I'm interested in the chemical synthesis of nucleoside phosphoramidites, but all the procedures I can find have "start with a protected nucleoside" as the first step. Searches for chemical synthesis of nucleosides all seem to find process that start with existing nucleobases, or in vivo processes, or speculative origin-of-life processes. Searching for chemical synthesis of nucleobases ONLY seems to find speculative origin-of-life processes.
What I'm looking for (just out of curiosity) are processes for chemical synthesis of nucleosides from a non-biological basis (not using pre-existing nucleobases) as used in typical commercial/industrial supply of nucleoside phosphoramidites. I'm starting to wonder if they even are chemically synthesized, or if they are instead harvested in some way, but it's hard to find a smoking gun that outright says it one way or the other without access to whitepapers anymore (no more university account).
Does anybody know?