Enantioselective synthesis of (R)-Sumanirole using organocatalytic asymmetric aziridination of an α,β-unsaturated aldehyde

Herein we report an enantioselective synthesis of (R)-Sumanirole wherein an organocatalytic asymmetric aziridination of 2-nitrocinnamaldehyde was the key step.     

Enantioselective formal synthesis of (−)-ovalicin using quinic acid as a chiral template

The key intermediate 18 for the synthesis of (−)-ovalicin was synthesized using (−)-quinic acid as the chiral source, through a series of stereocontrolled and efficient chemical reactions, thus establishing a new, formal synthesis of the natural target. The featuring spirocyclic epoxide function has been installed by internal Williamson ether synthesis using the functionalities originally present at C-1 of (−)-quinic acid after the appropriate adjustments required to introduce the necessary functionality at C-2.     

Enantioselective organocatalytic formal allylation of α-branched aldehydes

Heteroarylvinyl sulfone 1 has been successfully used as a new sulfonyl Michael acceptor in aminocatalytic reactions with branched aldehydes. Subsequent one-pot Julia-Kocienski olefination allows the challenging preparation of enantiomerically pure α-allylated aldehydes bearing C-α quaternary carbons.     

Synthesis of (−)-benzolactam-V8 by application of asymmetric aziridination

(?)-Benzolactam-V8, an artificially-designed cyclic dipeptide with strong tumor-promoter activity, was synthesized from benzyl (S)-N-(2-formylphenyl)-N-methylvalinate by application of guanidinium ylide-participated asymmetric aziridination followed by the reductive ring-opening reaction of 3-arylaziridine-2-carboxylate formed.     

Asymmetric total synthesis of (+)-isocryptotanshinone and formal synthesis of (−)-cryptotanshinone

The first asymmetric total synthesis of (+)-isocryptotanshinone was achieved in 12 linear steps with 12% overall yield from commercially available dihydrobenzopyrone. The key step was a base-mediated cyclization reaction. In addition, the synthetic strategy included the formal synthesis of (−)-cryptotanshinone.     

An efficient and practical asymmetric synthesis of (−)-tasimelteon

Tasimelteon is a useful drug for the treatment of Non-24 in totally blind individuals. An efficient and convenient asymmetric synthesis of tasimelteon has been developed from 4-vinyl-2,3-dihydrobenzofuran through six steps in 53% overall yield using the Corey–Bakshi–Shibata (CBS0 asymmetric reduction of ketone as a key step.     

An expeditious formal synthesis of (−)-epibatidine

A short route to (−)-epibatidine is described. It includes a hetero-Diels–Alder reaction of a chiral non-racemic acyl-nitroso derivative and 2-t-butyldimethylsilyloxycyclohexadiene followed by chemical manipulations to furnish a known precursor of the natural product in 11% overall yield.     

Enantioselective synthesis of (−)-γ-jasmolactone

The title compound was prepared in seven steps starting from the commercially available 4-ketopimelic acid. The key step features an enantioselective lactonization promoted by PPL.     

Enantioselective synthesis of β-hydroxy amines and aziridines using asymmetric transfer hydrogenation of α-amido ketones

A rapid, expedient and enantioselective method for the synthesis of β-hydroxy amines and monosubstituted aziridines in up to 99% e.e., via asymmetric transfer hydrogenation of α-amido ketones, is described.     

Enantioselective synthesis of gabapentin analogues via organocatalytic asymmetric Michael addition of α-branched aldehydes to β-nitroacrylates

Michael addition reaction of α-branched aldehydes to β-nitroacrylates was successfully carried out by using a mixed catalyst consisting of a primary amino acid, L-phenylalanine, and its lithium salt to give β-formyl-β'-nitroesters having a quaternary carbon centre in good yields (up to 85%) with high enantioselectivity (up to 98% ee). By using benzyl β-nitroacrylates as Michael acceptors, the obtained β-formyl-β'-nitroesters were converted into various 4,4-disubstituted pyrrolidine-3-carboxylic acids including analogues of gabapentin (Neurotin(?)) in one step from the Michael adducts in high yields.     

Enantioselective synthesis of the quinolizidine alkaloids (+)-myrtine and (−)-epimyrtine

The enantioselective synthesis of (+)-myrtine 1 and (−)-epimyrtine 2 is described starting from (S)-2-(2-hydroxypropyl)allyltrimethylsilane 4 using an intramolecular allylsilane N-acyliminium ion cyclization.     

De novo asymmetric synthesis of (−)-nanaomycin A

The de novo asymmetric total synthesis of (?)-nanaomycin A is described. The entirely linear route required only 13 steps from commercially available starting materials (3% overall yield). Key transformations include a Claisen rearrangement, an asymmetric dihydroxylation, a regioselective tosylation, a diastereoselective intramolecular Friedel-Crafts alkylation and a nitrile hydrolysis. As the route relies on a Sharpless asymmetric dihydroxylation it is equally amenable for the synthesis of (+)-nanaomycin A.     

Enantioselective organocatalytic α-sulfamidation of aldehydes using sulfonyl azides

Enantioselective organocatalytic α-sulfamidation of unbranched aldehydes is described using MacMillan’s second-generation imidazolidinone catalyst and o-nitrobenzenesulfonyl azide. The reactions are highly stereoselective (89.9–96.3% ee) with yields up to 71%. A strong correlation between aldehyde structure and product yield was found to exist, with 3-arylpropanals providing the best results. Application to functionalized amino acid synthesis is presented.     

Enantioselective synthesis of herbertane sesquiterpenes: synthesis of (−)-α-formylherbertenol

The synthesis of 4-hydroxy-3-[(1S)-1,2,2-trimethylcyclopentyl]benzaldehyde [(−)-α-formylherbertenol 1], a herbertane-type sesquiterpene isolated from the leafy liverwort Herberta adunca, from β-cyclogeraniol is described. The synthesis is based on the previously described preparation of an enantiopure 1,2,2-trimethylcyclopentane synthon from which the characteristic aromatic moiety of 1 is elaborated, using a Robinson annulation and a palladium-catalysed methoxycarbonylation of an aryl triflate as key synthetic steps. The synthesis of the natural sesquiterpene (−)-α-herbertenol, also a natural sequiterpene, using the same methodology is also described.     

Total synthesis of (−)-cleistenolide and formal synthesis of herbarumin I via a diastereoselective modulable allylation

A modulable tin based allylation method for the synthesis of 1,2,3-triols is described. The optimization of the reaction was aided by ¹H and ¹¹⁹Sn low temperature NMR spectroscopic investigations, which support the formation of two cyclic intermediates after transmetallation. Depending on the nature of the Lewis acid, either syn/anti or anti/syn configured triols could be obtained with good stereocontrol. To demonstrate the value of this methodology and the resulting scaffolds, they were used to install the signature triol motifs of (?)-cleistenolide and of herbarumin I.     

Enantioselective Synthesis and Racemization of (−)-Sinoracutine

Sinoracutine is a recently isolated alkaloid with unusual stereochemical and biological properties. It features an unprecedented tetracyclic 6/6/5/5 skeleton that bears an N-methylpyrrolidine ring fused to a cyclopentenone. Interestingly, both enantiomers of sinoracutine have been independently isolated from the same plant, yet the molecule does not appear to occur as a racemate. Here, we present a short synthesis of (−)-sinoracutine that relies on a highly diastereoselective Pauson–Khand reaction and a Mandai–Claisen reaction to install the quaternary stereocenter. Our work establishes the absolute configuration of the levorotatory isomer and suggests that the optical purity of sinoracutine varies in nature due to its gradual racemization. Experimental evidence supports this proposal, and a plausible mechanism for the racemization is provided.     

Total synthesis of (−) verbenalol and (−) epiverbenalol

The first asymmetric synthesis of (−) verbenalol and (−) epiverbenalol, starting from the organometallic complex (−) 1, is described.     

Enantioselective synthesis of γ-lactones from thioglycolic acid: Syntheses of (−)-muricatacin and 5-epi-(−)-muricatacin

An approach for the enantioselective synthesis of functionalized γ-lactones and its application to the syntheses of (−)-muricatacin 1a and 5-epi-(−)-muricatacin 1b is reported. A sequential oxidation of the intermediate 4 with m-chloroperoxybenzoic acid was conducted to realize the reaction mechanism.     

Nitrogen cation-π interactions in asymmetric organocatalytic synthesis

Cation-π interactions have been widely exploited and utilised in the structural biology arena, their fundamental importance in supramolecular chemistry and the pivotal role they play in host guest chemistry has rapidly expanded. In terms of organic synthesis π-π, CH-π and cation-π interactions are often invoked providing hypotheses for observed selectivities and reaction outcomes although fundamental studies of these interactions are less well reported, especially in the organic synthesis arena. This article considers cation-π interactions in the field of asymmetric organocatalysis and provides a summary of cases where such interactions may play an important role. Importantly this article sets out to highlight where such interactions could be operating in order to highlight the potential wealth of investigations to be had in this area rather than categorically claiming such interactions are in operation. For asymmetric catalysis this is particularly important as the geometry of a transition state dictates the stereochemical outcome of the reaction, this article provides a perspective on such phenomena.     

Synthesis of (−)-aphanorphine using a sulfur-directed aryl radical cyclization

Treatment of radical precursor 15a having a vinyl sulfide moiety with Bu₃SnH in the presence of AIBN in boiling benzene afforded exclusively the 6-exo cyclization product 16a, whereas similar treatment of the exo-methylene compound 15b gave a mixture of the 6-exo cyclization product 16b and the endo-olefin product 17 formed by a 1,5-hydrogen shift. Based on these findings, the synthesis of (−)-aphanorphine was achieved using a sulfur-directed 6-exo-selective aryl radical cyclization of 22.