General Approach to Enantiopure 1-Aminopyrrolizidines: Application to the Asymmetric Synthesis of the Loline Alkaloids.

The synthesis of a range of loline alkaloids is reported. The C(7) and C(7a) stereogenic centers for the targets were formed by the established conjugate addition of lithium (S)-N-benzyl-N-(α-methylbenzyl)amide to tert-butyl 5-benzyloxypent-2-enoate, ensuing enolate oxidation to give an α-hydroxy-ß-amino ester, and then formal exchange of the resultant amino and hydroxyl functionalities (via the intermediacy of the corresponding aziridinium ion) to give an α-amino-ß-hydroxy ester. Subsequent transformation gave a 3-hydroxyprolinal derivative which was converted to the corresponding N-tert-butylsulfinylimine. Mannich-type reaction with the enolate derived from O-Boc protected methyl glycolate then formed the remaining C(1) and C(2) stereogenic centers for the targets. The 2,7-ether bridge was formed by a displacement reaction, completing construction of the loline alkaloid core. Facile manipulations then gave a range of loline alkaloids, including loline itself.

Microgrewiapine C: Asymmetric Synthesis, Spectroscopic Data, and Configuration Assignment.

The first asymmetric synthesis of microgrewiapine C, a piperidine alkaloid isolated from Microcos paniculata, is reported. This synthesis prompted correction of the 1H and 13C NMR data for the natural sample of the alkaloid, which was achieved by reanalysis of the original spectra. The corrected data for the natural product were found to be identical to those of the synthetic sample prepared herein, thus confirming the structural and relative configurational assignment of microgrewiapine C. Although comparison of specific rotation values indicates that the (1R,2S,3S,6S) absolute configuration should be assigned to the alkaloid, consideration of potential common biosynthetic origins of microgrewiapine C and congeners suggests that further phytochemical investigations are warranted.

Synthesis and Configuration of O-Acetyl Microgrewiapine A: Phantomization of O-Acetyl 6-epi-Microgrewiapine A.

The formation of O-acetyl microgrewiapine A is investigated. NMR data for the authentic sample derived from the natural product are corrected. Wholly synthetic samples, produced from reductive N-methylation of synthetic microcosamine A (to give synthetic microgrewiapine A) followed by O-acetylation, exhibit NMR data that are identical to those of the authentic sample. The previous report that this two-step transformation proceeds with epimerization at C-6 is thus shown to be in error: the purported sample of O-acetyl 6-epi-microgrewiapine A is structurally misassigned and is, in fact, O-acetyl microgrewiapine A. A plausible rationale for the structural misassignment is advanced.

Mutual kinetic resolution: probing enantiorecognition phenomena and screening for kinetic resolution with racemic reagents.

Enantiorecognition between a racemic reagent and a racemic substrate can be a valuable process in organic synthesis. This review highlights representative examples of this phenomenon and the use of mutual kinetic resolution as a method for screening of kinetic and/or parallel kinetic resolutions.

SuperQuat chiral auxiliaries: design, synthesis, and utility.

The SuperQuat (4-substituted 5,5-dimethyloxazolidine-2-one) family of chiral auxiliaries was first developed by us in the 1990s to address the shortcomings of the Evans (4-substituted oxazolidin-2-one) family of chiral auxiliaries. The incorporation of geminal dimethyl substitution at C(5) has two effects: (i) it induces a conformational bias on an adjacent, otherwise conformationally labile C(4)-substituent so that it projects towards the N-acyl fragment, thus offering superior diastereofacial selectivity in a range of transformations; and (ii) it hinders nucleophilic attack at the endocyclic carbonyl group, facilitating recovery and recyclability of the auxiliary, with enhanced cleavage properties. This review summarises the development and some of the most common uses of the SuperQuat family of chiral auxiliaries, particularly in the synthesis of natural products or other targets having bioloigcal interest. Where possible, comparisons with the performances of the corresponding Evans auxiliaries are presented.

N-Acetylcolchinol Methyl Ether (a Natural Product); Suhailamine (a Phantom Natural Product).

The reported characterization data for the allocolchicinoid alkaloid suhailamine, isolated from Colchicum decaisnei and known to have an erroneous structure, have been reanalyzed. This analysis has led to the current proposal that suhailamine has the same structure as N-acetylcolchinol methyl ether (NCME), an assertion that is supported by comparison with previously reported data for NCME. Suhailamine is therefore a phantom natural product, while NCME represents a naturally occurring allocolchicinoid rather than a purely synthetic entity.

Diastereoselective Ammonium-Directed Epoxidation in the Asymmetric Syntheses of Dihydroconduramines (+)-C-2, (-)-C-2, (+)-D-2, (+)-E-2, (+)-F-2, and (-)-F-2.

Epoxidations (40% aq HBF4 then m-CPBA) of racemic cis-2-( N-benzylamino)cyclohex-3-en-1-ol and racemic cis-2-( N, N-dibenzylamino)cyclohex-3-en-1-ol proceed with very high levels of diastereoselectivity (>95:5 dr). The latter is in direct contrast to the epoxidation of the corresponding trans-diastereoisomer (which proceeds with essentially no selectivity), showing that the relative configuration of the substrate dramatically influences the diastereoselectivity in these instances. Meanwhile, epoxidations of enantiopure (1 R,2 S,α R)-2-[( N-α-methylbenzyl)amino]cyclohex-3-en-1-ol and (1 S,2 R,α R)-2-[( N-α-methylbenzyl)amino]cyclohex-3-en-1-ol [surrogates for the enantiomers of cis-2-( N-benzylamino)cyclohex-3-en-1-ol] proceed with complete diastereoselectivity (>95:5 dr) under the same conditions, showing that neither the presence of the α-methyl group nor the relative configuration of the α-methylbenzyl stereocenter have an effect upon the established level of diastereoslectivity in these cases. In contrast, epoxidations of enantiopure (1 R,2 S,α R)-2-[ N-benzyl- N-(α-methylbenzyl)amino]cyclohex-3-en-1-ol and (1 S,2 R,α R)-2-[ N-benzyl- N-(α-methylbenzyl)amino]cyclohex-3-en-1-ol [surrogates for the enantiomers of cis-2-( N, N-dibenzylamino)cyclohex-3-en-1-ol] proceed with lower diastereoselectivity (∼70:30 dr). Thus, the presence of the α-methyl group has a detrimental effect on the established level of diastereoselectivity in these cases (although again the relative configuration of the α-methylbenzyl stereocenter is unimportant). The diastereoselective epoxidation pathway is used to enable the asymmetric syntheses of six hitherto unknown, enantiopure dihydroconduramines (+)-C-2, (-)-C-2, (+)-D-2, (+)-E-2, (+)-F-2, and (-)-F-2 (>99% ee in each case).

The Hancock Alkaloids (-)-Cuspareine, (-)-Galipinine, (-)-Galipeine, and (-)-Angustureine: Asymmetric Syntheses and Corrected 1H and 13C NMR Data.

The asymmetric syntheses of all members of the Hancock alkaloid family based upon a 2-substituted N-methyl-1,2,3,4-tetrahydroquinoline core are delineated. The conjugate addition of enantiopure lithium N-benzyl- N-(α-methyl- p-methoxybenzyl)amide to 5-( o-bromophenyl)- N-methoxy- N-methylpent-2-enamide is used to generate the requisite C-2 stereogenic center of the targets, while an intramolecular Buchwald-Hartwig coupling is used to form the 1,2,3,4-tetrahydroquinoline ring. Late-stage diversification completes construction of the C-2 side chains. Thus, (-)-cuspareine, (-)-galipinine, (-)-galipeine, and (-)-angustureine were prepared in overall yields of 30%, 28%, 15%, and 39%, respectively, in nine steps from commercially available 3-( o-bromophenyl)propanoic acid in all cases. Unambiguously corrected 1H and 13C NMR data for the originally isolated samples of (-)-cuspareine, (-)-galipinine, and (-)-angustureine are also reported, representing a valuable reference resource for these popular synthetic targets.

Structural Revision of the Hancock Alkaloid (-)-Galipeine.

The 1H and 13C NMR data of synthetic samples of (S)-N(1)-methyl-2-[2'-(3″-hydroxy-4″-methoxyphenyl)ethyl]-1,2,3,4-tetrahydroquinoline, the originally proposed structure of the Hancock alkaloid (-)-galipeine, do not match those of the natural product. Herein, the preparation of the regioisomer (S)-N(1)-methyl-2-[2'-(3″-methoxy-4″-hydroxyphenyl)ethyl]-1,2,3,4-tetrahydroquinoline is reported, the 1H and 13C NMR data of which are in excellent agreement with those of (-)-galipeine. Comparison of specific rotation data enables assignment of the absolute (S)-configuration of the alkaloid, and together, these data engender the structural revision of (-)-galipeine to (S)-N(1)-methyl-2-[2'-(3″-methoxy-4″-hydroxyphenyl)ethyl]-1,2,3,4-tetrahydroquinoline.

Asymmetric Synthesis of the Tetraponerine Alkaloids.

The asymmetric syntheses of all eight tetraponerine alkaloids (T1-T8) were achieved using the diastereoselective conjugate additions of lithium amide reagents in the key stereodefining steps. Conjugate addition of either lithium (R)-N-allyl-N-(α-methylbenzyl)amide or lithium (R)-N-(but-3-en-1-yl)-N-(α-methylbenzyl)amide to tert-butyl sorbate was followed by ring-closing metathesis of the resultant N-alkenyl ß-amino esters, reduction to the corresponding aldehydes, and reaction with tert-butyl (triphenylphosphoranylidene)acetate. Subsequent conjugate addition of the requisite antipode of lithium N-benzyl-N-(α-methylbenzyl)amide to the resultant α,ß-unsaturated esters gave a range of diamines for elaboration to T1-T8 via a sequence involving reduction of the ester moiety to give the corresponding aldehyde, olefination, tandem hydrogenation/hydrogenolysis, and cyclization upon reaction with 4-bromobutanal to give the tricyclic skeleton.

Asymmetric Syntheses of 3-Deoxy-3-aminosphingoid Bases: Approaches Based on Parallel Kinetic Resolution and Double Asymmetric Induction.

The asymmetric syntheses of a range of N- and O-protected 3-deoxy-3-aminosphingoid bases have been achieved using two complementary approaches. dl-Serine was converted to a racemic N,N-dibenzyl-protected γ-amino-α,ß-unsaturated ester which was resolved using a parallel kinetic resolution (PKR) strategy upon reaction with a pseudoenantiomeric mixture of lithium (R)-N-benzyl-N-(α-methylbenzyl)amide and lithium (S)-N-3,4-dimethoxybenzyl-N-(α-methylbenzyl)amide, giving the corresponding enantio- and diastereoisomerically pure ß,γ-diamino esters. Alternatively, elaboration of l-serine gave the corresponding enantiopure N,N-dibenzyl-protected γ-amino-α,ß-unsaturated ester, and doubly diastereoselective conjugate addition of the antipodes of lithium N-benzyl-N-(α-methylbenzyl)amide was found to proceed under the dominant stereocontrol of the lithium amide reagent in both cases, thus augmenting the accessible range of ß,γ-diamino esters. Both of these protocols were expanded to include in situ oxidation of the enolate formed upon conjugate addition, giving access to the corresponding α-hydroxy-ß,γ-diamino esters. Elaboration of these ß,γ-diamino and α-hydroxy-ß,γ-diamino esters gave the protected forms of the 3-deoxy-3-aminosphingoid base targets.

Probing Competitive and Co-operative Hydroxyl and Ammonium Hydrogen-Bonding Directed Epoxidations.

The diastereoselectivities and rates of epoxidation (upon treatment with Cl3CCO2H then m-CPBA) of a range of cis- and trans-4-aminocycloalk-2-en-1-ol derivatives (containing five-, six-, and seven-membered rings) have been investigated. In all cases where the two potential directing groups can promote epoxidation on opposite faces of the ring scaffold, evidence of competitive epoxidation pathways, promoted by hydrogen-bonding to either the in situ formed ammonium moiety or the hydroxyl group, was observed. In contrast to the relative directing group abilities already established for the six-membered ring system (NHBn ≫ OH > NBn2), an N,N-dibenzylammonium moiety appeared more proficient than a hydroxyl group at directing the stereochemical course of the epoxidation reaction in a five- or seven-membered system. In the former case, this was rationalized by the drive to minimize torsional strain in the transition state being coupled with assistance from hydrogen-bonding to the ammonium moiety. In the latter case, this was ascribed to the steric bulk of the ammonium moiety disfavoring conformations in which hydrogen-bonding to the hydroxyl group results in direction of the epoxidation to the syn face. In cases where the two potential directing groups can promote epoxidation on the same face of the ring scaffold, an enhancement of epoxidation diastereoselectivity was not observed, while introduction of a second, allylic heteroatom to the substrate results in diminishment of the rate of epoxidation in all cases. Presumably, reduction of the nucleophilicity of the olefin by the second, inductively electron-withdrawing heteroatom is the dominant factor, and any assistance to the epoxidation reaction by the potential to form hydrogen-bonds to two directing groups rather than one is clearly unable to overwhelm it.

Asymmetric Syntheses of (+)-Preussin B, the C(2)-Epimer of (-)-Preussin B, and 3-Deoxy-(+)-preussin B.

Efficient de novo asymmetric syntheses of (+)-preussin B, the C(2)-epimer of (-)-preussin B, and 3-deoxy-(+)-preussin B have been developed, using the diastereoselective conjugate addition of lithium (S)-N-benzyl-N-(α-methylbenzyl)amide to tert-butyl 4-phenylbut-2-enoate and diastereoselective reductive cyclizations of γ-amino ketones as the key steps to set the stereochemistry. Conjugate addition followed by enolate protonation generated the corresponding ß-amino ester. Homologation using the ester functionality as a synthetic handle gave the corresponding γ-amino ketone. Hydrogenolytic N-debenzylation was accompanied by diastereoselective reductive cyclization in situ; reductive N-methylation then gave 3-deoxy-(+)-preussin B as the major diastereoisomeric product. Meanwhile, the same conjugate addition but followed by enolate oxidation with (+)-camphorsulfonyloxaziridine gave the corresponding anti-α-hydroxy-ß-amino ester. α-Epimerization by oxidation and diastereoselective reduction then gave access to the corresponding syn-α-hydroxy-ß-amino ester. Homologation of both of these diastereoisomeric α-hydroxy-ß-amino esters gave the corresponding ß-hydroxy-γ-amino ketones. N-Debenzylation and concomitant diastereoselective reductive cyclization, followed by reductive N-methylation, provided the C(2)-epimer of (-)-preussin B and (+)-preussin B as the major diastereoisomeric products, respectively. The overall yields (from phenylacetaldehyde) were 19% for 3-deoxy-(+)-preussin B over seven steps, 8% for the C(2)-epimer of (-)-preussin B over nine steps, and 7% for (+)-preussin B over eleven steps.

Asymmetric syntheses of nakinadine D, nakinadine E, and nakinadine F: confirmation of their relative (RS,SR)-configurations and proposal of their absolute (2S,3R)-configurations.

The syn- and anti-diastereoisomeric forms of the reported structures of the marine alkaloids nakinadines D-F have been synthesized, for the first time in all cases, via an approach involving asymmetric Mannich-type (imino-aldol) reactions of methyl phenylacetate with N-tert-butylsulfinyl imines as the key steps to control the stereochemistry. Comparison of the (1)H and (13)C NMR spectroscopic data reported for the natural materials with those acquired for these synthetic samples confirms the initially assigned relative (RS,SR)-configurations of these three alkaloids. In the absence of specific rotation (or other diagnostic) data for the natural materials, it is not possible to unambiguously assign their absolute configurations, although given the absolute (2S)-configurations assigned to nakinadines B and C, and the absolute (2S,3R)-configuration previously established for nakinadine A, the data herein uphold our proposal that nakinadines D-F share the absolute (2S,3R)-configuration.

Syntheses of Dihydroconduramines (±)-B-1, (±)-E-1, and (±)-F-1 via Diastereoselective Epoxidation of N-Protected 4-Aminocyclohex-2-en-1-ols.

Diastereoselective syntheses of dihydroconduramines (±)-B-1, (±)-E-1, and (±)-F-1 have been achieved from N-protected 4-aminocyclohex-2-en-1-ols via two complementary procedures for epoxidation as the key step. Treatment of either trans- or cis-4-N-benzylaminocyclohex-2-en-1-ol with Cl3CCO2H and then m-chloroperoxybenzoic acid (m-CPBA) resulted in initial formation of the corresponding ammonium species, followed by epoxidation on the face syn to the ammonium moiety exclusively; chemoselective N-benzylation then provided either (1RS,2SR,3RS,4RS)- or (1RS,2RS,3SR,4SR)-2,3-epoxy-4-N,N-dibenzylaminocyclohexan-1-ol, respectively. Treatment of either trans- or cis-4-N,N-dibenzylaminocyclohex-2-en-1-ol with m-CPBA resulted in initial formation of the corresponding N-oxide, followed by epoxidation on the face syn to the hydroxyl group exclusively; reduction then provided either (1RS,2RS,3SR,4RS)- or an alternative route to (1RS,2RS,3SR,4SR)-2,3-epoxy-4-N,N-dibenzylaminocyclohexan-1-ol, respectively. In all cases, S(N)2-type ring opening of these epoxides upon treatment with aqueous H2SO4 proceeded by nucleophilic attack with inversion at C(2) preferentially, distal to the in situ formed ammonium moiety. Hydrogenolytic N-deprotection then gave the corresponding dihydroconduramines (±)-B-1, (±)-E-1, and (±)-F-1.

Stereospecific cyclization strategies for α,ε-dihydroxy-ß-amino esters: asymmetric syntheses of imino and amino sugars.

A range of biologically significant imino and amino sugars [1,4-dideoxy-1,4-imino-D-allitol, 3,6-dideoxy-3,6-imino-L-allonic acid, (3R,4S)-3,4-dihydroxy-L-proline, 1,5-anhydro-4-deoxy-4-amino-D-glucitol, and 1,5-anhydro-4-deoxy-4-amino-L-iditol] has been prepared via stereospecific cyclization of α,ε-dihydroxy-ß-amino esters. These substrates are readily prepared via conjugate addition of lithium (S)-N-benzyl-N-(α-methylbenzyl)amide to enantiopure α,ß-unsaturated esters (ß-substituted with cis- and trans-dioxolane units) coupled with in situ enolate oxidation with camphorsulfonyloxaziridine (CSO). Activation of the ε-hydroxyl group allowed cyclization to either the corresponding pyrrolidine or the tetrahydropyran scaffold, with the course of the cyclization process being dictated by the relative configuration of the dioxolane unit. When the α,ε-dihydroxy-ß-amino ester bears a cis-dioxolane unit, cyclization occurs upon attack of the ß-amino substituent to give the corresponding pyrrolidine after in situ N-debenzylation. In contrast, when the α,ε-dihydroxy-ß-amino ester bears a trans-dioxolane unit, cyclization occurs upon attack of the α-hydroxyl substituent to give the corresponding tetrahydropyran.

Asymmetric syntheses of (-)-3-epi-Fagomine, (2R,3S,4R)-dihydroxypipecolic acid, and several polyhydroxylated homopipecolic acids.

A range of enantiopure polyhydroxylated piperidines, including (2R,3S,4R)-dihydroxypipecolic acid, (-)-3-epi-fagomine, (2S,3S,4R)-dihydroxyhomopipecolic acid, (2S,3R,4R)-dihydroxyhomopipecolic acid, and two trihydroxy-substituted homopipecolic acids, have been prepared using diastereoselective olefinic oxidations of a range of enantiopure tetrahydropyridines as the key step. The requisite substrates were readily prepared from tert-butyl sorbate using our diastereoselective hydroamination or aminohydroxylation protocols followed by ring-closing metathesis. After diastereoselective olefinic oxidation of the resultant enantiopure tetrahydropyridines and deprotection, enantiopure polyhydroxylated piperidines were isolated as single diastereoisomers (>99:1 dr) in good overall yield.

The asymmetric syntheses of pyrrolizidines, indolizidines and quinolizidines via two sequential tandem ring-closure/N-debenzylation processes.

Concise asymmetric syntheses of (-)-lupinine, (+)-isoretronecanol, (+)-5-epi-tashiromine and (R,R)-1-(hydroxymethyl)octahydroindolizine (the azabicyclic core within stellettamides A-C) have been achieved in 8 steps or fewer from commercially available starting materials. The key steps in these syntheses involved the preparation of enantiopure ß-amino esters, upon conjugate addition of lithium (R)-N-(p-methoxybenzyl)-N-(α-methyl-p-methoxybenzyl)amide to either ζ-chloro or ζ-hydroxy substituted tert-butyl (E)-hept-2-enoate, or ε-chloro or ε-hydroxy substituted tert-butyl (E)-hex-2-enoate. Activation of the ω-substituent as a leaving group led to SN2-type ring-closure, which occurred with concomitant N-debenzylation via an E1-type deprotection step, to give the corresponding pyrrolidine or piperidine in good yield. Subsequent alkylation of these enantiopure azacycles, followed by a second ring-closure/concomitant N-debenzylation step formed the pyrrolizidine, indolizidine or quinolizidine motif, and reduction with LiAlH4 gave the target compounds in diastereoisomerically and enantiomerically pure form.

Diastereoselective Ireland-Claisen rearrangements of substituted allyl ß-amino esters: applications in the asymmetric synthesis of C(5)-substituted transpentacins.

The diastereoselective Ireland-Claisen rearrangement of a range of substituted allyl ß-amino esters gave the corresponding enantiopure α-substituted-ß-amino esters with good diastereoselectivity. The application of this methodology in the asymmetric synthesis of a range of C(5)-substituted 1,2-anti-1,5-syn-transpentacins was demonstrated by the rearrangement of a range of ß-amino esters derived from sorbic acid, followed by esterification, ring-closing metathesis, hydrogenolytic deprotection/reduction, and hydrolysis, which gave the C(5)-substituted transpentacins in only 9 steps from commercially available starting materials.

Development of an inhibitor of the mutagenic SOS response that suppresses the evolution of quinolone antibiotic resistance.

Antimicrobial resistance (AMR) is a growing threat to health globally, with the potential to render numerous medical procedures so dangerous as to be impractical. There is therefore an urgent need for new molecules that function through novel mechanisms of action to combat AMR. The bacterial DNA-repair and SOS-response pathways promote survival of pathogens in infection settings and also activate hypermutation and resistance mechanisms, making these pathways attractive targets for new therapeutics. Small molecules, such as IMP-1700, potentiate DNA damage and inhibit the SOS response in methicillin-resistant S. aureus; however, understanding of the structure-activity relationship (SAR) of this series is lacking. We report here the first comprehensive SAR study of the IMP-1700 scaffold, identifying key pharmacophoric groups and delivering the most potent analogue reported to date, OXF-077. Furthermore, we demonstrate that as a potent inhibitor of the mutagenic SOS response, OXF-077 suppresses the rate of ciprofloxacin resistance emergence in S. aureus. This work supports SOS-response inhibitors as a novel means to combat AMR, and delivers OXF-077 as a tool molecule for future development.