Gen. George Doriot, chairman of American Research and Development Corp., recently signed a lease for office space at One Beacon St. With other tenant commitments to be announced soon, the building, owned by Commercial Union Assurance Cos., will be 80% leased according to Fred Stockwell, executive vice president, M.M. Bradley & Co., the leasing agent Cobroker on the lease to A.R.D.C. was David Almy of Leggat McCall & Werner.