@article {46132, title = {Fe(III)-Catalyzed diastereoselective friedel-crafts alkylation-hemiketalization-lactonization cascade for the synthesis of polycyclic bridged 2-chromanol lactones}, journal = {Organic Letters}, volume = {21}, year = {2019}, month = {APR }, pages = {2629-2633}, type = {Article}, abstract = {

An unprecedented Fe(III)-catalyzed Friedel-rafts alkylation-hemiketalization-lactonization cascade of electron-rich hydroxy arenes and distinctively functionalized unsaturated 4-keto esters is developed for the construction of polycyclic bridged 2-chromanol lactones. Following this simple and facile protocol, a broad range of products was prepared in good to excellent yields as a single diastereomer. An unusual conglomerate (enantiomerically pure polymorph) of 3ac is reported along with the absolute stereochemistry, and the remaining products were rigorously confirmed by single-crystal X-ray analysis and analogy.

}, issn = {1523-7060}, doi = {10.1021/acs.orglett.9b00614}, author = {Borade, Balasaheb R. and Nomula, Rajesh and Gonnade, Rajesh G. and Kontham, Ravindar} } @article {46650, title = {Total synthesis of beshanzuenone D and its epimers and abiespiroside A}, journal = {Organic Letters}, volume = {22}, year = {2020}, month = {NOV }, pages = {8561-8565}, type = {Article}, abstract = {

A unified and protecting-group-free six-step total synthesis of bisabolane-type sesquiterpenoid beshanzuenone D and its stereoisomers and abiespiroside A using S-(+)-carvone as a common chiral-pool building block is disclosed. This synthetic route features chemoselective allylic chlorination of carvone, Au(I)-catalyzed cydoisomerization induced construction of furan from homopropargylic diol, substrate-controlled selective hydroxylation using Davis-oxaziridine, and dye-sensitized photo-oxidation (through O-1(2)) of hydroxyalkyl tethered furan to access oxaspirolactone as key transformations. A comprehensive set of NMR data along with DFT calculations, ECD spectra, and optical rotation measurements of the synthesized beshanzuenone D and its epimers were obtained to confirm absolute configurations.

}, issn = {1523-7060}, doi = {10.1021/acs.orglett.0c03157}, author = {Borade, Balasaheb R. and Dixit, Ruchi and Kontham, Ravindar} } @article {47633, title = {Bismuth(iii)-catalyzed bis-cyclization of propargylic diol-esters: a unified approach for the synthesis of [5,5]- and [6,5]-oxaspirolactones}, journal = {Organic \& Biomolecular Chemistry}, volume = {19}, year = {2021}, month = {AUG }, pages = {6618-6622}, type = {Article}, abstract = {Herein we disclose an unprecedented intramolecular cascade strategy for the construction of alpha,beta-unsaturated [5,5]- and [6,5]-oxaspirolactones that capitalizes on the pi-electrophilic Lewis acid-catalyzed 5-exo-dig or 6-exo-dig mode of cyclization of propargylic diol esters, followed by dehydration and spirolactonization steps. Moreover, semi-protected substrates also delivered the respective oxaspirolactones with the same ease and in appreciable yields under optimal reaction conditions.}, issn = {1477-0520}, doi = {10.1039/d1ob00974e}, author = {Kambale, Digambar A. and Borade, Balasaheb R. and Kontham, Ravindar} } @article {48355, title = {Concise total synthesis of (+)-lanceolactone a: revision of absolute stereochemistry}, journal = {Journal of Organic Chemistry}, volume = {87}, year = {2022}, month = {OCT}, pages = {12867-12876}, type = {Article}, abstract = {

A chiral-pool protecting-group-free five-step total synthesis of tetranorsesquiterpenoide (+)-lanceolactone A and all of its four stereoisomers using (S)-(+)-, and (R)-(-)-linalool (coriandrol) as building blocks is disclosed. The key steps involved in this synthetic route are regioselective ozonolysis, Au(I)-catalyzed cycloisomerization-induced construction of furan from alleneone, and dye-sensitized photo-oxidation (through O-1(2) ; singlet oxygen) of hydroxyalkyl-tethered furan to access oxaspirolactone. After a thorough evaluation of electronic circular dichroism (ECD) and optical rotation data of all possible stereoisomers, the absolute configuration of natural lanceolactone A at the C4 and C7 positions has been assigned as (+)-(4S,7S), which is an enantiomer to the initially proposed structure (+)-(4R,7R). Further, these investigations led us to extend Feringa and Gawronski{\textquoteright}s CD correlation method to [5,5]- and [6,5]-oxaspirolactones.

}, issn = {0022-3263}, doi = {10.1021/acs.joc.2c01450}, author = {Borade, Balasaheb R. and Kontham, Ravindar} } @article {48114, title = {Enceleamycins A-C, furo-naphthoquinones from amycolatopsis sp. MCC0218: isolation, structure elucidation, and antimicrobial activity}, journal = {Journal of Natural Products}, volume = {85}, year = {2022}, month = {MAY}, pages = {1267-1273}, type = {Article}, abstract = {

Three novel furo-naphthoquinones, enceleamycins A-C (1-3), and a new N-hydroxypyrazinone acid (4) were identified from the strain Amycolatopsis sp. MCC 0218, isolated from a soil sample collected from the Western Ghats of India. Their chemical structure and absolute and relative configurations were established by 1D and 2D NMR spectroscopy, single-crystal X-ray crystallography, and high-resolution mass spectrometry. Compounds 1 and 3 were active against methicillin-susceptible and -resistant Staphylococcus aureus with MIC values of 2-16 mu g/mL.

}, issn = {0163-3864}, doi = {10.1021/acs.jnatprod.1c01160}, author = {Khan, Abujunaid and Said, Madhukar S. and Borade, Balasaheb R. and Gonnade, Rajesh and Barvkar, Vitthal and Kontham, Ravindar and Dastager, Syed G.} } @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 {48733, title = {Metabolites from lactococcus lactis subsp. lactis: isolation, structure elucidation, and antimicrobial activity}, journal = {ACS Omega}, volume = {8}, year = {2023}, month = {SEP}, pages = {36628-36635}, type = {Article}, abstract = {

Herein, we disclose the identification of novel metabolites from a potential probiotic strain, Lactococcus lactis subsp. lactis, obtained from traditional dairy milk samples collected in Maharashtra, India (in January 2021). Isolated metabolites include pyrazin-2-carboxamide [1, pyrazinamide, a potential antitubercular drug], 3,5-dihydroxy-6-methyl-2,3-dihydro-4H-pyran-4-one (2, DDMP), 2,4-di-tert-butylphenol (3), and hexadecanoic acid (4, palmitic acid). The chemical structures of these metabolites were elucidated through extensive 1D NMR (H-1 and C-13) and 2D NMR (HSQC, HMBC, and NOESY) analyses, high-resolution mass spectrometry, high-performance liquid chromatography, and single-crystal X-ray crystallography. Furthermore, these novel metabolites exhibited potent inhibitory activities against various bacteria, fungi, and yeast strains with minimum inhibitory concentrations ranging between 1.56 and 25 mu g/mL, and compounds 1 and 3 were found to be most active against a wide range of microbial strains tested.

}, issn = {2470-1343}, doi = {10.1021/acsomega.3c01662}, author = {Navale, Vishwambar D. and Borade, Balasaheb R. and Rama Krishna, Gamidi and Vamkudoth, Koteswara Rao and Kontham, Ravindar} } @article {48937, title = {Bi(OTf)3-promoted cascade annulation of hydroxy-pyranones and unsaturated γ-ketoesters for the construction of polycyclic bridged pyrano-furopyranones}, journal = {Organic and biomolecular chemistry}, volume = {22}, year = {2024}, month = {JAN }, pages = {496-500}, type = {Article}, abstract = {

An efficient protocol for constructing complex three dimensional polycyclic bridged chromano-furopyranones and pyrano-furopyranones (closely related to bioactive natural products) via bismuth(III)-catalyzed cascade annulation of hydroxy-pyranones and unsaturated gamma-ketoesters is presented. This process involves intermolecular Michael addition, intramolecular hemiketalization, lactonization, formation of one C-C bond and two C-O bonds, rings, and contiguous stereocenters.

}, keywords = {1 3-Dicarbonyl Compounds, alkynols, Coumarins, Fungus}, issn = {1477-0520}, doi = {10.1039/d3ob01862h}, author = {Rathod, Akshay B. and Borade, Balasaheb R. and Sambherao, Pooja I. and Kontham, Ravindar} } @article {49561, title = {Divergent access to α-carbonyl-α'-vinyl sulfoxonium ylides and polysubstituted furans from β-ketosulfoxonium ylides and ynone-esters}, journal = {Journal of Organic Chemistry}, volume = {90}, year = {2025}, month = {APR}, pages = {6186-6207}, type = {Article}, abstract = {

Here, we describe the synthesis of novel alpha-carbonyl-alpha {\textquoteleft}-vinyl sulfoxonium ylides under ambient, catalyst-free, and additive-free conditions, demonstrating broad substrate scope and scalability using beta-ketosulfoxonium ylides and alpha-ynone-esters. Furthermore, these ylides serve as versatile intermediates for the synthesis of highly substituted furans via Br \& oslash;nsted acid (p-TsOH) catalysis at 130 degrees C. This approach offers key advantages, including wide substrate compatibility, divergent product formation from common precursors, gram-scale feasibility, and good to excellent yields.

}, issn = {0022-3263}, doi = {10.1021/acs.joc.5c00245}, author = {Ingale, Sudhir R. and Sahu, Manas Ranjan and Borade, Balasaheb R. and Gamidi, Rama Krishna and Kontham, Ravindar} }