Transmetalation Reactions of Allylic Stannanes

The authors surmise that the trichlorostannyl intermediate 218 directs a chelation-controlled addition via 220, which may involve pseudo-axial complexation of the carbamate carbonyl. The stereoselectivity of the allylation is significantly altered by the use of 2.0 equivalents of SnCU to produce the corresponding i yn.i yn-isomer of 219 via the antiperiplanar transition state derived from the a-chelation model for addition of y-alkoxyallylstannanes. 

Williams and coworkers have expanded the utility of this methodology via the application to highly functionalized substrates. transmetalation with R.R)-ot (S,S)-228 is quantitative and dependable, with a variety of allylic stannanes with C-2 substitution. Carbon chains at C-2 of the allyl component may contain additional functionality, including benzyl and allyl ethers, silyl ethers, esters, alkenes, halogens, or thioacetals, as well as stereogeificity. Asymmetric induction in the condensation with aldehydes is dominated by the chiral auxiliary. In this fashion, the allylation reaction may be designed as a convergent 

---