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124 P. Beak, Tetrahedron Letters p. 863 (1963).
125 P. Beak, Tetrahedron, 20, 831 (1964).
126 K. Dimroth and P. Heinrich, Angew. Chem. Intern. Ed. English 5, 676 (1966'
127 K. Hafner and H. Kaiser, Ann. Chem. 618, 140 (1958).
equation with p = 1.128 The formation of other alkylmercaptopyrylium salts has been reported.129-131 4-Selenopyrones behave analogously yielding 4-alkylselenopyrylium salts (24, Y = Se, R = Alk) very easily.182
By O-acylation with 2-methyl-l,3-dioxolenium fluoroborate, which reacts as O-acetyl ethylene oxide, 2,6-dimethyl-4-pyrone is converted into 4-acetoxy-2,6-dimethylpyrylium fluoroborate133 (24, Y = 0, R = Ac, X = BF4). The alleged compound with this structure which has been obtained from 8 and acetyl fluoroborate184 is, in fact,133 the BF3-complex of the pyrone.
c. Nucleophilic Displacement of (Thio)alkoxy Groups from (Thio)-alkoxypyrylium Salts. 4-Alkoxy groups of pyrylium salts undergo nucleophilic displacement very easily, even with weak nucleophiles. On recrystallization of 2,6-dimethyl-4-methoxypyrylium perchlorate (22, X = C104) from ethanol, the methoxy group is exchanged by ethoxy; recrystallization of the latter salt from methanol leads back to 22 (X = C104).118 Water exchanges alkoxy groups by hydroxy, and hence hydrolyzes alkoxypyrylium salts to pyrones.22 Other nucleophilic reagents which may replace alkoxy groups are thiols such as benzylmercaptan and secondary amines such as piperidine or morpholine.118 Hydrogen sulfide, sodium hydrosulfide, or sodium sulfide react readily with 4-alkoxypyrylium salts (25, X = 0, R = 0Me) (pyrones react much more sluggishly) leading to 4-mer-captopyrylium salts (25, X = 0, R = SH) and hence to 4-thiopyrones (23, Y = S).130 Thus, an alternative route from pyrones to thiopyrones or thiopyrylium salts is available, by intermediate alkylation which causes an enhancement of the reactivity toward nucleophiles. Likewise, sodium hydrogen selenide or sodium selenide afford seleno-pyrones (23, Y = Se).132
Subsequent studies by King and Ozog135 showed that the ease with which the 4-substituent R is replaced by R' in 25 (X = 0)
128 F. J. Ozog, V. Comte, and L. C. King, J. Am. Chem. Soc. 74, 6225 (1952).
128 A. Hantzsch, Ber. Deut. Chem. Qes. 52, 1535 (1919).
130 G. Traverso, Ann. Chim. (Rome) 46, 821 (1956).
131 R. L. Letsinger and J. D. Jamison, J. Am. Chem. Soc. 83, 193 (1961).
132 G. Traverso, Ann. Chim. (Rome) 47, 1244 (1957).
133 H. Meerwein, K. Bodenbennor, P. Borner, F. Kunert, and K. Wunderlich, Ann. Chem. 632, 38 (1960).
134 F. Seel, Z. Anorg. Allgem. Chem. 250, 331 (see p. 349) (1943).
135 L. C. King and F. J. Ozog, J. Org. Chem. 20, 448 (1955).
decreases in the order R = MeO > MeS > Me2N, i.e., in the order of increasing basicity. 4-Alkoxypyrylium salts can be converted into alkylmercapto- or dialkylaminopyrylium, and alkylmercapto- into dialkylaminopyrylium salts, but the reverse processes are not possible, e.g., 4-dialkylaminopyrylium salts do not react with alcohols or thiols. Such exchange is also possible, but with a lower rate, for pyridinium salts (25, X = NMe) which exchange OR or SR groups with NR2 groups, but do not exchange OR by SR groups. With primary amines in excess, e.g., with methylamine, 4-methoxy-2,6-dimethylpyrylium perchlorate (22, X = C104) undergoes replacement of both the alkoxy group and the heteroatom yielding 2,6-dimethyl-4-metbylamino-iV-methylpyridinium perchlorate (25, X = NMe, R = NHMe). Because 22 with ammonia or ammonium carbonate yields 4-methoxy-2,6-lutidine22 by replacing only the heteroatom, it is plausible186 that the first rapid step is the formation of a pyridinium ion, followed by slow displacement of the alkoxy group.
A similar replacement of the oxygen heteroatom by sulfur to thia-pyrylium salts (25, X = S) can occur on treatment with Na2S or NaSH. By making use of the difference in reactivity between OAlk and SAlk groups and of the strong complexation of RSH with mercury salts, Arndt et al.,136 Traverso,130, 132,137 Wizinger and Ulrich,138,139 and Ohta and Kato140 with their co-workers succeeded in preparing
136 F. Arndt and N. Bekir, Ber. Deut. Chem. Ges. 63, 2393 (1930); F. Arndt and C. Martius, Rev. Fac.. Sv.i. Univ. Istanbul A13, 57 (1948); Chem. Abstr. 42, 4176 (1948).
137 G. Traverso, Chem. Ber. 91, 1224 (1958) and previous papers.
188 R. Wizinger and P. Ulrich, Helv. Chim. Acta 39, 207 (1956).
13® R. Wizinger and P. Ulrich, Helv. Chim. Acta 39, 217 (1956).
140 M. Ohta and H. Kato, Bull. Chem. Soc. Japan 32, 707 (1959).
thiopyrones (23, Y = S), thiapyrones (26,'X = S, Y = 0), and thiathio-pyrones (26, X = Y = S) starting from pyrones (26, X = Y = 0).
Ohta and Kato140 found that in the presence of bases the OMe group of 22 may be displaced by compounds with active methylene groups (ethyl cyanoacetate, malonodinitrile), yielding 27, which can be deprotonated to a methylenepyran derivative (28, R = CN, R'=CNor C02Et).