I did exactly what you're describing when leaving a privately-held company that was about to have the final 21% ownership acquired. You have to have enough cas

I did exactly what you're describing when leaving a privately-held company that was about to have the final 21% ownership acquired. You have to have enough cash-on-hand (and be willing to risk losing it) to purchase the options in your 90-day window. Once converted from options to stock, you have to hold it until the company can buy them back from you. The advice to consult a lawyer is relevant as I'm sure the contract language has several paragraphs related to this transaction.