@article {48722, title = {Divergent synthesis of oxepino-phthalides and [5,5]-oxaspirolactones through [2+2+2]- and [2+3]-annulation of alkynyl alcohols with a-ynone-esters}, journal = {Journal of Organic Chemistry}, volume = {88}, year = {2023}, month = {AUG }, pages = {12597-12612}, type = {Article}, abstract = {

Unmasking the synthetic potential of alkyne functional group of alkynyl alcohols as surrogates of carbonyl compounds, herein we present the first Bronsted acid (TfOH)-catalyzed [2 + 2 + 2]-annulation of 4-pentyn-1-ols (possessing terminal alkyne) with a-ynone-esters to access tricyclic tetrahydro-oxepino-phthalides. Besides, an unprecedented synthesis of a-acetoaryl or a-alkynyl [5,5]-oxaspirolactones has been demonstrated by employing 4-pentyn-1-ols (possessing an internal alkyne) as an annulation partner, which proceeds through a divergent [2 + 3]-annulation pathway.

}, issn = {0022-3263}, doi = {10.1021/acs.joc.3c01301}, author = {Kambale, Digambar A. and Borade, Balasaheb R. and Vinodkumar, Ramavath 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 {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} } @article {49256, title = {The first enantioselective total synthesis of the eremophilane-type sesquiterpenoid (-)-peniroqueforin C}, journal = {Tetrahedron Letters}, volume = {154}, year = {2025}, month = {JAN }, pages = {155386}, type = {Article}, abstract = {

Herein, we report the first stereoselective total synthesis of the eremophilane-type sesquiterpenoid (-)-peniroqueforin C using a chiral-pool strategy. This synthetic route features the use of readily available (S)-(+)-carvone as a chiral building block, Robinson annulation to construct the decalin system, substrate-controlled stereoselective methylation, single-step annulative construction of a tricyclic gamma-ylidene-butenolide with concomitant alkene transposition, and direct lactone-to-lactam conversion as key transformations.

}, keywords = {annulation reactions, Eremophilanes, Peniroqueforin C, Sesquiterpenoids, Total synthesis}, issn = {0040-4039}, doi = {10.1016/j.tetlet.2024.155386}, author = {Ingale, Sudhir R. and Vinodkumar, Ramavath and Kontham, Ravindar} }