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d. Nwcleophilic Displacements at Pyrones. In fact, this is a particular case of the former type, since pyrones, and even more thiopyrones, behave as pyrylium oxides and mercaptides, respectively (8b). A separate paragraph was deemed necessary, however, owing to the historical importance of the method and to its relative importance. The above reactivity sequence for the replacement of the R group in 25 can be completed R = MeO > MeS >Me2N> 0~; in fact, the nucleophilic displacements occur just in this order, the pyrone reacting more sluggishly and requiring more powerful nucleophiles than alkoxy, alkylmercapto, or dialkylaminopyrylium salts.
The usual carbonyl reagents (hydrazines, semicarbazone, hydro xyl-amine) do not give the normal derivatives, but lead to ring contraction and formation of pyrazoles or isoxazoles. However, a semicarbazone and an oxime of 2,6-diphenylpyrone has been obtained by Arndt et ai.,141 indirectly, through the intermediacy of the more reactive 4-thiopyrone.
Diphenylketene has been reported to react with 2,6-dimethyl-pyrone affording the diphenylmethylenepyran (28, R = R' = Ph) after splitting off carbon dioxide.142
Wizinger and co-workers143 found that 2,6-diphenyl-4-pyrone or
4,6-diphenyl-2-pyrone react with iV,i\7-dirnethylani 1 ine in the presence of P0C13 and PC15, yielding triarylpyrylium salts 29 and 30, respectively. With the same condensing agents, the above authors reacted
141 F. Arndt, G. .Ò. O. Merlin, and J. R. Partington, J. Chem. Soc. p. 602 (1935).
142 H. Staudinger and N. Kon, Ann. Chem. 384, 38 (see p. 129) (1911).
143 R. Wizinger, A. Grime, and E. Jacobi, Helv. Chim. Acta 39, 1 (1956).
4,6-diphenyl-2-pyrone with 1,1-diarylethylenes leading to 2-styryl-pyrylium salts (31, Ar = anisyl and/or phenyl). Even CH2= groups of methylenepyrans (anhydro bases) can act as nucleophiles. Indeed, in the presence of tertiary bases, 4-alkoxypyrylium salts undergo autocondensation to pyrylocyanines 32118~120; similarly, pyrones react with anhydro bases of 2,4,6-trialkylpyrylium salts on refluxing in acetic anhydride, affording colored symmetric (33) or asymmetric (34) pyrylocyanines76,77 (cf. Section II, B, l,a).
The more conventional nucleophiles obtained from compounds with active methylene groups, such as malonodinitrile or ethyl cyanoacetate give144-147 similar methylenepyrans (28, R = B/ = CN
144 L. L. Woods, J. Am. Chem. Soc. 80, 1440 (1958).
145 L. L. Woods, J. Org. Chem. 22, 341 (1957).
146 D. Farcasiu, Rev. Roumaine Chim. 9, 865 (1964).
147 S. Yamamura, K. Kato, and Y. Hirata, Tetrahedron Letters p. 1637 (1967);
J. A. Van Allan, G. A. Reynolds, and D. P. Maier, J. Org. Chem. 33, 4418
and/or COOEt) with 4-pyrones as with 4-alkoxypyrylium salts (the reaction is carried out in refluxing acetic anhydride, so that 4-acetoxy-pyrylium salts are plausible intermediates). The same reaction may be catalyzed by bases, and may be effected also with rhodamine,140 barbituric acid derivatives (cyclic malonodiamides), pyrazolones, etc.76'77'148 Nitromethane or cyclopentadiene do not react with pyrones,148'149 but tetrachloro- or tetramethoxycyclopentadienes afford cyclopentadienylidenepyrans.149 Tri- and tetraphenylcyclo-pentadienylidenepyrans150 and 9-fluorenylidenepyrans151 are also known. These compounds are heteroanalogs of sesquifulvalene.162
The best-known reaction pertaining to this type is the condensation of pyrones with Grignard reagents. One year after Gomberg and Cone153 had reported that they had observed no reaction between
2.6-dimethylpyrone and phenylmagnesium bromide, Baeyer and Piccard32 reacted 2,6-dimethylpyrone with methylmagnesium iodide obtaining 2,4,6-trimethylpyrylium perchlorate (2),48 and with phenylmagnesium bromide obtaining 2,6-dimethyl-4-phenylpyrylium perchlorate,48,68’75 the first pyrylium salts without alkoxy substituents to be identified. Since pyrylium salts also react with organo-magnesium derivatives (cf. Section II,B,l,e), it is necessary to mix rapidly at low temperature equimolar amounts of reagents and to pour the mixture after 1-2 minutes into an excess of cold concentrated acid. The procedure is delicate and the yields moderate (30-35%);
2.6-dimethylpyrone and isopropylmagnesium bromide76'77 or t-butylmagnesium chloride127 give pyrylium salts in 50% yield. Also,
2.6-diphenylpyrone reacts with methylmagnesium iodide normally154;
2.6-diphenylthiapyr-4-one reacts with methylmagnesium iodide affording 2,6-diphenyl-4-methylthiapyrylium.13 9
So far, this reaction finds application for the conversion of 4-pyronc into 4-substituted pyrylium salts with free à-positions, for which few
148 Eiden, Arch. J‘harm. 293,404 (1960); F. Eiden and H. Fenner, Chem. Ber. 101, 3403 (1968).
149 G. Seitz, Angew. Chem. 79, 96 (1967).
150 D. Lloyd and F. T. Wasson, ,J. Chem. Soc. C, 1086 (1966); Chem. Ind. (London) p. 1559 (1963).