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Advances in Heterocylic Chemestry. Vol 10 - Boulton A.J.

Boulton A.J. Advances in Heterocylic Chemestry. Vol 10 - Academic Press, 1969. - 347 p.
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E,2 or for the five-component synthesis F,1 is known and is unlikely to be found; in these cases at least three reagents are necessary, therefore complex mixtures are expected to result and the procedure is unpractical.
It will be observed that most syntheses yield pyrylium salts in which positions 2,4, and 6 are substituted. Since according to formulas lb-lc these positions have a partial positive charge, it can readilv be understood why electron-donating substituents (hydroxy, alkoxy, alkyl, or aryl) in these positions stabilize the pyrylium salts. Only three pyrylium salts which do not have substituents in either a-position have been reported and few unsubstituted in or in one -position; they are less stable toward hydrolysis, and in the case of perchlorates they explode more easily, than 2,4,6-trisubstituted compounds. In fact, the former are secondary, the latter tertiary carbonium ions. This fact also explains why the parent compound (1) was prepared only in 1953.
B. One-Oomponent Syntheses
1. From Compounds with Preformed Pyran Ring
a. From a Different Pyrylium Salt. To this section pertain all reactions which convert one pyrylium salt into another. This does not include, however, 2- or 4-pyrones which behave according to formula
51 The potential of the positive integer 5, i.e., the seven different ways it can be obtained as sum of positive integers, is represented by: 5 (type syntheses); 4+1, 3 + 2 (type C); 2 + 2+1, 3 + 1 + 1 (type D); 2 +1 + 1 + 1 (type E); and l + l + l + l + l (type F). (G-. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers, Chapt. 19, 3rd Ed. Oxford Univ. Press, London and New York, 1954.)
8b as pyrylium oxides, just like the phenolate anion which is a benzene oxide, or like tropone which is a tropylium oxide.
First, mention should be made of the metathetical reaction, replacing an anion of a pyrylium salt by another; when the solubility of the latter salt is lower than that of the former, the conversion is easy. In the opposite case, one has to find a solvent in which the solubilities are reversed (perchlorates are less soluble in water than chloroferrates or iodides, but in concentrated hydrochloric or hydroidic acids, respectively, the situation is reversed52, 53). For preparing chlorides which are usually readily soluble salts, one can treat the less soluble chloroferrates with hydrogen sulfide63 or hydroxylamine.54 Another method is to obtain the pseudo base in an organic solvent and to treat it with an anhydrous acid.58
Most reactions are carried out with perchlorates or fiuoroborates; methylene dichloride is a convenient aprotic solvent for salts with the former anion. Among the various anions, the strongest nucleophile yet reported to give ionized salts (in the case of an isobenzopyrylium salt) is the formate anion55 (cf. also Section II,B,2,c).
Second, there exists a number of reactions starting from pyrylium salts and involving substitution at the pyrylium ring, or modification of existing substituents. These reactions will be described here briefly; they will be discussed in more detail in the forthcoming second part of this review.
Only one electrophilic substitution56 at the pyrylium, ring has been described, namely the /3-deuteration of 2,4,6-trimethylpyryliumB7 or
2,4,6-triphenylpyrylium perchlorates58 on prolonged reflux in CHjCOOD; possibly, however, this reaction proceeds via the pseudo base. The nitration of 2,4,6-triphenylpyrylium does not affect the pyrylium ring, but attacks the 2- and 6-phenyl groups in the meta-, and the 4-phenyl group in the .-position, as was proved by
52 A. T. Balaban and C. D. Nenitzescu, J. Chem. Soc. p. 3553 (1961).
53 A. T. Balaban, Compt. Bend. 256, 4041 (1963).
64 N. . Khromov-Borisov and L. A. Gavrilova, Zh. Obshck. Khim. 31, 2192
55 M. Lempert-Sreter, Acta Chim. Acad. Sci. Hung. 50, 379 (1966).
66 A. R. Katritzky and C. D. Johnson, Angew. Chem. 79, 629 (1967).
57 E. Gard, A. Vasilescu, G. D. Mateescu, and A.T. Balaban, J\ Labelled Comds, 3, 196 (1967).
68 E. Gard, I. I. Stanoiu, F. Chiraleu, and A. T. Balaban. Rev. Roumaine Chim,. 14, 247 (1969).
Le Fevre, both by degradation and by synthesis.59-61 The course of the substitutions described above was accounted for by molecular orbital calculations.62,63
Less reactive electrophilic reagents like those involved in acylation or alkylation apparently do not react with phenyl-substituted pyrylium salts; the ^-acylation of a phenyl group in position 3 of the pyrylium salt obtained on diacylation of allylbenzene (Section
II,D,3,a), and the ^-Mjutylation of phenyl groups in -positions of pyrylium salts prepared by dehydrogenation of 1,5-diones by means of /-butyl cations (Section II, B, 2, f) probably occur in stages preceding the pyrylium ring closure.
Nucleophilic displacements at alkoxypyrylium salts will be described in Section II,B, l,c.
As modification of substituents, the simplest is the deuteration of alkyl groups bonded to positions 2, 4, and 6 of the pyrylium nucleus on recrystallization from deuterium oxide or from CHsCOOD. Since only benzylic hydrogens are exchanged, the intermediates63*1 are probably methylenepyrans (anhydro bases). A study by means of NMR spectroscopy showed that the 4-methyl group reacts 10 times faster than the 2- and 6-methyl groups in 2,4,6-trimethylpyrylium, allowing selective deuteration and/or dedeuteration,57, d8 and that diphenylmethylpyrylium salts are deuterated even faster.64 Molecular orbital calculations substantiate these findings.82'68 The deuterated or partly deuterated 2,4,6-trimt lylpyrylium salts are useful starting materials for the preparation of aromatic or heterocyclic compounds with deuterated side-chains difficultly obtainable by direct exchange. 65> 66 The easy formation of the anhydro bases from 2- and
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