Crotyltitanium reagents

Asymmetric reactions have also been developed. The reactions of allyltitaniums with chiral aldimines derived from optically active 1-phenylethylamine afford optically active homoallylic amines with excellent diastereofacial selectivities. Thus, the Cram syn addition products are obtained highly predominantly when using crotyltitanium reagent 33, as exemplified in Scheme 13.30 [61]. 

Scheme 13.30. Addition of crotyltitanium reagents to a chiral aldimine.

Table 5. Chemoselective Addition of Crotyltitanium Reagents to Carbonyl Compounds 90)...

Configurational lability is desirable in type III reagents, in as much as one geometric isomer is more reactive and/or highly favored at equilibrium. Ti -Crotyl-chromium, -titanium and -zirconium compounds apparently satisfy these requirements, - while q -crotyltitanium reagents such as dicyclopenta-dienylcrotyltitanium appear to be configurationally stable. ii -Cyclopentadienylcrotylmolybdenum complexes also appear to be stable. ... 

The reactions of V-crotyltitanium reagents (125) and aldehydes have been studied by Sato and coworkers.These reactions proceed in good yield and with high anti diastereoselectivity. For example, the reactions of (125 R = H) with propionaldehyde and benzaldehyde are 93-95% selective for the anfi-ho-moallyl alcohol. The stereochemical course is consistent with the reaction proceeding by way of the usual chair-like transition state (126). 

These methodological studies served to demonstrate that reactions of an a-oxygenated crotyltitanium reagent and aldehyde lead to anft-homoallylic alcohol with a Z-O-enecarbamate group in a one-step sequence. We could take advantage of this functionality to give access either to the natural product or to synthetic analogues. 

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