The mechanism is investigated for Wolff rearrangement/Staudinger [2+2] cycloaddition cascade and Pd-catalyzed, decarboxylative, formal [3+2] cycloaddition. Wolff rearrangement of 3-diazotetramic acid is determined to be rate-limiting step generates cyclic acyl ketene. The interaction of ketene with imine firstly results in zwitterion followed by conrotatory cyclization giving major cis-β-lactam. For synthesis of s-VECs, the epoxidation−cyclization cascade and hydrolysis give precursor of better performing exocyclic derivative. The reaction with 3-cyanochromone includes decarboxylation, nucleophilic attack and subsequent ring closure yielding 5-exo-trig furobenzopyranone. Based on the comparison between possible paths, the diastereoselectivity of cis over trans and regio-divergence of 5-exo-trig over 7-endo-trig are both kinetically controlled for [2+2] and [3+2] cycloaddition in common. The positive solvation effect is suggested by decreased absolute and activation energies in chlorobenzene and chloroform solution compared with in gas. These results are supported by Multiwfn analysis on FMO composition of specific TSs, and MBO value of vital bonding, breaking.

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