An Unusual Grob–type C–C/C‐O Bond Cleavage of 5‐Acyl‐2,3‐dihydro‐4 H ‐pyran‐4‐one Derivatives

A variety of 5‐aroyl/alkanoyl‐2,3‐dihydro‐4 H ‐pyran‐4‐ones have been synthesised by reaction between acyclic β‐diketones and α,β‐unsaturated acyl chlorides in the presence of CaCl 2 as catalyst. The reaction proceeds via domino C‐acylation/6π‐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 1 H ‐pyrazoles, under mild and metal free conditions. However, the corresponding alkyl‐substituted 2,3‐dihydropyranones underwent only C−O bond cleavage to furnish 4‐acyl‐3,5‐disubstituted‐1 H ‐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 diphenyl‐substituted DHP 3 aa and 2 equiv. of hydrazine support the formation of ynone III intermediate and its subsequent cyclization to form diphenyl pyrazole 7 aa . In the case of dialkyl‐substituted DHP 3 pa nucleophilic attack at the C4 position is too unfavorable on thermochemical grounds.