[SharesToCharity]

> can you offset capital gains against income tax in the US?

You can offset charitable donations against income tax. The personal capital gains is avoided by transferring shares directly, without selling. (The charity typically then pays the capital gains via unrelated business income tax.)

> Alternatively, have you tried selling part of your stake to the new investors in the round, at a discount?

Unfortunately, no. I tried :(

[charlesdm]

Another thing to consider if you do do this: if tax law changes are made, it could be that the carry forward mechanism no longer works indefinitely, but is capped at a certain amount of tax years before it is permanently lost (e.g. 5, 7, 9).

Just a thought: having a large carry forward tax loss in itself has value, and if it were a sure thing you would get the deduction (e.g. through an IRS revenue ruling), then if I were you I'd probably look for some way to team up with someone. Ethics aside, there are probably ways to help someone else avoid taxes using this loss. Maybe you can make a million dollars, by helping someone else save 5 or 10 million in taxes.