Ethyl 2,3-dihydroindole-2-carboxylate

Ethyl indole-2-carboxylate (45.2 g, 0.238 mmol) was dissolved in abs. EtOH (450 ml) in a 11 polyethylene container and cooled in a dry icc-cthanol bath. The solution was saturated with dry HCl gas until the volume increased to 875 ml, Granular tin metal (84.2g, 0.7l0mmol) was added to the slurry and 

Entry Indole substituents Reduction conditions Yield (%) Ref 

10 5-Hydroxy-6-l-(Methoxycarbonyl)- (pbenylsulfonyl)-8-metbyl- pyrrolo[3,2-e] EtjSiH. TEA 80 [13] 

A solution of benzyl 5-methoxyindole-3-propanoate (26Og, 0.084mol) in 

HOAc (500 ml) was cooled as NaBH3CN (26 g, 0.41 mol) was added. The resulting mixture was stirred at room temperature for 3,5 h and then poured into cold water (21). The mixture was extracted with several portions of 

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A traditional method for such reductions involves the use of a reducing metal such as zinc or tin in acidic solution. Examples are the procedures for preparing l,2,3,4-tetrahydrocarbazole[l] or ethyl 2,3-dihydroindole-2-carbox-ylate[2] (Entry 3, Table 15.1), Reduction can also be carried out with acid-stable hydride donors such as acetoxyborane[4] or NaBHjCN in TFA[5] or HOAc[6]. Borane is an effective reductant of the indole ring when it can complex with a dialkylamino substituent in such a way that it can be delivered intramolecularly[7]. Both NaBH -HOAc and NaBHjCN-HOAc can lead to N-ethylation as well as reduction[8]. This reaction can be prevented by the use of NaBHjCN with temperature control. At 20"C only reduction occurs, but if the temperature is raised to 50°C N-ethylation occurs[9]. Silanes cun also be used as hydride donors under acidic conditions[10]. Even indoles with EW substituents, such as ethyl indole-2-carboxylate, can be reduced[ll,l2]. 

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