Heteroatom exchange

Two benzoannelated analogues of the pyrylium cation are known ben-zo[b]pyrylium (chromylium or 1-benzopyrylium) and benzo[c]pyrylium (isochromylium or 2-benzopyrylium) salts. So far, the 1-benzopyrylium system is considered to be the more interesting (79MI2 84MI1) because it is the basic heterocyclic system of important plant pigments (antho-cyanins), and because there are major differences between properties of its heterocyclic ring and those of monocyclic pyrylium systems as a result of benzoannelation. Thus, for instance, 1-benzopyrylium salts do not possess the ability to have recyclization reactions with heteroatom exchange. 

A specific behavior in reactions with primary amines was described for l-aryl-3-carboxy-benzo[c]pyrylium salts 62 (89KGS454). While their monocyclic analogs undergo decarboxylation with heteroatom exchange,... 

The 2- or 4-hydroxypyridines [(81) and (82)] are tautomeric and in solution exist preferentially in the corresponding pyridone forms. Their synthesis from the corresponding pyrones by a heteroatom exchange reaction is possible. 

R ac.tL jity umpoiung and 6ynthztic e.quivaJte.ntA are very fruitful concepts in modern organic synthesis, in particular for the multistep preparation of natural compounds. In this area, organo-phosphorus compounds can be useful tools, particularly for reversible umpolung by heteroatom exchange (.1.-4). 

Both the hetero and carbonyl oxygen atoms of maltol, 3-hydroxy-2-methylpyran-4-one, are replaced by sulfur on treatment with Lawesson s reagent. While the ketone —> thione conversion can be achieved with various sulfur reagents, the heteroatom exchange is unusual and appears specific for Lawesson s reagent (Equation 179). This reaction does not occur with either related chromones or xanthones <2006CC206>. 

If use is made of the five-membered cyclic phosphorus derivatives, the geometry of which is more suitable than that of the triphenylphosphine, formation of the phos-phorane intermediate is more favored and the yield of olefin increases. Deoxygenation with KSeCN in mildly alkaline medium can similarly occur by heteroatom exchange retention is observed as a consequence of double inversion. Open-chain compounds react with good yields, but cyclic oxiranes yields vary considerably with the number of ring atoms, due to steric hindrance. Alkali metal 0,0-diethyl-phosphorotellurates are very useful reagents for the conversion of terminal oxiranes, Eq. 104 but the reaction does work for other types. In the case of oxiranes formally derived from acyclic olefins, the (Z) compounds react more readily than the (E)-oxiranes. ... 

Anew synthetic method involves reaction with an A -substituted iminophosphorane, which is accompanied by oxygen-nitrogen heteroatom exchange (Eq. 182). ... 

Oxygen-sulfur heteroatom exchange has been achieved with 3-methyl-benzene thiazole-2-thione in the presence of trifluoroacetic acid and with l-phenyl-5-mercaptotetrazole. Thiirane can be prepared from oxirane on a support impregnated with alkali metal salts, by decomposition of the dithiocarbon-ate formed with carbon disulfide. A macrocyclic ether, perhydrobenzo-18-crown-6, plays a role in the nucleophilic reaction of oxirane with KCNS, which leads to thiirane in good yield. ... 

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