Chelation 1,2 -Wittig rearrangement

As an extention of these studies, Maleczka and Geng have investigated a chelation effect in the diastereoselective [1,2]-Wittig rearrangement where a lithium-bearing terminus is enantiomerically defined. ... 

Polyols synthesis A two-directional application of chelation-controlled [1,2]-Wittig rearrangement has been used to prepare the all syn polyol 31 from 30 (equation 16) °. 

Maleczka, R. E. Geng, F. Stereospecificity of the 1,2-Wittig rearrangement how chelation effects influence stereochemical outcome. 

Wittig rearrangement of a-allyloxycarboxylic acid dianions and allyl propargylic dianions (Section 3.11.3.3) might be expected to proceed analogously. In fact, the same high preference for ( )-pro-ducts is observed, but the diastereoselectivity is reversed (Table 17, entries 5 versus 6). A chelated bicyclo [3.3.0] transition state readily explains the anti selectivity of (Z)-allylic ethers [Scheme 12, compare (R) with (T)]. The basis for syn selectivity observed with ( )-allylic ethers (Q) versus (S) is less clear. 

The synthesis of Cbz-protected D-valine methyl ester (296) (Scheme 40) begins with addition of an organometallic reagent to the ester function of 282. The resulting phosphonate 290 undergoes a Wittig reaction with isobutyraldehyde to afford 291. Chelation-controlled reduction of the ketone with zinc borohydride furnishes the a /-alcohol 292 (98% de). A [3,3] rearrangement of trifluoroacetimidate 293 produces allylic amine 294. Elaboration of the olefin to an ester furnishes the D-valine derivative 296 with 85% ee [101]. 

The [2,3]-Wittig-Still rearrangement of the phenyl sulfide 51 (n = 0) afforded a mixture of 52 and 53 from a and p approach in 8 92 ratio fScheme 17.9T From the corresponding sulfone 51 (n = 2), use of 2 equiv. n-BuLi selectively yielded 54 in 65% yield with the a stereochemistry, resulting from lithium-ion chelation with the sulfonyl methyl and oxymethyl lithium (51a). The synthesis was continued to attach the side chain, and alkylative macrocyclization gave diol 57 (Scheme 17.9). ... 

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