Carbon Double Bond Forming Reactions

Carbon-Carbon Double Bond Forming Reactions 

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The first diastereoselective total synthesis of ( )-guanacastepene A (rac-187) was published in a series of papers by Danishefsky and co-workers [107-110]. The tricyclic carbon framework was assembled by interplay of enolate alkylations and ring-closing carbon/carbon double bond forming reactions (Fig. 19). Commercially available cyclopentenone 90 was utilized as the A-ring precursor. 

Carbon-Carbon Double Bond Forming Reactions... 

Z)-Isomers of trisubstituted double bonds and of sterically unhindered disubstituted double bonds are sufficiently stable in solution to allow their isolation in the pure state. (Z)-Isomers of sterically hindered disubstituted double bonds are not formed during iodine-catalysed isomerization. When prepared by synthesis in a carbon-carbon double-bond forming reaction they readily isomerize to the corresponding ( )-isomer. Their thermodynamic instability therefore requires particular attention. 

Olefin Cross Metathesis. Olefin cross metathesis (CM) represents a valuable alternative to traditional carbon-carbon double bond forming reactions however, its practical utilization is sometimes limited by the generation of multiple homodimerization products (4 and 5) when reacting 1 and 2 to obtain the desired heterodimerization product 3 in solution phase (Scheme 6.1a). A solid support can induce chemoselectivity in CM reactions, as the immobilization of one olefin as 6 can effectively prevent its homodimerization (Scheme 6.1b). Moreover, the soluble olefin 2 can be added in excess to drive the reaction to completion. While the soluble olefin is capable of... 

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... 

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