Setting up synchronous/async transmission

let's assume you have the following connection:

Dte -> DCE -> DCE -> DCE -> DCE -> DTE

of course the two DCEs in the middle will be set to external clock, but i'm wondering about what happens with the dte and dce at the ends. if the dte sends async and the dce is working on sync, you'd have to set the dce to quadruple the sample rate of the dte's data rate to overclock the data. but what if the DTE is sending synchronously, and you want the DCE to receive asynchronously? so: DTE (sync) -> DCE (async). What special measures would you have to take if any to make this work and would it affect the clocking (int, ext, rcv) of the rest of the devices?

Thanks alot guys