Selective Synthesis of Cyclooctanoids by Radical Cyclization of Seven-Membered Lactones: Neutron Diffraction Study of the Stereoselective Deuteration of a Chiral Organosamarium Intermediate

Diastereoselective Radical 1,4-Ester Migration: Radical Cyclizations of Acyclic Esters with SmI2

Reductive cyclizations of carbonyl compounds, mediated by samarium(II) diiodide (SmI₂, Kagan’s reagent), represent an invaluable platform to generate molecular complexity in a stereocontrolled manner. In addition to classical ketone and aldehyde substrates, recent advances in radical chemistry allow the cyclization of lactone and lactam-type substrates using SmI₂. In contrast, acyclic esters are considered to be unreactive to SmI₂ and their participation in reductive cyclizations is unprecedented. Here, we report a diastereoselective radical 1,4-ester migration process, mediated by SmI₂, that delivers stereodefined alkene hydrocarboxylation products via radical cyclization of acyclic ester groups in α-carbomethoxy δ-lactones. Isotopic labeling experiments and computational studies have been used to probe the mechanism of the migration. We propose that a switch in conformation redirects single electron transfer from SmI₂ to the acyclic ester group, rather than the “more reactive” lactone carbonyl. Our study paves the way for the use of elusive ketyl radicals, derived from acyclic esters, in SmI₂-mediated reductive cyclizations.

Postfabrication Tethering of Molecular Gradients on Aligned Nanofibers of Functional Poly(ε-caprolactone)s

Substrates with combinations of topographical and biochemical cues are highly useful for a number of fundamental biological investigations. Tethered molecular concentration gradients in particular are highly desired for a number of biomedical applications including cell migration. Herein, we report a versatile method for the fabrication of aligned nanofiber substrates with a tunable concentration gradient along the fiber direction. 4-Dibenzocyclooctynol (DIBO) was used as an initiator for the ring-opening copolymerization of ε-caprolactone (εCL) and allyl-functionalized ε-caprolactone (AεPCL), which yielded a well-defined polymer with orthogonal functional handles. These materials were fabricated into aligned nanofiber substrates via touch-spinning. Fibers were modified post-spinning with a concentration gradient of fluorescently labeled dye via a light activated thiol-ene reaction through a photomask. As a demonstration, the cell adhesive peptide RGD was chemically tethered to the fiber surface at a second functionalization site via strain-promoted azide-alkyne cycloaddition (SPAAC). This novel approach affords fabrication of dual functional nanofiber substrates.

Biocatalytic Conversion of Cyclic Ketones Bearing α-Quaternary Stereocenters into Lactones in an Enantioselective Radical Approach to Medium-Sized Carbocycles

Cyclic ketones bearing α‐quaternary stereocenters underwent efficient kinetic resolution using cyclohexanone monooxygenase (CHMO) from Acinetobacter calcoaceticus. Lactones possessing tetrasubstituted stereocenters were obtained with high enantioselectivity (up to >99 % ee) and complete chemoselectivity. Preparative‐scale biotransformations were exploited in conjunction with a SmI₂‐mediated cyclization process to access complex, enantiomerically enriched cycloheptan‐ and cycloctan‐1,4‐diols. In a parallel approach to structurally distinct products, enantiomerically enriched ketones from the resolution with an α‐quaternary stereocenter were used in a SmI₂‐mediated cyclization process to give cyclobutanol products (up to >99 % ee).

Reductive cyclisations of amidines involving aminal radicals

The first general study of aminal radical cyclisations, triggered by reduction of amidines with SmI₂, delivers quinazolinones with complete diastereocontrol.

A green strategy for the synthesis of sulfone derivatives of p-methylaminophenol: Kinetic evaluation and antibacterial susceptibility

This is one of the few examples in which the diverse products have been synthesized just by changing the applied potential. The synthesis of sulfonyl derivatives of p-methylaminophenol were carried out by reaction of the electrogenerated p-methylquinoneimine with sulfinic acids. Various types of mono (MSP), bis (BSP) and tris (TSP) sulfonyl p-methyl aminophenols were obtained by changing the electrode potential, in one pot under green conditions. The mono sulfonyl-p-(methylamino)phenol derivatives (MSP) were assessed for their in vitro antibacterial activity against the gram positive (Staphylococcus aureus) and gram negative (Escherichia coli) strains. It was found that the tested compounds were more active against Staphylococcus aureus than Escherichia coli. We also found that the antimicrobial activity of MSP derivatives to vary in the order MSP₄ (R = CH₃) > MSP₁ (R = p-tolyl) ≈ MSP₂ (R = phenyl) > MSP₃ (R = p-ClC₆H₄). Moreover, the observed homogeneous rate constants (k_obs) of the reaction of p-methyl quinoneimine with sulfinic acids were estimated in various pH values, based on the EC and ECEC mechanisms, by comparing the simulated cyclic voltammograms with the experimental ones.

SmI2-mediated reductive cyclization of β-arylthio ketones: a facile and diastereoselective synthesis of thiochroman derivatives

SmI₂-mediated reductive cyclization of β-arylthio ketones afforded a general approach to dihydrothiochroman-ols in good yields and high degrees of diastereoselectivity.

Selective construction of quaternary stereocentres in radical cyclisation cascades triggered by electron-transfer reduction of amide-type carbonyls

Radical cyclisation cascades triggered by electron-transfer to amide-type carbonyls using SmI₂–H₂O–LiBr, result in the selective construction of quaternary carbon stereocentres.

Secondary Amides as Hydrogen Atom Transfer Promoters for Reactions of Samarium Diiodide

Two secondary amides (N-methylacetamide and 2-pyrrolidinone) were used as additives with SmI₂ in THF to estimate the extent of N-H bond weakening upon coordination. Mechanistic and synthetic studies demonstrate significant bond-weakening, providing a reagent system capable of reducing a range of substrates through formal hydrogen atom transfer.

Selective Synthesis of Cyclooctanoids by Radical Cyclization of Seven-Membered Lactones: Neutron Diffraction Study of the Stereoselective Deuteration of a Chiral Organosamarium Intermediate

Seven-membered lactones undergo selective SmI2 -H2 O-promoted radical cyclization to form substituted cyclooctanols. The products arise from an exo-mode of cyclization rather than the usual endo-attack employed in the few radical syntheses of cyclooctanes. The process is terminated by the quenching of a chiral benzylic samarium. A labeling experiment and neutron diffraction study have been used for the first time to probe the configuration and highly diastereoselective deuteration of a chiral organosamarium intermediate.