An unexpected one-pot synthesis of 7-isopropyl-3,3-dimethyl-10 H-spiro(indoline-2,9 -phenanthren)-10 -one

Abstract

The reaction of two commercially available compounds, phenanthrenequinone and 2,6-dialkylaniline (alkyl = Me, iPr), catalyzed by formic acid, in refluxing methanol, gave rise exclusively to 10-(2,6-dialkyl-phenylimino)-10H-phenanthren-9-one compounds (alkyl = Me (1), iPr (2)). In refluxing toluene, and when diisopropylaniline is employed, the heterocyclic compound 7-isopropyl-3,3-dimethyl-10'H-spiro(indoline-2,9'-phenanthren)-10'-one ( 3) was unexpectedly prepared as the major reaction product, in the presence of catalytic amounts of various Bronsted acids. DFT calculations indicate that the conversion of 2 into 3 involves a sigmatropic rearrangement as rate limiting step with a high energy barrier (25-28 kcal/mol), in agreement with the requirement of high reaction temperatures.