Functionalized ketones ketoamides

Ketoamides,33 ketonitriles54 and keto esters55 may all be reacted selectively at the ketone function (equations 14 16). In fact, esterification was found to be the best of several hydroxy-protecting methods for the transformation shown in Scheme 6. ... 

While only 1.0 equiv. of PMHS is needed to complete the reduction of some ketones e. g. a,a,a-trifluoroacetophenone (Ic) and methyl phenylglyoxylate (Id), excess PMHS is necessary in most cases. As shown in Table 1, inconplete conversions are mostly observed for the reduction of relatively acidic substrates i. e. y ketoesters If and Ig and /0-ketoamides Ih and li (pKa = 10-13). Therefore, a likely hypothesis is that the [Zn-diamine]/PMHS system is active not only for the reductive reaction of the carbonyl function, but also for the oxidative silylation of any enolisable group. Thus, the enol-silyl ether produced would hydrolyze back in methanol to the free enol, accounting for the consumption of extra equiv. of PMHS. Nevertheless, this hypothesis does not accotmt for the reduction of imine 3a, since no inqirovement in the conversion is noted on doubling the amount of PMHS (Scheme 3, Table 1). Other imines e. g. 3b, are readily reducible with the present Zn-diamine-methanol system. 

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