@article {44382, title = {Bismuth(III)-catalyzed cycloisomerization and (hetero)arylation of alkynols: simple access to 2-(hetero)aryl tetrahydrofurans and tetrahydropyrans}, journal = {Organic \& Biomolecular Chemistry}, volume = {16}, year = {2018}, month = {MAY}, pages = {3229-3240}, abstract = {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. This practical tandem process provides a means to build libraries related to pharmacologically active molecules and natural product like scaffolds.}, doi = {10.1039/c8ob00368h}, author = {Nakate, Ashwini K. and Pratapure, Madhukar S. and Kontham, Ravindar} } @article {47834, title = {Silver-catalyzed [3 + 3]-annulation cascade of alkynyl alcohols and α,β-unsaturated ketones for the regioselective assembly of chromanes}, journal = {Organic Chemistry Frontiers}, volume = {9}, year = {2022}, month = {JAN}, pages = {802-809}, type = {Article}, abstract = {An unprecedented Ag(i)-catalyzed [3 + 3]-annulation of alkynyl alcohols (5-hexyn-1-ols) and α,β-unsaturated ketones is reported to construct simple to complex chromanes. This transformation begins with hydroalkoxylation of alkynol through C{\textendash}C triple bond activation to give cyclic-enol ether, followed by intermolecular 1,4-addition and intramolecular 1,2-addition of enol ethers onto the enone and oxidative aromatization or Grob-type elimination steps. Facile reaction conditions, broad substrate scope, good to excellent yields, and atom economy are the salient features of this protocol. Isolation of the active pyran-tethered cyclohexadiene reaction intermediate, additional supporting experiments, and DFT calculations strongly support the experimental findings and corroborate our proposed mechanism.}, issn = {2052-4129}, doi = {10.1039/D1QO01643A}, author = {Nakate, Ashwini K. and Thorat, Sagar S. and Jain, Shailja and Rama Krishna, Gamidi and Vanka, Kumar and Kontham, Ravindar} } @article {49024, title = {Bronsted acid-promoted cyclodimerization of α,β-unsaturated γ-ketoesters: construction of fused pyrano-ketal-lactones and γ-ylidene-butenolides}, journal = {ACS Omega}, volume = {9}, year = {2024}, month = {APR }, pages = {19859-19878}, type = {Article}, abstract = {

Unprecedented MsOH-promoted diastereoselective cascade dimerization and intramolecular lactonization of readily accessible alpha,beta-unsaturated gamma-ketoesters are presented. The results obtained in this work, control experiments, and density functional theory (DFT) calculations suggested that the initial enolization and E to Z isomerization/equilibration of olefin (C=C) of substrate alpha,beta-unsaturated gamma-ketoesters give a Z-isomer preferentially over an E-isomer. Subsequently, the Z-isomer undergoes intermolecular annulation with alpha,beta-unsaturated gamma-ketoesters via domino Michael addition/ketalization/lactonization steps to furnish fused tetracyclic pyrano-ketal-lactone. However, the Z-isomer prefers intramolecular trans-esterification in a competing pathway and gives bicyclic gamma-ylidene-butenolide. The key features of this work include simple Br \& oslash;nsted acid catalysis, the formation of three bonds, two rings, and three contiguous stereogenic centers in a single step, DFT calculations, and the assignment of relative stereochemistry through X-ray diffraction (XRD) and two-dimensional (2D) nuclear magnetic resonance (NMR) analyses.

}, issn = {2470-1343}, doi = {10.1021/acsomega.3c08873}, author = {Vinodkumar, Ramavath and Nakate, Ashwini K. and Sharma, Himanshu and Vanka, Kumar and Rama Krishna, Gamidi and Kontham, Ravindar} } @article {49028, title = {Divergent access to polycyclic spiro- and fused-N,O-ketals through Bi(OTf)3-catalyzed [4+2]-annulation of cyclic N-sulfonyl ketimines and alkynols}, journal = {Chemical Communications}, volume = {60}, year = {2024}, month = {JAN}, pages = {1144-1147}, type = {Article}, abstract = {

Bismuth(iii) triflate-catalyzed [4+2]-annulation of cyclic N-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 N,O-ketals with excellent substrate scope, good isolated yields, and diastereoselectivities under mild reaction conditions.

}, issn = {1359-7345}, doi = {10.1039/d3cc05599j}, author = {Nakate, Ashwini K. and Kataria, Priyanka and Sambherao, Pooja I. and Krishna, Gamidi Rama and Kontham, Ravindar} } @article {49227, title = {Ready access to [5,6,5]-Trioxa-spiro and fused ketals via ag-catalyzed cascade annulation of 4-pentyn-1-ols and aldehydes}, journal = {Organic Letters}, volume = {26}, year = {2024}, month = {AUG}, pages = {7116-7121}, type = {Article}, abstract = {

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 pi-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.

}, issn = {1523-7060}, doi = {10.1021/acs.orglett.4c02357}, author = {Vinodkumar, Ramavath and Nakate, Ashwini K. and Gamidi, Rama Krishna and Kontham, Ravindar} }