Wittig-Oxy-Cope

S ily 1) ally lie ethers 234 readily undergo 2,3-Wittig rearrangement upon treatment with an amide base to give, after methylation, 235 (equation 192)37,343. Tandem Claisen-2,3-Wittig-oxy-Cope rearrangement has been observed when 236 is treated under similar... 

Ethyl lactate was used for diastereocontrol and asymmetric transmission in a sequential 2,3-Wittig-oxy-Cope rearrangement, affording product in 91% ee (eq 9). - 4 Excellent asymmetric induction has also been noted in the Lewis acid-mediated ene reaction of (5)-ethyl lactate-derived intermediates (eq 10). ... 

The [2,3] Wittig oxy-Cope reaction sequence proceeds with a high level of diastereoselectivity. Thus, aldehyde 14 a is obtained from 13 a (96% stereopurity) with 94% de and 99 % E. and 13 b (92% stereopurity) gives aldehyde 14b with 88% de and 97% E by the reaction sequence shown966. 

Mercury(n) triiluoroacetate has been shown to induce the oxy-Cope rearrangement of tertiary hexa-l,5-dien-3-ols at room temperature. Further details of the stereochemical consequences of sequential tandem Cope-Claisen rearrangements have been disclosed, and the use of the tandem [2,3]-Wittig-oxy-Cope rearrangement has been demonstrated in the synthesis of (E S-octenal, a precursor of exo-brevicomin [equation (36)]. Investigation of the trans-... 

Scheme 2.193. Domino [2,3]-Wittig/anionic-oxy-Cope rearrangement process...

An earlier example of this type of domino reaction was reported by Greeves and coworkers (Scheme 2.194) [443]. Treatment of either the ( )- or (Z)- allyl vinyl ether 2-870 with NaH initiates the [2,3]-Wittig rearrangement to afford 2-872 via 2-871. The subsequent oxy-Cope rearrangement led to the aldehyde 2-873, which was reduced with NaBH4 to give the alcohols 2-874. Both isomers of 2-870 predominantly generated the (/ )-xyu-product 2-874 in comparable ratios as the main product. 

Lastly it should be mentioned that a 2,3-Wittig rearrangement can be coupled with other sigmatropic processes in synthetically valuable tandem sequences. An illustrative example is a tandem 2,3-Wit-tig-oxy-Cope rearrangement, which provides a versatile route to 8,e-unsaturated carbonyl compounds (Scheme 46). ... 

It is remaikable that the ( )/(Z)-stereochemistry of the oxy-Cope product is not dependent on the erythrolthreo ratio of the 2,3-Wittig rearrangement but on the specific rearrangement procedures used thermal rearrangement exhibits higher ( )-selectivity (92-95%) than the anionic oxy-Cope and siloxy-Cope variants. 

Allylation of the 2,3-Wittig products (167) leads to diallylic ethers (171) in which one of the allylic double bonds is part of a 1,5-diene system. These ethers undergo 3,3-oxy-Cope rearrangement to afford vinyl allyl ethers (170), which rearrange in situ by a 3,3-Claisen process to yield the ( )-dienals (169). Some typical results are summarized in Table 14. The cyclopentanols (175) were shown to arise from aldehydes (169) by an intramolecular thermal ene reaction. ... 

For instance, the one-pot tandem reaction [2,3]-Wittig-anionic oxy-Cope rearrangement affords the unsaturated aldehydes 571 starting from the bis-allylic ethers 570 at a high level of stereocontrol (equation 224). It was shown that the efficiency of chirality transfer in anionic oxy-Cope rearrangements is determined only by the orientational preference of the oxyanionic bond in the precursors having a single carbinol carbon chiral center . 

---