Mesityl oxide, Diels-Alder reaction

The material is prepared by Diels-Alder reaction of 1,3-pentadiene and mesityl oxide and subsequent aldol condensation of the resulting acetyl trimethyl cyclohexene with acetaldehyde [99]. 5-Damascone is used in perfumes to create masculine notes. 

Diels-Alder catalyst. Unlike other Lewis acids, TiCl4 favors formation of the exo-isomer in the Diels-Alder reaction of cyclopentadiene and mesityl oxide (equation... 

C (10 mmHg). In CeD, the PNMR shift was <5 +159, entirely consistent with the presence of a C=P double bond, and this was confirmed by the C NMR spectrum, which showed phosphorus to be directly coupled to an sp carbon (<5 194.7, Jfc 68.7 Hz) and to an sp carbon 37.1, /pc 51.4 Hz). The 27f-phospholes where the 2-phenyl was replaced by 2,4,6-trimethylphenyl, 2,4,6-trimethoxyphenyl, and methyl were also made in good yield by this method. These 2 -phospholes are thermally stable but very sensitive to water and were hydrated on attempted chromatography on Kieselgel to form the phospholanone (204). Other reactions observed (Scheme 46) included a Diels-Alder cycloaddition with methyl propiolate (giving (205)), and a 1,3-dipolar cycloaddition with mesityl nitrile oxide that formed (206). 

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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