From 1,5-Dicarbonyl Compounds

5-Dicarbonyl compounds can be cyclised, with dehydration and in the presence of an oxidising agent. 

Mono-enolisation of a 1,5-diketone, then the formation of a cyclic hemiacetal, and its dehydration, produces 4//-pyrans, which require only hydride abstraction to arrive at the pyrylium oxidation level. The diketones are often prepared in situ by the reaction of an aldehyde with two moles of a ketone (compare Hantzsch synthesis, 8.14.1.2) or of a ketone with a previously prepared conjugated ketone - a chalcone in the case of aromatic ketones/aldehydes. It is the excess chalcone that serves as the hydride acceptor in this approach. 

Early work utilised acetic anhydride as solvent with the incorporation of an oxidising agent, often iron(ni) chloride (though it is beheved that the acylium cation is the hydride acceptor) latterly the incorporation of 2,3-dichloro-5,6-dicyano-l,4-benzoquinone, 2,6-dimethylpyrylium or, most often, the triphenyhnethyl cation have proved efhcient. In some cases the AH-pyxm is isolated then oxidised in a separate step.  

If an unsaturated dicarbonyl precursor is available, no oxidant needs to be added a synthesis of the perchlorate of pyrylium itself, shown below, falls into this category careful perchloric acid treatment of either glutaconaldehyde, or of its sodium salt, produces the parent salt. (CAUTION potentially explosive). 

---

The synthesis of 4//-thiopyrans from 1,5-dicarbonyl compounds consists of the cyclization of 1,5-dioxo precursors with hydrogen sulfide in the presence of protic acids or with various phosphorus polysulfides. 

Due to intramolecular aldol reaction and the following Schiff base formation prior to transfer hydrogenation, a 4 -3-substituted cyclohexylamines are obtained from 1,5-dicarbonyl compounds and ArNH2- The very crowded phosphate catalyst ID is used in this transformation. ... 

The Hantzsch synthesis of pyridines features 1,4-dihydropyridines in the first step. A variant of the method uses hydroxylamine (Table 17-5), which can be regarded as an oxidized version of ammonia. With this reagent, pyridines are formed directly from 1,5-dicarbonyl compounds, in turn readily made by Michael additions of enolates to a,j8-unsaturated aldehydes and ketones (Section 18-11). Formulate the mechanisms of the two steps in the pyridine synthesis shown. 

---