Kant: Synthetic judgments

Kant says that you can not use synthetic judgments because you will lapse in your principal purpose. An this will make metaphysics contradict itself. Since a synthetic judgment is something that is informative i think that right there Kant contradicts himself. He is implying that if you start to expand your mind and start to see more things then you will start to realize that all things over lap some place. I think that maybe Kant could have used Analytic judgments as an example. He could have said that if you listen to analytic judgments the things you are hearing may or may not be true and it also may not have any new meaning.