Wittig-Horner reaction Subject

Another variation of the Wittig reaction is the Wittig-Horner reaction, in which the anion generated ot- to phosphine oxide is used as a nucleophile to react with carbonyl compounds. The intermediate formed in this reaction, -hydroxyphosphine oxide, is isolable particularly when bases with lithium counterion are used for deprotonation. Since the j6-hydroxyphosphine oxides are diastereomers, they can be separated and subjected to elimination to form the corresponding alkenes. Since the elimination of phosphonate moiety is syn, stereospecific alkenes are obtained from the elimination step. As expected, the generation of erythro and threo isomers is dependent on the solvent and the reaction conditions. 

Ubukata s synthesis [51] started with ammonolysis followed by Swern oxidation of ( )-55 [52] to give nitrile aldehyde 56. Wittig-Horner reaction of 56 with phosphonate 58 derived from glutarimide acetic acid 57 [27] gave 59. The nitrile was converted to amide 60, which was subjected to selenolac-tamization [53] to afford 61. Oxidation-elimination of the selenenide moiety gave 48 a diastereomeric mixture. 

Moreover, vinyl-bridged Pc adducts with ferrocene derivatives have been described [78,79], Upon reaction of 3,4-dicyanobenzylphosphonate with formylfer-rocene under Wittig-Horner conditions with the corresponding ferrocene-containing mono- or bisphthalonitriles have been obtained, which were then subjected to statistical cyclotetramerization with 4-tert-butylphthalonitrile in the presence of the appropriate metal salt. Furthermore, a double bond has also been employed as spacer between the Pc and ruthenium(trisbipyridine) [47],... 

The Wittig-Horner olefination of the aldehyde 28 provided alkenes 29 which were subjected to radical cyclization leading to benzofused tricyclic j6-lactams 30, obtained as single diastereomers (Scheme 8) [36]. A convenient, direct regio- and stereoselective route to optically pure unusually fused or bridged tricyclic /3-lactams has been developed by the use of intramolecular nitrone-alkene cycloaddition reactions. For example, the aldehyde 21 can be transformed into nitrone 31 which subsequently was used for a variety... 

The Collins reagent in CH2CI2 oxidizes silylated primary alcohols in preference to the more hindered silylated secondary alcohols, as described for oxidation of the prostaglandin intermediate 2963 to the rather labile aldehyde 2964, which is immediately subjected to a Horner-Wittig-reaction to introduce the lower side chain [206] (Scheme 12.61). 

Other y-bromoesters such as 31 can be made by radical bromination13 with NBS (this too is a y reaction ) and used in similar Horner-Wadsworth-Emmons reactions. It does not matter if some reaction occurs at the y position of 29 or 33 as the Wittig elimination cannot occur on such an intermediate and it reverses. The products of these aldol reactions are dienes 30 and 34 and are usually made for use in Diels-Alder reactions. We shall return to this subject later in this chapter. 

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