Aldehydes vinylzinc

The aziridine-2-carboxaldehyde 56 can also serve as synthon for the synthesis of sphingosines, which are important biomembrane constituents [64]. One possible route involves the addition of an alanate to the aldehyde. In a later stage of this synthetic plan the aziridine can be opened, either via the intermediacy of an oxazoline or directly with dilute acid. Unfortunately, the reaction of aldehyde 56 with a vinylalanate has a poor diastereoselectivity of 3 2. Therefore, an alternative approach was considered, namely one involving the addition of a vinylzinc reagent to the aldehyde thereby employing our N-tritylaziridinediphenyl-methanol 51 as the chiral catalyst. Gratifyingly, only one diastereomer was obtained. Reductive removal of the trityl function, acetylation of the hydroxy... 

A few vinylzinc reagents have been added to aldehydes in the presence of chiral sulfur-containing ligands. As an example, Seto et al. reported, in 2005, the synthesis of dipeptide A-acylethylenediamine-based ligands by parallel... 

For enantioselective catalytic addition of vinylzinc reagents to aldehydes, see [52-70]. 

Recently, Soai and coworkers reported an enantioselective addition of diisopropenylzinc and an -branched vinylzinc reagent to aldehydes (equation 19)61. 

A series of /V-acylclhylcncdiaminc-based ligands have been synthesized from Boc-protected amino acids.105 The ligands have been screened for their ability to catalyse the asymmetric addition of vinylzinc reagents to aldehydes. The optimized ligand (39)... 

Diethylzinc catalyses allenylation of aldehydes and ketones by an allenyl boronate, via a site-selective B/Zn exchange a balance of allenyl and propargyl alcohols are produced. A mild enantioselective addition of vinylzinc reagents (derived from vinyl iodides and diethylzinc) has been reported, with wide tolerance of functional groups. 

A highly enantio-selective catalyst (20) for the reaction of vinyl iodides with aldehydes through the direct conversion of vinyl iodides into vinylzinc reagents in the presence of ZnEt2 has been discovered. This method allows the synthesis of substituted and functionalized allylic alcohols with high enantiomeric purity (up to 98% ee) under very mild reaction conditions. 

Walsh developed an efficient protocol for the asymmetric catalytic vinylation of a variety of aldehydes and ketones, using the vinylzinc species generated in situ by treatment of terminal alkynes with Schwartz s reagent for hydrozirconation, followed by transmetallation to zinc via reaction with Me2Zn. Under the catalysis of titanium complex of the chiral dihydroxyl bis (sulfonamide) ligand, the reaction provides access to the corresponding secondary or tertiary allylic alcohols in high yields and excellent enantiomeric excesses [65] (Scheme 14.13). 

The proposed mechanism involves the regioselective c/s-hydroboration of the 4-bromo-l,3-enyne as observed by Zweifel [2] followed by addition of hydride, originating from t-BuLi [3], to initiate a 1,2-metalate rearrangement forging the C-C bond with inversion at the vinylic center (Scheme 2). This key step enables the stereospecific character of the whole process. To circumvent the low reactivity of (Z)-vinylborane toward aldehydes, the corresponding (Z)-vinylzinc was prepared by transmetalation with diethylzinc and reacted successfully with carboxaldehyde. The isolation of aUylic alcohols in high yields was subordinated to a careful selection of the... 

---