Smaug had been on his own far too long, laying on and counting his treasures. Sauron had as yet nothing of value to offer Smaug he might have considered worth an alliance. And as with Ungoliant , unless Smaug saw some personal benefit, (like more gold) he had no need of Sauron and his petty conquering desires and illusions of grandeur. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.