I don't know how it morphed into Too-sawn, but the original pronunciation supposedly was more like Took-shown. It was an Indian name (perhaps To'hono Oodham) that possibly described a spring and a black rock (that was present on the original site of the village, or near it).

The already existing Indian village was re-settled by the Spanish back in 1776. Also, the original settlers that founded the village that became San Francisco mostly came from Tucson (led to the bay area by Juan Bautista de Anza). Bob