HIGHLY DIASTEREOSELECTIVE ADDITIONS OF ORGANOCOPPER REAGENTS TO 2-EXO-BORNYL CROTONATES

Iodotrimethylsilane-activated butylcopper adds to the chiral bornyl crotonates 1-3 in high yields and with good to excellent diastereoselectivities. In the present investigation, these reactions are compared with additions of lithium butylcuprates of various composition to the same esters. The cuprate additions show a remarkable dependence on the exact composition of the reagent, with the diastereoselectivity on addition to 1-naphthyl substituted ester 1 ranging from 57 % d.e. of the (S)-product using the reagent "LiBu3Cu2" to 90 % d.e. of the (R)-product using "Li2Bu3Cu". Diastereoselectivities of up to 98% can be obtained using BuCu.TMSI.