Horner method

Table 2 Wittig and Wittig-Horner Methods for the Preparation of 4-Aminobut-2-enoic Acids or Esters...

By application of the Horner method, developed originally for the preparation of stabilized phosphonium ylides, Emstbrunner and Lloyd ... 

The (Horner-)Wadsworth-Emmons reaction generally is superior to the Wittig reaction, and has found application in many cases for the synthesis of a ,/3-unsaturated esters, a ,/3-unsaturated ketones and other conjugated systems. Yields are often better then with the original Wittig procedure. However the Wadsworth-Emmons method is not suitable for the preparation of alkenes with simple, non-stabilizing alkyl substituents. 

There is a multiplicity of pathways for thermal dediazoniations. An analogous situation is to be expected for photochemical dediazoniations. Based on the general experience that light-sensitive reactions often involve free radical intermediates, it was commonly assumed that all photolytic dediazoniations are free radical reactions. Horner and Stohr s results (1952), mentioned above, could lead to such a conclusion. More sophisticated methods of photochemistry also began to be applied to investigations on arenediazonium salts, e. g., the study of photolyses by irradiation at an absorption maximum of the diazonium ion using broad-band or monochromatic radiation. This technique was advocated by Sukigahara and Kikuchi (1967 a, 1967 b,... 

This method, sometimes called the Horner-Emmons, Wadsworth-Emmons, or... 

The final example of a domino process under high pressure, to be discussed in this chapter, is a combination of a Horner-Wittig-Emmons (HWE) reaction with a Michael addition developed by Reiser and coworkers [5]. Hence, reaction of a mixture of an aldehyde such as 10-18, a phosphonate 10-19 and a nucleophile 10-20 in the presence of triethylamine at 8 kbar led to 10-21. By this method, (3-amino esters, 3-thio esters and 3-thio nitriles can be prepared in high yield (Scheme 10.4). Many of these transformations do not occur under standard conditions, thereby underlining the importance of high pressure in organic chemistry. 

The remaining section highlights various other methods of generating piperidine cores. The scheme below depicts a simple route to piperidines heterocycles, such as 196, via an intramolecular Horner-Emmons cyclization of phosphonate 197 <06JA12743>. 

Following Wilkinson s discovery of [RhCl(PPh3)3] as an homogeneous hydrogenation catalyst for unhindered alkenes [14b, 35], and the development of methods to prepare chiral phosphines by Mislow [36] and Horner [37], Knowles [38] and Horner [15, 39] each showed that, with the use of optically active tertiary phosphines as ligands in complexes of rhodium, the enantioselective asymmetric hydrogenation of prochiral C=C double bonds is possible (Scheme 1.8). 

High-level quantum mechanical calculations have been used to explore the Horner-Wandsworth-Emmons reaction in the gas phase and also with a solvation contribution evaluated using the PCM/DIR method. Ring closure of the P—O bond (TS2), to form oxaphosphetane, is rate determining in the absence of solvation however, the oxyanion becomes a true intermediate, at an energy minimum on the reaction path, only in response to the effects of solvation, whereupon its formation by carbonyl addition (TSl) becomes rate limiting. Formation of F-product is always... 

Other examples of Ugi reactions combined with RCM have been described in the literature. Hebach and Kazmaier reported the synthesis of conformationally fixed cyclic peptides [70] and Beck and Domling synthesized biaryl-containing natural product-like macrocycles using this method [41]. The same group also reported combination of Passerini and Horner-Wadsworth-Emmons reactions to obtain butenolides [67] and another variation for the combinatorial synthesis of thiazoles [69]. 

The Wittig-Horner (W-H) reaction is a versatile method for the synthesis of functionalized alkenes. The synthesis of 3-substituted ethyl acrylates and acrylonitriles, which are used as monomers in polymerization, were successfully carried out by the reaction of triethyl phosphonoacetate or cyanomethanephosphonate, respectively, with various aldehydes with activated Ba(OH)2 as the catalyst in the presence of dioxane solvent at 343 K (294). As was observed for other basic solid... 

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. 

The chemical shift differences of the diastereotopic hydrogens are listed in Table 17 they depend strongly on solvent effects, as expected for an ionic product. They are in the range of 8 = 0.01 -0.1, well suited for measurement of the enantiomeric purity of the phosphanes. An alternative method for the measurement of Horner phosphanes is by 13C-NMR spectroscopy of diastereomeric complexes formed with [>/3-( + )-0 7 ,57 )-pinenyl]nickel bromide dimer73. 

Horner, G. (1999) Qualitative and quantitative evaluation methods for sensor arrays. In Proceedings of the 6th International Symposium Olfaction and Electronic Nose, Tubingen, 20-22 September 1999. 

Diethyl [(2-tetrahydropyranyloxy)methyl]phosphonate is useful in the Wlttig-Horner synthesis of enol ethers, which are intermediates in one-carbon homologations of carbonyl compounds. This procedure is an adaptation of a general method for making dialkyl hydroxymethylphosphonates. An 0-tetra-hydropyranyl derivative also has been made from dibutyl hydroxymethyl -phosphonate, and diethyl hydroxymethylphosphonate has been O-silylated with tert-butylchiorodimethylsilane and imidazole. Another useful congener in this series has been prepared by an Arbuzov reaction of methoxyethoxymethyl (HEM) chloride and triethyl phosphite. 

This method, sometimes called the Horner-Emmons, Wadsworth-Emmons, or Wittig-Hor-ner reaction,658 has several advantages over the use of phosphoranes.659 These ylides are more reactive than the corresponding phosphoranes, and when R is an electron-withdrawing group, these compounds often react with ketones that are inert to phosphoranes. In addition, the phosphorus product is a phosphate ester and hence soluble in water, unlike PhjPO, which makes it easy to separate it from the olefin product. Phosphonates are also cheaper than phosphonium salts and can easily be prepared by the Arbuzov reaction ... 

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