Acetals 1.2 -Wittig rearrangement

The acetal [1,2]-Wittig rearrangement protocol is also applicable to the synthesis of medium-sized cyclic ethers. For example, a reaction of the 9-membered cyclic acetal 37 with lithium piperidide provides the 8-membered ring ether 38 in good yield along with high diastereoselectivity (equation 20) . 

Tomooka, K, Kikuchi, M, Igawa, K, Suzuki, M, Keong, P H, Nakai, T, Stereoselective total synthesis of zaragozic acid A based on an acetal [1,2] Wittig rearrangement, Angew. Chem. Int. Ed., 39, 4502-4505, 2000. 

The aza-[2,3] Wittig rearrangement of aziridines is an excellent method for the synthesis of substituted piperidines. The analogous reaction of an epoxide has recently been examined <06TL7281>. Reaction of divinyl epoxide 48 with /-butyl diazo acetate provides the ylide intermediate 49, which then undergoes the [2,3] Wittig rearrangement to 50, Several catalysts were examined as catalysts for the formation of 49. It is noteworthy that the copper catalyst performed much better than the more widely used rhodium catalysts. 

Scheme 4.60 Chiral base-induced [2,3]-Wittig rearrangement of a-(propargyloxy)acetic acid 234.

Recently, Gartner and colleagues have reported the [1,2]-Wittig rearrangement of camphor-derived acetal 39 to 40 (equation 21) . 

TABLE 7. Thia-[2,3]-Wittig rearrangements of stannylated precursors and i, i -acetals ... 

Wittig rearranged products derived from (Z)-substrates. It is possible that this rearrangement proceeded via a six-membered transition state in which a lithium atom co-ordinates to an oxygen atom of a methoxy group to afford (Z)-ketene silyl acetal preferentially [38]. (Fig. 3)... 

Table 8. Stereoselective Ester Enolate [2,3] Wittig Rearrangement of Substituted Methyl (Z)-2-[(4,4,4-Trifluorobut-2-enyl)oxy]acetates (Z)-25 to y.ti-U nsaturated anti-( )-a-Hydroxy-/3-(trifluoromethyl) Esters (E)-2617...

Silyl enolates generated from a-allyloxy esters undergo the [2,3]-Wittig rearrangement on treatment with Lewis base such as tetrabutylammonium acetate or tetrabuty-lammonium 4-methoxybenzoate (Scheme 10).14... 

The Still-Wittig rearrangement of stannylated cis-ethers (11) gives trans homoallylic alcohols (12) selectively (equation 5). If the underlying alcohols (R = Me) are rearranged under Biichi s conditions (DMF acetal, xylene, reflux), trans selectivity is achieved, too, but yields are lower (41-49%). ... 

A similar substrate-induced 1,2 and 1,3 diastereoselection has been described for the [2,3] Wittig rearrangement of a series of allyloxy acetates 18 80. As in the case of propargyl-stabilized systems, Z-olefins 18 undergo rearrangement with excellent simple and induced diastereoselection to afford major products which compose > 98 % of the product mixture. The correspond-... 

The auxiliary-mediated [2,3] Wittig rearrangement of nonracemic allyloxy acetates has to date received little attention. Enolates of the 8-phenylmenthyl esters 42 (R1 = CH3, Bu) undergo [2,3] rearrangement with good syn selectivity (>90%) and comparable levels of induced diastereose-lection85. 

Reagent-induced diastereoselectivity remains relatively unexplored for the [2,3] Wittig rearrangement. In the presence of the chiral amide base (S,S)-44, rearrangement of the propargyl-oxy acetic acid 45 affords a modest excess of the -alcohol 4676. 

Scheme 6.15. [2,3]-Wittig rearrangements of chiral propargyloxy acetates (a) Zirconium ester enolates [81,82]. (b) Lithium endiolates, S = solvent [81-83].

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