Temperature-Dependent Enantio- and Diastereodivergent Synthesis of Amino Acids with One or Multiple Chiral Centers

Cobalt(II)-Catalyzed Selective Three-Component Oxyalkylation of N-Aryl Glycinates: A Route to CF3-Labeled Threonine Analogues

Glycinates, protected enols, and an electrophilic trifluoromethylating reagent were employed to assemble CF3-labeled threonine analogues through a radical addition cascade. To suppress the competing oxidation of the oxyalkyl radical intermediate, various redox catalysts were evaluated and Co(II) exhibited supreme selectivity control with a proper counterion. A series of glycinate and related peptides were thus successfully modified under Co-catalysis. Mechanistic studies revealed that a N-aryl glycinate could be preferentially oxidized by cobalt over the oxyalkyl radical to generate an imine intermediate, and the key to this selectivity could be ascribed to the prechelation of glycinate, as well as a weakly basic carboxylate counterion.

Substrate-Controlled Diastereo- and Enantiodivergent Synthesis of Bis-Spirocyclopropyloxindoles from Available Isatin as a Single Starting Material

The first diastereo- and enantiodivergent asymmetric synthesis of new bis-spirocyclopropyloxindole scaffolds has been accomplished from the readily available isatin as a single starting material. Four rel-(1R,2R,3R), rel-(1S,2S,3R), rel-(1R,2R,3S), and rel-(1S,2S,3S) configurations of desired products were constructed in excellent enantiopurity via a simple switch in substrates using the chiral auxiliary-controlled method. The absolute configuration of cycloadducts with three contiguous quaternary/tertiary stereogenic centers was confirmed through X-ray diffraction analysis. A facile synthesis of versatile precursor 3-chlorooxindoles was also introduced.

Switchable, Reagent-Controlled Diastereodivergent Photocatalytic Carbocyclisation of Imine-Derived α-Amino Radicals

A reagent‐controlled stereodivergent carbocyclisation of aryl aldimine‐derived, photocatalytically generated, α‐amino radicals possessing adjacent conjugated alkenes, affording either bicyclic or tetracyclic products, is described. Under net reductive conditions using commercial Hantzsch ester, the α‐amino radical species underwent a single stereoselective cyclisation to give trans‐configured amino‐indane structures in good yield, whereas using a substituted Hantzsch ester as a milder reductant afforded cis‐fused tetracyclic tetrahydroquinoline frameworks, resulting from two consecutive radical cyclisations. Judicious choice of the reaction conditions allowed libraries of both single and dual cyclisation products to be synthesised with high selectivity, notable predictability, and good‐to‐excellent yields. Computational analysis employing DFT revealed the reaction pathway and mechanistic rationale behind this finely balanced yet readily controlled photocatalytic system.

Highly efficient and enantioselective syntheses of (2S,3R)-3-alkyl- and alkenylglutamates from Fmoc-protected Garner's aldehyde

A 6-step enantioselective synthesis of (2S,3R)-3-alkyl/alkenylglutamates, including the biologically significant amino acid, (2S,3R)-3-methylglutamate, protected for Fmoc SPPS, is reported. Overall yields range from 52-65%. Key to the success of these syntheses was the development of a high-yielding 2-step synthesis of Fmoc Garner's aldehyde followed by a Horner-Wadsworth-Emmons reaction to give the corresponding Fmoc Garner's enoate in a 94% yield. The diastereoselective 1,4-addition of lithium dialkylcuprates to the Fmoc Garner's enoate was explored. Significant decomposition occurred when using lithium diethylcuprate and conditions previously reported for the 1,4-addition of lithium dialkylcuprates to Boc or Cbz-protected Garner's enoate. An optimization study of this reaction resulted in a robust set of conditions that addressed the shortcomings of previously reported conditions. Under these conditions, highly diastereoselective (> 20:1 in most cases) 1,4-addition reactions of lithium dialkyl/dialkenylcuprates to the Fmoc Garner's enoate were achieved in 76-99% yield. The resulting 1,4-addition products were easily converted into the Fmoc-(2S,3R)-3-alkyl/alkenylglutamates in two steps.

Stereodivergent routes in organic synthesis: carbohydrates, amino acids, alkaloids and terpenes

The natural occurrence of enantiomers and diastereomers is often encountered.

Temperature-Controlled Stereodivergent Synthesis of 2,2'-Biflavanones Promoted by Samarium Diiodide

In this work, the first example of a radical stereodivergent reaction directed towards the stereoselective synthesis of both (R*,R*)- and (R*,S*)-2,2'-biflavanones promoted by samarium diiodide is reported. Control experiments showed that the selectivity of this reaction was exclusively controlled by the temperature. It was possible to generate a variety of 2,2'-biflavanones bearing different substitution patterns at the aromatic ring in good-to-quantitative yields, being both stereoisomers of the desired compounds obtained with total or high control of selectivity. A mechanism that explains both the generation of the corresponding 2,2'-biflavanones and the selectivity is also discussed. The structure and stereochemistry determination of each isomer was unequivocally elucidated by single-crystal X-ray diffraction experiments.