Math Challenge! Try it...‏

This is a mathematical challenge, and it's been said that:
If you're an engineer, you should be able to solve it in (under) three minutes (more like 30 seconds), if you're an architect, in three hours; if you're a doctor, in six hours; if you're an accountant, in three months and if you're a lawyer, probably never.

What is the missing number in this logical series?
1, 2, 6, 42, 1806, ____???