Total synthesis of mycestericin A and its 14-epimer

Recent advances in the total synthesis of polyhydroxylated alkaloids via chiral oxazines

This review summarizes recently established methodologies developed for the enantioselective and diastereoselective synthesis of chiral 1,3-oxazines. These compounds are of interest as advanced synthetic intermediates in the total synthesis of structurally complex and biologically active polyhydroxylated alkaloids such as (+)-1-deoxynojirimycin, (-)-anisomycin, (+)-castanospermine, (+)-casuarine, (-)-conduramine F-1, (-)-sphingofungin B, Neu5Ac methyl ester, and other natural products. The devised synthetic approach aims to offer a direct, efficient, and adaptable method for obtaining both pure enantiomers and pure diastereomers. It revolves around utilizing chiral building blocks like syn,syn-, syn,syn,anti-, syn,anti-, syn,anti,syn-, anti,syn-, anti,syn,syn-, and anti,syn,anti-oxazines. By integrating oxazine chemistry with established and innovative transformations, this approach enabled the synthesis of 30 polyhydroxylated amines across various studies conducted between 2007 and 2022.

Seven-Step Synthesis of All-Nitrogenated Sugar Derivatives Using Sequential Overman Rearrangements

All-nitrogenated sugars (ANSs), in which all hydroxy groups in a carbohydrate are replaced with amino groups, are anticipated to be privileged structures with useful biological activities. However, ANS synthesis has been challenging due to the difficulty in the installation of multi-amino groups. We report herein the development of a concise synthetic route to peracetylated ANSs in seven steps from commercially available monosaccharides. The key to success is the use of the sequential Overman rearrangement, which enables formal simultaneous substitution of four or five hydroxy groups in monosaccharides with amino groups. A variety of ANSs are available through the same reaction sequence starting from different initial monosaccharides by chirality transfer of secondary alcohols. Transformations of the resulting peracetylated ANSs such as glycosylation and deacetylation are also demonstrated. Biological studies reveal that ANS-modified cholesterol show cytotoxicity against human cancer cell lines, whereas each ANS and cholesterol have no cytotoxicity.

Synthesis and anticancer profile of novel sphingoid base-like compounds with a quaternary stereocentre

The synthesis of novel sphingoid base-like compounds with a quaternary stereocentre was achieved in a sequence featuring [3,3]-sigmatropic rearrangements and olefin cross-metathesis transformation as the key reaction steps, which were accompanied by the rational selection of suitable functional group transformations. The stereochemistry of the desired tetra-substituted carbon bearing nitrogen functionality was determined via NOESY experiments of the advanced oxazolidine-2-thiones. Cell viability experiments revealed significant antiproliferative/cytotoxic activity of the target compounds 7, ent-7 and 29 against the Jurkat cell line, with the IC₅₀ values of 6.6 μM, 5.6 μM and 6.1 μM, respectively.

Recent advances in the Overman rearrangement: synthesis of natural products and valuable compounds

This review documents the reports since 2005 on the Overman rearrangement, an important C–N bond forming reaction that has been profoundly used in the synthesis of natural products, synthetic intermediates, building blocks and valuable compounds.

Endophytic fungal compounds active against Cryptococcus neoformans and C. gattii

Infections with Cryptococcus are invasive mycoses associated with significant morbidity and mortality, mainly in immunosuppressed patients. Several drugs have been introduced to combat these opportunistic infections. However, resistance of this organism to antifungal drugs has increased, causing difficulties in the treatment. The goal of this work was to evaluate the antifungal activity of ethanol extracts from endophytic fungi isolated from plants collected from different Brazilian ecosystems and to perform the fractionation of the most promising extract. Four-hundred fungal extracts were investigated by microdilution broth assays against Cryptococcus neoformans and Cryptococcus gattii at a concentration of 500 μg ml(-1). Among them, the extract of Mycosphaerella sp. UFMGCB 2032, an endophytic fungus isolated from the plant Eugenia bimarginata DC. (Myrtaceae) exhibited outstanding antifungal activity against C. neoformans and C. gattii, with MIC values of 31.2 μg ml(-1) and 7.8 μg ml(-1), respectively. The fractionation of this extract using liquid-liquid partitioning and semi-preparative HPLC afforded two eicosanoic acids with antifungal activity, compound 1, (2S,3R,4R)-(E)-2-amino-3,4-dihydroxy-2-(hydroxymethyl)-14-oxoeicos-6,12-dienoic acid with MIC values ranging from 1.3-2.50 μg ml(-1), and compound 2, known as myriocin, with MIC values of 0.5 μg ml(-1) against C. neoformans and C. gattii. These compounds are reported for the first time in the Mycosphaerella genus.

Chirality extension of an oxazine building block en route to total syntheses of (+)-hyacinthacine A2 and sphingofungin B

Concise total syntheses of (+)-hyacinthacine A₂ and sphingofungin B were achieved by palladium(0)-catalyzed intramolecular oxazine formation and diastereoselective nucleophilic additions.

Cascade- and orthoamide-type Overman rearrangements of allylic vicinal diols

This article describes the details of two new types of Overman rearrangement from allylic vicinal diols. Starting from identical diols, both bis(imidate)s and cyclic orthoamides were selectively synthesized by simply changing the reaction conditions. Whilst exposure of the bis(imidate)s to thermal conditions initiated the double Overman rearrangement to introduce two identical nitrogen groups in a single operation (the cascade-type Overman rearrangement), the reaction of cyclic orthoamides resulted in a single rearrangement (the orthoamide-type Overman rearrangement). The newly generated allylic alcohols from the orthoamide-type reaction can potentially undergo a variety of further transformations. For instance, we demonstrated an Overman/Claisen sequence in one pot. The most conspicuous feature of this method is that it offers precise control over the number of Overman rearrangements from the same allylic vicinal diols. This method also excludes the tedious protecting-group manipulations of the homoallylic alcohols, which are necessary in conventional Overman rearrangements. All of the performed rearrangements proceeded in a completely diastereoselective fashion through a chair-like transition state.

A facile synthesis of D-ribo-C(20)-phytosphingosine and its C2 epimer from D-ribose

A facile synthetic route to d-ribo-C(20)-phytosphingosine 31 and its C2 epimer 32 is described. The Overman rearrangement of allylic trichloroacetimidates derived from the known ribose derivative 7 has been used as the key step. The subsequent functional group interconversions in rearranged products 14 and 15 followed by Wittig olefination, Pd/C-mediated reduction and the removal of protecting groups successfully constructed the final molecules.

Total synthesis of a protected form of sphingofungin E using the [3,3]-sigmatropic rearrangement of an allylic thiocyanate as the key reaction

An approach to the stereocontrolled synthesis of the protected form of sphingofungin E (32) starting from the known protected d-glucose derivative 3 is described herein. For the construction of a tetrasubstituted carbon atom that is substituted with nitrogen, the [3,3]-sigmatropic rearrangement of thiocyanate 8 was employed. Subsequent functional group interconversions afforded the highly functionalized fragment, allylic bromide 26. Its coupling reaction with the known C(12) hydrophobic segment 2, followed by further manipulation, completed the total synthesis.