Of quinolizines

The cathodic cyclization of several types of A -(5-oxoalkyl)pyridinium salts in 1 M sulfuric acid afforded quinolizine derivatives. For example, compound 256, prepared from 4-methylpyridine, gave a diastereomeric mixture of quinolizines 257 and 258. Differential pulse polarographic studies allowed a mechanism to be proposed, which is outlined in Scheme 53 <2003EJ02919>. 

Oxidation of quinolizines leads to a variety of products depending on the reagent and experimental conditions. Quinolizinium salts can be obtained from 4/f-quinolizine derivatives by oxidation with... 

Dipolar cycloaddition of 1,1-dimethoxyethene to the mesoionic pyrido[2,l-6][l,3]oxazoline 59 gave the adduct 60.The similar reaction with 61 afforded quinazolin-4-one (62). When 1-dimethylamino-l-methoxyeth-ylene was applied in the latter reaction, a mixture of quinolizin-4-one (63) and the ring-opened product 64 was obtained (88CB951). 

The reactions of l-azaquinolizin-4-ones resemble those of quinolizin-4-ones (73JHC821). For example, the oxygen of 2-methyl-l-azaquinolizin-4-one (247) may be replaced by sulfur by the action of P2Ss (Scheme 122). The resulting thione (248) reacts effectively with methyl iodide to produce 2-methyl-4-methyIthio-l-azaquinolizinium iodide (249). 

The scope of this process has been extended in a more detailed investigation to the synthesis of quinolizines [175] and the influence of alkyl substituents in various positions of the aminodialkene substrate on product diastereoselectivity was probed. Neodymium-based catalysts are particularly efficient for six-membered ring formation (40). The methodology has found further application in the synthesis of tri- and tetracyclic alkaloidal skeletons (41) [176]. 

Monographs on Contemporary Heterocyclic Chemistry and quinoline chemistry have appeared. The latter work contains reviews on alkyl- and aryl-quinolines, Reissert compounds and related dihydroquinolines, and quinoline N-oxides. The chemistry of quinolizines, 0 1, 7-naphtliyridines, azaphenalenes, 2... 

As already mentioned, most of the syntheses known for the quinolizinium system were suitably covered in CHEC-I and the field has shown little activity in the years following <81AQ225,85JHC681, 86JHC1151, 92T8459,94JOC8294>. Therefore, this section will be devoted primarily to the synthesis of quinolizines and quinolizidines. The routes are classified according to the number and position of the new bonds. 

Similar reactions can be employed for the preparation of quinolizines, usually from tetra-hydropyridines bearing a donor group on the C—N bond. Equation (30) summarizes two such syntheses, both employing ethyl 3-oxo-4-pentenoate (Nazarov s reagent) as the Michael acceptor... 

Azabicyclo[5,3,0]-l-decene derivatives (326) can be oxidized to the corresponding cyclic endo-peroxides (327), which rearrange to compounds (328), precursors of quinolizine derivatives (329) (Scheme 75) <82CB1626>. 

The past year has seen the publication of Comprehensive Organic Chemistry, one volume of which contains much information on the six-membered ring systems to be reviewed in this article a monograph on the chemistry of condensed pyrazines has also appeared. Reviews on 1,4-thiazines, l,3-benzothiazines," pyridazines, benzo[c]cinnolines, quinazolines, purines, pyrrolo[3,2-c]quino-lines, 1,10-phenanthroline and its complexes, polyaza-phenanthrenes, and 1,9- and 1,10-diaza-anthracenes have been published. Other specialist reviews are devoted to catalytic methods of obtaining pyridine bases pyridine N-oxides the stereochemistry of quinolizines, indolizines, and pyrrolizines benzothiazinone dioxides 2-quinazolones and their cyclic homologues (e.g. 

---