RAD001 (2) under bar, a new immunosuppressant, was labelled in its hydroxye
thyl side chain as well as in the rapamycin skeleton using carrier-free lit
hium triethylborotritide and tributyltin tritide (TBTT), respectively. The
tritium labelling of the rapamycin moiety followed Curran's strategy of int
ramolecular hydrogen transfer. Our studies demonstrated that base-free TBTT
is essential for a successful reaction.