Total synthesis of a novel 2-thiabicyclo[3.2.0]heptan-6-one analogue of penicillin N

Synthesis of α-Amino Cyclobutanones via Formal 1,3-Hydroxy Migration Triggered by Formation of α-Imino Rhodium Carbene

Formal intramolecular 1,3-OH migration of α-imino carbene was achieved producing a unique zwitterion, and the subsequent selective annulation afforded α-amino cyclobutanone. Features such as readily available substrates, mild reaction conditions, a time-saving procedure, excellent functional group compatibility, and valuable transformations of the products qualified this unique protocol as an efficient tool for the synthesis of strained cyclic compounds. Density functional theory calculations were in good agreement with experimental observations, and a plausible mechanism is presented.

Tandem four-component reaction for efficient synthesis of dihydrothiophene with substituted amino acid ethyl esters

The one-pot four-component reaction of aromatic aldehydes, malononitrile, 1,3-thiazolidinedione and ethyl glycinate hydrochloride in ethanol in the presence of triethylamine afforded trans-dihydrothiophene ureidoformamide derivatives in moderate to good yields. The other α-amino acid ethyl esters resulted in the corresponding diastereoisomeric dihydrothiophene derivatives with various molecular ratios. The functionalized thiophene derivatives were also successfully prepared by sequential dehydrogenation reaction with DDQ.

The one-pot four-component reaction of aromatic aldehydes, malononitrile, 1,3-thiazolidinedione and ethyl glycinate hydrochloride in ethanol in the presence of triethylamine afforded trans-dihydrothiophene.

Cyclobutanone Analogues of β-Lactam Antibiotics: β-Lactamase Inhibitors with Untapped Potential?

β-Lactam antibiotics have been used for many years to treat bacterial infections. However the effective treatment of an increasing range of microbial infections is threatened by bacterial resistance to β-lactams: the prolonged, widespread (and at times reckless) use of these drugs has spawned widespread resistance, which renders them ineffective against many bacterial strains. The cyclobutanone ring system is isosteric with β-lactam: in cyclobutanone analogues, the eponymous cyclic amide is replaced with an all-carbon ring, the amide N is substituted by a tertiary C-H α to a ketone. Cyclobutanone analogues of various β-lactam antibiotics have been investigated over the last 35 years, initially as prospective antibiotics in their own right and inhibitors of the β-lactamase enzymes that impart resistance to β-lactams. More recently they have been tested as inhibitors of other serine proteases and as mechanistic probes of β-lactam biosynthesis. Cyclobutanone analogues of the penam ring system are the first reversible inhibitors with moderate activity against all classes of β-lactamase; other compounds from this family inhibit Streptomyces R61 dd-carboxypeptidase/transpeptidase, human neutrophil elastase and porcine pancreatic elastase. But has their potential as enzyme inhibitors been fully exploited? Challenges in synthesising diversely functionalised cyclobutanone derivatives mean that only a limited number have been made (with limited structural diversity) and evaluated. This review surveys the different synthetic approaches that have been taken to these compounds, the investigations made to evaluate their biological activity and prospects for future developments in this area.

Synthetic scope, computational chemistry and mechanism of a base induced 5-endo cyclization of benzyl alkynyl sulfides

We present an experimental and computational study of the reaction of aryl substituted benzyl 1-alkynyl sulfides with potassium alkoxide in acetonitrile, which produces 2-aryl 2,3-dihydrothiophenes in poor to good yields. The cyclization is most efficient with electron withdrawing groups on the aromatic ring. Evidence indicates there is rapid exchange of protons and tautomerism of the alkynyl unit prior to cyclization. Theoretical calculations were also conducted to help rationalize the base induced 5-endo cyclization of benzyl 1-propynyl sulfide (1a). The potential energy surface was calculated for the formation of 2,3-dihydrothiophene in a reaction of benzyl 1-propynyl sulfide (1a) with potassium methoxide. Geometries were optimized with CAM-B3LYP/6-311+G(d,p) in acetonitrile with the CPCM solvent model. It is significant that the benzyl propa-1,2-dien-1-yl sulfane (6) possessed a lower benzylic proton affinity than the benzyl prop-2-yn-1-yl sulfane (8) thus favoring the base induced reaction of the former. From benzyl(propa-1,2-dien-1-yl sulfane (6), 2,3-dihydrothiophene can be formed via a conjugate base that undergoes 5-endo-trig cyclization followed by a protonation step.

A Cyclobutanone Analogue Mimics Penicillin in Binding to Isopenicillin N Synthase

A carbocyclic analogue of the beta-lactam antibiotic isopenicillin N (IPN) has been synthesised and cocrystallised with isopenicillin N synthase (IPNS), the central enzyme in the biosynthesis of penicillin antibiotics. The crystal structure of the IPNS-cyclobutanone complex reveals an active site environment similar to that seen in the enzyme-product complex generated by turnover of the natural substrate within the crystalline protein. The IPNS-cyclobutanone structure demonstrates that the product analogue is tethered to the protein by hydrogen bonding and salt bridge interactions with its carboxylate groups, as seen previously for the natural substrate and product. Furthermore, the successful cocrystallisation of this analogue with IPNS provides firm structural evidence for the utility of such cyclobutanone derivatives as hydrolytically stable analogues of bicyclic beta-lactams.

Design and synthesis of chiral N-chloroimidodicarbonates: application to asymmetric chlorination of silyl enol ethers

New chiral N-chloroimidodicarbonates, which function as efficient chiral chlorinating agents, were designed and synthesized. Among these, C(2)-symmetric (1R,2S,5R)-(-)-menthyl-N-chloroimidodicarbonate 2a provided moderate to good enantioselectivity (up to 40%) for the chlorination of silyl enol ethers to afford alpha-chloroketones only in the presence of a suitable Lewis acid such as Sm(OTf)(3).