A facile solid phase synthesis of tetramic acid

tert-Butyl 2-Amino-3-cyano-5-oxo-4-phenyl-5,7-dihydropyrano[2,3-c]pyrrole-6(4H)-carboxylate

Organocatalyzed synthesis of tert-butyl 2-amino-3-cyano-5-oxo-4-phenyl-5,7-dihydropyrano[2,3-c]pyrrole-6(4H)-carboxylate, prepared from Boc-tetramic acid and benzylidenemalononitrile, is disclosed. Two bifunctional noncovalent organocatalysts were employed, yielding the product as a racemic mixture in both cases. The structure of the new synthesized compound was confirmed by high resolution mass-spectrometry, 1H- and 13C-NMR, HSQC, and IR spectroscopy.

Integration of C-Acylation in the Solid-Phase Synthesis of Peptides and Peptidomimetics Employing Meldrum’s Acid, Phosphorus, and Sulfur Ylides

The modification of native peptides to peptidomimetics is an important goal in medicinal chemistry and requires, in many cases, the integration of C-acylation steps involving amino acids with classical peptide synthesis. Many classical C-acylation protocols involving Claisen condensations and the use of ylides are not compatible with peptide synthesis, mostly due to the requirements for strong bases leading to epimerization or deprotection of peptides. Meldrum’s acid as well as several specific phosphorus and sulfur ylides, however, are acidic enough to provide reactive C-nucleophiles under mildly basic conditions tolerated during peptide synthesis. This review provides an overview of peptide-compatible C-acylations using Meldrum’s acid and phosphorus and sulfur ylides, and their application in the medicinal chemistry of peptides.

1 Introduction

2 C-Acylation of Meldrum’s Acid

2.1 C-Acylation of Meldrum’s Acid on Solid Phase

3 Ylides as Substrates for C-Acylation

3.1 C-Acylation of Phosphorus Ylides in Solution Phase

3.2 C-Acylation of Solid-Supported Phosphorus Ylides

3.3 C-Acylation of Sulfur Ylides

3.4 C-Acylation of Solid-Supported Sulfur Ylides

4 Miscellaneous Ylides as Acyl Anion Equivalents

5 Summary

Traceless Solid-Phase Organic Synthesis

Traceless solid-phase synthesis represents an ultimate sophisticated synthetic strategy on insoluble supports. Compounds synthesized on solid supports can be released without a trace of the linker that was used to tether the intermediates during the synthesis. Thus, the target products are composed only of the components (atoms, functional groups) inherent to the target core structure. A wide variety of synthetic strategies have been developed to prepare products in a traceless manner, and this review is dedicated to all aspects of traceless solid-phase organic synthesis. Importantly, the synthesis does not need to be carried out on a linker designed for traceless synthesis; most of the synthetic approaches described herein were developed using standard, commercially available linkers (originally devised for solid-phase peptide synthesis). The type of structure prepared in a traceless fashion is not restricted. The individual synthetic approaches are divided into eight sections, each devoted to a different methodology for traceless synthesis. Each section consists of a brief outline of the synthetic strategy followed by a description of individual reported syntheses.

Metal-Catalyzed and Metal-Mediated Approaches to the Synthesis and Functionalization of Tetramic Acids

The heterocyclic ring of tetramic acids is found in naturally occurred biologically active products isolated from fungi, bacteria, molds, and sponges. Thus, these molecules have attracted significant attention as synthetic targets, and various synthetic paths have been developed. Over recent years, a growing number of catalytic approaches toward functionalized products have been established in order to overcome the limitations of the conventional methods. The present review describes the strategies for the metal-catalyzed and metal-promoted synthesis and further derivatization of tetramic acids, with emphasis on recent examples from the literature.

Nα-Amino acid containing privileged structures: design, synthesis and use in solid-phase peptide synthesis

Fmoc-protected Nα-amino acid containing heterocyclic privileged structures, O-(1-methyl-5-oxo-2,5-dihydro-1H-pyrrol-3-yl)-l-serine and O-((S)-5-oxo-2,3,5,7a-tetrahydro-1H-pyrrolizin-7-yl)-l-serine, were synthesized on the solid phase from simple commercially available building blocks under mild conditions. The amino acid side-chain is composed of tetramic acid, a natural product derived privileged structure. The key transformation was the formation of cyclic enol ethers via nonclassical Wittig olefinations of the esters. Solid-phase synthesis represents a method of choice, particularly for the synthesis of peptides. This route is compatible with traditional Merrifield solid-phase peptide synthesis (SPPS), as documented on the preparation of the pentapeptide Leu-enkephalin amide H-Tyr-Gly-Gly-Phe-Leu-NH2 with Phe or Tyr replaced by a novel amino acid.

Bioactive Microbial Metabolites

The short history, specific features and future prospects of research of microbial metabolites, including antibiotics and other bioactive metabolites, are summarized. The microbial origin, diversity of producing species, functions and various bioactivities of metabolites, unique features of their chemical structures are discussed, mainly on the basis of statistical data. The possible numbers of metabolites may be discovered in the future, the problems of dereplication of newly isolated compounds as well as the new trends and prospects of the research are also discussed.