the oxidative transposition of tertiary allylic alcohols to give enones. The mechanism of this process likely depends on the acidity of the chromium reagent. Acidic reagents such as PCC may cause ionization and recombination of the chromate ester (path A), while the basic reagents (Collins) likely undergo direct allylic transposition via sigmatropic rearrangement (path B). (4) Oxidative cyclizations of olefinic alcohols to cyclic ethers may occur via [3+2], [2+2], or epoxidation mechanisms. Insights into the mechanism is provided by structure-reactivity, implicating direct epoxidation by the chromate ester. Subsequent epoxide opening and release of chromium leads to the observed products. (5) 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. (6) Oxidative cyclization can be used to prepare substituted tetrahydrofurans. Cyclization of dienols leads to the formation of two tetrahydrofuran rings in a syn fashion. (7) 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. (8) Suitably substituted olefinic alcohols undergo oxidative cyclization to give tetrahydrofurans. Further oxidation of these compounds to give tetrahydropyranyl carbonyl compounds then occurs. (9) In addition to the limitations described above, chromium(VI) reagents are often unsuccessful in the oxidation of substrates containing heteroatoms (particularly nitrogen). Coordination of the heteroatoms to chromium (with displacements of the amine ligand originally attached to the metal) leads to deactivation and eventual decomposition of the oxidizing agent. Comparison with other methods Methods employing dimethyl sulfoxide (the Swern and Moffatt oxidations) are superior to chromium(VI)-amines for oxidations of substrates with heteroatom functionality that may coordinate to chromium. Dess-Martin periodinane (DMP) offers the advantages of operational simplicity, a lack of heavy metal byproducts, and selective oxidation of complex, late-stage synthetic intermediates. Additionally, both DMP and manganese dioxide (MnO2) can be used to oxidize allylic alcohols to the corresponding enones without allylic transposition. When allylic transpositions is desired, however, chromium(VI)-amine reagents are unrivaled. Catalytic methods employing cheap, clean terminal oxidants in