Electrochemical Synthesis of Substituted Morpholines via a Decarboxylative Intramolecular Etherification

Electrochemical Synthesis of Unnatural Amino Acids via Anodic Decarboxylation of N-Acetylamino Malonic Acid Derivatives

Broad application of α,α-disubstituted cyclic amino acid derivatives in medicinal chemistry urges for analogue design with improved pharmacokinetic properties. Herein, we disclose an electrochemical approach toward unnatural THF- and THP-containing amino acid derivatives that relies on anodic decarboxylation-intramolecular etherification of inexpensive and readily available N-acetylamino malonic acid monoesters under Hofer-Moest reaction conditions. The decarboxylative cyclization proceeds under constant current conditions in an undivided cell in an aqueous medium without any added base. A successful bioisosteric replacement of the 1-aminocyclohexane-1-carboxylic acid subunit by the THP-containing amino acid scaffold in cathepsin K inhibitor balicatib helped to reduce lipophilicity while retaining low nanomolar enzyme inhibitory potency and comparable microsomal stability.

Zinc chloride-catalyzed cyclizative 1,2-rearrangement enables facile access to morpholinones bearing aza-quaternary carbons

Morpholines and morpholinones are important building blocks in organic synthesis and pharmacophores in medicinal chemistry, however, C3-disubstituted morpholines/morpholinones are extremely difficult to access. Here we show the ZnCl2-catalyzed cyclizative 1,2-rearrangement for the efficient synthesis of morpholinones bearing aza-quaternary stereocenters. A series of structurally diverse C3-disubstituted morpholin-2-ones which are difficultly accessible by existing methods were efficiently constructed from readily available two achiral linear compounds. Notably, mechanistic studies reveal that this reaction proceeds via an unusual sequence of direct formal [4 + 2] heteroannulation regioselectively delivering specific α-iminium/imine hemiacetals followed by a 1,2-esters or amides shift process, which is different from the reported mechanism of the aza-benzilic ester rearrangements.

Selective Electrochemical Halogenation of Functionalized Quinolone

This work describes an effective C3-H halogenation of quinoline-4(1H)-ones under electrochemical conditions, in which potassium halides serve as both halogenating agents and electrolytes. The protocol provides expedient access to different halogenated quinoline-4(1H)-ones with unique regioselectivity, broad substrate scope, and gram-scale synthesis employing convenient, environmentally friendly electrolysis, in an undivided cell. Mechanism studies have shown that halogen radicals can promote the activation of N-H bonds in quinolones.

Syntheses of 2- and 3-Substituted Morpholine Congeners via Ring Opening of 2-Tosyl-1,2-Oxazetidine

Diastereoselective and diastereoconvergent syntheses of 2- and 3-substituted morpholine congeners are reported. Starting from tosyl-oxazatedine 1 and α-formyl carboxylates 2, base catalysis is utilized to yield morpholine hemiaminals. Their further synthetic elaborations allowed the concise constructions of conformationally rigid morpholines. The observed diastereoselectivities and the unusual diastereoconvergence in the photoredox radical processes seem to be the direct consequence of the avoidance of pseudo A^1,3 strain between the C-3 substituent and the N-tosyl group and the anomeric effect of oxygen atoms.

Rhodium-catalyzed diastereoselective synthesis of highly substituted morpholines from nitrogen-tethered allenols

Rhodium-catalyzed intramolecular cyclization of nitrogen-tethered allenols was investigated for the synthesis of functionalized morpholines. By using this strategy, various N-protected 2,5- and 2,6-disubstituted as well as 2,3,5- and 2,5,6-trisubstituted morpholines were obtained via an atom-economic pathway with high to excellent yields, diastereo- and enantioselectivities (up to 99% yield, up to >99 : 1 dr and up to >99.9 ee). The utilities of the synthesized morpholines in ozonolysis, hydration, metathesis and epoxidation reactions were also investigated.