Recent advances in cyclization reactions of conjugated ynals.

Conjugated ynals, typically alk-2-ynals bearing an aldehyde group conjugated with an alkyne, have emerged as versatile synthons in annulation chemistry. Their compact structure combines an electrophilic formyl group with an activated C-C triple bond, enabling diverse activation modes under transition-metal, N-heterocyclic carbene, Lewis acid and base-mediated or photoredox conditions. In recent years, these features have been exploited to construct a wide range of heterocycles, carbocycles, fused ring systems and chiral frameworks with high regio-, chemo- and stereocontrol. This review summarizes recent advances in cyclization reactions of conjugated ynals, with emphasis on asymmetric annulations, two-component cyclizations and multicomponent cascade processes. Particular attention is paid to catalyst-controlled divergent reactivity, the formation of key intermediates such as alkynyl acyl azoliums, allenolates, alkynyl iminium ions and radical/electrophilic species, as well as the synthetic utility of the resulting cyclic products. Current limitations and future opportunities in this rapidly developing field are also discussed.