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309 G. I. Zhungietu and E. M. Perepelitsa, Zh. Obshch. Khim. 36, 1858 (1966).
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oxide, the equilibrium concentration is ca. 9%.815,316 The isomers can be separated319 by careful fractionations avoiding acidic or alkaline catalysts which immediately restore the equilibrium. Praill and Whitear322 showed that by acetylating (Ac20 + HC104) 4-methyI-4-penten-2-one (isomesityl oxide) (80) instead of mesityl oxide (78), the yield of trimethylpyrylium perchlorate was raised from ca. 40 to 87%, which strongly suggests that 80 is the active form (the C=CH2 group, not being conjugated with the carbonyl group, behaves normally). A case in which a /?,y-unsaturated isomer prevails owing to conformational reasons is that of cyclohexenylacetone (81, R=-Me) or cyclohexenylacet6phenone (81, R = Ph) which lead to Bz-tetrahydroisochromylium salts (82) in 50-90% yields, as shown by Dorofeenko and co-workers.823-327 By the irreversible formation of the resonance-stabilized pyrylium cation, the equilibrium (78 ^ 80) is shifted to the right. This method allowed the prepration of 5,6,7,8-tetrahydroisobenzopyrylium salts (82) with p-anisyl substituents328; by starting from 5- or 7-membered cycloalkanones, pyrylium salts with fused saturated rings were prepared.S29> 330 Pulegone also
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330 Y. A. Zhdanov, G. N. Dorofeenko, and V. A. Palchkov, Khim. Geterosikl. Soedin. Akad. Nauk Latv. SSB p. 812 (1965).
affords on acetylation a pyrylium salt.331 ň„-A Iky 1 - i ruloly I - acetones and similar glycosidic derivatives are acylated to indole[2,3-c]-pyrylium salts.831ŗ,ķ
Acylation of indenylindenones leads to indenopyrylium salts.332 A related reaction is the acylation of 3,4-disubstituted benzyl ketones to isobenzopyrylium salts833-337 ; the unsubstituted compounds yield
4-pyrones as shown in Section II,D,3,a. Another related acylation converts unsaturated esters into 2-pyrones, e.g., ethyl /3,/3-dimethyl-acrylate into 4,6-dimethyl-2-pyrone.109
The anhydride or acyl chloride and the catalyst (proton acid or Lewis acid) interact leading to the acylating agent [formulated here for brevity as an acyl cation (83)].13,14,282,338,389
(RóC0)20 + HX -----------RóCO+X- + RCOOH
(RóC0)20 + MHalĄ --------Ľ¶ RóC0++ RCOOMHaln-
RóCOHal + MHaln ---------> RóCO+ + MHalJ+i
RóCOOH + 2HX ------------> RóCO+X- + HaO+X-
b. From Unsaturated Ketones and Aldehydes, with Dehydrogenation. This reaction leads to 2,4-pentadien-l-ones (type in Section II,B,2,e)
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