Wittig-Homer condensation

Finally, the necessity arose for the synthesis of pentulose 21, labeled with, 3C on the central carbons, C-2 and C-3, for an independent biosynthetic study, which is reported in Section III.5.27 The doubly labeled ester 34 (Scheme 14) is readily available by a Wittig- Homer condensation of benzyloxyacetaldehyde with commercially available triethylphosphono-(l,2-l3C2)acetate. Chirality was introduced by the reduction of ester 34 to the allylic alcohol, which produced the chiral epoxide 35 by the Sharpless epoxidation procedure. This was converted into the tetrose 36, and thence, into the protected pentulose 37 by the usual sequence of Grignard reaction and oxidation. 

Figure 19.1 Synthetic scheme for example pheromone components. Abbreviations al and a2 = Wittig-Homer condensations with triethyl 2-phosphonopropionate and triethyl 2-phosphonobutyrate, respectively b = reduction of ester with lithium aluminum hydride c = partial oxidation of alcohol with manganese dioxide to aldehyde dl and d2 = Wittig condensations with ethyltriphenylphosphonium bromide and propyltriphenylphosphonium bromide, respectively e = condensation with dimethylhydrazone phosphonate reagent f = hydrolysis under acidic conditions. Compound numbers are as in Table 19.1.

The Wittig-like reactions are well known for C=C bond formation. An intramolecular process can serve as a ring closure and has been generally employed in the synthesis of numerous macrocyclic natural products [150]. Oishi and coworkers [151] applied this strategy to the synthesis of the aglycone (252) of the antibiotics venturicidins A and B. Thus, as shown in Scheme 84, the aldehyde phosphate 250 was subjected to the modified intramolecular Wittig-Homer condensation with a mild base, yielding the macrocycle 251 in 48% yield. 

Synthesis from xylose The xylose derivative 47, obtained from 5,5-bis-benzyloxy-7-oxa-bicyclo[2.2.1]hept-2-ene (45), has been used in the synthesis of (-P)-castanospermine (1) (Scheme 6). Bromination of 45 occurred exclusively on the less hindered convex face of 45, followed by stereoselective migration of the endo OBn group of the acetal to give 46, which subsequently converted to 47. Mesylation of 47 followed by cyclization with ammoifia gave 48, whose protection, hydrolysis, acetylation and cyclization by an intramolecular Wittig-Homer condensation gave 49. Conversion of 49 into epoxide 50... 

Other examples concerned the use of macroscopic systems such as Syn-phase lanterns. Lippens et compared the results they obtained using Wittig-Homer condensation on a macroscopic support with earlier results of the same reaction on a standard resin. As in all of the examples shown above, gel-phase HR-MAS NMR spectroscopy proved to characterize better the supported intermediates and final products and provide better understanding of the interactions between the polymer and the reagents. This is an important development in supported organic chemistry. 

Asymmetric synthesis of a chroman, Solladie and Moine1 have effected an en-antiospecific synthesis of the chroman-2-carboxaldehyde 7, a key intermediate in the synthesis of a-tocopherol, from (R)-( + )-l. The phosphonate 2, derived from 1, undergoes a Wittig-Homer reaction with the dimethyl ketal of pyruvaldehyde to afford the optically active vinyl sulfoxide 3. Condensation of the aldehyde 4 with the lithio derivative of 3 affords, after silyl deprotection, the allylic alcohol 5 as the only diastereoisomer. This... 

The Wittig-Homer reaction between C-metallated a-phosphoryl sulfides and carbonyl compounds provides a common route to a,P-unsaturated sulfides, but one which usually results in poor yields of the desired product when enolisable ketones are used. Stephan et al. have investigated the role of boron trifluoride in promoting the Wittig-Horner reaction of enolisable ketones, which usually follows a two-step process the initial condensation of the anion followed by an elimination step. It was found that when the second step occurred at a slow rate then addition of boron triflouride was necessary for completion. ... 

Aldehyde functions have been used as such to synthesize Schiff bases °, various stilbene derivatives by Wittig-Homer-type olefination or cinnamic acids or by Knoevenagel condensation with malonic acid . They can also be reduced to or oxidized to As examples for derivatives obtained via alcohol functions,... 

No reaction occurs without sonication. Quantitative yields are obtained after only a 5-min exposure to ultrasoimd. The catalyst contains reducing and basic sites. Selective poisoning of the former by m-dinitrobenzene demonstrated the high probability of a SET mechanism. The same group also concluded on the preferential SET mechanisms for Michael, Wittig-Homer, and Claisen-Schmidt condensations in the presence of the same catalyst. 

Ba(OH)2 is known to cattdyze several base-catalyzed organic reactions in the solid form, Of the reactions, aldol condensation is the most common. In recent years, several organic reactions besides aldol condensation have been found to be effectively catalyzed by Ba(OH)2. These reactions are the Claisen-Schmidt reaction, esterification of acid chlorides, Williamson s ether synthesis, benzil-benzilic acid rearrangement, the synthesis of A -pyrazolines by the reaction of a,/3-unsaturated ketone with PhNHNHz Wittig-Homer reaction, and Michael addition. For these reactions, the Ba(OH)2 catalyst prepared from Ba(0H)2-8H20 by cidcination at 473 K shows the highest activity. 

A tandem enzymatic aldol-intramolecular Homer-Wadsworth-Emmons reaction has been used in the synthesis of a cyclitol.310 The key steps are illustrated in Scheme 8.33. The phosphonate aldehyde was condensed with dihydroxyacetone phosphate (DHAP) in water with FDP aldolase to give the aldol adduct, which cyclizes with an intramolecular Horner-Wadsworth-Emmons reaction to give the cyclo-pentene product. The one-pot reaction takes place in aqueous solution at slightly acidic (pH 6.1-6.8) conditions. The aqueous Wittig-type reaction has also been investigated in DNA-templated synthesis.311... 

Pauson-Khand cycloaddition Pictet-Spengler reaction Ugi four-component condensation Wittig reaction (and Homer-Emmons condensation)... 

Pommer et al.146) reported a synthesis of a stereoisomer mixture of JH1 using a Homer-Wadsworth-Emmons olefmation for introducing the double bond at C-2 of 184146). Three Wittig olefinations were applied in the condensation of C4 + C6 + + C5 + C2 synthons 147). The isomers obtained were separated by distillation (Scheme 35). 

---