Synthesis Through Elimination Reactions

The early literature describes examples of elimination reactions of a rather forcing nature which have not been explored further. For example, the elimination of HCl from (2-chloroethyl)phosphonic dichloride occurs over BaCl2 at 330 and dechlorination of (l,2-dichloroethyl)phosphonic diesters occurs on heating with zinc dust. Dehydrochlorination of a (2-chloroalkyl)phosphonic acid occurs on simple pyrolysis but the preferred procedure consists in the treatment of the acid diester with Et3N in warm benzene, a procedure also used for analogous (2-chloroethyl)phosphinic esters . The dehydro-halogenation of isopropyl (2-haloethyl)phenylphosphinate by a chiral tertiary amine, such as quinine, quinidine, 1 -phenylethylamine or A-methylephedrine, in a less than equivalent quantity, affords an enrichment of one enantiomer of the ethenylphenylphosphinic 

The elimination of FlCl from the diester 331 with base afforded a mixture of the diesters 332 and 333, both as E-Z mixtures, together with 334. During the course of contact with the base, the composition of the product mixture was determined by proton NMR spectroscopy and GLC. For the 0L,p- and j5,y-unsaturated esters, plots of composition vs time cross, showing their interconvertibility. Formation of the product 332 is kinetically controlled, and both 333 and 334 are the thermodynamically controlled products. At final equilibrium, the mixture of unsaturated esters 332-333-334 had the composition 12 84 4 Similar prototropic changes had been observe earlier during the dehydrochlorination of diethyl (2-chloropentyl)phosphonate.  

The addition of PCI5 to alka-1,3-dienes has already been discussed, and attention has been drawn to the disputed nature of the products. Irrespective of whether, in the product phosphonic dichlorides, chlorine resides on C(2) or on C(4), their reaction with Et3N results in dehydrochlorination to the (alka-l,3-dienyl)phosphonic dichloride If the phosphonic dichloride is initially converted into the phosphonic diester, the dehydrochlorination can be carried out with KOH in Dehydrochlorination of 

Dehydration of the sodium salts of 1-hydroxyalkylbisphosphonic acids (335) occurs at 400 °C to give alken-l,l-diylbisphosphonic acids Esters of this last acid have been 

Pyrolysis (in boiling toluene) of the sulphoxide obtained from a dialkyl (l-phenylth ioalkyl)phosphonate (339) and 3-chloroperoxybenzoic acid affords an alkenephosphonic diester, and subsequent work showed that the sequence was adaptable to the production of chiral esters of alkenylphosphonic diester with optical purities of not less than 93%. Use of the ( S )p-phosphonic amide 340 afforded a mixture of the (E)-(S)p- and (Z)-(S)p-stereoisomers (341), separable by chromatographic methods.  

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Another interesting example of Ugi-Michael process is represented by the synthesis of pyridones 145 (Fig. 28), which originate from an intramolecular domino addition-elimination reaction of the active methylene group proceeding through intermediate 144 [120]. 

Vic-diols can thus be easily converted to alkenes through their reaction with A, A -thiocarbonyldimidazule. The reported synthesis of trans-cyclooctene is illustrative of the method [219]. It should be noted that continuous elimination of rrans-cyclooctene by a stream of argon was necessary to avoid isomerization to the cis isomer. The conversion of cis-cyclooctene to ww-cyclooctene through a trithiocarbonate is described in the same paper. 

As a further stereoselective organic synthesis [40-47] using reactive sp2 carbon-centered radicals, eq. 10.23 shows the preparation of chiral 4-te/7-butylcyclohexene (49) from the optically pure o-bromophenyl sulfoxide (48) through 1,5-H shift by sp2 carbon-centered radical, followed by (3-elimination. This reaction looks like a thermal concerted intramolecular elimination reaction (Ei). 

EPSP synthase catalyzes the synthesis of EPSP by an addition-elimination reaction through the tetrahedral intermediate shown in Fig. 2a. This enzyme is on the shikimate pathway for synthesis of aromatic amino acids and is the target for the important herbicide, glyphosate, which is the active ingredient in Roundup (The Scotts Company EEC, Marysville, OH). Transient-state kinetic studies led to proof of this reaction mechanism by the observation and isolation of the tetrahedral intermediate. Moreover, quantification of the rates of formation and decay of the tetrahedral intermediate established that it was tmly an intermediate species on the pathway between the substrates (S3P and PEP) and products (EPSP and Pi) of the reaction. The chemistry of this reaction is interesting in that the enzyme must first catalyze the formation of the intermediate and then catalyze its breakdown, apparently with different requirements for catalysis. Quantification of the rates of each step of this reaction in the forward and reverse directions has afforded a complete description of the free-energy profile for the reaction and allows... 

Japanese workers have developed a new synthesis of ureas and thioureas through the reaction of carbon dioxide or carbon disulphide with diphenyl phosphite and primary amines. The reaction is thought to take place via an intermediate (85), followed by elimination to give isocyanate or direct displacement by amine. The principles of this new synthesis have also been applied to the preparation of peptides and amino-acid esters. 

Cycloalkeno-l,2,3-selenadiazoles (114) have been converted into compounds 153 [Eq. (37)]. Subsequent reaction of the latter with triethylphos-phine or triethyl phosphite gave sym-diselenadithiafulvalenes of type 154. Similarly, the synthesis of a tetraselenafulvene (156) has been reported through the reaction of the selenadiazole 155 with carbon diselenide and subsequent elimination of selenium with triethyl phosphite [Eq. (38)]. 

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