Key steps in two alternative approaches to the C10-C21 fragments 4 and
17 of the immunosuppressant rapamycin employ catalytic asymmetric all
ylation and catalytic asymmetric hydrogenation to introduce the first
stereogenic centre at C14. Appendage of C1-C9 via trapping of an a-oxo
ketene intermediate generates the C1-C21 fragment 5.