Double bond formation synthesis

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

Synthesis of hetarylethylenes using novel methods for C=C double-bond formation 98ACR584. 

One could plunge into the steric problems posed by the mechanism of protein synthesis on the ribosome 25 26)> or consider the steric fit of the hormone insulin to its acceptor in the cell membrane 27>. Or one could delve into the beautiful intricacy of terpenoid, squalene and steroid metabolism, or get lost in double bond formation, or in the steric problems posed by the branched chain fatty acids and their derivatives 28-34). 

Furstner et al. synthesized lamellarin O dimethyl ether following their previous research on carbon-carbon double bond formation from carbonyl compounds by catalytic titanium coupling reactions <94JOC5215, 95JA4468>. A new titanium-mediated approach to pyrrole synthesis, based on the cyclization of a 3-acylamino-enone, was reported <95JOC6637>. 

While the Wessling-Zimmermann route is a typical method of polymer synthesis, both PPVs and their corresponding oligomers can be synthesized by the extension of methods used for the synthesis of the easiest building block, stilbene 70 (scheme 13). Conceptionally, this is possible by (i) carbon-carbon double bond formation, for whieh synthetic organic chemistry provides a great number of methods [116], and (ii) by aryl-vinyl coupling [117] some examples of both methods are outlined in scheme 13. 

A major task in carotenoid synthesis is the construction of the polyene chain. The coupling reactions commonly used for carbon-carbon double bond formation are dealt with in the several parts of Chapter 2. 

We named this mechanism carbene mechanism because of the transition metal carbene complexes present in the three cycles. If it is true, then the reactions can be named metathesis reactions -OM, carbonyl-olefin metathesis (COM) and carbonyl metathesis (CM), respectively. All of them are double bond formation reactions. It is easy to recognize in this chart not only the catalytic cycle of OM but the Grubbs strategy for the synthesis of cycloalkenes via alkylidene-mediated OM and carbonyl olefination [15] as well (Chart 4a). 

Probably the most popular carbon-carbon double-bond-forming reaction involving sulphur proceeds via the elimination of a sulphur-oxygen species. This is illustrated by a synthesis of 1,5-unsaturated dicarbonyl compounds (236) which proceeds by phenylthioalkylation of enolates (236a), using the phenyl-thioalkene (237), followed by ozonolysis and elimination of the sulphoxide moiety to provide the double bond. An alternative method for double-bond formation is shown in the preparation of alkenes R CH=CHR by reductive cleavage of the sulphide (238) with titanium salts, and demonstrates the versatility of sulphur in such double-bond formations. In the latter example... 

Klamer, G., Former, C., Martin, K. etal. (1998) Connective CC double bond formation for the synthesis of donor- and acceptor-substituted poly(p-phenylenevinylene)s. Macromolecules, 31,3571. 

Stereoselective synthesis of tetrasubstituted alkenes remains a challenging task. A unique solution to this problem is to use torquoselectivity-controlled oletina-tion of carbonyl compounds with ynolates, which is summarized in Chapter 1. In general, the stereoselective synthesis of Z-alkenes, which are thermodynamically less favorable, is more difficult than the synthesis of corresponding -isomers. In Chapter 2, various methods for stereoselective synthesis of Z-alkenes are reviewed. Finally, the C=C double bond formation through catalytic carbene transformation has recently emerged as a new approach toward olefin synthesis. Two chapters covering olefin synthesis based on catalytic carbene transformations are included (Chapter 5 and Chapter 8). 

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