Still-Gennari olefination stereoselectivity

Fig. 11.15. Analysis of the overall stereoselectivity of a Still—Gennari olefination such as the one in Figure 11.13 simple diastereoselectivity of the formation of the alkoxide intermediate from the achiral phosphonate A and the achiral aldehyde B. For both reagents the terms "back face" and "front face" refer to the selected projection.

Scheme 3.20 High stereoselectivity (>98%) observed in the synthesis of (2Z,4 )-ethyl undeca-2,4-dienoic esters. Scope and limitations of the Still—Gennari olefination [104].

In C.J. Forsyth s total synthesis of phorboxazole A, the intramolecular version of the Still-Gennari modified HWE olefination was used to affect the macrocyclization of a complex it>/s(trifluoroethoxy) phosphonate-aldehyde precursor. The precursor was dissolved in toluene and was exposed to K2CO3 in the presence of 18-crown-6. The desired C1-C3 (Z)-acrylate moiety was formed in 77% yield with a 4 1 (Z ) ratio. Interestingly, when the same cyclization was carried out with the regular /)/s(dimethoxy) phosphonate, the macrocyclization was markedly slower, but the stereoselectivity was the same (4 1). 

SG A olefination (Still-Gennari and Ando Generally not applicable Generally not applicable Can be high-yielding and highly (>98%) selective Not stereoselective (Eqs. (6) and (7),... 

The Homer-Emmons olefination shows a preference for the formation of the more stable ( )-olefins. On the other hands. Still and Gennari modified HWE olefination shows high (Z)-stereoselectivity and high yield in the preparation of unsaturated esters, 45(Z) 46( ) = > 50 1, from a variety of aromatic, saturated and unsaturated aliphatic aldehydes. ... 

---