Wittig alkenylation

Since electron-donating substituents at the phosphorus atom favor addition reactions over olefination reactions, addition of 9 to aldehydes leads to the exclusive formation of the silyl-pro-tected allylic alcohols 10. No reaction products arising from Wittig alkenylation could be detected. The ylides (R,S)-9 and (S.S)-9 and their enantiomers were prepared from the corresponding optically pure l-[2-(diphenylphosphino)ferrocenyl]-A,A -dimethylethanamine diastereomers 7 via the phosphonium salts 8. 

In one example of application of the RCM strategy, summarized in Scheme 107, the key intermediate 454 was obtained from 451 by sequential Wittig methylenation to 452, reductive iV-deprotection to 453, and introduction of an alkenyl chain onto the secondary amine. The RCM reaction of 454 to 455 proceeded in quantitative yield in the presence of the second-generation Grubbs catalyst <20040L965>. 

The migration of a C=C bond to form a C=N bond was also observed with hydro-xylamine [78, 79], hydrazine [80, 81] and primary amines [82]. The /f iminylphos-phine oxide formed in the reaction may serve as a Wittig reagent in the presence of a base to react with a ketone or an aldehyde leading to ,/fun saturated alkenyl-imines 153 (Scheme 10.74). The phosphorus group can be a phosphonium salt as well as a phosphonate. 

Taylor and colleagues have shown the sequential carbolithiation/[l,4]-Wittig rearrangement of styryl ethers 105 (e.g., 105a) which provide appropriately functionalized 2-alkylphenol 106 (e.g., 106a) in good yield . In this reaction, the alkenyl transfer occurs with retention of configuration (equations 59 and 60). 

Table 2 Examples of Wittig alkenylations (Scheme 11) <2003060664,1999J(P1)163>...

Durch Wittig-Olefinierung lassen sich 1-Nitromethyl-cycloalkene mit Triphenyl- oder Tri-butyl-phosphan in Gegenwart von Tetrakis-[triphenylphosphan]-palladium(0) und an-schlieBender Basen-katalysierter Reaktion mit Aldehyden in einer Eintopfreaktion zu 1 -(1-Alkenyl)-cycloalkenen ( >95%) umwandeln3 ... 

A Wittig reaction, for instance, transforms 30a in 30b M. Thus, acylation of 22 followed by Wittig reactions and hydrolysis offers the possibility to prepare a variety of optically active a-alkenyl alanines of type 28. 

The Trost synthesis of calcitriol (3) is shown in Scheme 5. In a Wittig reaction, the hydroxylated Grundmann s ketone 26 (obtained by partial synthesis) is transformed into the alkenyl bromide 27 with astonishingly high diastereoselec-tivity ElZ >50 1). The chiral enyne 29, which is needed as the second building block for the coupling step, is prepared from the aldehyde 28... 

The stereoselective synthesis of alkenyl halides is also possible via Julia-Lythgoe alkenation. The other two methods for the preparation of alkenyl halides - Wittig reaction (using... 

Alkynols complexed to cobalt can be oxidized to alkynals without decomplexation. Propargyl aldehydes are protected from polymerization upon complexation with Co2(CO)6. These aldehydes smoothly undergo Wittig-type reactions. Carbonyl-ene reactions have been demonstrated (Scheme 194). Complexation to cobalt protected the enyne in complex (132) from Michael-type reactions (Scheme 195). Alkenyl-substituted complexes undergo [3 + 2]cycloadditions with nitrile A-oxides (Scheme 196). 

---