Enantioselective syntheses of 2-alkyl- and 2,6-dialkylpiperidine alkaloids: preparations of the hydrochlorides of (-)-coniine, (-)-solenopsin A, and (-)-dihydropinidine

Introducing a sulfone-embedded anhydride to the anhydride-imine reaction for the modular synthesis of N-heterocyclic sulfones bearing vicinal stereocenters

N-heterocyclic sulfones constitute the core of several pharmaceuticals, including the antityrpanosomal drug Nifurtimox. Their biological relevance and architectural complexity makes them valued targets and inspires the development of more selective and atom-economical strategies for their construction and post-modification. In this embodiment, we describe a flexible approach to sp³-rich N-heterocyclic sulfones, which hinges on the efficient annulation of a novel sulfone-embedded anhydride with 1,3-azadienes and aryl aldimines. Further elaboration of the lactam esters has facilitated the construction of a library of vicinally functionalized sulfone-embedded N-heterocycles.

Enhancement of l-Pipecolic Acid Production by Dynamic Control of Substrates and Multiple Copies of the pipA Gene in the Escherichia coli Genome

l-Pipecolic acid is an important rigid cyclic nonprotein amino acid, which is obtained through the conversion of l-lysine catalyzed by l-lysine cyclodeaminase (LCD). To directly produce l-pipecolic acid from glucose by microbial fermentation, in this study, a recombinant Escherichia coli strain with high efficiency of l-pipecolic acid production was constructed. This study involves the dynamic regulation of the substrate concentration and the expression level of the l-lysine cyclodeaminase-coding gene pipA. In terms of substrate concentration, we adopted the l-lysine riboswitch to dynamically regulate the expression of lysP and lysO genes. As a result, the l-pipecolic acid yield was increased about 1.8-fold as compared with the control. In addition, we used chemically inducible chromosomal evolution (CIChE) to realize the presence of multiple copies of the pipA gene on the genome. The resultant E. coli strain XQ-11-4 produced 61 ± 3.4 g/L l-pipecolic acid with a productivity of 1.02 ± 0.06 g/(L·h) and a glucose conversion efficiency (α) of 29.6% in fermentation. This is the first report that discovered multiple copies of pipA gene expression on the genome that improves the efficiency of l-pipecolic acid production in an l-lysine high-producing strain, and these results give us new insight for constructing the other valuable biochemicals derived from l-lysine.

Rhodium-Catalyzed Asymmetric Hydrogenation of 3-Benzoylaminocoumarins for the Synthesis of Chiral 3-Amino Dihydrocoumarins

An asymmetric hydrogenation of 3-benzoylaminocoumarins was achieved for the first time using our BridgePhos-Rh catalytic system, providing chiral 3-amino dihydrocoumarins in high yields (up to 98 %) and with excellent enantioselectivities (up to 99.7 % ee). The relationship between the enantioselectivities of the hydrogenations and the dihedral angles and the resulting π-π stacking effects of the BridgePhos-Rh complexes, which were determined by X-ray diffraction analysis, are discussed. The corresponding hydrogenated products allow for many transformations, providing several chiral skeletons with important physiological and pharmacological activities.

Structural Basis for Recognition of L-lysine, L-ornithine, and L-2,4-diamino Butyric Acid by Lysine Cyclodeaminase

L-pipecolic acid is a non-protein amino acid commonly found in plants, animals, and microorganisms. It is a well-known precursor to numerous microbial secondary metabolites and pharmaceuticals, including anticancer agents, immunosuppressants, and several antibiotics. Lysine cyclodeaminase (LCD) catalyzes β-deamination of L-lysine into L-pipecolic acid using β-nicotinamide adenine dinucleotide as a cofactor. Expression of a human homolog of LCD, μ-crystallin, is elevated in prostate cancer patients. To understand the structural features and catalytic mechanisms of LCD, we determined the crystal structures of Streptomyces pristinaespiralis LCD (SpLCD) in (i) a binary complex with NAD+, (ii) a ternary complex with NAD+ and L-pipecolic acid, (iii) a ternary complex with NAD+ and L-proline, and (iv) a ternary complex with NAD+ and L-2,4-diamino butyric acid. The overall structure of SpLCD was similar to that of ornithine cyclodeaminase from Pseudomonas putida. In addition, SpLCD recognized L-lysine, L-ornithine, and L-2,4-diamino butyric acid despite differences in the active site, including differences in hydrogen bonding by Asp236, which corresponds with Asp228 from Pseudomonas putida ornithine cyclodeaminase. The substrate binding pocket of SpLCD allowed substrates smaller than lysine to bind, thus enabling binding to ornithine and L-2,4-diamino butyric acid. Our structural and biochemical data facilitate a detailed understanding of substrate and product recognition, thus providing evidence for a reaction mechanism for SpLCD. The proposed mechanism is unusual in that NAD+ is initially converted into NADH and then reverted back into NAD+ at a late stage of the reaction.

Expanding metabolic pathway for de novo biosynthesis of the chiral pharmaceutical intermediate L-pipecolic acid in Escherichia coli

BACKGROUND

The six-carbon circular non-proteinogenic compound L-pipecolic acid is an important chiral drug intermediate with many applications in the pharmaceutical industry. In the present study, we developed a metabolically engineered strain of Escherichia coli for the overproduction of L-pipecolic acid from glucose.

RESULTS

The metabolic pathway from L-lysine to L-pipecolic acid was constructed initially by introducing lysine cyclodeaminase (LCD). Next, L-lysine metabolic flux from glucose was amplified by the plasmid-based overexpression of dapA, lysC, and lysA under the control of the strong trc promoter to increase the biosynthetic pool of the precursor L-lysine. Additionally, since the catalytic efficiency of the key enzyme LCD is limited by the cofactor NAD⁺, the intracellular pyridine nucleotide concentration was rebalanced by expressing the pntAB gene encoding the transhydrogenase, which elevated the proportion of LCD with bound NAD⁺ and enhanced L-pipecolic acid production significantly. Further, optimization of Fe²⁺ and surfactant in the fermentation process resulted in 5.33 g/L L-pipecolic acid, with a yield of 0.13 g/g of glucose via fed-batch cultivation.

CONCLUSIONS

We expanded the metabolic pathway for the synthesis of the chiral pharmaceutical intermediate L-pipecolic acid in E. coli. Using the engineered E. coli, a fast and efficient fermentative production of L-pipecolic acid was achieved. This strategy could be applied to the biosynthesis of other commercially and industrially important chiral compounds containing piperidine rings.

Expedient and modular access to 2-azabicyclic architectures by iron-catalyzed dehydrative coupling of alcohol-bearing allylic lactams

A high-yielding, cost-effective, modular and environmentally benign approach to [5,5]- or [6,5]-2-azabicyclic architectures bearing vicinal stereocenters, by Fe-catalyzed dehydrative coupling of readily affordable alcohol-bearing allylic lactams is presented.

One-pot multiple reactions: asymmetric synthesis of 2,6-cis-disubstituted piperidine alkaloids from chiral aziridine

A divergent, new, and highly stereoselective synthesis of cis-2,6-disubstituted piperidine natural products including isosolenopsins, deoxocassine, and spectaline was achieved from chiral aziridine decorated with appropriate alkyl chains for isosolenopsins or alkynyl groups for deoxocassine and spectaline at C2. The characteristic feature of this synthesis is one-pot sequential reactions under atmospheric hydrogen including the reduction of alkyne (for deoxocassine and spectaline), reductive ring-opening of aziridine, debenzylation, and intramolecular reductive amination in high yields. The prerequisite aziridines were elaborated from commercially available (2S)-hydroxymethylaziridine through oxidation, Wittig olefination, and the Grignard reaction for isosolenopsins or substrate-controlled lithium alkynylate addition for deoxocassine and spectaline.

Enantioselective synthesis and antioxidant activity of 3,4,5-substituted piperidine derivatives

In this study, 3,4,5-trisubstituted piperidines were synthesized enantioselectively, and their antioxidant activity was evaluated. The 3,4,5-trisubstituted piperidines containing TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) and a spatially proximal hydroxy group showed good antioxidant activity. Some of these compounds showed IC50 values in a nanomolar range, comparable to that of TEMPO. Probably the TEMPO generated from the homolysis of the CON bond of 3,4,5-trisubstituted piperidines functions as a radical-scavenging entity, and the hydroxy group of piperidines has a synergistic effect to the antioxidant activity.

The design of 8-hydroxyquinoline tetracyclic lactams as HIV-1 integrase strand transfer inhibitors

A novel series of HIV-1 integrase strand transfer inhibitors were designed using the venerable two-metal binding pharmacophore model and incorporating structural elements from two different literature scaffolds. This manuscript describes a number of 8-hydroxyquinoline tetracyclic lactams with exceptional antiviral activity against HIV-1 and little loss of potency against the IN signature resistance mutations Q148K and G140S/Q148H.

Stereocontrolled synthesis of vicinally functionalized piperidines by nucleophilic β-addition of alkyllithiums to α-aryl substituted piperidine enecarbamates

Substituted piperidines are emerging as important medicinally-active structural motifs. Here, we report highly stereoselective carbolithiation reactions of α-aryl piperidine enecarbamates that offer direct access to vicinally-substituted piperidine compounds. We have also demonstrated that the carbanion intermediates can be trapped with a carbon electrophile.

Stereochemical Course of Deprotonation-Acylation of N-Boc- and N-Carbamoyl-2-cyano-6-methylpiperidines

The stereochemical course of electrophilic substitution of α-nitrile metallocarbanions generated by deprotonation from N-Boc- and N-carbamoyl-2-cyano-6-methylpiperidines was investigated. Deprotonation in the presence of an electrophile taking advantage of the high acidity of α-nitrile protons allowed examination of the effects of a chelating group on the nitrogen atom, a countercation, and the reactivity of an electrophile on the steric course. Analyses of reactions using aroyl chlorides and methyl iodide revealed the following: (1) the substitution reactions basically proceed with retention of configuration, (2) the extent of an inversion product increases with decreasing chelating ability of the N-substituent and with increasing leaving ability (ionic character) of a countercation (Li, Na, K) of the anionic species, and (3) the use of a more reactive electrophile results in an increase of the retention product.

One-shot access to α,β-difunctionalized azepenes and dehydropiperidines by reductive cross-coupling of α-selenonyl-β-selenyl enamides with organic bromides

The synthesis of α- and α,β-functionalized azepenes and dehydropiperidines from readily prepared α-selenonyl eneformamides or enecarbamates has been achieved through Fe-catalyzed α-substitutive deselenonation, β-regioselective lithiation/trapping, and Co-catalyzed reductive cross-coupling protocols.

Solenopsin A and analogs exhibit ceramide-like biological activity

BACKGROUND

(-)-Solenopsin A is a piperidine alkaloid that is a component of the venom of the fire ant Solenopsis invicta. Previously, we have demonstrated that solenopsin exhibit anti-angiogenic activity and downregulate phosphoinositol-3 kinase (PI3K) in the p53 deficient renal cell carcinoma cell line 786-O. Solenopsin has structural similarities to ceramide, a major endogenous regulator of cell signaling and cancer therapy induced apoptosis.

METHODS

Different analogs of solenopsin were synthesized in order to explore structure-activity relationships. The anti-proliferative effect of solenopsin and analogs was tested on six different cell lines, including three tumor cell lines, two normal cutaneous cell lines, and one immortalized hyperproliferative cell line. FRET-based reporters were used to study the affect of solenopsin and analogs on Akt activity and PDK1 activation and sucrose density gradient fractionation was performed to examine recruitment of PTEN to membrane rafts. Western-blotting was used to evaluate the affect of solenopsin and analogs on the Akt and the MAPK 44/42 pathways in three different tumor cell lines. Measurement of cellular oxygen consumption rate together with autophagy staining was performed to study mitochondrial function. Finally, the affect of solenopsin and analogs on ROS production was investigated.

RESULTS

In this paper we demonstrate that solenopsin analogs with potent anti-proliferative effects can be synthesized from inexpensive dimethylpyridines. To determine whether solenopsin and analogs act as ceramide analogs, we examined the effect of solenopsin and analogs on two stereotypic sites of ceramide activity, namely at lipid rafts and mitochondria. We found that native solenopsin, (-)-solenopsin A, inhibits functional Akt activity and PDK1 activation in lipid rafts in a similar fashion as ceramide. Both cis and trans analogs of solenopsin reduce mitochondrial oxygen consumption, increase reactive oxygen, and kill tumor cells with elevated levels of Akt phosphorylation. However, only solenopsin induces mitophagy, like ceramide.

CONCLUSIONS

The requirements for ceramide induced mitophagy and inhibition of Akt activity and PDK1 activation in lipid rafts are under strict stereochemical control. The naturally occurring (-)-solenopsin A mimic some of the functions of ceramide and may be therapeutically useful in the treatment of hyperproliferative and malignant disorders of the skin, even in the presence of elevated levels of Akt.

Generation and electrophile trapping of N-Boc-2-lithio-2-azetine: synthesis of 2-substituted 2-azetines

s-BuLi-induced α-lithiation–elimination of LiOMe from N-Boc-3-methoxyazetidine and further in situ α-lithiation generates N-Boc-2-lithio-2-azetine which can be trapped with electrophiles, either directly (carbonyl or heteroatom electrophiles) or after transmetalation to copper (allowing allylations and propargylations), providing a concise access to 2-substituted 2-azetines.

Control of asymmetry in the radical addition approach to chiral amine synthesis

The state-of-the-science in asymmetric free radical additions to imino compounds is presented, beginning with an overview of methods involving stereocontrol by various chiral auxiliary approaches. Chiral N-acylhydrazones are discussed with respect to their use as radical acceptors for Mn-mediated intermolecular additions, from design to scope surveys to applications to biologically active targets. A variety of aldehydes and ketones serve as viable precursors for the chiral hydrazones, and a variety of alkyl iodides may be employed as radical precursors, as discussed in a critical review of the functional group compatibility of the reaction. Applications to amino acid and alkaloid synthesis are presented to illustrate the synthetic potential of these versatile stereocontrolled carbon-carbon bond construction reactions. Asymmetric catalysis is discussed, from seminal work on the stereocontrol of radical addition to imino compounds by non-covalent interactions with stoichiometric amounts of catalysts, to more recent examples demonstrating catalyst turnover.

Stereoselective syntheses of L-pipecolic acid and (2S,3S)-3-hydroxypipecolic acid from a chiral N-imino-2-phenyl-1,2-dihydropyridine intermediate

Stereoselective syntheses of L-pipecolic acid and (2S,3S)-3-hydroxypipecolic acid were achieved from a chiral N-imino-2-phenyl-1,2-dihydropyridine intermediate. The 3-hydroxy substituent of the latter amino acid was introduced by hetero-Diels-Alder reaction of singlet oxygen with the 1,2-dihydropyridine.

The Pauson-Khand Reaction as a New Entry to the Synthesis of Bridged Bicyclic Heterocycles: Application to the Enantioselective Total Synthesis of (-)-Alstonerine

The first application of the Pauson-Khand reaction (PKR) to the synthesis of azabridged bicyclic structures is described. Compounds containing azabicyclo[3.3.1]nonane and azabicyclo[3.2.1]octane rings fused to cyclopentenones were efficiently constructed via the PKR of cis-2,6-disubstituted N-acyl piperidine enyne substrates, many of which can be readily prepared from 4-methoxypyridine in a few steps. Moreover, the PKR of cis-2,6-disubstituted piperazine enynes allowed the preparation of diazabicyclo[3.3.1]nonanes fused to cyclopentenones. This new strategy for the synthesis of azabridged bicyclic frameworks was exploited as a key step in a concise, enantioselective total synthesis of the macroline alklaoid (-)-alstonerine.

Asymmetric alpha-alkynylation of piperidine via N-sulfinyliminium salts

Piperidine was stereoselectively alpha-alkynylated in a four-step sequence made up of transformation to a chiral nonracemic N-sulfinylpiperidine, anodic oxidation to N-sulfinyliminium ion equivalent, alkynylation through addition of a mixed organoaluminum derivative, and final acidic deprotection of the sulfoxide. Overall yields are around 50%, and the diastereoselectivity of the nucleophilc addition was between 92 and 99% de, allowing isolation of the final product with 99% enantiomeric purity.