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261 W. Dilthey andH. Kaffer. Ber. Deut. Chem. Ges. 55, 1276 (1922).
262 M. Simalty, J. Carretto, and R. Fugnitto, Bull. Soc. Chim. France p. 2969 (1966).
263 c. F. H. Allen and H. R. Sallans, Can. J. Res. 9, 574 (1933).
264 G. N. Dorofeenko, Z. N. Nazarova, and N. N. Novikov, Zh. Obshch. Khim. 34, 3918 (1964).
265 G. N. Dorofeenko, G. A. Korolchenko, and S. V. Krivun, Khim. Geterotsikl. Soedin. Akad. Nauk Latv. SSR p. 817 (1965).
266 a. Treibs and H. Bader, Ghem. Ber. 90, 789 (1957).
267 R. Lombard and J. P. Stephan, Bull. Soc. Chim. France p. 1458 (1958).
pyrylium salts268 from the 1,5-diones obtained by condensing aldehydes with two moles of cvclanones,269~271 and does not affect sensitive substituents like furyl.205 Balaban272 showed that similar conversions may be effected with f-butyl chloride and aluminum chloride, but f-butylation of phenyl substituents occurs as a side process.278
2,4,6-Tri-i-butylpyrylium can be obtained by dehydrogenating the corresponding 1,5-diketone with triphenylmethyl fluoroborate.278a It was shown by Farcasiu that 1,5-diketones can also be dehydrogenated and dehydrated to pyrylium salts by triphenylmethyl hexachloroantimonate generated in situ from chlorotriphenyl-methane and antimony pentachloride. Even pentaphenylpyrylium may thus be prepared at room temperature.278b
It is known that tropylium may be prepared from tropylidene via hydride abstraction by Ph3C+274 or Me3C+275 carbonium ions; therefore, it is very likely that here too the dehydrogenation is a hydride transfer from the 1,5-dione to an acceptor. A similar dehydrogenation of chromanones to chromones, with triphenylmethyl perchlorate was reported.27e> 276a A study of the electrooxidation of 1,5-diones on a rotating platinum electrode277 showed that 1,5-diaryl-substituted diones afford pyrylium salts in these conditions and that the half-wave potentials correlate with yields in chemical dehydrogenations.
268 A. T. Balaban and N. S. Barbulescu, Rev. Roumaine Chim. 11, 109 (1966).
299 N. S. Barbulescu, G. Badita, and M. N. Tilichonko, Zh. Obshch. Khim. 33, 4027 (1963).
270 M. N. Tiliohenko, V. G. Kharchenko, and Ò. I. Krupina, Zh. Obshch. Khim.
34, 2721 (1964); M. N. Tilichenko, Zh. Obshch. Khim. 25, 2503 (1955); V. A. Khaminski and M. N. Tilichenko, Khim. Gelerotsikl. Soedin. Aka/d. Nauk Latv. SSR p. 708 (1967).
271 G. Ó. Pavel andM. N. Tilichenko, Zh. Organ. Khim. 2, 2262 (1966).
272 A. T. Balaban, Compt. Rend. 256, 4239 (1963).
273 A. T. Balaban, A. R. Katritzky, and B. Semple, Tetrahedron 23, 4001 (1967). 273a ê. Dimroth and W. Mach, Angew. Chem. 80, 489 (1968); W. Rundel, Chem.
Ber. 102, 374 (1969).
27çü d. Farcasiu, Tetrahedron 25, (1969) in press.
274 H. J. Dauben, F. A. Gadecki, Ê. M. Harmon, and D. R. Pearson, J. Am. Chem. Soc. 79, 4557 (1957).
275 1). Bryce-Smith and N. A. Perkins, J. Chem. Soc. p. 2320 (1961).
276 A. Schonberg and G. Schiitz, Chem. Ber. 93, 1466 (1960).
2764 B. D. Tilak and Z. Mulijiani, Tetrahedron 24, 949 (1968).
277 C. Bratu and A. T. Balaban, Rev. Roumaine Chim. 10, 1001 (1965).
As will be discussed in the next section, 1,5-pentanediones are obtained by Michael addition of acetophenones to chalcones. The addition and cyclization may be merged in one step (see Section
II,C,2,g). When acetophenone was condensed with chalcone (74) in the presence of BP3-Eta0267,278 or of HC104,272 /9-phcnylpropio-phenone (76) was obtained as by-product; its formation is due to hydride transfer to the conjugate acid of chalcone (75), which is the acceptor (experimental data and theoretical calculations279 show that chalcones are protonated at the oxygen atom). Balaban272 obtained a 72% yield in the conversion 70 37 using as acceptor chalcone and
as catalysts perchloric or sulfuric acids (i.e., 75). The formation of /3-p henylpropioplienone (76) in the Chichibabin synthesis of pyridines from chalcones and ketones in the presence of ammonium acetate,280 and in the pyrimidine synthesis from chalcones and amidines281 is undoubtedly due to a similar hydride transfer.
(70) + (75)
-> (71) +
Boron trifluoride etherate,279a is also a good catalyst for this hydride transfer to chalcone. Unlike triphenylmethyl perchlorate, however, chalcone is able to enter Michael additions with the 1,5-diketone followed by eliminations leading to unexpected products, e.g., 3-benzyl-2,4,6-triphenylpyrylium from 2-carbethoxy-l,3,5-tri-phenylpentane-l,5-dione and chalcone; the benzyl group originates from chalcone, the elimination product being ethyl benzoylacetate.279*
278 R. Lombard and J. P. Stephan, Bull. Soc. Chim. France p. 1369 (1957).
279 Ì. H. Palmer and D. S. Urch, J. Chem. Soc. p. 174 (1963); N. C. Deno,