Complex sentence: It is not known to which master he was apprenticed, but he certainly took notice of the work of some of the Sienese artists active in San Sepolcro during his youth; e.g. Sassetta.
Split into simpler sentences: It is not known to which master he was apprenticed. But he took certainly notice of the work of some of the Sienese artists active in San Sepolcro during his youth; e.g. Sassetta.