New amino acids derived from L-pyroglutamic acid: Synthesis of Trans-4-benzyl-cis-5-phenyl-L-proline, L-α-(2-benzyl-3-phenylpropyl)-glycine and L-α-(3-phenylpropyl)-glycine

Selective C-N Bond Cleavage in Unstrained Pyrrolidines Enabled by Lewis Acid and Photoredox Catalysis

Cleavage of inert C-N bonds in unstrained azacycles such as pyrrolidine remains a formidable challenge in synthetic chemistry. To address this, we introduce an effective strategy for the reductive cleavage of the C-N bond in N-benzoyl pyrrolidine, leveraging a combination of Lewis acid and photoredox catalysis. This method involves single-electron transfer to the amide, followed by site-selective cleavage at the C2-N bond. Cyclic voltammetry and NMR studies demonstrated that the Lewis acid is crucial for promoting the single-electron transfer from the photoredox catalyst to the amide carbonyl group. This protocol is widely applicable to various pyrrolidine-containing molecules and enables inert C-N bond cleavage including C-C bond formation via intermolecular radical addition. Furthermore, the current protocol successfully converts pyrrolidines to aziridines, γ-lactones, and tetrahydrofurans, showcasing its potential of the inert C-N bond cleavage for expanding synthetic strategies.

Synthesis of Complex Diarylamines through a Ring-Opening Difunctionalization Strategy

The diarylation and skeletal diversification of unstrained cyclic amines was exploited to expand and modify the favorable properties of this important substrate class with pivotal roles in drug discovery. Cyclic amines were employed in the synthesis of a novel class of amino-substituted diaryliodonium salts, which were converted to highly functionalized diarylamines through an atom-efficient one-pot N-arylation/ring opening reaction with external nucleophiles. The reaction proceeds through in situ formation of a diarylammonium intermediate that undergoes a nucleophilic ring opening by cleavage of the strong C-N bond. A wide variety of diarylamines was obtained through introduction of two different aryl groups of varied electronics, and the retained iodo-substituent enables downfield diversifications of the products. More than 20 nucleophiles, including amines, phenols, carboxylic acids, thiols and halides, were alkylated with high functional group tolerance, and the strategy proved efficient also in in late-stage functionalization of natural products and pharmaceuticals.

B(C6 F5 )3 -Catalyzed Regioselective Ring Opening of Cyclic Amines with Hydrosilanes

Opening the ring of cyclic amines by regioselective fission of one of the carbon-nitrogen bonds greatly expands the repertoire of available nitrogen-containing skeletons. Unlike approaches starting from cyclic tertiary amines, methods that can directly open secondary amines are still scarce. The present work discloses an efficient reductive ring opening of either of these cyclic amines using PhSiH₃ under B(C₆ F₅ )₃ catalysis. By this, the direct transformation of unstrained cyclic amines into the corresponding acyclic amines is achieved in a simple one-pot operation. A stepwise mechanism proceeding through the intermediacy of silylammonium ions followed by reductive cleavage of a carbon-nitrogen bond was experimentally verified.

Ring-opening functionalizations of unstrained cyclic amines enabled by difluorocarbene transfer

Chemical synthesis based on the skeletal variation has been prolifically utilized as an attractive approach for modification of molecular properties. Given the ubiquity of unstrained cyclic amines, the ability to directly alter such motifs would grant an efficient platform to access unique chemical space. Here, we report a highly efficient and practical strategy that enables the selective ring-opening functionalization of unstrained cyclic amines. The use of difluorocarbene leads to a wide variety of multifaceted acyclic architectures, which can be further diversified to a range of distinctive homologative cyclic scaffolds. The virtue of this deconstructive strategy is demonstrated by successful modification of several natural products and pharmaceutical analogues.

Synthetic approaches to peptides containing the l-Gln-l-Val-D(S)-Dmt motif

The pseudoprolines S-Dmo (5,5-dimethyl-4-oxaproline) and R-Dmt (5,5-dimethyl-4-thiaproline) have been used to study the effects of forcing a fully cis conformation in peptides. Synthesis of peptides containing these (which have the same configuration as L-Pro) is straightforward. However, synthesis of peptides containing S-Dmt is difficult, owing to the rapid cyclisation of L-Aaa-S-Dmt amides and esters to form the corresponding diketopiperazines (DKP); thus the intermediacy of L-Aaa-S-Dmt amides and esters must be avoided in the synthetic sequence. Peptides containing the L-Gln-L-Val-D(S)-Dmt motif are particularly difficult, owing to the insolubility of coupling partners containing Gln. Introduction of Gln as N-Boc-pyroglutamate overcame the latter difficulty and the dipeptide active ester BocPygValOC(6)F(5) coupled in good yield with S-DmtOH. BocPygVal-S- DmtNH(CH(2))(2)C(6)H(4)NO(2) was converted quantitatively to BocGlnVal-S-DmtNH(CH(2))(2)C(6)H(4)NO(2) with ammonia, demonstrating the utility of this approach. Two peptide derivatives (CbzSerLysLeuGlnVal-S-DmtNH(CH(2))(2)C(6)H(4)NO(2) and CbzSerSerLysLeuGlnVal-S- DmtNH(CH(2))(2)C(6)H(4)NO(2)) were assembled, using these new methods of coupling a dipeptide acid active ester with S-DmtOH and introduction of Gln as Pyg, followed by conventional peptide couplings. The presence of the Val caused these peptides to be cleaved very slowly by prostate-specific antigen (PSA) at Leu Gln, rather than the expected Gln Val.

2,5-Disubstituted pyrrolidine carboxylates as potent, orally active sphingosine-1-phosphate (S1P) receptor agonists

A series of 2,5-cis-disubstituted pyrrolidines were synthesized and evaluated as S1P receptor agonists. Compounds 15-21 were identified with good selectivity over S1P3 which lowered circulating lymphocytes after oral administration in mice.

Novel Approach to 5-Substituted Proline Derivatives Using a Silver-Catalyzed Cyclization as the Key Step

A novel synthetic approach to the synthesis of enantiomerically pure 2,5-disubstituted pyrrolines is described. The methodology involves a Ag-catalyzed 5-endo-dig cyclization of enantiopure aryl-substituted acetylene-containing amino acids. It has also been shown that the obtained pyrrolines can be efficiently transformed into the corresponding saturated 5-aryl-substituted proline derivatives.

Masked oxo sulfinimines (N-sulfinyl imines) in the asymmetric synthesis of proline and pipecolic acid derivatives

[structure: see text]. On addition of Et2AlCN/i-PrOH, masked oxo sulfinimines give alpha-amino nitriles that afford oxo alpha-amino acids on hydrolysis. These amino acids cyclize and are reduced to cis proline and cis pipecolic acids derivatives in high ee and good yield. This new procedure avoids many of the limitations related to the preparation of oxo amino acids from proteinogenic amino acids.