Horner-Emmons olefination procedur

Similarly, starting from 2,3 5,6-di-0-isopropylidene-D-mannofuranose 72, a one-pot procedure that implies the synthesis of glycosyl sulfones by Horner-Wadsworth-Emmons olefination with phosphonate 73, followed by subsequent Michael addition and Ramberg-Backlund reaction92 gives compound 74 in 78% yield93 (Scheme 24). 

Fig. 11.28. Aldehyde alkyne chain elongation via [1 -rearrangement of a vinyl carbene (Seyferth procedure). First, a Horner-Wadsworth-Emmons olefination of the aldehyde is carried out to prepare alkene A. Upon warming to room temperature, alkene A decomposes and gives the vinyl carbene B. From that, the alkyne is formed by way of a [1,2]-rearrangement.

An important addition to the Wittig tran -olefination procedure is the introduction of phosphonate-stabilized carbanions as olefin-forming reagents, referred to as the Horner-Wadsworth-Emmons or HWE reaction. The HWE olefination offers several advantages over the Wittig reaction using stabilized ylides ... 

The synthesis of the Cl-Cl5 segment (116) [198] of streptovaricin D (109) has been carried out from the aldehyde (115) [199], an enantiomer of an intermediate used by the same authors in the synthesis of the rifamycin ansa chain. Compound (105) was submitted to the Evans aldol procedure to give an aldol, which was transformed into the corresponding acetonide. Ozonolysis to the expected aldehyde and crotylmetallation with chromium(II) chloride provided a 5 1 mixture of diastereoisomers. The major component was acylated, giving only one diastereoisomer after ozonolysis and crotylboration. Final introduction of the corresponding (2 ,4Z)-dienoate unit by two subsequent Horner-Wadsworth-Emmons olefinations afforded compound (116) (Scheme 51). 

Olefination. Although the conventional procedure for the Horner-Emmons-Wadsworth reaction calls for enolate generation with NaH in an aprotic solvent, more complex substrates tend to give unsatisfactory results due to elimination and/or epimerization. The Ba(OH)2/aqueous THE system is uniquely effective at room temperature in such cases. Usually 0.3 equivalent of activated (by heating 100-140°C for... 

The strategy proposed by Kang for achieving a synthesis of lasonohde A is outlined in Scieme 15. For the introduction of the side chain, Lee s procedure was adopted. Macrocychzation was performed via the Horner-Emmons reaction, and the construction of the C12-C28 triene unit was similar to Lee s protocol using Juha-type olefination and Still s ds-olefination. The A-ring 66 was formed by iodoetherification of the alkenyl alcohol 67, the tertially asymmetric centers of which were formed by asymmetric allylations. The construction of the key quaternary asymmetric carbon was carried out by the imique procedure of the thermodynamically controlled transacetahzation. The B-ring 68... 

A simple procedure for the preparation of trifluoromethylated vinyl- and dienyl-phosphonates with y-alkoxycarbonyl moiety of exclusively or predominantly (Z)-configuration (201) has been described. It involves acylation of ethyl-1,1-bisphosphonate (202) with trifluoroacetic anhydride, addition of selected Reformatsky reagents to the resulting 1-trifluoroacetyl-1,1-ethyl bisphos-phonates (203) and finally spontaneous Horner-Wadsworth-Emmons (HWE) olefination of the adducts (Scheme 55). " ... 

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