Enantioselective synthesis of β-amino acids. Part 11: Diastereoselective alkylation of chiral derivatives of β-aminopropionic acid containing the α-phenethyl group

Stereodivergent Mannich reaction of bis(trimethylsilyl)ketene acetals with N-tert-butanesulfinyl imines by Lewis acid or Lewis base activation, a one-pot protocol to obtain chiral β-amino acids

We report a one-pot synthesis of chiral β^2,2,3-amino acids by the Mannich addition of bistrimethylsilyl ketene acetals to N-tert-butanesulfinyl imines followed by the removal of the chiral auxiliary. The synthesis and isolation of pure β-amino acid hydrochlorides were conducted under mild conditions, without strong bases and this method is operationally simple. The stereoselective reaction was promoted by two different activation methods that lead to different stereoisomers: (1) Lewis Acid (LA) catalysis with boron trifluoride diethyl etherate and (2) Lewis Base (LB) catalysis with tetrabutylammonium difluorotriphenylsilicate. The reaction presented good diastereoselectivity with LB activation and moderate to good dr with LA catalysis. The exceptions in both protocols were imines with electron donating groups in the aromatic ring.

Remarkable magnitude of the self-disproportionation of enantiomers (SDE) via achiral chromatography: application to the practical-scale enantiopurification of β-amino acid esters

We report the best performance yet for the self-disproportionation of enantiomers (SDE) via achiral chromatography as typically used in laboratories for the isolated yield of the excess enantiomer using N-acetyl β-amino acid ethyl esters. The results are the most convincing ever demonstration of the capability of the SDE for practical-scale enantiopurification as comparable, or even superior for some systems, to that of recrystallization. For example, from a sample of 94.4 % ee, a yield of 71 % of enantiopure material was isolated in a single chromatographic run. Moreover, the lack of an esoteric structural entity, e.g. strongly polarizing groups, such as, for instance CF3, highlights the fact that the phenomenon is not dependent on the presence of such and thus the process is relevant to any usual-type structure. In contrast to recrystallization, the procedure is predictable, general, and dependable, boding well for its widespread application in routine laboratory settings.

Asymmetric synthesis of β2-amino acids: 2-substituted-3-aminopropanoic acids from N-acryloyl SuperQuat derivatives

Conjugate addition of lithium dibenzylamide to (S)-N(3)-acryloyl-4-isopropyl-5,5-dimethyloxazolidin-2-one (derived from l-valine) and alkylation of the resultant lithium beta-amino enolate provides, after deprotection, a range of (S)-2-alkyl-3-aminopropanoic acids in good yield and high ee. Alternatively, via a complementary pathway, conjugate addition of a range of secondary lithium amides to (S)-N(3)-(2'-alkylacryloyl)-4-isopropyl-5,5-dimethyloxazolidin-2-ones, diastereoselective protonation with 2-pyridone, and subsequent deprotection furnishes a range of (R)-2-alkyl- and (R)-2-aryl-3-aminopropanoic acids in good yield and high ee. Additionally, the boron-mediated aldol reaction of beta-amino N-acyl oxazolidinones is a highly diastereoselective method for the synthesis of a range of beta-amino-beta'-hydroxy N-acyl oxazolidinones.

Efficient Synthesis of β2-Amino Acid by Homologation of α-Amino Acids Involving the Reformatsky Reaction and Mannich-Type Imminium Electrophile

Development of new methods for the synthesis of beta-amino acids is important as polymers of these compounds are promising peptidomimetic candidates in medicinal chemistry. We report here our findings on a new and highly efficient general strategy for the synthesis of beta2-amino acids by homologation of alpha-amino acids, involving the Reformatsky reaction and Mannich-type imminium electrophile.

Stereoselective synthesis of beta-amino acids

A convenient method for the stereoselective syntheses of beta-amino acids with alpha-substitutions has been developed. This synthetic route involves the preparation of isoxazolidinones through hydroxylamine addition to unsaturated esters and subsequent hydrogenation. This procedure is also useful for the stereoselective syntheses of alpha-deuterated beta-amino acids.

Asymmetric alkylation of dimethoxyphosphoryl-N-[1-(S)-alpha-methylbenzyl]acetamide enolates. Synthesis of both stereoisomers from the same source of chirality by changing the equivalents of LDA

A new methodology has been developed for the synthesis of both stereoisomers from a single chiral source.

Asymmetric synthesis of 2-alkyl- and 2-aryl-3-aminopropionic acids (β2-amino acids) from (S)-N-acryloyl-5,5-dimethyloxazolidin-2-one SuperQuat derivatives

Conjugate addition of lithium amides to (S)-N-acryloyl- or (S)-N-2'-alkylacryloyloxazolidinones and alkylation or protonation of the resulting enolates with 2-pyridone respectively provides a highly stereoselective and product complementary route to a range of (R)- and (S)-2-alkyl-3-aminopropionic acids in good yield and in high ee.

?2-amino acids?syntheses, occurrence in natural products, and components of ?-peptides1,2

Although they are less abundant than their alpha-analogues, beta-amino acids occur in nature both in free form and bound to peptides. Oligomers composed exclusively of beta-amino acids (so-called beta-peptides) might be the most thoroughly investigated peptidomimetics. Beside the facts that they are stable to metabolism, exhibit slow microbial degradation, and are inherently stable to proteases and peptidases, they fold into well-ordered secondary structures consisting of helices, turns, and sheets. In this respect, the most intriguing effects have been observed when beta2-amino acids are present in the beta-peptide backbone. This review gives an overview of the occurrence and importance of beta2-amino acids in nature, placing emphasis on the metabolic pathways of beta-aminoisobutyric acid (beta-Aib) and the appearance of beta2-amino acids as secondary metabolites or as components of more complex natural products, such as peptides, depsipeptides, lactones, and alkaloids. In addition, a compilation of the syntheses of both achiral and chiral beta2-amino acids is presented. While there are numerous routes to achiral beta2-amino acids, their EPC synthesis is currently the subject of many investigations. These include the diastereoselective alkylation and Mannich-type reactions of cyclic- or acyclic beta-homoglycine derivatives containing chiral auxiliaries, the Curtius degradation, the employment of transition-metal catalyzed reactions such as enantioselective hydrogenations, reductions, C-H insertions, and Michael-type additions, and the resolution of rac. beta2-amino acids, as well as several miscellaneous methods. In the last part of the review, the importance of beta2-amino acids in the formation of beta-peptide secondary structures is discussed.