Alkaline or alkaline earth octahydrotriborate M(B3H8)x and dodecahydro-closo-dodecaborate MxB12H12 (M = Li, Na, Mg or Ca with x = 1 or 2) have recently attracted a lot of interest for hydrogen storage and solid electrolyte applications. Nevertheless, their syntheses are still a roadblock for large scale applications. In this paper we propose a novel approach for their syntheses starting from the cheapest borohydride NaBH4. The process involves first the solvothermal synthesis of tetrabutylammonium octahydrotriborate (C4H9)4NB3H8 (TBAB3H8) being the basis for the syntheses of the others boranes. Starting from TBAB3H8, we have synthesized pure and unsolvated NaB3H8 by salt metathesis reaction with sodium tetraphenylborate. Then, we have successfully obtained Na2B12H12 by solvothermal decomposition of NaB3H8. This approach has shown to be quantitative and reproducible, which could lead to the development of these boranes in real life applications.