Reduction of hemiaminal by triethylsilane / boron trifluoride
My organic chemistry is rusty by now, but the full mechanism for the reduction itself is likely something along these lines:

In a &quot;typical&quot; lactone / ester, the alkyl oxygen can't be lost so easily, but in this case there's a nitrogen lone pair that can assist in that. The resulting iminium ion is reduced by the silane. The acid at the end will remove the remaining boron trifluoride.