Alkenyl, substrate controlled addition

Substrate Controlled Addition of Allyl, Alkenyl, and Alkynyl Groups. 255... 

A number of new methods for the homologation of esters have been reported. Methyl bis(ethylthio)acetate, derived from dichloroacetate and the sodium salt of ethanethiol, can be smoothly alkylated under standard conditions to the corresponding a-alkyl (or alkenyl) derivatives, in excellent yield. The use of DBU as base to effect high yield dialkylations of Knoevenagel type substrates with alkyl bromides has been described. Directed additions of a-substituted ester enolates to cyclohexenone can be achieved by careful control of temperature. Conjugate addition of diphenylmethide to acrylate esters has been achieved after much trial and error substituted acrylates lead to the best yields. 

In contrast to many other methods commonly employed for stereoselective alkene synthesis such as elimination, alkenylation, alkene metathesis, alkyne addition, the JuUa olefination, and the Peterson olefination [1-3], the olefination reactions of phosphorus-stabilized carbon nucleophiles remain very powerful for modem stereoselective alkene synthesis owing to their convenience, complete positional selectivity, and generally high levels of geometrical control. However, further modifications of these olefination reactions are definitely needed to broaden substrate scope, enhance stereoselectivity, and improve environmental impacts. 

The Zr-Zn transmetalation, aldehyde addition strategy has also been used in the total syntheses of (-)-ratjadone [85] and fostriecin. In the latter example, the substrate was a chiral epoxyketone, which was converted to a tertiary alcohol in excellent diastereoselectivity by chelation control [86]. More recently, the reaction scope has been extended to the addition to C=N electrophiles to form allylic amides [87] as well as allyl hydroxylamines [88], and to the preparation of frans-1,2-disubstituted cyclopropanes [87, 89]. Powell and Rych-novsky found that the BF3-mediated addition to in situ formed oxacarbenium ions led to a mixture of alkyl and alkenyl ligand transfers [90] (Scheme 18). 

---