Horner-Wadsworth-Emmons Still-Gennari variant

The Ando variant is an alternative to the Still-Gennari variant of the Horner-Wadsworth-Emmons reaction. Here, phosphonates are employed that contain two aryloxy residues, for example, the ortho-tolyloxy residues of the phosphonate A. The Ar-O groups in this reaction... 

Fig. 11.14. Preparation of tmns- or f-configured a,/3-unsaturated esters by the Horner-Wadsworth-Em mons reaction (left) or preparation of their cis- or Z-isomers by the Still-Gennari variant of it (right). 18-Crown-6 is a so-called crown ether containing a saturated 18-membered ring that is made up from six successive —CH2—0—CH2-units. 18-Crown-6 dissociates the K ions of the Horner-Wadsworth-Emmons reagent by way of complexation.

The stereostructure of the alkoxide intermediate of a Horner-Wadsworth-Emmons reaction which finally leads to the trans-o cim was recorded in Figure 9.14 (as formula A). The Still-Gennari variant of this reaction (Figure 9.15) must proceed via an alkoxide with the inverse stereostructure because an olefin with the opposite configuration is produced. According to Figure 9.16, this alkoxide is a 50 50 mixture of the enantiomers C and ent-C. Each of these enantiomers contributes equally to the formation of the finally obtained cw-configured acrylic ester D. 

In the Horner-Wadsworth-Emmons (HWE) reaction, a lithium, sodium or potassium salt of a P-keto-phosphoric acid dialkyl ester or an a-(alcoxycarbonyl) phosphoric acid dialkyl ester is reacted with an aldehyde to form an a,p-unsaturated ketone or an a,p-unsaturated ester as product and a phosphoric acid dialkyl ester as byproduct. The product of a Horner-Wadsworth-Emmons reaction possesses a new -configured C=C double bond. To obtain products with Z-confi-guration of the C=C double bond the Still-Gennari variant or the Ando variant can be employed. The Still-Gennari variant uses two partly fluorinated F3C-CH2-O groups at the phosphorus and the Ando variant uses two aryloxy residues and the transferable alcoxycarbonyl group at the phosphorus. 

This is the Still-Gennari variant of the Horner-Wadsworth-Emmons reaction. 

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