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Six‐Membered Rings with One Oxygen: Pyrylium Ion, Related Systems and Benzo‐Derivatives

“…For instance, the reaction of secondary amines with trialkylpyrylium salts gives rise to N,N-dialkylanilines 78 through the formation of an intermediate enamine (77) (Scheme 18.20) [37].…”

“…The cyclization proceeds in the presence of either protic or Lewis acids. A review that covers the different variations of this reaction is available [77]. In many cases, the dicarbonyl compound is prepared in situ by Michael addition of an active methylene or an enolate to an a,b-unsaturated carbonyl compound [78].…”

A new reaction of the phenylhydrazones of 1,5-diketones. Synthesis of dihydropyran derivatives

“…Thus, the synthesis of dihydropyrans from ε-keto acids by the action of acetic anhydride is well known [10]. Oxopropylpyrazolones undergo cyclization during ionic hydrogenation with triethylsilane and trifluoroacetic acid in the presence of catalytic amounts of boron trifluoride etherate with the formation of substituted 5,6-dihydropyrano[3,2-d]-pyrazoles [11]. Although the synthesis of dihydropyrans from the 1,5-diketones themselves has not been described, various triketones of the oxo-1,5-diketone type were converted by ionic or catalytic hydrogenation over Raney nickel or by borohydride reduction into dihydropyran derivatives containing a keto group conjugated with the double bond of the dihydropyran ring [11].…”

“…Oxopropylpyrazolones undergo cyclization during ionic hydrogenation with triethylsilane and trifluoroacetic acid in the presence of catalytic amounts of boron trifluoride etherate with the formation of substituted 5,6-dihydropyrano[3,2-d]-pyrazoles [11]. Although the synthesis of dihydropyrans from the 1,5-diketones themselves has not been described, various triketones of the oxo-1,5-diketone type were converted by ionic or catalytic hydrogenation over Raney nickel or by borohydride reduction into dihydropyran derivatives containing a keto group conjugated with the double bond of the dihydropyran ring [11]. At the same time during catalytic hydrogenation over Raney nickel at 150°C in acetic acid 2-(3-oxo-1,3-diphenylpropyl)tetral-1-one (1d) undergoes reduction of the carbonyl groups and heterocyclization, leading to the corresponding tetrahydropyran [11].…”

“…Although the synthesis of dihydropyrans from the 1,5-diketones themselves has not been described, various triketones of the oxo-1,5-diketone type were converted by ionic or catalytic hydrogenation over Raney nickel or by borohydride reduction into dihydropyran derivatives containing a keto group conjugated with the double bond of the dihydropyran ring [11]. At the same time during catalytic hydrogenation over Raney nickel at 150°C in acetic acid 2-(3-oxo-1,3-diphenylpropyl)tetral-1-one (1d) undergoes reduction of the carbonyl groups and heterocyclization, leading to the corresponding tetrahydropyran [11]. The transformation of the monophenylhydrazones of 1,5-dicarbonyl compounds into dihydropyran derivatives was previously unknown.…”