Is it possible to run a Buck converter with rectified AC supply voltage?
Most switching power supply adapters (such as your cell phone charger) will rectify right off the power line and then use a high voltage capacitor to smooth out the resulting rectified voltage. As long as the smoothing reduces the ripple to a small enough amount then there is no problem for the succeeding buck regulator to operate and produce its designed output voltage. 

Some ripple at the input can come through to the output if the output section of the switching regulator is not filtered adequately. For certain you want to make sure that the input capacitor smooths the DCin so that the lowest voltage of the input waveform is still well over the intended output voltage.