J. M. DeSimone Associates Inc., a Smithtown, N.Y.-based real estate and financial services company, has signed a letter of intent to buy the bank for about $3.6 million. DeSimone has an exclusive right until Friday to enter into a formal, binding agreement with the privately held bank. The bank posted a $424,000 profit for the first quarter of this year, said C. Ellis Porter, the bank's president and chief executive. Porter would not talk about the tentative sale. John C. Coelho, the bank's chairman and largest shareholder, could not be reached for comment.