I'm betting that ekinaz is confused by your question. He is talking about "synthesis" from the standpoint of developing processes in the laboratory to replicate compounds found in nature.
I'm betting you are talking about the reaction type where two reactants form a single product as in
H2 + Cl2 --> 2HCl
or 2H2 + O2 --> 2H2O
or 4Fe + 3O2 --> 2Fe2O3
The last one is certainly found in "nature". Throw an iron wrench out in the yard and come back in a week. It will be nice and rusty.
Burn some coal:
C + O2 --> CO2
plus there will be some sulfur in the coal
S + O2 --> SO2
There are lots of other examples of oxidation found in nature.
Lime, CaO, found in dried clay, reacts with carbon dioxide in the air to make limestone, calcium carbonate.
CaO + CO2 --> CaCO3
The Haber process is an important reaction for making ammonia for fertilizer and other applications.
N2 + 3H2 --> 2NH3
It was very important in World War I, because it allowed Germany to make munitions long after the British blockade cut off the supply of ammonium nitrate from Chile.