Treatment of the 4-silyloxy-N-Boc-L-proline methyl ester 1 with allyl bromi
de in the presence of LDA in THF at, -78 degreesC gives a mixture of the (2
S,4R)- and (2R,4R)-2-(prop-2-enyl)-4-silyloxy-N-Boc-L-proline esters 2a and
2b in 75% combined yield in the ratio 53:47. In contrast, similar treatmen
t of the (-)- and (+)-menthyl esters 5 and 6 gives a mixture of allylated p
roducts 7a,b and 8a,b in the ratio 75:25 and 89:11, respectively. Reaction
of 6 with methyl iodide and propyl iodide also give the corresponding 2-alk
ylated esters 16a and 16b in a highly diastereoselective manner (94:6 and 9
3:7, respectively).