Keto reductive, hydrogen-mediated

Table 22.2 Catalytic hydrogen-mediated reductive aldol cyclization of keto-enones.

This reaction type has also been used for the diastereoselective reduction of a variety of structurally related o -amino-/3-keto esters (equations 9 and 10). Treatment of rac-73 with a substoichiometric amount of chiral mediator 72, using formic acid and triethylamine as the hydrogen source, gave the enantiomerically pure a-amino-/ -hydroxy ester (R,S)-syn-74 in 100% yield with 95 5 dr and 99% ee (equation 9), via the diastereoselective reduction of (S)-73. The unreacted R)-73 efficiently racemizes through tautomerization under these reaction conditions. 

More recently, the related indium-mediated radical reactions have been widely studied (Scheme 7.13).19 Indium iodide-mediated radical cyclisation was first reported by Cook et al.20 The indium-mediated 1,4-addition of alkyl radicals to (F)-but-2-enenitrile was investigated by using 1-ethylpiperidinum hypophosphite (EPHP) as a hydrogen donor in aqueous media (Scheme 7.13).21 Atom transfer radical cyclisation and reductive radical cyclisation were studied using indium and iodine.22 Indium mediated alkyl radical addition to dehydroamino acid derivatives was also reported.23 The indium-mediated radical ring expansion of a-halomethyl cyclic (3-keto esters, shown in Scheme 7.13, was achieved in aqueous alcohols.24... 

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