Me. Pierce et al., ON THE MECHANISM OF THE REDUCTION OF 1-PHENYL-3-(4-PYRIDINYLMETHYLENE)-2-OXINDOLE WITH SODIUM-BOROHYDRIDE IN METHANOL, Journal of heterocyclic chemistry, 31(1), 1994, pp. 17-23
Sodium borohydride reduction of 1-phenyl-3(4-pyridinylmethylene)-2-oxi
ndole 1 in methanol gives a high yield of the oxindole 2, a precursor
to linopirdine. The reduction is accelerated by methanol and the major
by-products during this reduction are the diastereomeric Michael addu
cts between 1 and 2 and the overreduced products, 9 and 11. Surprising
ly, indole 9 is not derived from further reduction of 2 but is formed
concurrently, whereas the ring opened product 11 is the unexpected red
uction product derived from 2.