Terra Mystica - Taking a bonus card with coins on the final turn as an Alchemist
No, you can't do this.

When you pass at the end of the last round, you do not take a bonus card. From the rules about passing:


  ...immediately return your Bonus card and take one of the three available ones. ... (Exception: Do not take any Bonus cards in the last round of the game - round 6.) ...


So you don't even get a bonus card in the last round.

Any coins you do have at the end of the game, however you got them, can be converted into VP. Everyone can do this at 3 coins per VP, and the Alchemists can do it at 2 coins per VP. But you can't get those coins in the way you mention in the question anyway.