β-Hydroxy-α-amino acids are valuable constituents of complex natural products and medicinally important compounds. For instance, the naturally occurrence β-hydroxyleucine is a representative of this class and has attracted considerable attention as an unusual amino acid constituent of cyclodepsipeptides such as azinothricin, A 83586 C, papuamides, polyoxypeptins and GE 3. As a consequence of the importance of β-hydroxy-α-amino acids in the biological activities as well as their usefulness as synthetic building blocks, a number of useful strategies have been devised for better method of the synthesis of β-hydroxy-α-amino acids. This review summarizes recent progress in asymmetric synthesis of β-hydroxy-α-amino acids.
As found in biological systems, proper combinations of stereo-controlled ligands and functional metal centers are demonstrated to generate sophisticated functions in several non-asymmetric processes. This review highlights stereochemical approaches to the synthetic ligands effective for efficient cation recognition and selective luminescence anion sensing processes. A new series of Ag+ ion-specific ligands was designed, in which three pyridine donors selectively bound the Ag+ ion in a tridentate fashion. In addition to such unique coordination geometry, the stereo-controlled substitution on the podand-type ligand significantly enhanced Ag+ ion selectivity in binding, extraction, and membrane transport processes. Chiral versions of tripode ligands were also synthesized in a stereo-controlled fashion. They formed stable complexes with several lanthanide cations in which three sidearm donors and tertiary nitrogen atom were cooperatively involved. Their Eu3+ and Tb3+ complexes gave characteristic luminescence upon ligand chromophore excitation, and the observed luminescence intensity sensitively depended on the natures of external guest anions. In these lanthanide luminescence sensory processes, the use of stereo-controlled ligands remarkably increased the anion-responsibility.
Silk-fibroin supported palladium catalyst (Pd/Fib) was prepared by incipient wetness impregnation with a rust-colored MeOH solution of Pd (OAc)2. After the palladium (II) was absorbed completely in the fibroin, the zero-valent Pd was formed on the fibroin fiber via reduction by MeOH as a reductant. The Pd/Fib catalyst, black-colored yarn, shows efficient chemoselectivity in hydrogenation of acetylene, olefin and azide functionalities in the presence of aromatic ketones, aldehydes and halides, benzyl esters and N-Cbz protective groups which are readily reduced under the Pd/C or Pd/C(en)-catalyzed hydrogenation conditions.
4-Nitroisoxazolin-5 (2H) -ones 1 and 2 behave as excellent precursors for the syntheses of various kinds of polyfunctionalized compounds. Dianionic compounds, cyano-aci-nitroacetates 7 and dinitroglutaronitriles 9, are prepared from pyridinium salt 1 by the ring opening reaction and the succeeding condensation with ketones or aldehydes. When glutaronitriles 9 are treated with acetyl chloride, intramolecular ring transformation proceeds to afford polyfunctionalized isoxazoline 10 and isoxazoles 11. Further chemical transformation of 11 is possible, and leading to isoxazole derivatives 12-16. On the other hand, N-methylisoxazolone 2 is converted to functionalized units 17-19 upon treatment with amines or water. Polyfunctionalized pyrroles 23 are also prepared by the ring transformation of 2 with enolates of active methylene compounds.
Ligand effects on the structure and reactivity of copper complexes have been studied systematically by using several types of pyridylalkylamine ligands. Special attention has been focused on the dioxygen activation chemistry by copper (I) complexes aiming at understanding the catalytic mechanism of copper oxidases and copper monooxygenases. Different types of copper-dioxygen complexes such as a (μ-η2 : η2-peroxo) dicopper (II) complex, a (μ-η1 : η2-peroxo) dicopper (II) complex, a bis (μ-oxo) dicopper (III) complex, and a mixed-valent bis (μ3-oxo) tricopper (II, II, III) complex have been assessed by the reaction of O2 and copper (I) complexes supported by a series of pyridylalkylamine ligands. Reactivity of the generated active oxygen complexes has been also investigated in detail to get insights into the catalytic mechanism of copper enzymes such as tyrosinase and particulate methane monooxygenase. Mononuclear copper (II) -hydroperoxo complexes as well as bis (μ-oxo) dinickel (III) complexes have been also obtained using the same series of pyridylalkylamine ligands. Furthermore, redox interconversion between disulfide (RS-SR) and thiolate (2RS-) has been finely controlled by the copper complexes of pyridylalkylamine derivatives. Ligand effects on the respective reactions have been discussed based on the structures and physicochemical properties of the complexes.
Recent development of asymmetric synthesis is strongly dependent on the design of the chiral ligand molecule. Numbers of chiral catalysts and reagents have skillfully incorporated the enantiopure 1, 2-diamines and their derivatives into their chiral ligand structure. Studies of polymer-supported chiral ligands are always important mainly due to their recyclability. Although 1, 2-diamines have been efficient chiral ligand, the polymer-supported version has not been studied. In this context we concentrated on the preparation of optically active 1, 2-diamine which could be immobilized to polymer. New chiral 1, 2-diamine having phenolic OH groups was synthesized and attached to Merrifield type resins. Another way to obtain polymeric chiral 1, 2-diamine is polymerization of the corresponding chiral 1, 2-diamine monomer. These chiral 1, 2-diamine polymers were used as chiral ligand of the catalyst for asymmetric hydrogenation of aromatic ketones to yield the alcohols with high level of enantioselectivities. These results opened the possibility of a new synthetic methodology in asymmetric reactions using polymeric chiral 1, 2-diamine ligand.