(Adds executive comment from interview) SINGAPORE, Jan 21 (Reuters) - Private exchange iSTOX, backed by Singapore Exchange and other investors, said on Thursday it is raising $50 million in its first major funding round, led by two Japanese state-backed firms. The financing will enable iSTOX to expand in Asia and offer a range of investment products, Oi Yee Choo, chief commercial officer of the Singapore-based regulated trading platform told Reuters in an interview. Choo, formerly the Singapore investment banking chief of UBS till late 2019, said iSTOX expects more than 20 listings on its pla...