A general strategy for the synthesis of 2-substituted cyclobutanone sulphides via a tandem Brønsted acid catalysed nucleophile addition/ring-contraction/C3-C4 ring-expansion reaction sequence has been exploited. The procedure led to a wide panel of four membered cyclic ketones in good to excellent yields and with broad substrate scope. Mechanistic aspects and kinetic parameters were investigated by quantum chemical DFT calculations allowing us to rationalize the different reactivity of 2-aryl- and 2-alkyl- substituted 2-hydroxycyclobutanones towards thiol nucleophiles in reactions mediated by sulphonic acids. NMR and in situ Raman techniques, were employed to better understand the reaction kinetics and parameters that affect the desired outcome.

The straightforward, continuous-flow synthesis of cyclopropyl carbaldehydes and ketones has been developed starting from 2-hydroxycyclobutanones and aryl thiols. This acid-catalyzed mediated procedure allows access to the multigram and easily scalable synthesis of cyclopropyl adducts under mild conditions, using reusable Amberlyst-35 as a catalyst. The resins, suitably ground and used for filling steel columns, have been characterized via TGA, ATR, SEM and BET analyses to describe the physical–chemical properties of the packed bed and the continuous-flow system in detail. To highlight the synthetic versatility of the arylthiocyclopropyl carbonyl compounds, a series of selective oxidation reactions have been performed to access sulfoxide and sulfone carbaldehyde cyclopropanes, oxiranes and carboxylic acid derivatives.