Allylation of Erythromycin Derivatives: Introduction of Allyl Substituents into Highly Hindered Alcohols

New Bicyclic Azalide Macrolides Obtained by Tandem Palladium Catalyzed Allylic Alkylation/Conjugated Addition Reaction

Unprecedented tandem allylic alkylation/intermolecular Michael addition was used in the preparation of novel bicyclic azalides. NMR spectroscopy was used not only to unambiguously determine and characterize the structures of these unexpected products of chemical reaction but also to investigate the effect the rigid bicyclic modification has on the conformation of the whole molecule. Thus, some of the macrolides prepared showed antibacterial activity in the range of well-known antibiotic drug azithromycin.

O-Allylated Pudovik and Passerini Adducts as Versatile Scaffolds for Product Diversification

The palladium-catalyzed O-allylation of α-hydroxyphosphonates and α-hydroxyamides obtained from Pudovik and Passerini multicomponent reactions has allowed interesting and highly straightforward access to a variety of building blocks for product diversification. These post-functionalizations include a selective base- or ruthenium hydride-mediated isomerization/Claisen rearrangement cascade and a ring-closing metathesis that allows access to a variety of diversely functionalized phosphono-oxaheterocycles.

Design, synthesis and structure-bactericidal activity relationships of novel 9-oxime ketolides and reductive epimers of acylides

Erythromycin was long viewed as a bacteriostatic agent. The erythromycin derivatives, 9-oxime ketolides have a species-specific bactericidal profile. Among them, the 3'-allyl version of the 9-oxime ketolide 1 (Ar=3-quinolyl; 17a) is bactericidal against Streptococcus pneumoniae and Streptococcus pyogenes. In contrast, the 2-fluoro analogs of 1, 13a (Ar=6-quinolyl), 13b (Ar=3-quinolyl) and 24a (Ar=4-isoquinolyl), show bactericidal activities against S. pneumoniae, Staphylococcus aureus and Moraxella catarrhalis, while the 2-fluoro analogs 13c (Ar=3-aminopyridyl) and 24b (Ar=3-carbamoylpyridyl) are only bactericidal against S. pneumoniae and Haemophilus influenzae. Reduction of the ketolides led to novel epiacylides, the 3-O-epimers of the acylides. Alteration of linker length (30b vs. 30a), 2-fluorination (33 vs. 30a) and incorporation of additional spacers at the 9-oxime or 6-OH (35, 40 vs. 30a) did not restore the epiacylides back to be as active as the acylide 31. Molecular docking suggested that epimerization at the 3-position reshapes the orientation of the 3-O-sidechain and leads to considerably weaker binding with bacterial ribosomes.

Base-mediated cascade rearrangements of aryl-substituted diallyl ethers

Two base-mediated cascade rearrangement reactions of diallyl ethers were developed leading to selective [2,3]-Wittig-oxy-Cope and isomerization-Claisen rearrangements. Both diaryl and arylsilyl-substituted 1,3-substituted propenyl substrates were examined, and each exhibits unique reactivity and different reaction pathways. Detailed mechanistic and computational analysis was conducted, which demonstrated that the role of the base and solvent was key to the reactivity and selectivity observed. Crossover experiments also suggest that these reactions proceed with a certain degree of dissociation, and the mechanistic pathway is highly complex with multiple competing routes.

High yielding allylation of a chiral secondary alcohol containing base sensitive functional groups

Inhibitors of neuronal nitric oxide synthase, based on a chiral pyrrolidine scaffold, show promise for the treatment of certain neurodegenerative diseases. We recently reported the synthesis of a series of selective inhibitors, but the method was limited at a key step of formingan allyl ether intermediate. Yields for this step were very inconsistent, and the presence of base sensitive functional groups limited the range of available methods for forming this ether bond. This work describes a novel application of palladium catalyzed decarboxylativeallylation, consistently resulting in 90% isolated yields, which is crucial for the synthesis of the critical allyl ether late stage intermediate. We also report a new quantitative yielding and straightforward synthesis of the allyl-t-butylcarbonate precursor.

Asymmetric synthesis: From transition metals to organocatalysis

Umpolung in the allylation reaction is discussed with examples drawn from transition-metal-catalyzed allylic substitution (with the allylic unit acting as an electrophile) and Lewis base-catalyzed allylation of aldehydes with allyltrichlorosilane (with the allyl acting as a nucleophile). Iridium-catalyzed electrophilic allylation of O-nucleophiles has been employed in our new approach to C-nucleoside analogs, where the C-O bond (rather than C-C) was constructed stereospecifically. Variation of the absolute configuration in the starting segments allowed the synthesis of all four combinations of D/L-α/β-ribosides. In the nucleophilic allylation of aldehydes, chiral pyridine-type N-oxide catalysts are presented, in particular QUINOX and METHOX, and the intriguing behavior of QUINOX is discussed. Here, the π-π interactions between the substrate aldehyde and the catalyst are suggested to rationalize the experimental observations. Good correlation between the calculated energies for the transition states and the experimentally observed enantioselectivities has been obtained.

Synthesis of Novel 6,11-O-Bridged Bicyclic Ketolides via a Palladium-Catalyzed Bis-allylation

A bridging chemistry process was developed to form an ether bridge between 6-O and 11-O of erythromycin A via a tandem or stepwise palladium-catalyzed bis-pi-allylation. By applying this bridging process, new 6,11-O-bridged bicyclic ketolides (BBKs) were synthesized. These BBKs showed good antibacterial activities against the macrolide-susceptible strains as well as mef-resistant strains and served as a good core for further modifications to study the structure-activity relationship (SAR) and to overcome bacterial resistance. [reaction: see text]

Novel ketolide antibiotics with a fused five-membered lactone ring––synthesis, physicochemical and antimicrobial properties

In an effort to find novel semisynthetic macrolides with extended antibacterial spectrum and improved activity we prepared a series of compounds based on commercially available clarithromycin, a potent and safe antimicrobial agent of outstanding clinical and commercial interest. According to the literature, improvement of antibacterial activity of erythromycin type antibiotics can be achieved by introduction of fused heterocycles such as cyclic carbonates or carbamates at positions 11 and 12 (such as in telithromycin). In the course of the work presented here, a similar, hitherto unprecedented set of compounds bearing a five-membered lactone ring fused to positions 11 and 12 was prepared based on carbon-carbon bond formation via intramolecular Michael addition of a [(hetero)arylalkylthio]acetic acid ester enolate to an alpha,beta-unsaturated ketone as the key step. Some of the ketolide compounds described in this paper were highly active against a representative set of erythromycin sensitive and erythromycin resistant test strains. The best compound showed a similar antimicrobial spectrum and comparable activity in vitro as well as in vivo as telithromycin. Furthermore, some physicochemical properties of these compounds were determined and are presented here. On the basis of these results, the novel ketolide lactones presented in this paper emerged as valuable lead compounds with comparable properties as the commercial ketolide antibacterial telithromycin (Ketek).

Synthesis and Antibacterial Activity of a Novel Class of 4‘-Substituted 16-Membered Ring Macrolides Derived from Tylosin

Novel 4'-substituted 16-membered ring macrolides were synthesized by the cleavage of the mycarose sugar of tylosin and subsequent modification of 4'-hydroxyl group. This new class of macrolide antibiotics exhibited potent activity against some key erythromycin-resistant pathogens.