Hydroxyketones special

This classification is illustrated in Scheme 370. The traditional method of three-component condensation of dicarbonyl compounds (or ot-hydroxyketones), amines (including ammonia) and aldehydes are still widely utilized to prepare poly-substituted imidazoles. In a special case where glyoxal and ammonia are the reactants, 2-substituted imidazoles such as 1429, 1430 are the products (Scheme 371) <1997TL1275, 1999JME1587, 2002JME2173>. 

One special case with a non-stabilised Wittig reaction is the remarkably Z-selective reaction6 with THP-protected a-hydroxyketones 37. These are among the most Z-selective Wittig reactions known and it is the influence of the a-OTHP group that is decisive. The product 38 is of course a protected allylic alcohol. In chapter 33 we shall see how to make these a-hydroxyketones stereo- and regiospecifically. 

The scope of synthetically interesting acceptors that can be functionalized in the rhodium/diene-catalyzed conjugate addition has been expanded considerably since the initial reports (Figure 8.10). Today, the range comprises not only a,P-unsaturated esters 105, which are particularly well suited for heterocychc-substi-tuted substrates [91], but also a,p-unsaturated Weinreb amides 106, which have found wide appHcation [92]. Both classes of acceptors allow straightforward modification of the resulting adducts. The use of P-silyl-substituted a,P-unsaturated carbonyl compounds 107 as acceptors is of special interest as these compounds can be transformed to P-hydroxyketones via Tamao-Fleming oxidation [93]. 

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