Non-Classical Polycyclic β-Lactams

One-Pot Construction of Diverse β-Lactam Scaffolds via the Green Oxidation of Amines and Its Application to the Diastereoselective Synthesis of β-Amino Acids

In this study, a simple one-pot construction of β-lactam scaffolds was successfully achieved via 4,6-dihydroxysalicylic acid-catalyzed organocatalytic oxidation of amines to imines using molecular oxygen. Although some imines are highly unstable and difficult to isolate by conventional methods, the organocatalytic oxidation of amines described herein, followed by their direct reaction with acyl chlorides in the presence of a base, afforded a series of new β-lactam derivatives with excellent cis selectivity, which could not be synthesized and isolated by previously reported methods. Thus, this one-pot protocol will be one of the powerful methods applicable to the synthesis of various potential drug candidates and functional molecules. Furthermore, the subsequent hydrolysis of these β-lactams successfully afforded the corresponding β-amino acids as almost single diastereomers in up to 99% yields.

Evaluation of benzthiazolidine-based formamidinium salts for the synthesis of penem-type β-lactams by uncatalysed carbonylation of acyclic diaminocarbenes

The reaction of [iPr2N=CHCl]Cl with 2-phenylbenzthiazolidine (Ph-BtzH) and subsequent anion metathesis afforded the formamidinium salt [(Ph-Btz)CH(NiPr2)][PF6] ([1aH][PF6]). The reaction of [1aH][PF6] with (Me3Si)2NNa in the presence of grey selenium furnished the selenourea derivative 1aSe as trapping product of the acyclic diaminocarbene 1a. In the absence of selenium, a 2H-1,4-benzothiazine derivative (2) was obtained, which is an isomer of 1a, whose formation is plausibly initiated by deprotonation of [1aH]+ not at the cationic N2CH unit, but at the neighbouring PhCH position. CO was ineffective as trapping reagent for 1a. [1aH][PF6], 1aSe and 2 have been structurally characterised by X-ray diffraction.

A metal- and base-free domino protocol for the synthesis of 1,3-benzoselenazines, 1,3-benzothiazines and related scaffolds

Efficient protocols have been described for the synthesis of 1,3-benzoselenazines, 1,3-benzothiazines, 2-aryl thiazin-4-ones and diaryl[b,f][1,5]diazocine-6,12(5H,11H)-diones. These transformations were successfully driven towards the product formation under mild acid catalyzed reaction conditions at room temperature using 2-amino aryl/hetero-aryl alkyl alcohols and amides as substrates. The merits of the present methods also rely on the easy access of rarely explored bioactive scaffolds like 1,3-benzoselenazine derivatives, for which well-documented methods are rarely known in the literature. A broad range of substrates with both electron-rich and electron-deficient groups were well-tolerated under the developed conditions to furnish the desired products in yields up to 98%. The scope of the devised method is not only restricted to the synthesis of 1,3-benzoselenazines, but it was also further extended towards the synthesis of 1,3-benzothiazines, 1,3-benzothiazinones and the corresponding eight membered N-heterocycles such as diaryl[b,f][1,5]diazocine-6,12(5H,11H)-diones.

Spectroscopic analyses on interaction of Amantadine-Salicylaldehyde, Amantadine-5-Chloro-Salicylaldehyde and Amantadine-o-Vanillin Schiff-Bases with bovine serum albumin (BSA)

In this work, three Tricyclo [3.3.1.1(3,7)] decane-1-amine (Amantadine) Schiff-Bases, Amantadine-Salicylaldehyde (AS), Amantadine-5-Chloro-Salicylaldehyde (AS-5-C) and Amantadine-o-Vanillin (AS-o-V), were synthesized by direct heating reflux method in ethanol solution and characterized by infrared spectrum and elementary analysis. Fluorescence quenching was used to study the interaction of these Amantadine Schiff-Bases (AS, AS-5-C and AS-o-V) with bovine serum albumin (BSA). According to fluorescence quenching calculations the bimolecular quenching constant (K(q)), apparent quenching constant (K(SV)), effective binding constant (K(A)) and corresponding dissociation constant (K(D)), binding site number (n) and binding distance (r) were obtained. The results show that these Amantadine Schiff-Bases can obviously bind to BSA molecules and the binding strength order is AS<AS-5-C=AS-o-V. Synchronous fluorescence spectroscopy reveals that these Amantadine Schiff-Bases adopt different way to bind with BSA molecules. That is, the AS and AS-5-C are accessibility to tryptophan (Trp) residues more than the tyrosine (Tyr) residues, while the AS-o-V is equally close to the Tyr and Trp residues.

Dicobalt hexacarbonyl complexes of alkynyl imines in a sequential Staudinger/Pauson-Khand process. A route to new fused tricyclic beta-lactams

Dicobalt hexacarbonyl complexes of alkynyl imines were allowed to react with ketenes via Staudinger reaction. Sequential [2 + 2] cycloaddition/Pauson-Khand reaction led to structurally new fused-tricyclic beta-lactams and fused-azabicyclic cyclopentenones. Chemoselectivity, scope, and limitation of the process were investigated.

The practice of ring constraint in peptidomimetics using bicyclic and polycyclic amino acids

Medicinal chemistry has witnessed major advances with the discovery of small synthetic molecules that mimic natural peptidic substrates. These small synthetic mimics do not undergo proteolytic degradation, an advantage they hold over their natural counterparts. Small synthetic molecules make up a number of life-saving marketed drugs that inhibit certain physiologically relevant proteases. The advent of sophisticated instrumental methods, such as X-ray crystallography and high-field NMR, has played a pivotal role in the design of structure-based enzyme inhibitors. Highly stereocontrolled methods of synthesis have led to a variety of functionally diverse molecules that function as peptidomimetics because they have isosteric subunits not affected by proteolytic enzymes. Further studies to optimize biological activity and achieve desirable pharmacokinetic profiles can eventually lead to drug substances. The practice of constraining natural amino acids like their conformationally rigid counterparts has been highly successful in the design and synthesis of peptidomimetic molecules. With some notable exceptions, structural information gathered from protein X-ray crystallography of therapeutically relevant target enzymes, alone or in complex forms with inhibitor molecules, has been instrumental in the design of peptidomimetics. For example, a significant number have become marketed drugs as antihypertensives and antivirals. Natural products have also been a source of inspiration for the design and synthesis of truncated analogues with the intention of maintaining, or even improving, their biological activities. However, lower molecular weight peptides are not suitable as therapeutic agents because they are subject to rapid amide proteolysis. They are poorly transported to the brain and rapidly excreted through the liver and kidney. Thus, lower molecular weight peptides are eliminated as potential drug substances in clinical practice. A synthetic peptidomimetic is needed that is resistant to cleavage but maintains its biological activity. Conformationally constrained monocyclic and bicyclic unnatural amino acids can be directly incorporated in a potential inhibitor molecule as part of the design element. In this Account, we describe our efforts in the synthesis of constrained azacycles that contain proline or pipecolic acid as an integral part of bicyclic and polycyclic amino acids. We devised syntheses of conformationally biased monocyclic, bicyclic, and polycyclic amino acid analogues, into which pharmacologically or structurally relevant functional groups were incorporated. Stereocontrolled reactions for C-C, C-N, and C-O bond formation had to be implemented on appropriately protected amino acid frameworks. A number of these frameworks provided access to functionally diverse scaffolds for further use as core subunits in more elaborated structures. Specific applications as peptidomimetics of natural substrates for relevant enzymes, such as thrombin, were also pursued, resulting in highly active inhibitors in vitro.

Synthesis of Highly Functionalized Macrocycles by the Peripheral Functionalization of Macrocyclic Diimines

The easily available macrocyclic diimines 4-7 can be stereoselectively transformed to macrocyclic bis-beta-amino acids 13-17, macrocyclic bisazetidines 18-20, and macrocyclic bisamides 21 and 22 by means of the corresponding bis-beta-lactam scaffolds 8-12. These key intermediates are available through standard Staudinger reaction and obtained as the cis-cis diastereomers, exclusively. An interesting relation between the proximity of the reactive C=N bonds and the selectivity in the formation of the bis-beta-lactams 8-12 is observed. Thus, diimine 4 leads to low selectivities, producing a 1:1 mixture of cis-syn-cis and cis-anti-cis diastereomers, while diimines 5-7 having the diimine sites more separated lead almost exclusively to the cis-anti-cis diastereomers. The stereochemistry of all the products was unambiguously assigned by X-ray diffraction analysis of compounds cis-syn-cis 8 and cis-anti-cis 12-Co2CO6 complex.