Synthesis of CC biaryl segment of complestatin and chloropeptin: Approach to the right hand CEF-ring system of complestatin

Preparation of Cyclic Peptide Alkaloids Containing Functionalized Tryptophan Residues

This review covers the synthesis of various cyclic peptide natural products possessing highly functionalized tryptophan residues, focusing on the examples of diazonamide A, the TMC-95 compounds, the celogentin/moroidin family and the complestatin/chloropeptin system. Recent efforts toward the preparation of these modified-tryptophan-containing peptides will be outlined, focusing primarily on the novel methods for the assembly of the highly functionalized indole/tryptophan moieties at the core of these structures.

A Pd(0)-mediated indole (macro)cyclization reaction

Herein we report a systematic study of the Larock indole annulation designed to explore the scope and define the generality of its use in macrocyclization reactions, its use in directly accessing the chloropeptin I versus II DEF ring system as well as key unnatural isomers, its utility for both peptide-derived and more conventional carbon-chain based macrocycles, and its extension to intramolecular cyclizations with formation of common ring sizes. The studies define a powerful method complementary to the Stille or Suzuki cross-coupling reactions for the synthesis of cyclic or macrocyclic ring systems containing an embedded indole, tolerating numerous functional groups and incorporating various (up to 28-membered) ring sizes. As a result of the efforts to expand the usefulness and scope of the reaction, we also disclose a catalytic variant of the reaction, along with a powerful Pd(2)(dba)(3)-derived catalyst system, and an examination of the factors impacting reactivity and catalysis.

Synthesis and stereochemical determination of complestatin A and B (neuroprotectin A and B)

Recently, we reported the first total synthesis of chloropeptin II (1, complestatin), the more strained and challenging of the two naturally occurring chloropeptins. Central to the design of the approach and by virtue of a single-step, acid-catalyzed ring expansion rearrangement of chloropeptin II to chloropeptin I, the route also provided a total synthesis of chloropeptin I. Herein, we report a complementary and divergent oxidation of chloropeptin II (1, complestatin) to either complestatin A (2, neuroprotectin A) or complestatin B (3, neuroprotectin B), providing the first synthesis of the natural products and establishing their remaining stereochemical assignments. Key to the approach to complestatin A (2, neuroprotectin A) was the development of two different single-step indole oxidations (HCl-DMSO and NBS, THF-H(2)O) that avoid the rearrangement of chloropeptin II (1) to chloropeptin I (4), providing the 2-oxindole 2 in superb yields (93% and 82%). With a mechanistic understanding of features that impact the latter oxidation and an appreciation of the intrinsic reactivity of the chloropeptin II indole, its modification (NCS, THF-H(2)O; Cs(2)CO(3), DMF-H(2)O) provided a two-step, single-pot oxidation of chloropeptin II (1) to afford directly the 3-hydroxy-2-oxindole complestatin B (3, neuroprotectin B). Extensive studies conducted on the fully functionalized synthetic DEF ring system of chloropeptin II were key to the unambiguous assignment of the stereochemistry as well as the exploration and subsequent development of the mild oxidation conditions used in the synthesis of complestatin A and B.

Recent advances in the synthesis of new glycopeptide antibiotics

The vancomycin family of glycopeptide antibiotics has been inspiring research in the field of synthetic chemistry since the 1980s. Recent studies have moved away from the focus of total synthesis into new territory: the design and evaluation of novel compounds based on the natural products which exhibit improved antibacterial activity. Modern approaches to drug synthesis draw together investigations into the nature of the binding environment, and innovative synthetic methodologies which provide solutions to the challenging structural features and stereochemistry associated with this intriguing class of compounds. New analogues, derivatives and dimers of the natural products, as well as recent successes in the total synthesis of the complestatins are described in this tutorial review, covering literature from the last decade.

Total synthesis of complestatin: development of a Pd(0)-mediated indole annulation for macrocyclization

Full details of the initial development and continued examination of a powerful intramolecular palladium(0)-mediated indole annulation for macrocyclization closure of the strained 16-membered biaryl ring system found in complestatin (1, chloropeptin II) and the definition of factors impacting its intrinsic atropodiastereoselectivity are described. Its examination and use in an alternative, second-generation total synthesis of complestatin are detailed in which the order of the macrocyclization reactions was reversed from our first-generation total synthesis. In this approach and with the ABCD biaryl ether ring system in place, the key Larock cyclization was conducted with substrate 36 (containing four phenols, five secondary amides, one carbamate, and four labile aryl chlorides) and provided the product 37 (56%) exclusively as a single atropisomer (>20:1, detection limits) possessing the natural (R)-configuration. In this instance, the complexity of the substrate and the reverse macrocyclization order did not diminish the atropodiastereoselectivity; rather, it provided an improvement over the 4:1 selectivity that was observed with the analogous substrate used to provide the isolated DEF ring system in our first-generation approach. Just as significant, the atroposelectivity represents a complete reversal of the diasteroselectivity observed with analogous macrocyclizations conducted using a Suzuki biaryl coupling.

Total synthesis of chloropeptin II (complestatin) and chloropeptin I

The first total synthesis of chloropeptin II (1, complestatin) is disclosed. Key elements of the approach include the use of an intramolecular Larock indole synthesis for the initial macrocyclization, adopting conditions that permit utilization of a 2-bromoaniline, incorporating a terminal alkyne substituent (-SiEt(3)) that sterically dictates the indole cyclization regioselectivity, and benefiting from an aniline protecting group (-Ac) that enhances the atropdiastereoselectivity and diminishes the strained indole reactivity toward subsequent electrophilic reagents. Not only did this key reaction provide the fully functionalized right-hand ring system of 1 in superb conversion (89%) and good atropdiastereoselectivity (4:1 R:S), but it also represents the first reported example of what will prove to be a useful Larock macrocyclization strategy. Subsequent introduction of the left-hand ring system enlisting an aromatic nucleophilic substitution reaction for macrocyclization with biaryl ether formation completed the assemblage of the core bicyclic structure of 1. Intrinsic in the design of the approach and by virtue of the single-step acid-catalyzed conversion of chloropeptin II (1) to chloropeptin I (2), the route also provides a total synthesis of 2.

Complestatin synthetic studies; the effect of the amino acid configuration on peptide backbone conformation in the common western BCD macrocycle

The synthesis and structural analysis, involving X-ray crystallographic, nuclear magnetic resonance, and computational studies of four diastereomers of the common western BCD diarylether macrocycle of the complestatins, a family of HIV entry inhibitors, has been achieved exploiting a ruthenium-promoted intramolecular S(N)Ar reaction. The stereogenicity of the individual phenylglycines (residues C and D) results in remarkable effects on the backbone conformation.

Two syntheses of the 16- and 17-membered DEF ring systems of chloropeptin and complestatin

[formula: see text] Two syntheses of a model system of the DEF ring system of complestatin and chloropeptin are described. The key step in both of these syntheses involves the formation of the biaryl linkage using a palladium-catalyzed Suzuki cross-coupling reaction and a catalytic enantioselective ene reaction to form the 6-bromo-D-tryptophan. Additionally, ring contraction of the 17-membered DEF ring system of complestatin generates the 16-membered DEF ring system of chloropeptin in a biomimetic fashion.