Mechanism and Stereochemical Features in Asymmetric Deprotonation Using RLi/(–)‐Sparteine Bases

Metabolic engineering of narrow-leafed lupin for the production of enantiomerically pure (-)-sparteine

The protein crops known as lupins have been bred to accumulate low levels of antinutritional alkaloids, neglecting their potential as sources of valuable metabolites. Here, we engineered narrow-leafed lupin (NLL) to accumulate large amounts of a single alkaloid of industrial interest called (-)-sparteine. While (-)-sparteine is recognized as a key auxiliary molecule in chiral synthesis, its variable price and limited availability have prevented its large-scale use. We identified two enzymes that initiate the conversion of (-)-sparteine to a variety of alkaloids accumulating in NLL. The first one is a cytochrome P450 monooxygenase belonging to family 71 (CYP71D189), and the second one is a short-chain dehydrogenase/reductase (SDR1). We screened a non-GMO NLL mutant library and isolated a knockout in CYP71D189. The knockout displayed an altered metabolic profile where (-)-sparteine accounted for 96% of the alkaloid content in the seeds (GC-MS basis). The (-)-sparteine isolated from the mutant seeds was enantiomerically pure (99% enantiomeric excess). Apart from the altered alkaloid profile, the mutant did not have any noticeable phenotype. Our work demonstrates that (-)-sparteine is the precursor of most QAs in NLL and expands the current uses of NLL as a crop.

The Enantioselective Total Synthesis of Bisquinolizidine Alkaloids: A Modular "Inside-Out" Approach

Bisquinolizidine alkaloids are characterized by a chiral bispidine core (3,7-diazabicyclo[3.3.1]nonane) to which combinations of an α,N-fused 2-pyridone, an endo- or exo-α,N-annulated piperidin(on)e, and an exo-allyl substituent are attached. We developed a modular "inside-out" approach that permits access to most members of this class. Its applicability was proven in the asymmetric synthesis of 21 natural bisquinolizidine alkaloids, among them more than ten first enantioselective total syntheses. Key steps are the first successful preparation of both enantiomers of C₂ -symmetric 2,6-dioxobispidine by desymmetrization of a 2,4,6,8-tetraoxo precursor, the construction of the α,N-fused 2-pyridone by using an enamine-bromoacrylic acid strategy, and the installation of endo- or, optionally, exo-annulated piperidin(on)es.

Discovery of temperature-dependent, autoinductive reversal of enantioselectivity: palladium-mediated [3+3]-annulation of 4-hydroxycoumarins

An unusual temperature-dependent autoinductive reversal of enantioselectivity (TARE) was discovered in an asymmetric palladium-mediated [3+3]-annulation of 4-hydroxycoumarin with Morita–Baylis–Hillman acetate.

The Use of (-)-Sparteine/Organolithium Reagents for the Enantioselective Lithiation of 7,8-Dipropyltetrathia[7]helicene: Single and Double Kinetic Resolution Procedures

The effect of organolithium reagent (RLi: R=nBu, iPr, sBu, tBu), solvent (diethyl ether, diethyl ether/THF and MTBE), and stoichiometry on the (-)-sparteine-mediated silylation of 7,8-dipropyltetrathia[7]helicene shows that, unusually, substantially more than 0.5 equivalent of RLi (R=iPr, sBu, tBu) and a large excess of (-)-sparteine (R=nBu, sBu) is often needed to achieve substantial conversions and good ee values. With nBuLi, however, just one equivalent of the organolithium reagent is sufficient to obtain high conversions. Our best results were obtained using the convenient tBuLi/(-)-sparteine adduct with which the need for a high (-)-sparteine/RLi ratio can be avoided. Single- and double-kinetic resolution (KR) procedures give enantiopure samples of 2-trimethylsilyl- and 2,13-di(trimethylsilyl)-7,8-dipropyltetrathia[7]helicene and two-step double-KR combining (-)-sparteine/sBuLi and chiral formamides affords the synthetically valuable 2-formyl-7,8-dipropyltetrathia[7]helicene. This is the first use of (-)-sparteine for the enantioselective lithiation of helicenes and the first report of tBuLi outperforming sBuLi in a (-)-sparteine-mediated procedure.

“The Great Beauty” of organolithium chemistry: a land still worth exploring

This Frontier highlights and analyses, from the authors’ perspective, some recent selected contributions from the organolithium field with an eye towards new challenging research directions.

Preparation and reactions of enantiomerically pure α-functionalized Grignard reagents

A strategy for the generation of enantiomerically pure α-functionalized chiral Grignard reagents is presented. The approach involves the synthesis of α-alkoxy and α-amino sulfoxides in ≥99:1 dr and ≥99:1 er via asymmetric deprotonation (s-BuLi/chiral diamine) and trapping with Andersen's sulfinate (menthol derived). Subsequent sulfoxide → Mg exchange (room temperature, 1 min) and electrophilic trapping delivers a range of enantiomerically pure α-alkoxy and α-amino substituted products. Using this approach, either enantiomer of products can be accessed in 99:1 er from asymmetric deprotonation protocols without the use of (-)-sparteine as the chiral ligand. Two additional discoveries are noteworthy: (i) for the deprotonation and trapping with Andersen's sulfinate, there is a lack of stereospecificity at sulfur due to attack of a lithiated intermediate onto the α-alkoxy and α-amino sulfoxides as they form, and (ii) the α-alkoxy-substituted Grignard reagent is configurationally stable at room temperature for 30 min.

Dynamics of catalytic resolution of 2-lithio-N-Boc-piperidine by ligand exchange

The dynamics of the racemization and catalytic and stoichiometric dynamic resolution of 2-lithio-N-Boc-piperidine (7) have been investigated. The kinetic order in tetramethylethylenediamine (TMEDA) for both racemization and resolution of the title compound and the kinetic orders in two resolving ligands have been determined. The catalytic dynamic resolution is second order in TMEDA and 0.5 and 0.265 order in chiral ligands 8 and 10, respectively. The X-ray crystal structure of ligand 10 shows it to be an octamer. Dynamic NMR studies of the resolution process were carried out. Some of the requirements for a successful catalytic dynamic resolution by ligand exchange have been identified.