An unusually facile SnAr 14-membered biaryl ether macrocyclization reaction suitable for preparation of the cycloisodityrosine subunit of bouvardin, deoxybouvardin and related agents

Total Syntheses of Vancomycin-Related Glycopeptide Antibiotics and Key Analogues

A review of efforts that have provided total syntheses of vancomycin and related glycopeptide antibiotics, their agylcons, and key analogues is provided. It is a tribute to developments in organic chemistry and the field of organic synthesis that not only can molecules of this complexity be prepared today by total synthesis but such efforts can be extended to the preparation of previously inaccessible key analogues that contain deep-seated structural changes. With the increasing prevalence of acquired bacterial resistance to existing classes of antibiotics and with the emergence of vancomycin-resistant pathogens (VRSA and VRE), the studies pave the way for the examination of synthetic analogues rationally designed to not only overcome vancomycin resistance but provide the foundation for the development of even more powerful and durable antibiotics.

Total synthesis of [Ψ[C(═S)NH]Tpg4]vancomycin aglycon, [Ψ[C(═NH)NH]Tpg4]vancomycin aglycon, and related key compounds: reengineering vancomycin for dual D-Ala-D-Ala and D-Ala-D-Lac binding

The total synthesis of [Ψ[C(═S)NH]Tpg(4)]vancomycin aglycon (8) and its unique AgOAc-promoted single-step conversion to [Ψ[C(═NH)NH]Tpg(4)]vancomycin aglycon (7), conducted on a fully deprotected substrate, are disclosed. The synthetic approach not only permits access to 7, but it also allows late-stage access to related residue 4 derivatives, alternative access to [Ψ[CH(2)NH]Tpg(4)]vancomycin aglycon (6) from a common late-stage intermediate, and provides authentic residue 4 thioamide and amidine derivatives of the vancomycin aglycon that will facilitate ongoing efforts on their semisynthetic preparation. In addition to early stage residue 4 thioamide introduction, allowing differentiation of one of seven amide bonds central to the vancomycin core structure, the approach relied on two aromatic nucleophilic substitution reactions for formation of the 16-membered diaryl ethers in the CD/DE ring systems, an effective macrolactamization for closure of the 12-membered biaryl AB ring system, and the defined order of CD, AB, and DE ring closures. This order of ring closures follows their increasing ease of thermal atropisomer equilibration, permitting the recycling of any newly generated unnatural atropisomer under progressively milder thermal conditions where the atropoisomer stereochemistry already set is not impacted. Full details of the evaluation of 7 and 8 along with several related key synthetic compounds containing the core residue 4 amidine and thioamide modifications are reported. The binding affinity of compounds containing the residue 4 amidine with the model D-Ala-D-Ala ligand 2 was found to be only 2-3 times less than the vancomycin aglycon (5), and this binding affinity is maintained with the model d-Ala-d-Lac ligand 4, representing a nearly 600-fold increase in affinity relative to the vancomycin aglycon. Importantly, the amidines display effective dual, balanced binding affinity for both ligands (K(a)2/4 = 0.9-1.05), and they exhibit potent antimicrobial activity against VanA resistant bacteria ( E. faecalis , VanA VRE) at a level accurately reflecting these binding characteristics (MIC = 0.3-0.6 μg/mL), charting a rational approach forward in the development of antibiotics for the treatment of vancomycin-resistant bacterial infections. In sharp contrast, 8 and related residue 4 thioamides failed to bind either 2 or 4 to any appreciable extent, do not exhibit antimicrobial activity, and serve to further underscore the remarkable behavior of the residue 4 amidines.

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.

Synthesis of (S,S)-Isodityrosine by Dötz Benzannulation

[reaction: see text] A synthesis of (S,S)-isodityrosine 1, a naturally occurring, key structural subunit of numerous biologically active macromolecules, is described. A formal [3 + 2 + 1] cycloaddition (Dötz benzannulation) approach was utilized to simultaneously construct an aromatic ring and the diaryl ether linkage in one step. This key step was extended to the synthesis of (S,S)-isodityrosine in two separate convergent synthetic routes. This method demonstrates a novel and mild method for the synthesis of diaryl ethers.

Vancomycin, teicoplanin, and ramoplanin: synthetic and mechanistic studies

Vancomycin, teicoplanin, and ramoplanin are potent glycopeptide antibiotics that act by inhibiting bacterial cell wall biosynthesis. The former are used clinically as the antibiotics of last resort for the treatment of methicillin-resistant Staphylococcus aureus and the latter is a promising new antibiotic that is not susceptible to the emerging bacterial resistance to vancomycin and teicoplanin. A summary of our recent total synthesis of the vancomycin aglycon, our first and second generation total syntheses of the teicoplanin aglycon, and our progress on the total synthesis of the ramoplanins is presented. This work lays the foundation for ongoing structure-function studies on the antibiotics that may clarify or define their site and mechanism of action leading to the development of improved or reengineered antibiotics.

On-resin macrocyclization of peptides via intramolecular SnAr reactions

On-resin macrocyclization via an SNAr reaction is employed in the synthesis of tocinoic acid analogs. Specifically, an N-terminal nitrofluorobenzene is attacked by a nucleophilic C-terminal sidechain. The remaining nitro group can be reduced and acylated. NMR is used to compare the conformation of the new macrocyclic peptides to tocinoic acid.

Synthesis of beta-hydroxy-beta-(fluoronitrophenyl)alanines: vital components in the assembly of biologically active cyclic peptides

[formula: see text] Numerous biologically active cyclic peptides, such as the antibiotic vancomycin, contain amino acid residues connected through side-chain biaryl or aryl-alkyl ether linkages. Nucleophilic aromatic substitution reactions have recently been shown to provide a general method for the formation of such ether linkages, and consequently the synthesis of functionalized fluoronitro-substituted aromatic amino acids is of great interest. Herein, a method for the stereospecific synthesis of 3-fluoro-4-nitro- and 4-fluoro-3-nitro-threo-beta-hydroxyphenylalanine is described.

Synthesis of (9R,12S)- and (9S,12S)-Cycloisodityrosine and Their N-Methyl Derivatives

Full details of the synthesis of (9R,12S)- and (9S,12S)-cycloisodityrosine and their N-methyl derivatives are detailed based on an intramolecular nucleophilic aromatic substitution reaction for formation of the key biaryl ether with 14-membered ring macrocyclization. Their comparison with prior samples and the documentation of a facile C9 epimerization within the natural 9S series are described.