Tandem oxidative processes

When the Neunhoffer double-condensation approach is applied to a-ketoaldehydes, the so-formed 3,5-disubstituted-l,2,4-triazines are often produced with excellent regioselectivity. However, a-ketoaldehydes are notoriously difficult to handle due to their high reactivity (85JOC2198). To circumvent these issues, the Taylor group (06TL3865) has developed methodology to allow the utilisation of a-hydroxyketones 14 in a one-pot tandem oxidation process (TOP), in which the a-ketoaldehyde in generated and trapped in situ (Scheme 5). 

Tandem oxidation processes using manganese dioxide 05ACR851. 

Tandem Oxidation Processes (TOP). Tandem oxidation processes that employ manganese dioxide as the oxidant have recently been reviewed. Oxidation-Wittig tandem sequences, as described above, are the most explored process of this type. Although recent applications have appeared in the literature, only a few constitute improvements or enhancements to what is known. Notable among these is the conversion of activated alcohols into 1,1-dibromoalkenes, a very mild process analogous to a Corey-Fuchs approach to this type of compound (eq 100). ... 

The tandem oxidation processes employing Mn02 continue to be applied to functional group interconversions. A new entry in this area is the direct conversion of activated alcohols to nitriles via an intermediate aldehyde followed by its corresponding imine (eq 104). 52... 

Tandem oxidation processes have also been applied to five-and six-membered heterocycle synthesis. New examples of six-membered rings include pyrimides and p)mdines. In one approach, an cK-arylpropargyl alcohol is oxidized to the corresponding ynone, which then reacts with ethyl a-aminocrotonate or an amidine to afford pyridines or p)Timidines, respectively (eqs 105 and 106). In both cases, the reactions were limited in scope, but the former was subsequently inproved by implementation of 8-keto esters and ammonium acetate in place of the a-amino cro-tonate s)mthon (eq 107). ... 

This reaction has been improved by the treatment of aldehydes with trimethylsilyl cyanide to give trimethylsilyl cyanohydrins, which are subsequently hydrolyzed to cyanohydrins under weakly acidic conditions and oxidized by Mn02 to acyl cyanides. Such improvement works under the condition that the original reaction protocol fails. In addition, Mn02 has been implemented to a tandem oxidation process (TOP) recently because of its mild oxidation ability. ... 

It was shown, that a tandem oxidation process occurs, in which initially the aUylic alcohol is oxidized in situ by Mn02 to the corresponding aldehyde 33 ... 

Substituted quinoxalines can be also prepared by a one-pot process commencing from hydroxy ketones using a Mn02 (Raw et al. 2004), FeCla/morphoIine (Song et al. 2012), and silica gel (Jeena and Robinson 2014)-mediated tandem oxidation process (Table 2.1). 

Soule J-F, Miyamura H, Kobayashi S (2013) Direct amidation from alcohols and amines through a tandem oxidation process eatalyzed by heterogeneous-polymer-incarcerated gold nanopartieles under aerobic conditions. Chem Asian J 8(ll) 2614—2626... 

Foot JS, Kanno H, Gibhn GMP, Taylor RJK (2003) Esters and amides from activated alcohols using manganese(lV) dioxide tandem oxidation processes. Synthesis 07 1055-1064... 

McAllister GD, Wilfred CD, Taylor RJK (2002) Tandem oxidation processes the direct conversion of activated alcohols into nitriles. Synlett 8 1291-1292... 

Very recently, Gu, Li, and coworkers described an iron-catalyzed tandem oxidative process for the synthesis of benzoxazoles from easily available substrates, in which an alkyl C(sp )-H bond adjacent to an oxygen atom was directly aminated with a N-H bond (Scheme 9.9) [11]. In addition, excellent functional group tolerance under relatively mUd conditions and great regioselectivity were exhibited. 

---