# Mechanism of enolether formation with ethyl orthoformate in estr-4-ene-3,17-dione (Djerassi's norethisterone synthesis)

The second step of Djerassi's synthesis of 19-nor-17α-ethynyltestosterone (norethindrone), involves the protection of the ketone in position 3 via enolether formation, as in the picture:

What is the mechanism of this reaction? I couldn't find it anywhere. I suppose it should be similar to an ether synthesis, but I'm not sure about the precise mechanism. (On a side note, I assume that the other ketone in 17 remains intact because of controlled reaction conditions.)

$$\ce{HC(OEt)_3 + H2O -> HCO2Et + 2EtOH}$$