Wittig reagents stereochemistry

Consider the problem of making cis enone (8), a structure found in insect pheromones, flavourings, and perfumes. A Wittig reaction would give the right stereochemistry (Chapter 15) but would require the selectively protected keto aldehyde (9), as a Wittig reagent is probably too reactive to show the required chemoselectivity. 

Tables 6 and 7 summarize results from stereochemical equilibration studies performed over the past decade by MaryanofT et al. (22,23), and Vedejs et al. (20, 21c, 39-42). A few other convincing examples are included to expand the scope of the systems covered. Table 6 lists those examples where control experiments establish at least 90% retention of stereochemistry from intermediates-to alkene products. As already discussed, the percentage of equilibration represents the upper limit for loss of stereochemistry from all possible pathways in the control experiments. No attempt has been made to determine whether the minor levels of stereochemical leakage in Table 6 occur at the stage of oxaphosphetanes, betaines, or other potential intermediates. Table 6 includes entries corresponding to all of the principal families of Wittig reagents nonstabilized ylides (entries 1-12, 24, 25, 29, and 30), benzylic ylides (entries 13-17 and 28), allylic ylides (entries 22, 23, 26, and 27), and ester-stabilized ylides (entries 18-21). The corresponding Wittig reactions must take place under dominant kinetic control.

The synthesis below offers a nice illustration of the contrasting selectivity of the two classes of Wittig reagent. The female silkworm moth attracts mates by producing a pheromone known as bombykol. Bombykol is an ,Z-diene, and in this synthesis two successive Wittig reactions use first a stabilized and second an unstabilized ylid to control the stereochemistry of the product. 

PhSe,HsOAc (SO) furanoses by treatment with Wittig reagents. In the first of these the 2-a-lkene gave a -product with specific stereochemistry at the iodine-carrying atom whereas the E-isomer afforded a specific a-... 

The di-, tri-, and tetra-methyl-3-thietanones have been prepared by the ring-closure of cr,a -dibromo-ketones with sulphide ion. The stereochemistry of the dimethyl compounds was determined by reduction to the thietanols. 3-Thietanones can also be prepared by the reaction of ketones with thionyl chloride. The Wittig reagent (85) has been prepared. ... 

Intracluster reactions are often induced by photons, as discussed in several chapters in this volume. One reason for the strong interest in such processes is that the relative geometries of the reagents are dictated by the cluster geometry so that one can, in principle, control the stereochemistry between them as if carrying out bimolecular collisions with oriented reactants (Wittig et al. 1988). 

The action of a chiral Homer-Wittig type reagent on a racemic aldehyde has been mentioned in Section 2.3.7. In the E/Z mixture of products arising from olefination of 109, the -stereoisomer is mainly formed from one enantiomer of the aldehyde 109 while the Z-isomer is derived from the other enantiomer. Here, each enantiomer gives a defined stereochemistry for the double bond that is created (69). A new stereogenic unit is formed, which is conceptually similar to the formation of a new asymmetric centre in a racemic substrate. 

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