Coupling reactions Wittig

Ley, S.V., Lygo, B., Organ, H.M., and Wonnacott, A., Wittig and Homer-Wittig coupling reactions of 2-substituted cyclic ethers and their application to spiroketal synthesis, Tetrahedron, 41, 3825, 1985. 

The general process involved in a bis-Wittig coupling reaction (shown in Scheme 11) is the condensation of a bis-Wittig reagent with a suitable dicarbonyl compound (usually a dialdehyde) to produce a ring. The resulting products are normally uniquely accessible by... 

Cyclopropanone ethyl hemiketal (167) has also been used in the synthesis of cyclopropanespiro-2-bis-norpenicillanic acidsnovel analogs of the natural penicillins. In this case, the use of cyclopropanone in a Wittig coupling reaction was nicely illustrated. The intermediate (168) was generated in this process and then carried on to the )8-lactam derivative as illustrated in Scheme 62. 

C-analogs of the a-(l- 5)-arabinofuranosyl repeating motifs found in the polysaccharidic chains of mycobacterial cell wall were also prepared using this approach by Dondoni and Marra [43]. This route is based on the use of a bifunctional unit 54 with regeneration of the formyl group in one step after the Wittig coupling reaction (O Scheme 11). 

The total synthesis of 5(5)-hydroxy-12-oxo-20 3(6Z,8.E,14Z) involving Wittig coupling reactions of tailored intermediates was described (38). 10(5)-Hydroxy-20 4(5Z,8Z,11Z,14Z) was synthesized in eight steps starting from enantiomerically pure (7 )-glyceraldehyde acetonide (39). Chiral adducts from Grignard or allylsilane... 

The olefin metathesis of 3-hydroxy-4-vinyl-l,2,5-thiadiazole 112 and a McMurry coupling reaction (Ti3+ under reductive conditions) of the aldehyde 114 were both unsuccessful <2004TL5441>. An alternative approach via a Wittig reaction was successful. With the use of the mild heterogenous oxidant 4-acetylamino-2,2,6,6-tetramethyl-piperidine-l-oxoammonium perfluoroborate (Bobbitt s reagent), the alcohol 113 was converted into the aldehyde 114. The phosphonium salt 115 also obtained from the alcohol 113 was treated with the aldehyde 114 to give the symmetrical alkene 116 (Scheme 16) <2004TL5441>. 

Polyenes are most often synthesized by cross-coupling reactions between unsaturated systems. Typically these reactions require an activated carbon, often in the form of an organometallic reagent. Enolates and phosphonium ylides, Wittig-type reagents, are also commonly employed in carbon-carbon bond formation. Pericyclic rearrangements also result in the generation of new carbon-carbon bonds and will be treated separately. 

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