Parikh-Doering reagent

Palladium(II) chloride-thiourea, 397 Palladium-poly(ethylenimine), 393 Pallescensin E, 144 (3-Panasinsene, 343, 344 Paniculide, 594-595 Parikh-Doering reagent, 216 Pentaalky (guanidines, 105 Pentacarbonyl(methoxyphenylcarbene)chromium (0), 397-401... 

Peptide aldehydes 1 can be synthesized effectively by the oxidation of peptide alcohols 15, which are readily available without racemization by reduction of peptide esters 9 with sodium borohydride-lithium chloride (Scheme 5). The peptide alcohols 15 can be readily oxidized to afford enantiomerically pure aldehydes using Parikh-Doering or Dess-Martin reagents. This route is less popular than the previously described reductive methods due to (1) the sensitivity of the aldehydes to further oxidation, (2) racemization under the reaction conditions, and (3) instability of the products under the reaction conditions. 

During the oxidation, an acid-catalyzed cyclization of the product by attack of the nitrogen atoms on the ketone, leading to three different aminals, must be avoided. A Parikh-Doering oxidation gives a good yield of the desired ketone, while PCC, Dess-Martin reagent and Jones oxidation deliver diverse amounts of aminals. 

The reagent of Parikh and Doering was used successfully for oxidation of tricarbonyl-(7-norbomadienol)iron (1) to tricarbonyl(7-norbornadienone)iron (2). In this case use of chromium trioxide-pyridine or of the Pfitzner-Moffatt procedure resulted in fragmentation 10 benzaldehyde and benzene. 

Oxidation of carbohydrate deriratwes. Oxidation of partially acetylated carbohydrates with the reagent of Parikh and Doering leads to both oxidation and elimination of the elements of acetic acid, and thus provides a high-yield route to novel unsaturated carbohydrates.3 Apparently the oxidation step is followed by spontaneous elimination of a /f-acctoxy group. Hydroxy groups protected by benzyl or trimethylsilyl groups are not eliminated. 

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