Oxidation with chromium(VI) complexes: Difference between revisions

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==Scope and Limitations==

Buffering agents may be used to prevent acid-labile protecting groups from being removed during chromium(VI)-amine oxidations. However, buffers will also slow down oxidative cyclizations, leading to selective oxidation of alcohols over any other sort of oxidative transformation. Citronellol, for instance, which cyclizes to pugellols in the presence of PCC, does not undergo cyclization when buffers are used.<ref>Fieser, L. F.; Fieser, M. ''Reagents for Organic Synthesis''; Wiley-Interscience, New York, 1979, '''7''', 309.</ref><ref name=whatup>Babler, J. H.; Coghlan, M. J. ''Synth. Commun.'' '''1976''', ''6'', 469.</ref>

<span style="float:right;padding-right:50px;padding-top:30px;">'''''(5)'''''</span><center>[[File:ChroScope1.png]]</center>

Oxidative cyclization can be used to prepare substituted tetrahydrofurans. Cyclization of dienols leads to the formation of two tetrahydrofuran rings in a ''syn'' fashion.<ref>McDonald, F. E.; Towne, T. B. ''J. Am. Chem. Soc.'', '''1994''', ''116'', 7921.</ref>

<span style="float:right;padding-right:50px;padding-top:30px;">'''''(6)'''''</span><center>[[File:ChroScope2.png]]</center>

Enones can be synthesized from tertiary allylic alcohols through the action of a variety of chromium(VI)-amine reagents. The reaction is driven by the formation of a more substituted double bond. (''E'')-Enones form in greater amounts than (''Z'') isomers because of chromium-mediated geometric isomerization.<ref>Majetich, G.; Condon, S.; Hull, K.; Ahmad, S. ''Tetrahedron Lett.'', '''1989''', ''30'', 1033.</ref><ref name=whatup></ref>

<span style="float:right;padding-right:50px;padding-top:10px;">'''''(7)'''''</span><center>[[File:ChroScope3.png]]</center>

Suitably substituted olefinic alcohols undergo oxidative cyclization to give tetrahydrofurans. Further oxidation of these compounds to give tetrahydropyranyl carbonyl compounds then occurs.<ref>Schlecht, M. F.; Kim, H.-J. ''Tetrahedron Lett.'', '''1986''', ''27'', 4889.</ref>