Olefinations Still-Gennari

HORNER-WADSWORTH-EMMONS OLEFINATION - STILL-GENNARI MODIFICATION... 

Horner-Wadsworth-Emmons Olefination (Still-Gennari modification) 214... 

Related reactions , Horner-Wadsworth-Emmons olefination - Still-Gennari modification, Julia-Lithgoe olefination, Peterson... 

Related reactions Horner-Wadsworth-Emmons olefination, Homer-Wadsworth-Emmons olefination - Still-Gennari modification, Julia-Lithgoe olefination, Takai-Utimoto olefination, Tebbe oiefination, Wittig reaction, Wittig reaction - Schiosser modification ... 

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 trisubstituted (Z)-olefin was introduced by Still-Gennari HWE olefination, as precedented by Schreiber [43, 44, 106], and following silyl protection provided 124. Conversion into the iodide 125 was followed by alkylation with the lithium enolate of aryl ester 126, to complete the C9-C16 subunit 121. The synthesis of the C17-C24 subunit 98 from 120 began with a four-step sequence involving protecting group manipulations and oxidation at C21 to provide aldehyde 127, converging with the earlier route to 98 [55-57],... 

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.

Fig. 11.16. Analysis of the simple diastereoselectivity of a Still-Gennari olefination that starts from the enantiomeri-cally pure phosphonate A and the achiral aldehyde B.

Fig. 11.19. Still-Gennari olefination of a racemic a-chi-ral aldehyde with an enan-tiomerically pure phosphonate as kinetic resolution I—Loss of the unreactive enantiomer ent-B of the aldehyde (R stands for the phenylmenthyl group in the Horner-Wadsworth-Emmons products the naming of the products in this figure is in agreement with the nomenclature of Figures 11.17 and 11.18).

A completely analogous kinetic resolution succeeds with the Still-Gennari olefination of Figure 11.20. Here the racemic substrate is a different oc-chiral aldehyde. It carries a sulfon-... 

The Still-Gennari olefination in Figure 11.21 is recommended to anyone who wants to enjoy a third stereochemical dehcacy. The substrate is a dialdehyde that contains oxygenated stereocenters in both a-positions. Nevertheless, this aldehyde is achiral because it has a mirror plane and thus represents a mew-compound. Meso-compounds can sometimes be con-... 

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. 

The Still-Gennari olefinations of Figures 9.17-9.22 start from similar substrates as those shown in Figure 9.16. However, at least one of them is chiral. Since each of these... 

We now analyze the Still-Gennari olefination of Figure 9.18. The reagents there are the enantiomerically pure chiral phosphonate A, with which you are familiar from Figure 9.17, and an enantiomerically pure a-chiral aldehyde B. The diastereoselectivity of the formation of the crucial alkoxide intermediate(s) is in this case determined by the interplay of three factors ... 

The mismatched case of a Still-Gennari olefination, which corresponds to the matched case of Figure 9.18, can be found in Figure 9.19. There, reagent control and substrate control act against each other. The substrate in Figure 9.19 is the enantiomer... 

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