Since no one has bothered to actually answered your question, I will try --

Start with United - go to the United website - www.united.com - and take a look at the big box along the left side of the screen. Enter whether you are looking for one way or roundtrip. Enter your date(s) of travel, # of people traveling, under "cabin" choose first class, and click search. The next screen will show you the nonstop and connecting flights available and the fare for first class tickets.

Repeat the same procedure with American Airlines, Delta Airlines, and USAirways. Those are the airlines that offer 1st class seating and fly between Denver and Boston. Among those, only United has nonstop flights.