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*[[pyridinium dichromate]] (PDC) is the pyridium salt of dichromate, [Cr<sub>2</sub>O<sub>7</sub>]<sup>2-</sup>.
*[[pyridinium chlorochromate]] (PCC) is the pyridinium salt of [CrO<sub>3</sub>Cl]<sup>−</sup>.
These salts are less reactive, more easily handled, and more selective than Collins reagent in oxidations of alcohols. These reagents, as well as other, more exotic adducts of nitrogen heterocycles with chromium(VI), facilitate a number of oxidative transformations of organic compounds, including cyclization to form [[tetrahydrofuran]] derivatives and [[Babler oxidation|allylic transposition]] to afford enones from [[allyl]]ic alcohols.
==Practical considerations==
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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;">'''''(7)'''''</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, in a reaction known as the [[Babler oxidation]]. 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 name=whatup /><ref>Majetich, G.; Condon, S.; Hull, K.; Ahmad, S. ''Tetrahedron Lett.'', '''1989''', ''30'', 1033.</ref>
<span style="float:right;padding-right:50px;padding-top:10px;">'''''(8)'''''</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>
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