Horner condensation

Wittig or Wittig-Horner condensation between substituted terephthalaldehydes and p-xylene-diylphosphonium salts is also extensively used in the preparation of alternating PPV copolymers, e.g., containing different substituents in adjacent phenylene units (Scheme 2.8) [48]. 

Fermentation obviously does not afford the quantities required for clinical tests, so that a total synthesis was inevitable. Many of the methods described in earlier sections have been employed. The synthesis is convergent and combines the fragments (C-34-C-21) and (C-10-C-18) via a Wittig-Horner condensation. 

Emmons-Horner condensation of (Et0)2P(0)CH2CN with 21-acetoxy-21-oxo-compounds gave the Z-cyano-olefins (337) which on treatment with H -EtOH gave the cardenolides (338).Similar earlier work had reported the E-configuration for one of the cyano-olefins (337). Acetyldigitoxigenin (339) was... 

Diiodoalkenes. Diethyl diiodo(lithio)methylphosphonate is formed in situ from MePO(OEt)2, LiHMDS, and iodine. The Horner condensation products can also be dehydroiodinated to give iodoacetylenes. 

In the reaction between the (2-oxopropyl)phosphonic diesters and the aminals (167) in the presence of monochloroacetic or trifluoroacetic acid, the Wittig-Horner condensation is entirely suppressed and the (acetylstyryl)phosphonic diesters (168) can be obtained in good to excellent yields. The use of acetic acid as the catalyst led to poor results. Variations in the yields of products are to be found associated with the nature of group NO2. CN and CF3 groups favour the reaction, but reduced yields were found for 2-NO2 and 2- and 4-MeO substituents. The products (168) consist of (E) Z )mixtures of isomers, this ratio being commonly 80-90 20-10, and only rarely is the (Z ) isomer in excess. 

Intramolecular phosphonate-ketone condensation (Horner- Wadsworth-Emmons)... 

A second example of step-growth polycondensations with formation of the ole-finic double-bond are Wittig- and Wittig-Horner-type condensations. The Wittig-type polycondensations involve AA/BB-type reactions of aromatic bisal-dehydes with bisphosphonium ylides [99,100] with formation of PPV derivatives (75) and lead to products of only moderate molecular weight (DP 10-20). 

Recently, Meier et al. synthesized the most extended OP Vs (81) known to date [104]. Four different synthetic pathways were used for the generation of these OP Vs the final step involves the formation of one or two olefinic double bonds via a Wittig-Horner-type, a Siegrist-type or a McMurry-type condensation. 

The Orthogonal Approach (Combined Heck- and Wittig-Horner-Type Condensations) to PPM Oligomers... 

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... 

The synthesis of quadrupolar chromophores has also been achieved from 2,6-DTT-dicarboxaldehyde 117. Push-push (i.e., bis-donor) compound 118 was prepared via a double Wittig reaction carried out under solid-liquid phase transfer conditions. Pull-pull (i.e., bis-acceptor) compounds 119 were obtained from a symmetrical bis-aldehydes via a double Horner-Emmons-Wittig condensation (Scheme 9) <2002SM17, 1999CC2055>. 

Kim et al. have introduced silicon atoms in PPV block copolymers to confine the conjugation length and achieve blue EL materials. Copolymers 188-190 [215] and 191 [216] have been synthesized by Wittig-Horner and Knoevenagel condensation, respectively. The emission band in this series can be tuned between 410 and 520 nm, and ITO/polymer/Al PLEDs with turn-on voltages 7 V have been reported (Chart 2.41). 

Sharpless asymmetric dihydroxylation procedure was applied to the synthesis of the side chain of azinomycin A (equation 26)43. Horner-Emmons condensation of phospho-nate 36 with a /J-aziridine substituted acrolein afforded dehydroamino acid diene 37. Treatment of the diene with catalytic amounts of an osmium reagent and dihydroquini-dine (DHQD) p-chlorobenzoate resulted in asymmetric dihydroxylation, producing diol 38. Diol 38 was further converted to the naphthyl ester. 

The condensation of aldehydes and ketones with aminoacetonitriles, although it requires more vigorous soliddiquid catalytic conditions to produce the cyano-enamines, is preferable in many respects to the traditional Wittig-Horner or Peterson procedures [45]. Hydroxyalkylphosphonates are obtained from the catalysed aldol condensation of nitromethane with acylphosphonates [46]. 

The catalyst surface contains both reducing and basic sites and experiments involving selective poisoning of the former by m-dinitrobenzene demonstrated the high probability of a SET mechanism. The same group also concluded that an SET mechanism was probably involved for Michael, Wittig-Horner, and Claisen-Schmidt condensations in the presence of the same catalyst [139,140]. 

Scheme 8 summarizes the introduction of the missing carbon atoms and the diastereoselective epoxidation of the C /C double bond using a Sharpless asymmetric epoxidation (SAE) of the allylic alcohol 64. The primary alcohol 62 was converted into the aldehyde 63 which served as the starting material for a Horner-Wadsworth-Emmons (HWE) reaction to afford an E-configured tri-substituted double bond. The next steps introduced the sulfone moiety via a Mukaiyama redox condensation and a subsequent sulfide to sulfone oxidation. The sequence toward the allylic alcohol 64 was com-... 

Other nucleophilic additions conducted in aqueous media can be found in the literature (Lubineau et al., 1994 Strauss, 1999). These reactions will include the benzoin condensation, the Prins reaction, and the Wittig-Horner reaction, and the Baeyer-Villi-ger oxidation. 

The preparation of 1 started with the addition of lithiated 4 to the enantiomcrically-pure epoxide 5, which was prepared from the racemate using the Jacobsen protocol. Reduction followed by selective protection of the primary alcohol gave the monosilyl ether, which was further protected with MOM chloride to give 7. Pd-mediated oxidation to the methyl ketone followed by condensation with the Horner-Emmons reagent gave the unsaturated ester 8 as an inconsequential mixture of geometric isomers. Oxidation then set the stage for the crucial cyclization. 

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