide resulted in the formation of 6 as the sole product.…”

“…Fluorohydrins in the BF 3 ⋅Et 2 O-catalysed rearrangement of 5 [5] Since involvement of fluorohydrin-like intermediates is precluded with the heterogeneous catalysts, rearrangement of pulegone oxide induced by solid acids will, therefore, probably occur via an unfavourable transition state. In such cases, the rate of substituent migration is probably much faster than the rate of rotation to an optimal conformation, and the result is therefore likely to be a divergent package of products.…”

“…Synthesis of 2,2,5-Trimethylcycloheptane-1,3-dione (6): The homogeneous rearrangement of pulegone oxide was based on a method of Bach and Klix. [5] BF 3 ⋅Et 2 O (32 mmol) was added to a solution of pulegone oxide (16 mmol) in 50 mL of benzene. The resulting solution was refluxed for 1 min.…”

“…Acyl migration results in ring contraction and is favoured over hydrogen migration when BF 3 ⋅Et 2 O is used as a catalyst, particularly if an additional substituent is present at the β-position. [3,5,6] Conversely, 1,2-diones were found to be the major products upon treatment of cyclic α,β-epoxy ketones with aqueous solutions of hydrochloric acid [7] or sulfuric acid. [8] Probably, hydrolysis of the epoxide to its corresponding diol precedes the actual rearrangement under these conditions.…”