Having had no responses to this post, I thought I would offer a scheme in an effort to answer the question about the existence of the oxy Claisen rearrangement. α-Diketone 1[1] was chosen as a starting material owing to the equivalency of the two carbonyls and to insure that the all substituents in aldehyde 7 are equatorial. Selective monoketalization of α-diketone 1 with β-bromoethanol affords cyclohexanone 2. Over ketalization is unlikely owing to steric considerations. Wittig reactions of the type 2 $\rightarrow$ 3 have been conducted successfully.[2]
The elimination of HBr from 3-(2-bromoethoxy)prop-1-ene with powdered KOH to form 3-(vinyloxy)prop-1-ene (allyl vinyl ether) was conducted by Hurd and Pollack.[3] Ideally, allyl vinyl ether 4 would be a more suitable precursor for an alkoxy Claisen rearrangement than divinyl ether 5, given the competing resonance stabilization introduced into the structure by the additional vinyl ether group. Nonetheless, vinyl ethers 4 and 5 may lead to rearrangement products 8 and 6, respectively. Acidic hydrolysis of either of these aldehydes under equilibrating conditions would afford keto aldehyde 7. While the attainment of keto aldehyde 7 would be an achievement, its realization does not determine whether the product is obtained by a concerted rearrangement, or by dissociation into an acetaldehyde enolate/oxyallylcarbocation pair of ions followed by recombination. Barring an intimate ion pair, a crossover experiment is required.
A crossover experiment requires isotopic labeling in both residues involved in the transformation. Deuterium may be incorporated judiciously in both moieties through the use of 2-bromoethan-1,1-d2-1-ol[4] and (methylene-d2)triphenyl-λ5-phosphane leading to substrate 10. If rearrangement is concerted, then keto aldehyde 14 will contain three non-exchangeable deuteria.
In the event that a dissociative mechanism applies, then a near statistical mixture of keto aldehydes
16-d
2 and
17-d
1 is expected when an equal mixture of
3-d
0 and
10-d
6 undergo the rearrangement process. Both
1H NMR and mass spectrometry would be critical in an analysis of labeling patterns. The presence of the methyl groups in the deuterated products provides an internal standard for NMR integration. These keto aldehydes will likely lend themselves to McLafferty rearrangements.
- R. Lenz, S. V. Ley, D. R. Owen, S. L. Warriner, Tetrahedron: Asymmetry, 1998, 8, 2471.
- M. Pellet, F. Huet, J. M. Conia, Tetrahedron Lett., 1977, 39, 1979.
- C. D. Hurd, M. A Pollack, J. Am. Chem. Soc., 1938, 60, 1905.
- I. Bird, P. B. Farmer, J. Labelled Compounds and Radiopharmaceuticals, 1989, 27, 199.
