Practical, asymmetric synthesis of aromatic-substituted bulky and hydrophobic tryptophan and phenylalanine derivatives

1-Aminoalkylphosphonium Derivatives: Smart Synthetic Equivalents of N-Acyliminium-Type Cations, and Maybe Something More: A Review

N-acyliminium-type cations are examples of highly reactive intermediates that are willingly used in organic synthesis in intra- or intermolecular α-amidoalkylation reactions. They are usually generated in situ from their corresponding precursors in the presence of acidic catalysts (Brønsted or Lewis acids). In this context, 1-aminoalkyltriarylphosphonium derivatives deserve particular attention. The positively charged phosphonium moiety located in the immediate vicinity of the N-acyl group significantly facilitates Cα-P+ bond breaking, even without the use of catalyst. Moreover, minor structural modifications of 1-aminoalkyltriarylphosphonium derivatives make it possible to modulate their reactivity in a simple way. Therefore, these types of compounds can be considered as smart synthetic equivalents of N-acyliminium-type cations. This review intends to familiarize a wide audience with the unique properties of 1-aminoalkyltriarylphosphonium derivatives and encourage their wider use in organic synthesis. Hence, the most important methods for the preparation of 1-aminoalkyltriarylphosphonium salts, as well as the area of their potential synthetic utilization, are demonstrated. In particular, the structure-reactivity correlations for the phosphonium salts are discussed. It was shown that 1-aminoalkyltriarylphosphonium salts are not only an interesting alternative to other α-amidoalkylating agents but also can be used in such important transformations as the Wittig reaction or heterocyclizations. Finally, the prospects and limitations of their further applications in synthesis and medicinal chemistry were considered.

Modifications of amino acids using arenediazonium salts

Aryl transfer reactions from arenediazonium salts have started to make their impact in chemical biology with initial forays in the arena of arylative modifications and bio-conjugations of amino acids, peptides and proteins.

Asymmetric Synthesis of Non-Natural Amino Acid Derivatives: (2R/3S) and (2S/3R) 2-(Tert-Butoxycarbonylamino)-3-Cyclohexyl-3-Phenyl Propanoic Acids

Highly conformationally-constrained novel α-amino acid derivatives ((2R/3S) and (2S/3R)-2-(tert-butoxycarbonylamino)-3-cyclohexyl-3-phenylpropanoic acids) have been synthesised with high stereoselectivity (> 90% de) and in 36–37% overall yields. In the synthesis, Evans’ auxiliary (4(R/S)-4-phenyl-oxzaolidin-2-one) was used to control the stereoselectivity via the key reactions of asymmetric Michael addition, azidation and catalytic hydrogenolysis.

Metal-free trifluoromethylation of aromatic and heteroaromatic aldehydes and ketones

The ability to convert simple and common substrates into fluoroalkyl derivatives under mild conditions remains an important goal for medicinal and agricultural chemists. One representative example of a desirable transformation involves the conversion of aromatic and heteroaromatic ketones and aldehydes into aryl and heteroaryl β,β,β-trifluoroethylarenes and -heteroarenes. The traditional approach for this net transformation involves stoichiometric metals and/or multistep reaction sequences that consume excessive time, material, and labor resources while providing low yields of products. To complement these traditional strategies, we report a one-pot metal-free decarboxylative procedure for accessing β,β,β-trifluoroethylarenes and -heteroarenes from readily available ketones and aldehydes. This method features several benefits, including ease of operation, readily available reagents, mild reaction conditions, high functional-group compatibility, and scalability.

The Use of Cyclohexyl Isocyanide in the Esterification of N-Benzoyl α-Amino Acids Derivatives

The N-benzoyl of α,β-dehydroamino acids that were obtained by condensation of N-benzoylglycine and an aldehyde, were esterified with an alcohol in high yield using cyclohexyl isocyanide to activate the carboxylic acid under neutral conditions.

Organic chemistry and biology: chemical biology through the eyes of collaboration

From a scientific perspective, efforts to understand biology including what constitutes health and disease has become a chemical problem. However, chemists and biologists "see" the problems of understanding biology from different perspectives, and this has retarded progress in solving the problems especially as they relate to health and disease. This suggests that close collaboration between chemists and biologists is not only necessary but essential for progress in both the biology and chemistry that will provide solutions to the global questions of biology. This perspective has directed my scientific efforts for the past 45 years, and in this overview I provide my perspective of how the applications of synthetic chemistry, structural design, and numerous other chemical principles have intersected in my collaborations with biologists to provide new tools, new science, and new insights that were only made possible and fruitful by these collaborations.

Syntheses of optically pure, conformationally constrained, and highly hydrophobic unusual amino acids: 2-amino-3, 3-diarylpropionic acids*

A series of optically pure, conformationally constrained, and highly hydrophobic unusual aromatic amino acids, 2-amino-3,3-diarylpropionic acids, were synthesized via asymmetric 1,4-Michael addition reaction/azidation reactions in seven steps with overall yields of 20-30%.

Asymmetric synthesis of chiral 4-substituted 5,5-diethyl oxazolidin-2-ones as potential effective chiral auxiliaries

Four new chiral 4-substituted 5,5-diethyl oxazolidin-2-ones (5a-5d) as potential effective chiral auxiliaries were synthesized from readily available amino acids via asymmetric etherification, Boc protection, Grignard reaction and cyclization reactions in four steps with overall yields of 50-60%.

1H NMR spectroscopic criteria for the configuration of N-acyl-alpha,beta-dehydro-alpha-amino acid esters

The diagnostic values of the following three spectral criteria for the configuration of N-acyl-alpha,beta-dehydro-alpha-amino acid esters were examined: (i) the proton at the beta-position at the double bond of a Z-isomer is shielded if compared with the respective E-isomer (delta(beta)Z < delta(beta)E); (ii) the proton at the nitrogen atom is shielded in a Z-isomer in comparison with the corresponding E-isomer (delta(NH)Z < delta(NH)E); and (iii) changing of the solvent from CDCl3 to deuterated trifluoroacetic acid (TFA) causes shielding of the H(beta) vinylic proton of an E-isomer or deshielding of the respective proton of the Z-isomer (delta(CDCl3)E > delta(TFA)E or delta(CDCl3)Z < delta(TFA)Z). The investigations were based on a set of 22 (Z)- and (E)-N-acyl-alpha,beta-dehydro-alpha-amino acid esters of diverse structures, with aliphatic, aromatic and heteroaromatic substituents at the vinylic beta-carbon; most of the examined compounds were hitherto unknown. The application of the substituent effect additivity rule given by Pascual et al. for olefinic protons leads to evidently erroneous configuration assignments of N-acyl-alpha,beta-dehydro-alpha-amino acid esters. The considered criteria were fulfilled for all the examined cases with one exception [the second criterion for the alpha-pivaloylamino-beta-(2-furyl)acrylates]. The comparison of changes in the chemical shifts of H(beta) vinylic protons in CDCl3 and deuterated TFA seems to be the most reliable and useful configuration criterion, as it can be used in the case of a single isomer.

Catalytic Asymmetric Synthesis of the Central Tryptophan Residue of Celogentin C

[reaction: see text] Chiral phase-transfer catalyst 5 containing an electron-deficient trifluorobenzyl moiety promoted the alkylation of glycine derivative 6 with propargyl bromide 7a in good yield and excellent ee. The resulting propargyl glycine 8 was converted to 14, the central tryptophan residue of Celogentin C, in two steps, with the Pd-catalyzed heteroannulation as the key transformation. This method promises to be an efficient route for the preparation of tryptophan derivatives possessing substitution on the indole ring.