The short answer is NO.
Adobe does not sell CS5.x nor CS5.x Extended anymore, so you cannot buy the upgrade to CS5.1 Extended now. Too late,
However, you are entitled to upgrade to CS6 Extended. Contact Adobe Customer Service by phone or go straight through the Adobe store,
It will cost you the same as what you would have paid to upgrade to CS 5.1 Extended last month, when it was still being supported and sold.
Thank you very much indeed.