Toward synthesis of carbasugars (+)-gabosine C, (+)-COTC, (+)-pericosine B, and (+)-pericosine C

Synergy of R-(-)carvone and cyclohexenone-based carbasugar precursors with antibiotics to enhance antibiotic potency and inhibit biofilm formation

The widespread use of antibiotics in recent decades has been a major factor in the emergence of antibiotic resistances. Antibiotic-resistant pathogens pose increasing challenges to healthcare systems in both developing and developed countries. To counteract this, the development of new antibiotics or adjuvants to combat existing resistance to antibiotics is crucial. Glycomimetics, for example carbasugars, offer high potential as adjuvants, as they can inhibit metabolic pathways or biofilm formation due to their similarity to natural substrates. Here, we demonstrate the synthesis of carbasugar precursors (CSPs) and their application as biofilm inhibitors for E. coli and MRSA, as well as their synergistic effect in combination with antibiotics to circumvent biofilm-induced antibiotic resistances. This results in a biofilm reduction of up to 70% for the CSP rac-7 and a reduction in bacterial viability of MRSA by approximately 45% when combined with the otherwise ineffective antibiotic mixture of penicillin and streptomycin.

Enantiomeric composition of natural pericosine A derived from Periconia byssoides and α-glycosidase inhibitory activity of (-)-enantiomer

Chiral high-performance liquid chromatography (HPLC) analysis of natural pericosine A, which appeared in literature first in 1977, from Periconia byssoides was conducted using a column CHIRALPAK® AD-H to determine the enantiomeric composition of the original mixture which was found to be 68: 32 mixtures of (+)- and (-)-enantiomer, respectively. Furthermore, two independently isolated samples of pericosine A from the same fungus were also analyzed to show the two peaks in the HPLC charts at approximate 1:1 ratio. These results concluded that pericosine A derived from Periconia byssoides was indeed an enantiomeric mixture. Synthesized enantiomers were subjected to evaluation of antitumor activity against three kinds of tumor cells (p388, L1210, HL-60), indicating moderate cytotoxicity against all three kinds of tumor cell lines, but significant difference in potency between the enantiomers was not observed. In contrast, when both the enantiomers of pericosine A were evaluated against five kinds of glycosidases-inhibitory activities (α- and β-glucosidases, α- and β-galactosidases, and α-mannosidase), an apparent difference on anti-glycosidase assay was found between the enantiomers: (-)-pericosine A inhibited α-glucosidase at IC₅₀ : 2.25 mM, and β-galactosidase at IC₅₀ : 5.38 mM, albeit the (+)-enantiomer showed inactivity against these five enzymes.

Synthesis of 6-Halo-Substituted Pericosine A and an Evaluation of Their Antitumor and Antiglycosidase Activities

The enantiomers of 6-fluoro-, 6-bromo-, and 6-iodopericosine A were synthesized. An efficient synthesis of both enantiomers of pericoxide via 6-bromopericosine A was also developed. These 6-halo-substituted pericosine A derivatives were evaluated in terms of their antitumor activity against three types of tumor cells (p388, L1210, and HL-60) and glycosidase inhibitory activity. The bromo- and iodo-congeners exhibited moderate antitumor activity similar to pericosine A against the three types of tumor cell lines studied. The fluorinated compound was less active than the others, including pericosine A. In the antitumor assay, no significant difference in potency between the enantiomers was observed for any of the halogenated compounds. Meanwhile, the (−)-6-fluoro- and (−)-6-bromo-congeners inhibited α-glucosidase to a greater extent than those of their corresponding (+)-enantiomers, whereas (+)-iodopericosine A showed increased activity when compared to its (−)-enantiomer.

Current Synthetic Approaches to the Synthesis of Carbasugars from Non-Carbohydrate Sources

Carbasugars are a group of carbohydrate derivatives in which the ring oxygen is replaced by a methylene group, producing a molecule with a nearly identical structure but highly different behavior. Over time, this definition has been extended to include other unsaturated cyclohexenols and carba-, di-, and polysaccharides. Such molecules can be found in bacterial strains and the human body, acting as neurotransmitters (e.g., inositol trisphosphate). In science, there are a wide range of research areas that are affected by, and involve, carbasugars, such as studies on enzyme inhibition, lectin-binding, and even HIV and cancer treatment. In this review article, different methods for synthesizing carbasugars, their derivatives, and similar cyclohexanes presenting comparable characteristics are summarized and evaluated, utilizing diverse starting materials and synthetic procedures.

The Chemistry and Pharmacology of Fungal Genus Periconia: A Review

Periconia is filamentous fungi belonging to the Periconiaceae family, and over the last 50 years, the genus has shown interest in natural product exploration for pharmacological purposes. Therefore, this study aims to analyze the different species of Periconia containing natural products such as terpenoids, polyketides, cytochalasan, macrosphelides, cyclopentenes, aromatic compounds, and carbohydrates carbasugar derivates. The isolated compound of this kind, which was reported in 1969, consisted of polyketide derivatives and their structures and was determined by chemical reaction and spectroscopic methods. After some years, 77 compounds isolated from endophytic fungus Periconia were associated with eight plant species, 28 compounds from sea hare Aplysia kurodai, and ten from endolichenic fungi Parmelia sp. The potent pharmacological agents from this genus are periconicin A, which acts as an antimicrobial, pericochlorosin B as an anti-human immunodeficiency virus (HIV), peribysin D, and pericosine A as cytotoxic agents, and periconianone A as an anti-inflammatory agent. Furthermore, information about taxol and piperine from Periconia producing species was also provided. Therefore, this study supports discovering new drugs produced by the Periconia species and compares them for future drug development.

Enantioselective Synthesis of a New Non-Natural Gabosine

The preparation of a new non-natural gabosine is reported, in which the chirality is transferred from the toluene’s biotransformed metabolite (1R,2S)-3-methylcyclohexa-3.5-diene-1,2-diol. Further chemical transformations to introduce additional functionality and chirality to the molecule were also accomplished.

Syntheses and Glycosidase Inhibitory Activities, and in Silico Docking Studies of Pericosine E Analogs Methoxy-Substituted at C6

Inspired by the significant -glucosidase inhibitory activities of (+)- and (-)-pericosine E, we herein designed and synthesized 16 analogs of these marine natural products bearing a methoxy group instead of a chlorine atom at C6. Four of these compounds exhibited moderate -glucosidase inhibitory activities, which were weaker than those of the corresponding chlorine-containing species. The four compounds could be prepared by coupling reactions utilizing the (-)-pericosine B moiety. An additional in silico docking simulation suggested that the reason of reduced activity of the C6-methoxylated analogs might be an absence of hydrogen bonding between a methoxy group with the surrounding amino acid residues in the active site in -glucosidase.

A divergent strategy to synthesize gabosines featuring a switchable two-way aldol cyclization

Gabosines and their natural analogues, belonging to C7 carbasugars, have attracted great attention in synthesis due to their rich structural variety and promising biological activities. A new diversity-oriented approach for the gabosine-type carbasugars based on a tunable regioselective aldol cyclization of flexible precursor 2 is explored. Two cyclization modes (A and B) of the precursor can be well controlled by switching promoters to selectively produce two resulting cyclohexa(e)nones 3 and 10, both of which are versatile intermediates for various C7 carbasugars. After the conversion of 3 to eight natural carbasugars, the utility of intermediate 10 is illustrated by the first synthesis of (-)-gabosine L, as well as the new synthesis of (-)-gabosine A, (-)-gabosine B, (-)-gabosine N and (-)-gabosine O. The chemical structure and the absolute configuration of (-)-gabosine L are confirmed by its total synthesis.

Gram-scale carbasugar synthesis via intramolecular seleno-Michael/aldol reaction

Carbasugars represent an important category of natural products possessing a broad spectrum of biological activities. Lots of effort has been done to develop gram scale synthesis. We are presenting a new approach to gram scale synthesis of the carbasugar skeleton via intramolecular seleno-Michael/aldol reaction. The proposed strategy gave gram amounts of 6-hydroxy shikimic ester in a tandem process in 36% overall yield starting from d-lyxose. We have attempted to demonstrate the synthetic utility of 6-hydroxyshikimic acid derivatives by covering the important synthetic modifications and related applications, namely synthesis of protected (−)-gabosine E, (−)-MK7606, (−)-valienamine and finally unprotected methyl (−)-shikimate.

A new approach to gram scale synthesis of carbasugar derivatives via intramolecular seleno-Michael/aldol reaction.

Antileishmanial Carbasugars from Geosmithia langdonii

Two new carbasugar-type metabolites, (1 S,2 R,3 R,4 R,5 R)-2,3,4-trihydroxy-5-methylcyclohexyl-2',5'-dihydroxybenzoate (1) and (1 S,2 S,3 S,4 R,5 R)-4-[(2',5'-dihydroxybenzyl)oxy]-5-methylcyclohexane-1,2,3-triol (2), were isolated from the filamentous fungus Geosmithia langdonii isolated from cotton textiles from Assiut, Egypt. The structures of 1 and 2 were elucidated based on comprehensive 1D and 2D NMR and MS data. Compounds 1 and 2 showed antileishmanial activity against Leishmania donovani with IC₅₀ values of 100 and 57 μM, respectively. The (1 S,2 R,3 R,4 R,5 R) absolute configuration of carbasugar 1 was assigned via 2D NMR and experimental and calculated electronic circular dichroism (ECD) data. Similarly, the tentative structure of compound 2 was shown to possess a (1 S,2 S,3 S,4 R,5 R) absolute configuration via comparing its experimental ECD data and the specific rotation with 1 as well as examining the energy-minimized 3D computational models of compounds 1 and 2.

Total Syntheses of (+)-Gabosine P, (+)-Gabosine Q, (+)-Gabosine E, (-)-Gabosine G, (-)-Gabosine I, (-)-Gabosine K, (+)-Streptol, and (-)-Uvamalol A by a Diversity-Oriented Approach Featuring Tunable Deprotection Manipulation

A new diversity-oriented approach to C7-cyclitols, which possess a broad spectrum of biological activities, is developed. The key polyoxygenated intermediates with different O-protecting groups were accessed by an intramolecular aldol-cyclization of a diketone derived from δ-d-gluconolactone. The versatile intermediates can be easily transformed into structurally different carbasugars based on control of deprotection manipulation. The utility of the robust approach is illustrated by the first syntheses of (+)-gabosines P and Q, as well as the syntheses of several other gabosines and related analogues viz. (+)-gabosine E, (-)-gabosine G, (-)-gabosine I, (-)-gabosine K, (+)-streptol, and (-)-uvamalol A. In addition, the absolute configuration of (-)-uvamalol A is assigned by its total synthesis.

Synthesis of Natural O-Linked Carba-Disaccharides, (+)- and (-)-Pericosine E, and Their Analogues as α-Glucosidase Inhibitors

Pericosine E (6), a metabolite of Periconia byssoides OUPS-N133 was originally isolated from the sea hare Aplysia kurodai, which exists as an enantiomeric mixture in nature. The enantiospecific syntheses of both enantiomers of Periconia byssoides OUPS-N133 has been achieved, along with six stereoisomers, using a common simple synthetic strategy. For these efficient syntheses, highly regio- and steroselective processes for the preparation of bromohydrin and anti-epoxide intermediates were applied. In order to access the unique O-linked carbadisaccharide structure, coupling of chlorohydrin as a donor and anti-epoxide as an acceptor was achieved using catalytic BF₃·Et₂O. Most of the synthesized compounds exhibited selectively significant inhibitory activity against α-glycosidase derived from yeast. The strongest analog showed almost 50 times the activity of the positive control, deoxynojirimycin.

Kinetic and mechanistic investigations of the Baylis–Hillman reaction in ionic liquids

We report here a quantitative study of the kinetics and mechanism of the Baylis–Hillman reaction in the presence of ionic liquids as solvent media.

Synthesis of marine natural product (-)-pericosine E

The first synthesis of (-)-pericosine E (6), a metabolite of the Periconia byssoides OUPS-N133 isolated originally from the sea hare Aplysia kurodai, has been achieved. Efficient and regioselective synthetic procedures for the synthesis of key intermediates, anti- and syn-epoxides 9 and 10, were developed using an anti-epoxidation of diene 12 with TFDO and a bromohydrination of 12 with NBS in CH(3)CN/H(2)O (2:3), respectively. In addition, comparison of the specific optical rotations between synthetic 6 and natural 6 elucidated that the naturally preferred enantiomer of pericosine E had the same absolute configuration as (-)-6 synthesized from chlorohydrin (-)-8 and anti-epoxide (+)-9.