Gribble indole reduction

Reduction of the indole double bond using sodium cyanoborohydride in glacial acetic acid. The use of sodium borohydride leads to reduction and N-alkylation. 

Reduction of diaryl ketones and diarylmethanols to diarylmethanes using sodium borohydride in trifluoroacetic acid. Also applicable to diheteroaryl ketones and alcohols. 

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For a review of the reduction of enamines and indoles with NaBH, and a carboxylic acid, see Gribble Nutaitis Org. Prep. Proced. Int. 1985, 17, 317-384. Enamines can also be reduced by formic acid sec Nilsson Carlson Acta Chem. Scand. Sect. B 1985, 39, 187. 

Gribble and Saulnier (79) have extended their ellipticine synthesis 43) to the synthesis of 9-methoxyellipticine (2) (Scheme 24). One of the key features of this approach is the regioselective nucleophilic addition to the C-4 carbonyl group of pyridine anhydride 28. The other noteworthy transformation is the conversion of keto lactam 142 to the diol 143 with methyllithium, a process that presumably involves cleavage of the initial adduct to a methyl ketone which undergoes cyclization at the C-3 position of the indolyl anion. Reduction of 143 with sodium borohydride completes the synthesis of 2, in 47% overall yield from 5-methoxyindole (139). Gribble and students 80) have also used this method to synthesize 8-methoxyellipticine (134), 9-fluoroellipticine (144), and the previously unknown 7,8,9,10-tetrafluorellipticine (145), each from the appropriate indole. 

For a review of the reduction of enamines and indoles with NaBH4 and a carboxylic acid, see Gribble,... 

Following the discovery of the ability of NaBHVRCC H to effect the reduction and N-alkylation of indoles,12 Gribble and co-workers found that diaryl ketones are smoothly reduced to diarylmethanes using NaBFL/TFA.1 A year earlier it was found that diarylmethanols and triarylmethanols are similarly reduced to the respective hydrocarbons.2... 

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