Ready Access\nto [5,6,5]-Trioxa-spiro and Fused Ketals\nvia Ag-Catalyzed Cascade Annulation of 4‑Pentyn-1-ols and Aldehydes

In this study, we unveil the versatility of 4-pentyn-1-ols\nas carbonyl\nsurrogates for the unprecedented synthesis of diverse oxygen heterocycles,\nincluding [5,6,5]-bis-spiroketals (trioxadispiroketals) and [5,6,5]-furano-spiroketals\nrelated to bioactive natural products. These reactions commence with\nthe π-activation-induced intramolecular hydroalkoxylation of\n4-pentyn-1-ols, yielding cyclic enol ethers, which undergo subsequent\nthree-component annulation with aldehydes in a [2+2+1+1] fashion,\nresulting in the formation of [5,6,5]-bis-spiroketals. Notably, the\ndistinctive steric features of alkynyl alcohols, particularly those\nwith a secondary or tertiary alcohol functionality, dictate divergent\nreaction pathways, leading to the formation of [5,6,5]-furano-spiroketals.