Enantioselective Preparation of β^²-Amino Acid Derivatives for β-Peptide Synthesis

Abstract

β-Amino acids with a single side chain in the α-position (β^²-amino acids or H-β^²hXaa(PG)-OH; i.e., homo-amino acids with proteinogenic side chains) have turned out to be important components in β-peptides. They contribute to unique secondary structures, they are required for mimicking the structure and the activity of β-turn-forming α-peptides, and they can be used for protecting α-peptides against attack by aminopeptidases. In contrast to β^³-homo-amino acids, the β^²-isomers cannot be obtained simply by enantiospecific homologation of the (natural) α-amino acids, but have to be prepared by enantioselective reactions or sequences of transformations, which are presented herein. The various preparative methods are ordered according to the bond at the stereogenic center, which is formed in the stereoselective step, with the four strategic bonds being the C(2)-C(3) backbone bond, the C(2)-side-chain bond, the C(2)-H bond, and the C(1)-C(2) bond between the carboxylate and the α-carbon. In the most frequently employed methods, a chiral auxiliary group is attached at the carboxyl C(1) atom or at the nitrogen in the 3-position, but there are also a number of enantioselective catalytic processes, including the hydrogenation of suitable acrylates. The alternative of stereoselective synthesis, namely resolution of racemic mixtures (for instance by biocatalysis), is also discussed. A critical comparison of the various methods and strategies is presented. For the peptide chemist, a list is included with the Cbz-, Boc-, and Fmoc-protected β^²-amino acid building blocks, ready for peptide coupling. In addition, the search strategy for nonracemic β^²-amino acids and their precursors from the databases is described in detail.

1 Introduction

2 Why β^²-Amino Acids?

3 Literature Search

4 Retrosynthetic Analysis

5 β^²-Amino Acids by Formation of the C(2)-C(3) Bond

5.1 Is There a Stereospecific Route from α-Amino Acids?

5.2 Chiral Auxiliaries and Catalysts for C(2)-C(3) Bond ­Formation

6 β^²-Amino Acids by Formation of the C(2)-R Bond

6.1 α-Alkylations of Chiral Enolates Derived from β-Aminopropanoic Acid

6.2 C(2)-R Bond Formation by Nucleophilic Addition and Substitution

7 β^²-Amino Acids by Stereoselective Formation of the C(2)-H Bond

7.1 Protonation of Enols or Enolates Derived from 3-Aminopropanoic Acid

7.2 Enantioselective Hydrogenation of Acrylates and Nitro­olefins with Formation of β^²-Amino Acid Derivatives

8 Preparation of β^²-Amino Acids with Formation of the ­Strategic C(1)-C(2) Bond

9 β^²-Amino Acids by Resolution?

10 Detailed Search Strategy

11 Conclusions and a Table with β^²-Amino Acid Building Blocks for Peptide Synthesis

Key words

β^²-amino acids - β-peptide - retrosynthetic analysis - overall enantioselective synthesis - chiral auxiliaries - enantioselective catalysis - database literature search strategies

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6

Others groups studied β-peptides with ‘unnatural’ side chains or with cyclic β-amino acids as building blocks (see section 8 in an extensive review article on β-peptides^²d,e and references 2b and 2f).

15

See Figure  [4] c in reference 10.

64

The sultam D is an irritant which may prevent its application on large scale or in an industrial process.

71

An amino group itself is a poor leaving group (R₂N^-) from an enolate. On the other hand, phthalimido or sulfonylamido groups [R(R"SO₂)N^-] would be expected to be good leaving groups.

98

Seebach, D.; Marti, R.; Beck, A. K.; Sprecher, H.; Pletscher, S.; Möri, M. hitherto unpublished results; ETH Zürich and Zürcher Hochschule für Angewandte Wissenschaften 2006-2008.

135

In many cases, the reported preparations ended with ‘reasonable’ precursors to properly protected β^²-amino acids for peptide synthesis.

136

In one case discussed (Scheme  [¹¹] , c), an approximately 1:1 mixture of diastereoisomers was actually separated chromatographically to eventually provide the enantiomeric β^²-amino acid derivatives.^¹0²

137

For a large-scale production of an enantiopure drug or pesticide, on the other hand, it is not acceptable to lose 50% of material through resolution, after a multistep synthesis.

139

Lukaszuk, A.; Tourwé, D. hitherto unpublished results; Department of Organic Chemistry Vrije Universiteit Brussel, Belgium.

144

A notable exception is Gmelin, for which the version available at the host STN International has not been updated since 1997. For other databases, there may be significant differences in item coverage; for example, the Science Citation Index database version available under Web of Knowledge/Web of Science goes back to 1900, while the same database at the host STN International extends only back to 1974.

146

STN International: http://www.stn-international.de/ (last accessed 27.8.2008).

147

STN Messenger Commands: http://www.stn-international.de/training_center/rl/commands/Contents.htm (last accessed 27.8.2008).

148

STN Registry: http://www.stn-international.de/stndatabases/databases/registry.html (last accessed 27.8.2008).

149

STN CASREACT: http://www.stn-international.de/stndatabases/databases/casreact.html html (last accessed 27.8.2008).

150

So far, for any such ‘AutoFix’ problem in SciFinder Scholar referred to us, the undesired limit could be eliminated by searching in STN, but at the extra cost involved in those ‘pay-per-use’ searches at this host.

152

This reliance on CASREACT is not without problems, both regarding reaction selection policies of CAS, and particularly the time coverage of this database: specified by CAS as covering the literature back to 1840 (http://www.cas.org/expertise/cascontent/casreact.html), full coverage provided by CAS literature exception starts only in 1985 (for reactions from patents, only in 1991). Before that time, coverage is almost entirely provided by non-CAS sources: ZIC/VINITI, and INPI (French Patent Office).

153

CAplus: http://www.cas.org/expertise/cascontent/caplus/index.html (last accessed 27.8.2008).

154

Some publications appear in more than one of the categories shown.

155

SciFinder search (as of November 2008) provides more than 500 hits of commercially available enantiopure β^²-amino acids, but only 16 hits for β^²-amino acids, which are offered up to the 10-kg scale.

156

For instance, Phoenix Chemicals or Novasep.