Tetramate Derivatives by Chemoselective Dieckmann Ring Closure of threo-Phenylserines and Their Antibacterial Activity

A general route, which provides direct access to substituted bicyclic tetramates, making use of Dieckmann cyclization of oxazolidines derived from threo-arylserines, is reported; the latter were found to be available by an efficient aldol-like reaction of glycine with some substituted benzaldehydes under alkaline conditions. The tetramates were found to release chelated metal cations acquired during chromatographic purification by mild acid wash. Some compounds in the library showed good antibacterial activity against Gram-positive bacteria. Cheminformatic analysis demonstrates that the most active compounds were Ro5-compliant and occupy a narrow region of chemical space, distinct from that occupied by other known antibiotics, with the most potent compounds having 399 < M_w < 530 Da; 3.5 < cLogP < 6.6; 594 < MSA <818 Ų; 9.6 < rel. PSA <13.3%. MIC values were shifted to higher concentrations when tested in the presence of HSA or blood, but was not completely abolished, consistent with a plasma protein binding (PPB) effect.

Synthetic Access to 3-Substituted Pyroglutamic Acids from Tetramate Derivatives of Serine, Threonine, allo-Threonine, and Cysteine

A general route which provides direct access to pyroglutamates from tetramates, making use of Suzuki coupling on an enol mesylate, followed by reduction, is reported. This work permits direct scaffold hopping from tetramate to substituted pyroglutamates. Some compounds in the library showed modest antibacterial activity against Gram-positive bacteria.

[Development of Synthetic Strategies for Densely Oxygenated Natural Products: Total Synthesis of Lactacystin and Zaragozic Acid C Using Photochemical C(sp3)-H Functionalization]

This review describes two novel synthetic routes from (S)-pyroglutaminol to (+)-lactacystin, a potent inhibitor of the 20S proteasome and from d-gluconolactone derivative to zaragozic acid C, a potent squalene synthase inhibitor. In lactacystin synthesis, the photoinduced intermolecular C(sp³)-H alkynylation and intramolecular C(sp³)-H acylation chemoselectively and stereoselectively constructed the tetrasubstituted and trisubstituted carbon centers, respectively. In the synthesis of zaragozic acid C, the stereoselective installation of the two contiguous tetrasubstituted carbons was achieved by the photochemical intramolecular C(sp³)-H acylation of a densely oxygenated intermediate.

Mimics of pramanicin derived from pyroglutamic acid and their antibacterial activity

Mono and dihydroxypyrrolidinones are readily available by direct oxygenation of a pyroglutamate-derived bicyclic lactam with high diastereoselectivity, and these may be manipulated further in protected or unprotected form by Grignard addition to a pendant Weinreb amide to give acylhydroxypyrrolidinones.

Lithium Enolates Derived from Pyroglutaminol: Mechanism and Stereoselectivity of an Azaaldol Addition

A lithium enolate derived from an acetonide-protected pyroglutaminol undergoes a highly selective azaaldol addition with (E)-N-phenyl-1-[2-(trifluoromethyl)phenyl]methanimine. The selectivity is sensitive to tetrahydrofuran (THF) concentration, temperature, and the presence of excess lithium diisopropylamide base. Rate studies show that the observable tetrasolvated dimeric enolate undergoes reversible deaggregation, with the reaction proceeding via a disolvated-monomer-based transition structure. Limited stereochemical erosion stems from the intervention of a trisolvated-monomer-based pathway, which is suppressed at low THF concentrations and elevated temperature. Endofacial selectivity observed with excess lithium diisopropylamide (LDA) is traced to an intermediate dianion formed by subsequent lithiation of the monomeric azaaldol adduct, which is characterized as both a dilithio form and a trilithio dianion-LDA mixed aggregate.

Lithium Enolates Derived from Pyroglutaminol: Aggregation, Solvation, and Atropisomerism

Lithium enolates derived from protected pyroglutaminols were characterized by using (6)Li, (13)C, and (19)F NMR spectroscopies in conjunction with the method of continuous variations. Mixtures of tetrasolvated dimers and tetrasolvated tetramers in different proportions depend on the steric demands of the hemiaminal protecting group, tetrahydrofuran concentration, and the presence or absence of an α-fluoro moiety. The high steric demands of the substituted bicyclo[3.3.0] ring system promote dimers to an unusual extent and allow solvents and atropisomers in cubic tetramers to be observed in the slow-exchange limit. Pyridine used as a (6)Li chemical shift reagent proved useful in assigning solvation numbers.

Biomimetic synthesis, antibacterial activity and structure-activity properties of the pyroglutamate core of oxazolomycin

Biomimetic intramolecular aldol reactions on oxazolidine templates derived from serine may be used to generate densely functionalised pyroglutamates, which are simpler mimics of the right hand side of oxazolomycin. Some of the compounds from this sequence exhibit in vivo activity against S. aureus and E. coli, suggesting that pyroglutamate scaffolds may be useful templates for the development of novel antibacterials, and cheminformatic analysis has been used to provide some structure-activity data.

(3R,6S,7aS)-3-Phenyl-6-(phenyl-sulfan-yl)perhydro-pyrrolo[1,2-c]oxazol-5-one

Mol­ecules of the title compound [systematic name: (2R,5S,7S)-2-phenyl-7-phenyl­sulfanyl-1-aza-3-oxa­bicyclo­[3.3.0]octan-8-one], C₁₈H₁₇NO₂S, form high quality crystals even though they are only packed using C—H⋯O(carbon­yl) and weak C—H⋯S inter­actions. The dihedral angle between the aromatic rings is 85.53 (5)°. The fused rings adopt envelope and twist conformations.

Alkylation of Phenylglycinol-Derived Oxazolopiperidone Lactams. Enantioselective Synthesis of β-Substituted Piperidines

The stereochemical outcome of the alkylation of a variety of phenylglycinol-derived oxazolopiperidone lactams is studied. The influence of the configuration of the C-8a stereocenter and the effect of the substituents at the C-8 and C-8a positions on the stereoselectivity of the reaction are discussed. The synthetic utility of these alkylation reactions is illustrated with the synthesis of cis and trans 3,5-disubstituted, 2,5-disubstituted, and 2,3,5-trisubstituted enantiopure piperidines, and the indole alkaloids 20R- and 20S-dihydrocleavamine.

Stereoselective synthesis of conformationally constrained ω-amino acid analogues from pyroglutamic acid

Bicyclic lactams derived from pyroglutamic acid provide a useful scaffold for synthesis of conformationally restricted analogues of lysine, ornithine and glutamine, as well as an Ala-Ala dipeptide analogue. Amino alcohol and carboxylic acid derivatives are accessible from a common intermediate. In this strategy, the bicyclic lactam system not only controls, but also facilitates the determination of the stereochemistry of the synthetic intermediates.

Diastereoselective syntheses of deoxydysibetaine, dysibetaine, and its 4-epimer

(+/-)-Deoxydysibetaine 2 and 4-epi-dysibetaine 3 were prepared in a few steps from methyl pyroglutamate through a regioselective Mannich reaction at C-2. Natural (2S,4S)-dysibetaine 1, a sponge metabolite isolated from Dysidea herbacea, and (2S)-2 were synthesized from enantiopure (S)-pyroglutaminol with very high stereoselectivity. The key steps were an original formation of stereogenic quaternary center C-2 and the diastereoselective hydroxylation at C-4.

Enantioselective synthesis of cis- and trans-3,5-disubstituted piperidines. Synthesis of 20S- and 20R-dihydrocleavamine

[reaction: see text] Diastereoselective alkylation at the carbonyl alpha-position of chiral nonracemic lactams 2 and 3, the former prepared by cyclocondensation of (R)-phenylglycinol with a racemic gamma-substituted delta-oxoester in a process that involves a dynamic kinetic resolution and the latter by a subsequent equilibration, provides access to enantiopure cis- and trans-3,5-disubstituted piperidines. The usefulness of the approach is illustrated with the synthesis of the alkaloids 20S- and 20R-15,20-dihydrocleavamine.

Pyrrolidinones derived from (S)-pyroglutamic acid: penmacric acid and analogues

Alkylation reactions using alpha-halolactams or lactam enolates derived from bicyclic lactam templates can proceed with high endo- or exo- diastereoselectivity respectively. In the latter case, stereochemical correction by means of enolate generation and hindered phenol quench is possible with moderate efficiency. This protocol has been applied to the synthesis of protected penmacric acid and its analogues.