P-Stereogenic Phosphorus Ligands in Asymmetric Catalysis

Chiral phosphorus ligands play a crucial role in asymmetric catalysis for the efficient synthesis of useful optically active compounds. They are largely categorized into two classes: backbone chirality ligands and P-stereogenic phosphorus ligands. Most of the reported ligands belong to the former class. Privileged ones such as BINAP and DuPhos are frequently employed in a wide range of catalytic asymmetric transformations. In contrast, the latter class of P-stereogenic phosphorus ligands has remained a small family for many years mainly because of their synthetic difficulty. The late 1990s saw the emergence of novel P-stereogenic phosphorus ligands with their superior enantioinduction ability in Rh-catalyzed asymmetric hydrogenation reactions. Since then, numerous P-stereogenic phosphorus ligands have been synthesized and used in catalytic asymmetric reactions. This Review summarizes P-stereogenic phosphorus ligands reported thus far, including their stereochemical and electronic properties that afford high to excellent enantioselectivities. Examples of reactions that use this class of ligands are described together with their applications in the construction of key intermediates for the synthesis of optically active natural products and therapeutic agents. The literature covered dates back to 1968 up until December 2023, centering on studies published in the late 1990s and later years.

Synthesis and applications of symmetric amino acid derivatives

Symmetric α-amino acid derivatives can be used for the synthesis of intermolecularly linked peptides such as dimer-type peptides, and modified peptides in which two amino acids are intramolecularly linked. They are also synthetic intermediates for the total synthesis of natural products and functional molecules. These symmetric amino acid derivatives must be prepared based on organic synthesis. It is necessary to develop an optimal synthetic strategy for constructing the target symmetric amino acid derivative. In this review, we will introduce strategies for synthesizing symmetric amino acid derivatives. Additionally, selected applications of these amino acids in the life sciences will be described.

Synthesis and Characterization of Propeller- and Parallel-Type Full-Length Amyloid β40 Trimer Models

Oligomers of the amyloid β (Aβ) protein play a critical role in the pathogenesis of Alzheimer's disease. However, their heterogeneity and lability deter the identification of their tertiary structures and mechanisms of action. Aβ trimers and Aβ dimers may represent the smallest aggregation unit with cytotoxicity. Although propeller-type trimer models of E22P-Aβ40 tethered by an aromatic linker have recently been synthesized, they unexpectedly exhibited little cytotoxicity. To increase the flexibility of trimeric propeller-type models, we designed and synthesized trimer models with an alkyl linker, tert-butyltris-l-alanine (tButA), at position 36 or 38. In addition, we synthesized two parallel-type trimer models tethered at position 38 using alkyl linkers of different lengths, α,α-di-l-norvalyl-l-glycine (di-nV-Gly) and α,α-di-l-homonorleucyl-l-glycine (di-hnL-Gly), based on the previously reported toxic dimer model. The propeller-type E22P,V36tButA-Aβ40 trimer (4), which was designed to mimic the C-terminal anti-parallel β-sheet structures proposed by the structural analysis of 150 kDa oligomers of Aβ42, and the parallel-type E22P,G38di-nV-Gly-Aβ40 trimer (6) showed significant cytotoxicity against SH-SY5Y cells and aggregative ability to form protofibrillar species. In contrast, the E22P,G38tButA-Aβ40 trimer (5) and E22P,G38di-hnL-Gly-Aβ40 trimer (7) exhibited weak cytotoxicity, though they formed quasi-stable oligomers observed by ion mobility-mass spectrometry and native polyacrylamide gel electrophoresis. These results suggest that 4 and 6 could have some phase of the structure of toxic Aβ oligomers with a C-terminal hydrophobic core and that the conformation and/or aggregation process rather than the formation of stable oligomers contribute to the induction of cytotoxicity.