Thanks for your question.
Was this back tax for income taxes or payroll taxes? Are you an officer of the corp? Is this an S-corp or C-corp?
Sorry but I do not have good news. Unfortunately, you may not deduct the sales taxes paid on behalf of another entity per page A-2 of the Schedule A instructions. http://www.irs.gov/pub/irs-pdf/i1040sca.pdf This is also not a valid business expense since it not an ordinary expense that would be paid by a principle.
States will hold officers of corporations responsible for unpaid sales and employee withholding taxes because these are considered trust fund taxes. The tax when assessed to a principle is generally called a "Trust Fund Recovery Penalty". So since it is actually a penalty, penalties are never deductible for taxes.
If your corp was still in existence, you could have written up a formal loan document to fund the company for the taxes. Then you could have later wrote it off as a bad debt if the company became insolvent. However, it is too late at this point to do that.
Hope this helps,