Stereoselectivity triphenylphosphonium ylides

Tab. 11.1 Triphenylphosphonium Ylides Nomenclature, Preparation, and Stereoselectivity of Their Wittig Reactions...

Wittig reactions of diethyl oxalate with a variety of arylmethylene-triphenylphosphonium ylides under mild conditions provide a highly (Z)-stereoselective synthesis of ethyl 3-aryl-2-ethoxyacrylates (69). Wittig reactions of the a-keto amino acid derivative (70) with stabilized, semi-stabilized and reactive ylides have been used to prepare the synthetically... 

In 1963, Bestmann et al. disclosed that treatment of A-benzyhdeneaniline with semistabilized triphenylphosphonium ylides at 150-180 °C afforded alkenes, but with nonstabilized triphenylphosphonium ylides at 130-150 °C afforded allenes [219, 220]. For a long time this protocol had not been improved and developed into a useful stereoselective alkene synthesis, probably owing to the high reactiOTi temperature and inconvenient operation. Recently, Tian et al. have developed a highly trmable stereoselective olefination reaction of imines with triphenylphosphonium ylides at low temperature by employing sulfonyl groups to activate the imines [221-223]. 

LDA has been identified as the base of choice for the olefination of A-sulfonyl imines with semistabilized triphenylphosphonium ylides [221], A range of A-(p-toluenesulfonyl) aromatic imines undergo olefination with benzylidenetriphenylphosphoranes to give Z-stilbene derivatives with greater than 99 1 stereoselectivity (Scheme 51). The exclusive Z selective olefination reaction... 

The other stereoselective synthesis/281 shown in Scheme 8, foresees conversion of Boc-L-Asp-OtBu 20 into the related (3-aldehyde 22 via the Weinreb amide 21 and its reduction with diisobutylaluminum hydride (DIBAL-H). Wittig condensation of 22 with the ylide derived from (3-carboxypropyl)triphenylphosphonium bromide using lithium hexamethyldisilaza-nide at —78 to 0°C, produces the unsaturated compound 23 which is catalytically hydrogenated to the protected L-a-aminosuberic acid derivative 24. Conversion of the co-carboxy group into the 9-fluorenylmethyl ester, followed by TFA treatment and reprotection of the M -amino group affords Boc-L-Asu(OFm)-OH (25). 

---