The head of Boston Properties says he began to realize the housing market wasn’t sustainable at the beginning of 2007. “The same thing was true in commercial real estate, so we sold more than .25 billion of properties at the end of 2006 and 2007, realizing that prices were extraordinary, and unsustainable.” And they bought the .8 billion trophy Midtown GM building. He is keeping his money in cash for now. “I have been an optimist all of my life, and I hate to be a bear, but this is going to be a very serious recession.”