Silylcarbocyclisation-desilylation reactions of N-tosyl-2-ethynylaniline: a new protocol for the synthesis of 2-hydroxyindoline derivatives

2-Hydroxyindolines are useful building blocks for the preparation of pharmaceutical and biological-ly active compounds. (1) However, to the best of our knowledge only few synthetic procedures have been developed; most of them are based on the reduction of corresponding lactams or on cy-clisation processes of variously substituted anilines. (2) We found that β-lactams and β-lactones can be easily obtained from propargyl amides and propargyl alcohols by means of rhodium-catalysed silylcarbocyclisation reactions with dimethylarylsilanes; subsequent treatment with TBAF promotes a desilylation step, consisting in a 1,2-migration of aryl group from the silyl moiety to the adjacent carbon atom. (3) Very recently, we extended our silylcarbocyclisation-desilylation protocol to the preparation of various 4-(arylmethyl) isochroman-3-ones starting from 2-ethynylbenzyl alcohol and arylsilanes with different steric properties. (4) Here we report that silylcarbocyclisations of N-tosyl-2-ethynylaniline with dimethylarylsilanes, car-ried out with catalytic Rh+[(C7H8)(BPh4)]- (Rhsw, 0.3 mol%) under CO pressure (30 atm) at 100°C, generate (Z)-1-tosyl-3-((dimethylarylsilyl)methylene)indolin-2-ols with good yields (51-68%). These compounds can be submitted to a facile TBAF-promoted desilylation step with migration of the aryl group, affording the corresponding N-tosyl-3-(methylaryl)indolin-2-ols quantitatively and with very high diastereoselectivity (anti > 90%).