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Ilangovan, Andivelu; Sakthivel, Palaniappan; Zipse, Hendrik (2016): An Unusual Grob-type C-C/C-O Bond Cleavage of 5-Acyl-2,3-dihydro-4H-pyran-4-one Derivatives. In: Chemistryselect, Vol. 1, No. 5: pp. 1109-1116
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Abstract

A variety of 5-aroyl/alkanoyl-2,3-dihydro-4H-pyran-4-ones have been synthesised by reaction between acyclic beta-diketones and alpha, beta-unsaturated acyl chlorides in the presence of CaCl2 as catalyst. The reaction proceeds via domino C-acylation/6 pi-oxaelectrocyclization. The aryl-substituted 2,3-dihydropyranones, on treatment with hydrazine hydrate, underwent an unusual, sequential, C-C and C-O bond cleavage via tandem nucleophilic 1,2-addition/Grob-type fragmentation to provide 3,5-disubstituted 1H-pyrazoles, under mild and metal free conditions. However, the corresponding alkyl-substituted 2,3-dihydropyr-anones underwent only C-O bond cleavage to furnish 4-acyl-3,5- disubstituted-1H-pyrazoles. The reaction free energies have been calculated at the B3LYP/6-31 + G( d) level of theory for the formation of differently substituted pyrazoles. The value -215.0 kJ/mol, observed for overall reaction between diphenylsubstituted DHP 3aa and 2 equiv. of hydrazine support the formation of ynone III intermediate and its subsequent cyclization to form diphenyl pyrazole 7aa. In the case of dialkyl-substituted DHP 3pa nucleophilic attack at the C4 position is too unfavorable on thermochemical grounds.