AgOTf-catalyzed cascade annulation of 5-hexyn-1-ols and aldehydes: enabling the diastereoselective synthesis of [6,6,6]-trioxa-fused ketals and hexahydro-2-chromenes.

We report the unprecedented diastereoselective synthesis of novel [6,6,6]-trioxa-fused ketals AgOTf-catalyzed cascade annulation of 5-hexyn-1-ols (with primary or secondary hydroxyl groups) and aldehydes through a [2+2+1+1] pathway. In contrast, 5-hexyn-1-ols with tertiary hydroxyl groups yield hexahydro-2-chromenes a [3+1+1+1] pathway.

Ready Access to [5,6,5]-Trioxa-spiro and Fused Ketals via Ag-Catalyzed Cascade Annulation of 4-Pentyn-1-ols and Aldehydes.

In this study, we unveil the versatility of 4-pentyn-1-ols as carbonyl surrogates for the unprecedented synthesis of diverse oxygen heterocycles, including [5,6,5]-bis-spiroketals (trioxadispiroketals) and [5,6,5]-furano-spiroketals related to bioactive natural products. These reactions commence with the π-activation-induced intramolecular hydroalkoxylation of 4-pentyn-1-ols, yielding cyclic enol ethers, which undergo subsequent three-component annulation with aldehydes in a [2+2+1+1] fashion, resulting in the formation of [5,6,5]-bis-spiroketals. Notably, the distinctive steric features of alkynyl alcohols, particularly those with a secondary or tertiary alcohol functionality, dictate divergent reaction pathways, leading to the formation of [5,6,5]-furano-spiroketals.

Brønsted Acid-Promoted Cyclodimerization of α,β-Unsaturated γ-Ketoesters: Construction of Fused Pyrano-ketal-lactones and γ-Ylidene-butenolides.

Unprecedented MsOH-promoted diastereoselective cascade dimerization and intramolecular lactonization of readily accessible α,β-unsaturated γ-ketoesters are presented. The results obtained in this work, control experiments, and density functional theory (DFT) calculations suggested that the initial enolization and to isomerization/equilibration of olefin (C=C) of substrate α,β-unsaturated γ-ketoesters give a -isomer preferentially over an -isomer. Subsequently, the -isomer undergoes intermolecular annulation with α,β-unsaturated γ-ketoesters via domino Michael addition/ketalization/lactonization steps to furnish fused tetracyclic pyrano-ketal-lactone.

Divergent access to polycyclic spiro- and fused-,O-ketals through Bi(OTf)-catalyzed [4+2]-annulation of cyclic -sulfonyl ketimines and alkynols.

Bismuth(III) triflate-catalyzed [4+2]-annulation of cyclic -sulfonyl ketimines (derived from saccharin) and alkynyl alcohols (4-pentyn-1-ols and 5-hexyn-1-ols) has been reported. This cascade annulation provides a diverse array of polycyclic spiro-and-fused ,-ketals with excellent substrate scope, good isolated yields, and diastereoselectivities under mild reaction conditions.

Bismuth(iii)-catalyzed cycloisomerization and (hetero)arylation of alkynols: simple access to 2-(hetero)aryl tetrahydrofurans and tetrahydropyrans.

2-(Hetero)aryl tetrahydrofurans and tetrahydropyrans were successfully synthesized using Bi(OTf)3-catalyzed hydroalkoxylation (cycloisomerization) of alkynols (via 5 or 6 exo-dig cyclization) and intermolecular (hetero)arylation. This reaction involves a highly efficient cascade process, where initially the alkynol undergoes a cycloisomerization step via activation of the triple bond and generates the oxocarbenium ion, which subsequently participates in the (hetero)hydroarylation step with electron-rich arenes. Simple to complex suitably functionalized alkynols (4-pentyn-1-ols and 5-hexyn-1-ols) and electron-rich aromatic compounds were found to be reliable substrates in this cascade transformation and furnished a wide range of oxygen heterocycles.