β-Lactam synthesis: Chemospecific sulfonation and cyclization of the β-hydroxyvaline nucleus

Synthetic Approaches toward Monocyclic 3-Amino-β-lactams

Due to the emerging resistance against classical β-lactam-based antibiotics, a growing number of bacterial infections has become harder to treat. This alarming tendency necessitates continued research on novel antibacterial agents. Many classes of β-lactam antibiotics are characterized by the presence of the 3-aminoazetidin-2-one core, which resembles the natural substrate of the target penicillin-binding proteins. In that respect, this Review summarizes the different synthetic pathways toward this key structure for the development of new antibacterial agents. The most extensively applied methods for 3-amino-β-lactam ring formation are discussed, in addition to a few less common strategies. Moreover, approaches to introduce the 3-amino substituent after ring formation are also covered.

SAR and Structural Analysis of Siderophore-Conjugated Monocarbam Inhibitors of Pseudomonas aeruginosa PBP3

A main challenge in the development of new agents for the treatment of Pseudomonas aeruginosa infections is the identification of chemotypes that efficiently penetrate the cell envelope and are not susceptible to established resistance mechanisms. Siderophore-conjugated monocarbams are attractive because of their ability to hijack the bacteria's iron uptake machinery for transport into the periplasm and their inherent stability to metallo-β-lactamases. Through development of the SAR we identified a number of modifications to the scaffold that afforded active anti-P. aeruginosa agents with good physicochemical properties. Through crystallographic efforts we gained a better understanding into how these compounds bind to the target penicillin binding protein PBP3 and factors to consider for future design.

Serine as chiral educt for the practical synthesis of enantiopure N-protected beta-hydroxyvaline

N-tert-Butyloxycarbonyl- and N-benzenesulfonyl-beta-hydroxyvalines 1a and 1b were, respectively, synthesized in enantiomerically pure form by a two-step protocol from their enantiomeric N-protected serine methyl esters 2a and 2b. The addition of CH(3)MgBr to 2a and 2b provided diols 3a and 3b, respectively as major products in 83% and 81% yields. Selective oxidation of diols 3a and 3b was performed using a TEMPO, NaClO(2), NaOCl cocktail in 96% and 93% respective yields. This two-step process effectively furnished multigram amounts of enantiopure N-Boc-beta-hydroxyvaline 1a.

Contribution to the synthesis of aureobasidin A. Synthesis of cyclopeptolides containing the sequence leucyl-N-methyl-beta-hydroxyvalyl-(2R)-oxy-(3R)-methyl-pentanoi c acid

The efficient antimycotic agent aureobasidin A, isolated from the culture broth of Aureobasidium pullulans R 106, and the [(R)-Pro9]-aureobasidin A were prepared starting from benzyl N-Boc-N-methyl-(S)-beta-triethylsiloxyvalinate, the synthesis of which is described here. The easy accessibility of the tripeptolide benzyl Boc-leucyl-N-methyl-beta-hydroxyvalyl-(2R)-oxy-(3R)-methylpentanoa te [Boc-Leu-HOMeVal-(R)-HMP-OBn] facilitates the construction of the cyclopeptolides 28, 34, 45 and 47. The peptide bonds of the N-methylamino acids were formed with the help of O-(7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophophate. The rings of [(R)-Pro9]-aureobasidin A and of cyclopeptolides 28, 34, 45 and 47 were closed by way of pentafluorophenyl esters. The ring of aureobasidin A could only be formed with bromo-tris-pyrrolidinophosphonium hexafluorophosphate.

Construction of Quaternary Stereogenic Centers via [2 + 2] Cycloaddition Reactions. Synthesis of Homochiral 4,4-Disubstituted 2-Azetidinones and Imine Substituent Effects on beta-Lactam Formation

A study on the asymmetric construction of quaternary stereogenic centers via [2 + 2] cycloaddition reaction of ketenes with ketimines is described. Reaction of achiral ketenes and chiral alpha-alkoxy ketone-derived imines resulted in formation of new beta-lactams as single diastereomers. The cycloaddition was extended to pyruvate imines, aralkyl ketone-derived imines, and dialkyl ketimines. In these cases the asymmetric induction was satisfactorily achieved using beta-silylalkanoyl ketenes and the Evans-Sjögren ketene. C,C-Bis (trimethylsilyl)methylamine ketimines derived from enolizable dialkyl ketones cleanly led to the corresponding C(4) disubstituted beta-lactams without deprotonation. Therefore, a general methodology for a convergent asymmetric synthesis of beta-lactams in which C(4) exists as a quaternary carbon is provided.