Formal total synthesis of shikonin via Dötz benzannulation

Review of Shikonin and Derivatives: Isolation, Chemistry, Biosynthesis, Pharmacology and Toxicology

Shikonin and its derivatives, isolated from traditional medicinal plant species of the genus Lithospermum, Alkanna, Arnebia, Anchusa, Onosma, and Echium belonging to the Boraginaceae family, have numerous applications in foods, cosmetics, and textiles. Shikonin, a potent bioactive red pigment, has been used in traditional medicinal systems to cure various ailments and is well known for its diverse pharmacological potential such as anticancer, antithrombotic, neuroprotective, antidiabetic, antiviral, anti-inflammatory, anti-gonadotropic, antioxidants, antimicrobial and insecticidal. Herein, updated research on the natural sources, pharmacology, toxicity studies, and various patents filed worldwide related to shikonin and approaches to shikonin’s biogenic and chemical synthesis are reviewed. Furthermore, recent studies to establish reliable production systems to meet market demand, functional identification, and future clinical development of shikonin and its derivatives against various diseases are presented.

Benzannulation Reactions: A Case for Perspective Change From Arene Decoration to Arene Construction

Benzannulation reactions involve construction of a benzene ring from acyclic precursors. This class of reactions offer a versatile and often superior alternative to aromatic substitution for construction of substituted arenes. Selected pioneering and recent reports of various benzannulation reactions are categorised and discussed in this review.

Total synthesis of landomycins Q and R and related core structures for exploration of the cytotoxicity and antibacterial properties

Herein, we report the total synthesis of landomycins Q and R as well as the aglycone core, namely anhydrolandomycinone and a related core analogue. The synthesis features an acetate-assisted arylation method for construction of the hindered B-ring in the core component and a one-pot aromatization-deiodination-denbenzylation procedure to streamline the global functional and protecting group manuipulation. Subsequent cytotoxicity and antibacterial studies revealed that the landomycin R is a potential antibacterial agent against methicillin-resistant Staphylococcus aureus.

Transition metal mediated carbonylative benzannulations

This review summarizes novel building blocks recently developed for transition metal-catalyzed carbonylative benzannulations.

Studies towards the synthesis of medermycin via Dötz benzannulation

The C-arylglycosides are available in enantiomerically pure form via the Dötz benzannulation reaction between Fischer alkenyl chromium carbene complexes and alkynes; it also could be converted to a precursor of medermycin by O-carbamate directed ipso bromination and nitrile substitution in good overall yields.

Synthesis of bioactive speciosins G and P from Hexagonia speciosa

The first total synthesis of speciosins P and G, previously isolated from Hexagonia speciosa, is reported. These compounds have been synthesized by Sonogashira coupling from readily available starting materials. Siccayne was also synthesized from the same starting material in two steps along with a number of other derivatives. The compounds were tested in the wheat coleoptile bioassay. The most active compound was the intermediate 18, followed by 29 and 17. The structural requirements for activity in these compounds are the presence of methoxy groups in the aromatic ring and a formyl or hydroxy group in the side chain.

Identification of shikonin and its ester derivatives from the roots of Echium italicum L

Nine shikonin pigments: shikonin (S), acetylshikonin (AS), propionylshikonin (PS), isobutyrylshikonin (IBS), tiglylshikonin (TS), 3,3-dimethylacrylshikonin (DAS), angelylshikonin (ANS), 2-methyl-n-butyrylshikonin (MBS) and isovalerylshikonin (IVS) were identified in the root epidermis of Echium italicum L. for the first time. A new thin-layer chromatographic (TLC) method for the separation of enantiomers alkannin and shikonin proved only shikonin after saponification of the root extract, and was afterwards esterified with the corresponding acyl chloride to acquire seven standard compounds (all except ANS). The developed isocratic high-peformance liquid chromatographic (HPLC) methods with VIS and mass spectrometry (MS) detection, allowed for the first time simultaneous separation of all nine compounds with similar structures including positional and geometric isomers in a short time. Structures of the main five compounds (AS, IBS, ANS, MBS, IVS) isolated from the extract by a new semi-preparative HPLC on C18 have additionally been confirmed by (1)H and (13)C nuclear magnetic resonance spectra, which were reported for AS and MBS for the first time.

Synthesis of arylbromides from arenes and N-bromosuccinimide (NBS) in acetonitrile — A convenient method for aromatic bromination

Regioselective and chemoselective electrophilic bromination of a wide series of activated arenes using N-bromosuccinimide (NBS) in acetonitrile occurs readily. Environmentally friendly conditions, large substrate scope, and ease of synthesis enhance the utility of this method over other electrophilic bromination conditions.

Synthesis of pyrroles: reaction of chromium N-alkylaminocarbene complexes with α,β-unsaturated aldehydes

N-alkylaminocarbene complexes of chromium were found to react with alpha,beta-unsaturated aldehydes to give pyrroles in good yields.

Construction of the Benzindenoazepine Skeleton via Cyclopentannulation of Fischer Aminocarbene Complexes: Total Synthesis of Bulgaramine

A total synthesis of bulgaramine has been accomplished with a longest linear sequence of eight steps and an overall yield of 23% from commercially available 3,4-dimethoxyphenethyl alcohol. An intramolecular cyclopentannulation reaction of a Fischer aminocarbene complex provided the key step and occurred under significantly milder conditions and in higher yields than those of other reported examples of this reaction type. The reaction solvent was a critical factor in the cyclopentannulation reaction, with measurable amounts of the desired product observed only when THF was utilized. The product yield could be further enhanced by the addition of two-electron donor ligands, demonstrating the first example of this effect on the thermal reaction of aminocarbene complexes with alkynes.