Synthesis of sparteine-like chiral diamines and evaluation in the enantioselective lithiation–substitution of N-(tert-butoxycarbonyl)pyrrolidine

A safe and efficient synthesis of N-Boc-β3-amino acid methyl esters from α-amino acids: applications in the formal synthesis of sedum alkaloids

β3-Amino acids are essential components in the synthesis of biologically active compounds. However, obtaining them in enantiomerically pure forms remains challenging. This study investigates a safe and efficient method for synthesizing enantiopure N-Boc-β3-amino acid methyl esters, incorporating both natural and unnatural side chains. The procedure avoids the use of expensive and toxic reagents, providing a safer alternative to the hazardous Arndt-Eistert homologation and cyanation reactions, which typically begin with enantiopure α-amino acids. The practical value of this transformation was demonstrated in the formal synthesis of sedum alkaloids.

The Chemistry of the Defensive Secretions of Three Species of Millipedes in the Genus Brachycybe

Millipedes have long been known to produce a diverse array of chemical defense agents that deter predation. These compounds, or their precursors, are stored in high concentration within glands (ozadenes) and are released upon disturbance. The subterclass Colobognatha contains four orders of millipedes, all of which are known to produce terpenoid alkaloids-spare the Siphonophorida that produce terpenes. Although these compounds represent some of the most structurally-intriguing millipede-derived natural products, they are the least studied class of millipede defensive secretions. Here, we describe the chemistry of millipede defensive secretions from three species of Brachycybe: Brachycybe producta, Brachycybe petasata, and Brachycybe rosea. Chemical investigations using mass spectrometry-based metabolomics, chemical synthesis, and 2D NMR led to the identification of five alkaloids, three of which are new to the literature. All identified compounds are monoterpene alkaloids with the new compounds representing indolizidine (i.e. hydrogosodesmine) and quinolizidine alkaloids (i.e. homogosodesmine and homo-hydrogosodesmine). The chemical diversity of these compounds tracks the known species phylogeny of this genus, rather than the geographical proximity of the species. The indolizidines and quinolizidines are produced by non-sympatric sister species, B. producta and B. petasata, while deoxybuzonamine is produced by another set of non-sympatric sister species, B. rosea and Brachycybe lecontii. The fidelity between the chemical diversity and phylogeny strongly suggests that millipedes generate these complex defensive agents de novo and begins to provide insights into the evolution of their biochemical pathways.

Enantioselective Synthesis of Phosphine Boranes via Crystallization-Induced Dynamic Resolution of Lithiated Intermediate by Understanding the Underlying Epimerization Process

Described herein is the successful crystallization-induced dynamic resolution (CIDR) of an α-lithiated phosphine borane utilizing the easily accessible and inexpensive ligand (R,R)-TMCDA. Starting from the essential P-prochiral building block dimethyl phenyl phosphine borane we were able to obtain phosphine boranes in yields up to 80 % and e.r. up to 98 : 2 by crystallization of the lithiated intermediate prior to the trapping reaction. NMR-based deuterium labeling experiments indicate that the epimerization in solution is based on the intermolecular proton transfer between nonlithiated phosphine borane and the corresponding lithiated intermediate, rendering the presence of the remaining starting compound in an optimized solvent mixture the main factor for successful enantioselective synthesis. Quantum chemical calculations using different model systems based on solid state structures confirm these experimental results. By gaining insights into the epimerization mechanism, essential principles for CIDR of lithiated phosphine boranes are elucidated that may be expanded to other important P-stereogenic compounds and simple chiral amines.

Gosodesmine, a 7-Substituted Hexahydroindolizine from the Millipede Gosodesmus claremontus

Millipedes (Diplopoda) are well known for their toxic or repellent defensive secretions. Here we describe gosodesmine (1), 7-(4-methylpent-3-en-1-yl)-1,2,3,5,8,8a-hexahydroindolizine, a unique alkaloid with some terpene character found in the chemical defense secretions of the millipede Gosodesmus claremontus Chamberlin (Colobognatha, Platydesmida, Andrognathidae). The structure of 1 was suggested by its mass spectra and GC-FTIR spectra and established from its ¹H, ¹³C, and 2D NMR spectra and 1D NOE studies. The 7-substituted indolizidine carbon skeleton of 1 was confirmed by unambiguous synthesis. This is the first report of an alkaloid from a platydesmid millipede and the first report of a 7-substituted indolizidine from an arthropod.

Gram-Scale Synthesis of the (-)-Sparteine Surrogate and (-)-Sparteine

An 8-step, gram-scale synthesis of the (-)-sparteine surrogate (22 % yield, with just 3 chromatographic purifications) and a 10-step, gram-scale synthesis of (-)-sparteine (31 % yield) are reported. Both syntheses proceed with complete diastereocontrol and allow access to either antipode. Since the syntheses do not rely on natural product extraction, our work addresses long-term supply issues relating to these widely used chiral ligands.

Revisiting the sparteine surrogate: development of a resolution route to the (-)-sparteine surrogate

The improved performance of the sparteine surrogate compared to sparteine in a range of applications has highlighted the need to develop an approach to the (-)-sparteine surrogate, previously inaccessible in gram-quantities. A multi-gram scale, chromatography-free synthesis of the racemic sparteine surrogate and its resolution via diastereomeric salt formation with (-)-O,O'-di-p-toluoyl-l-tartaric acid is reported. Resolution on a 10.0 mmol scale gave the diastereomeric salts in 33% yield from which (-)-sparteine surrogate of 93 : 7 er was generated. This work solves a key limitation: either enantiomer of the sparteine surrogate can now be readily accessed.

Synthesis and asymmetric hydrogenation of (3E)-1-benzyl-3-[(2-oxopyridin-1(2H)-yl)methylidene]piperidine-2,6-dione

The synthesis of (3E)-1-benzyl-3-[(2-oxopyridin-1(2H)-yl)methylidene]piperidine-2,6-dione 5 from N-benzylglutarimide was achieved in three steps. The asymmetric hydrogenation of 4 gave either the product of partial reduction (10) or full reduction (13), depending on the catalyst which was employed, in high ee in each case. Attempts at asymmetric transfer hydrogenation (ATH) of resulted in formation of a racemic product.

Synthesis of new fluorinated, 2-substituted 5-pyrrolidinylsulfonyl isatin derivatives as caspase-3 and caspase-7 inhibitors: nonradioactive counterparts of putative PET-compatible apoptosis imaging agents

Downstream caspases-3 and -7 are essential to execute the programmed type I cell death (apoptosis). In order to better understand their role, specific inhibitors of these enzymes are required, which after radiolabeling can be applied to non-invasively visualize and monitor apoptotic pathways in vivo using Positron Emission Tomography (PET). Therefore, 2-methoxyethyl-, 2-methoxypropyl-, 2-ethoxymethyl-, 2-(2-fluoroethoxymethyl)-, and 2-(2,2,2-trifluoroethoxymethyl)pyrrolidinyl analogues of (S)-5-[1-(2-methoxymethylpyrrolidinyl)sulfonyl]isatin (2) were prepared and their in vitro binding affinities towards caspases-1, -3, -6 and -7 were evaluated and compared to that of the lead structure 2. While the inhibition potencies against caspases-1 and -6 were in the micromolar range, all synthesized compounds exhibited excellent and selective inhibition of caspases-3 and -7 in the nanomolar range up to IC50=4.79 nM and 7.47 nM, respectively. These highly potent 2-substituted analogues of 2 might be developed as anti-apoptosis agents and some selected fluorinated inhibitors might be useful as potential PET radiotracers for apoptosis imaging after (18)F-labeling.

Optically active (aR)- and (aS)-linear and vaulted biaryl ligands: deracemization versus oxidative dimerization

The copper-mediated deracemization of the C2-symmetric vaulted biaryl ligands VANOL and VAPOL has been investigated. In the course of the studies that have led to a more reliable procedure for this process, an unprecedented oxidative dimerization of these ligands has been uncovered. The structures of these oxidative dimerization products were elucidated by a series of NMR experiments, and these assignments were supported by other spectroscopic techniques as well as their chemical reactivity. This oxidative dimerization process was not observed for the linear biaryl ligands BANOL and BINOL, although the new deracemization procedure was effective for the generation of BINOL with high optical purity. The (aS)-enantiomers of BINOL, VANOL and VAPOL were accessible with a copper complex of (-)-sparteine, and the (aR)-enantiomeric series were accessible with a copper complex of O'Brien's diamine. Both (-)-sparteine and O'Brien's diamine give higher optical purities with VANOL and VAPOL than with BINOL, and this is consistent with the steric congestion present in the matched and mismatched copper complexes of these diamines with the biaryl ligands.

Structural features and reactivity of (sparteine)PdCl2: a model for selectivity in the oxidative kinetic resolution of secondary alcohols

The chiral ligand (-)-sparteine and PdCl(2) catalyze the enantioselective oxidation of secondary alcohols to ketones and thus effect a kinetic resolution. The structural features of sparteine that led to the selectivity observed in the reaction were not clear. Substitution experiments with pyridine derivatives and structural studies of the complexes generated were carried out on (sparteine)PdCl(2) and indicated that the C(1) symmetry of (-)-sparteine is essential to the location of substitution at the metal center. Palladium alkoxides were synthesized from secondary alcohols that are relevant steric models for the kinetic resolution. The solid-state structures of the alkoxides also confirmed the results from the pyridine derivative substitution studies. A model for enantioinduction was developed with C(1) symmetry and Cl(-) as key features. Further studies of the diastereomers of (-)-sparteine, (-)-alpha-iso- and (+)-beta-isosparteine, in the kinetic resolution showed that these C(2)-symmetric counterparts are inferior ligands in this stereoablative reaction [Mohr, J. T., Ebner, D. C., and Stoltz, B. M. Org. Biomol. Chem. 2007, 5, 3571-3576].

Total syntheses of the tylophora alkaloids cryptopleurine, (-)-antofine, (-)-tylophorine, and (-)-ficuseptine C

A concise, efficient and modular approach to the tylophora alkaloids is described, a family of potent cytotoxic agents that are equally effective against drug sensitive and multidrug resistant cancer cell lines. The advantages of the chosen route are illustrated by the total syntheses of the phenanthroquinolizidine cryptopleurine (1) and the phenanthroindolizidines (-)-antofine (2), (-)-tylophorine (3), and their only recently isolated congener (-)-ficuseptine C (4). The key steps consist in a Suzuki cross-coupling between a (commercial) boronic acid and a simple aryl-1,2-dihalide followed by elaboration of the resulting products into the corresponding 2-alkynyl-biphenyl derivatives 27, 33, 41 and 46. The latter undergo PtCl2-catalyzed cycloisomerizations with formation of the functionalized phenanthrenes 28, 34, 42 and 47, which were transformed into the targeted alkaloids by a deprotection/Pictet-Spengler annulation tandem. Due to the flexibility and robust character of this approach, it might enable a systematic exploration of the pharmacological profile of this promising class of bioactive natural products.

Reactivity series for s-BuLi/diamine-mediated lithiation of N-Boc pyrrolidine: applications in catalysis and lithiation of N-Boc piperidine

Using competition experiments between a range of ligands and (-)-sparteine, a reactivity series for N-Boc pyrrolidine lithiation using s-BuLi/diamines has been constructed; the results indicate that the s-BuLi/(+)-sparteine surrogate complex is more reactive than s-BuLi/(-)-sparteine and this has been exploited in the selection of ligand pairs for ligand exchange catalytic asymmetric lithiation of N-Boc pyrrolidine and lithiation of N-Boc piperidine.

Stereocontrolled Total Synthesis of (−)-Kainic Acid. Regio- and Stereoselective Lithiation of Pyrrolidine Ring with the (+)-Sparteine Surrogate

[reaction: see text] A stereocontrolled total synthesis of (-)-kainic acid is described. cis-3,4-Disubstituted pyrrolidine ring was constructed by [3 + 2] cycloaddition of azomethine ylide with chiral butenolide. The crucial introduction of carboxyl group at the C-2 position was executed by regio- and stereoselective lithiation of the pyrrolidine ring in the presence of a (+)-sparteine surrogate followed by trapping with carbon dioxide.

Dynamic kinetic resolution of N-Boc-2-lithiopyrrolidine

Asymmetric substitution of the organolithium derived either from N-Boc-2-tributylstannylpyrrolidine by tin-lithium exchange or from N-Boc-pyrrolidine by deprotonation occurs in the presence of a commercially available chiral diamine ligand with high levels of enantioselectivity by a dynamic kinetic resolution pathway.

A synthesis of (±)-sparteine

In a synthesis of racemic sparteine, Diels-Alder reaction between dimethyl bromomesaconate 14 and dicyclopentenyl 4, followed by cyclopropane formation, set up the stereochemistry at C-1 and C-5 as S and R, respectively, in a meso intermediate 8. The stereochemistry at C-2 and C-4 was then secured by a moderately diastereoselective protonation of the bis-enolate 17 derived from the diester 8 by reductive cleavage with lithium in liquid ammonia. The C=C in the racemic diester 19 was ozonolysed and the diketone converted by Beckmann rearrangement into the bis-lactam . Reduction of the bis-lactam with lithium aluminium hydride and intramolecular nucleophilic displacement gave racemic sparteine 1. Some ideas for making this synthesis amenable to a synthesis of enantiomerically enriched sparteine are presented.

An Experimental and Computational Study of the Enantioselective Lithiation of N-Boc-pyrrolidine Using Sparteine-like Chiral Diamines

The enantioselective lithiation of N-Boc-pyrrolidine using sec-butyllithium and isopropyllithium in the presence of sparteine-like diamines has been studied experimentally and computationally at various theoretical levels through to B3P86/6-31G*. Of the (-)-cytisine-derived diamines (N-Me, N-Et, N-(n)Bu, N-CH(2)(t)Bu, N-(i)Pr) studied experimentally, the highest enantioselectivity (er 95:5) was observed with the least sterically hindered N-Me-substituted diamine, leading to preferential removal of the pro-R proton i.e., opposite enantioselectivity to (-)-sparteine. The experimental result with the N-Me-substituted diamine correlated well with the computational results: at the B3P86/6-31G* level, the sense of induction was correctly predicted; the lowest energy complex of isopropyllithium/diamine/N-Boc-pyrrolidine also had the lowest activation energy (DeltaH++ = 11.1 kcal/mol, DeltaG++= 11.5 kcal/mol) for proton transfer. The computational results with the N-(i)Pr-substituted diamine identified a transition state for proton transfer with activation energies of DeltaH++= 11.7 kcal/mol and DeltaG++= 11.8 kcal/mol (at the B3P86/6-31G* level). Although comparable to (-)-sparteine and the N-Me-substituted diamine, these DeltaH++ and DeltaG++ values are at odds with the experimental observation that use of the N-(i)Pr-substituted diamine gave no product. It is suggested that steric crowding inhibits formation of the prelithiation complex rather than increasing the activation enthalpy for proton transfer in the transition state. Three other ligands (N-H and O-substituted as well as a five-membered ring analogue) were studied solely using computational methods, and the results predict that the observed enantioselectivity would be modest at best.