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1. Diastereodivergent synthesis of 4-oxocyclohexanecarbaldehydes by using the modularly designed organocatalysts upon switching on their iminium catalysis

   https://doi.org/10.1039/D1CC01020D  

   Bora, P.; Jakkampudi, S.; Parella, R.; Sakkani, N.; Dai, Q.; Bihani, M.; Arman, H. D.; Zhao, J. C.-G. (January 2021, Chemical communications)

   null (Ed.)

   The cinchona thiourea moiety in the self-assembled modularly designed organocatalysts (MDOs) switches off the iminium catalysis of these catalysts. In this study, it was found that the inhibited iminium catalysis could be switched on by using an appropriate weak acid and that, once the iminium catalysis was switched on, these catalysts could be applied for the highly stereoselective and diastereodivergent synthesis of 4-oxocyclohexanecarbaldehydes via a domino reaction between ketones and α,β-unsaturated aldehydes. 

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2. The mechanism of the triple aryne–tetrazine reaction cascade: theory and experiment

   https://doi.org/10.1039/C8SC01796D  

   Suh, Sung-Eun; Chen, Shuming; Houk, K. N.; Chenoweth, David M. (October 2018, Chemical Science)

   This article describes an experimental and computational investigation on the possible aryne reactivity modes in the course of the reaction of two highly energetic molecules, an aryne and a 1,2,4,5-tetrazine. Beyond the triple aryne–tetrazine (TAT) reaction, it was observed that combinations of several reactivity modes afford several heterocyclic compounds. Density Functional Theory (DFT) calculations of competition between a second Diels–Alder reaction and the nucleophilic addition pathways indicates the latter to be more favorable. Crossover experiments and computational study of the proton transfer step reveal that the reaction proceeds intermolecularly with the assistance of a water molecule, rather than intramolecularly. The resulting enamine intermediate was found to undergo either a stepwise formal [2 + 2] or [4 + 2] cycloaddition, and their energetic profiles were compared against each other. Isolation of an ene-product and a rearranged product shows the potential competition with oxidation/desaturation. These studies show how multiple arynes react with a highly reactive starting material and provide guidance for future applications of aryne-based multicomponent cascade reactions. 

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3. Synthesis of alpha-Gal C-disaccharides

   https://doi.org/10.1016/j.bmc.2024.117903  

   Ann, Alex; Truong, Steven; Peters, Jiwani; Mootoo, David R (October 2024, Bioorganic & Medicinal Chemistry)

   The synthesis of C-disaccharides of α-D-galactopyranosyl-(1 → 3)-D-galactopyranose (α-Gal), potential tools for studying the biology of α-Gal glycans, is described. The synthetic strategy, centers on the reaction of two easily available precursors 1,2-O-isopropylidene-D-glyceraldehyde and an α-C-glactosyl-E-crotylboronate, which affords a mixture of two diastereomeric anti-crotylation products. The stereoselectivity of this reaction was controlled with (R)- and (S)-TRIP catalysts, and the appropriate diastereomer was transformed to C-linked disaccharides of α-Gal, in which the aglycone segment comprised O-, C- and S-glycoside entities that can enable glycoconjugate synthesis. 

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4. Remote Fluorination of α,β‐Unsaturated Carbonyls via Silyl Dienol Ethers

   https://doi.org/10.1002/chem.202400493  

   Galaktionova, Daria V; Siddiqui, Sara Y; Mohr, Justin T (May 2024, Chemistry – A European Journal)

   Abstract We report a general, regioselective, and metal free γ‐fluorination of α,β‐unsaturated carbonyls via silyl dienol ethers that are readily prepared from simple ketones and aldehydes. The transformation displays broad scope including 27 cyclic and acyclic siloxydienes providing γ‐fluoro compounds in 28–91 % yield. Notably, the reported conditions are also suitable for the synthesis of challenging tertiary fluorides. The regioselectivity of the reaction was studied on a series of acyclic siloxydienes and was observed to be sensitive to the conformational flexibility of the substrate. Diversification of the γ‐fluorocarbonyls demonstrates the promise of fluorine as a stereocontrol element. 

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5. Generation and Application of Homoallylic α,α‐Diboryl Radicals via Diboron‐Promoted Ring‐Opening of Vinyl Cyclopropanes: cis ‐Diastereoselective Borylative Cycloaddition**

   https://doi.org/10.1002/chem.202303175  

   Vyas, Het; Gangani, Ashvin J.; Mini, Aiswarya; Lin, Shengjia; Chu, Jia‐Min; Agee, Caitlyn O.; Gabriel, Justin; Williamson, R. Thomas; Zhang, Yong; Sharma, Abhishek (November 2023, Chemistry – A European Journal)

   Abstract Carbon‐centered radicals stabilized by adjacent boron atoms are underexplored reaction intermediates in organic synthesis. This study reports the development of vinyl cyclopropyl diborons (VCPDBs) as a versatile source of previously unknown homoallylic α,α‐diboryl radicals via thiyl radical catalyzed diboron‐directed ring opening. These diboryl stabilized radicals underwent smooth [3+2] cycloaddition with a variety of olefins to provide diboryl cyclopentanes in good to excellent diastereoselectivity. In contrast to thetrans‐diastereoselectivity observed with most of the dicarbonyl activated VCPs, the cycloaddition of VCPDBs showed a remarkable preference for formation ofcis‐cyclopentane diastereomer which was confirmed by quantitative NOE and 2D NOESY studies. Thecis‐stereochemistry of cyclopentane products enabled a concise intramolecular Heck reaction approach to rare tricyclic cyclopentanoid framework containing the diboron group. The mild reaction conditions also allowed a one‐pot VCP ring‐opening, cycloaddition‐oxidation sequence to afford disubstituted cyclopentanones. Control experiments and DFT analysis of reaction mechanism support a radical mediated pathway and provide a rationale for the observed diastereoselectivity. To the authors’ knowledge, these are the first examples of the use of geminal diboryl group as an activator of VCP ring opening and cycloaddition reaction of α‐boryl radicals. 

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