Dilthey synthesis

When reacted with aryl methyl ketones in acetic anhydride in the presence of a hydride acceptor, e.g. FeCl3, chalcones 32 yield trisubstituted pyrylium salts 34 Dilthey synthesis) ... 

Primary products are pentane-1,5-diones 36 resulting from Michael addition of the methyl ketone to the enone moiety. In a recent modification of the Dilthey synthesis [5], two molecules of an aryl methyl ketone are cyclocondensed directly with an aldehyde... 

Dakin-West reaction 172 Darzens reaction 21 Dilthey synthesis (pyrylium ion) 302 Dimroth rearrangement 261 Doebner-Miller synthesis (isoquinoline) 400... 

The synthesis of pyrylium salts, discovered in a classical study by Dilthey (20JPR(l0l)l77>, involves the facile acid-catalyzed cyclization of a 1,5-dicarbonyl compound. The value of this route stems from the different structural features which are acceptable in the five-carbon unit and from the variety of methods available for their synthesis. The reaction is the reverse of the synthetically useful ring opening of pyrylium salts by nucleophiles. 

The synthesis of compd VI, already mentioned, was described earlier by Stoll Scher-rer (Ref 2) and Dilthey et al (Ref 4). The synthesis of Cyclohexanone Monoperoxide,... 

The synthesis of oxazoles from a-hydroxyimino ketones was originally described by Diels and Riley,143 and the method was later extended by Dilthey and Friedrichsen,144 and Selwitz and Kosak.145 The method involved condensation of a-hydroxyimino ketones with aromatic aldehydes in the presence of dry hydrogen chloride to form oxazole iV-oxides (67) which could easily be reduced with zinc in acetic acid to the free base (68). Catalytic reduction (Ni/H2) is also effective for the final stage.146... 

By 1916 Dilthey had synthesized 2,4,6-triphenylpyrylium ferrichlo-ride (tetrachloroferrate) from acetophenone and benzaldehyde in acetic anhydride in the presence of ferric chloride. He discovered other syntheses of pyrylium salts having aryl and alkyl substituents, as aryl groups enhanced the stability of pyrylium salts. Most often, the anions were perchlorate and tetrachloroferrate, but tetrafluoroborate began to be appreciated. A simple synthesis of alkyl-substituted pyrylium salts had to wait till the treatment of mesityl oxide with acetic anhydride was described by Schneider and Sack using sulfoacetic acid, and by Diels and Alder using perchloric acid. Then a lack of interest followed till the late 1950s, when Karl Dimroth and Klaus Hafner in Germany described new reactions of pyrylium salts, and new synthetic approaches were developed in Bucharest and in London. 

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