I've run uCs off 3 AA batteries, ~4.5V, even a '2560 based board @ 16 MHz.
I've also run 3.3V Promini @ 8 MHz from a single LiPo battery.
I don' think I saw what the application is here. Haven't seen what the need is for boost converters unless the goal is to run from a single AAA or AA size battery.