Total Syntheses of (−)-Spirooliganones A and B and Their Diastereoisomers: Absolute Stereochemistry and Inhibitory Activity against Coxsackie Virus B3

Total Syntheses of Spirooliganones A and B

This paper describes the total syntheses of spirooliganones A and B, which provides control over the stereochemistry of the spiro center via a cascade reaction involving the in situ formation of an o-quinone methide and a hetero-Diels–Alder reaction.

Synthesis of 2H-chromenes: recent advances and perspectives

As one type of chromene, 2H-chromenes represent important oxygen heterocycles, which not only widely exist in natural products, pharmaceutical agents and biologically relevant molecules, but have also been used broadly in materials science and organic synthesis. Two major synthetic strategies have been developed towards such compounds. This review mainly focuses on the recent advances in this field, including benzopyran ring formation involving cyclization reactions and the late-stage functionalization of the parent 2H-chromenes.

Prenylated C6-C3 derivatives from the stems and branches of Illicium ternstroemioides A. C. Smith with antiviral activity

Thirteen compounds were isolated from the lipid-soluble extracts of Illicium ternstroemioides A. C. Smith, including eleven previously undescribed prenylated C₆-C₃ compounds, a previously undescribed prenylated C₆-C₃ derivative-abscisic acid ester hybrid, and a known compound (4S)-illicinone I. Their structures and configurations were mainly elucidated by spectroscopic analyses, CD experiments and X-ray crystallography. (2S,4R,11S)-4-O-methyl-12-chloroillifunone C, (2S,4R,11R)-2,3-dihydro-4-O-methyl illioliganfunone D, and illiternfunol A were found to exhibit weak activity against Coxsackievirus B3, with IC₅₀ values ranging from 27.8 to 33.3 μM. Illiternone B exhibited more potent activities against Coxsackievirus B3 and influenza virus A than did its geometric isomer illiternone A, with IC₅₀ values of 7.7 μM and 2.5 μM, respectively. None of these compounds displayed cytotoxic activities.

Library construction of stereochemically diverse isomers of spirooliganin: their total synthesis and antiviral activity

The construction of libraries of stereoisomers of natural products serves as an important approach to investigating the correlation between the stereostructure and biological activity. However, the total synthesis and isomerzation of polycyclic scaffolds with multiple chrial centers are rare. Spirooliganin (1), a new skeleton natural product isolated from the plant Illicium oligandrum, was structurally characterized by comprehensive analysis of NMR spectroscopic data and ECD which revealed an unprecedented 5-6-6-6-7 polycyclic framework with six chiral centers. Here we report a 17-step total synthesis to prepare a library of stereochemically diverse isomers of spirooliganin, including 16 diastereoisomers and 16 regioisomers. In addition to a regioselective hetero-Diels-Alder cycloaddition, the synthetic strategy involves a photo-induced stereoselective Diels-Alder reaction, which gives only the abnormal trans-fused product as rationalized by density functional theory calculations. Preliminary biological evaluation showed that spirooliganin and regioisomers 39 exhibited potent inhibition of Coxsackievirus B3. It also revealed the pharmacophore effect of the D-ring (16R,18R,24R, and 26R) for their antiviral activities.

Pivotal Reactions in the Creation of the Polycyclic Skeleton of Cryptotrione

Three pivotal reactions, namely, enyne cycloisomerization, polyene cyclization, and quinone methide formation, are applied to synthesize the complex polycyclic skeleton of cryptotrione. This review summarizes the most prominent applications of these three reactions to the total syntheses of natural products, covering results published in the literature between 2011 and 2020.

1 Introduction

2 Three Pivotal Reactions Applied to Create the Polycyclic Framework of Cryptotrione

2.1 Enyne Cycloisomerization

2.2 Polyene Cyclization

2.3 Quinone Methide Formation

3 Conclusion

Sweritranslactone D, a hepatoprotective novel secoiridoid dimer with tetracyclic lactone skeleton from heat-transformed swertiamarin

Swertia mileensis, known as Qing-Ye-Dan (QYD), has been documented in Chinese Pharmacopoeia to cure hepatitis. Interestingly, its announced main active component, swertiamarin, could not be detected in the decoction, which indicated that the efficacy of QYD might be attributed to heat-transformed products of swertiamarin (HTPS). Further investigation on HTPS led to the isolation of sweritranslactone D (1), a novel secoiridoid dimer possessing a tetracyclic lactone skeleton, with better hepatoprotective activity than N-acetyl-L-cysteine in vitro.

Bioactive prenylated C6-C3 derivatives from the stems and leaves of Illicium fargesii

Seventeen new prenylated C₆-C₃ derivatives, namely, illifargeins A-M (1-13), including three pairs of enantiomers (1, 5, and 12) and one norillifargeal A (14), together with eight known analogues (15-22), were isolated from the stems and leaves of Illicium fargesii. The structures of the new compounds were elucidated using spectroscopic data (UV, IR, 1D and 2D NMR, and HRESIMS). Their absolute configurations were determined by using experimental and calculated ECD data analysis, as well as a modified Mosher's method. Compounds 1a, 1b, 2, 3, 5a, 7, 10, 11, 15, 16, 19, and 20 showed potential activity against Coxsackie virus B3, with IC₅₀ values ranging from 6.23 to 33.33 µM. Compounds 9 and 15 exhibited potential activity against influenza virus A, with IC₅₀ values of 11.11 and 19.24 µM, respectively. Compounds 2, 3, and 18 exhibited potential anti-oxidant activity, with IC₅₀ values ranging from 1.43 to 6.71 µM.

Burchellin and its stereoisomers: total synthesis, structural elucidation and antiviral activity

Burchellin and its analogues are a class of neolignan natural products containing a rare core with three contiguous stereogenic centers. In previous reports, racemic burchellin was synthesized without accessing each of the enantiomers. In this paper, a concise and efficient total synthetic route to divergently access the enantiomers of burchellin and those of its 1'-epi-diastereoisomer over six steps for each is disclosed, where each of the enantiomers was obtained by preparative chiral phase HPLC purification. The key steps include the construction of a 2,3-dihydrobenzofuran moiety by two Claisen rearrangements and a one-step rearrangement/cyclization and subsequent tandem ester hydrolysis/oxy-Cope rearrangement/methylation to furnish the basic skeleton of burchellin. The structures and absolute configurations of the four stereoisomers were determined using spectroscopic data analyses and comparison of experimental and calculated electronic circular dichroism data. These stereoisomers were found to have potent antiviral effects against coxsackie virus B3, and is the first time that bioactivity has been reported for these compounds.

Naturally occurring [4 + 2] type terpenoid dimers: sources, bioactivities and total syntheses

This review article highlights recent progress on their sources, bioactivities, biosynthetic hypotheses and total chemical syntheses of naturally occurring [4 + 2] type terpenoid dimers.

Iodine-Mediated Direct Generation of o-Quinone Methides at Room Temperature: A Facile Protocol for the Synthesis of ortho-Hydroxybenzyl Thioethers

An iodine-mediated preparation of ortho-quinone methides (o-QMs) from ortho-hydroxybenzyl alcohols by a C-O bond scission strategy is described. The in situ generated o-QMs were then employed for C-S bond formation by thio-Michael addition of thiols to provide the ortho-hydroxybenzyl thioethers (o-HBT) in moderate to excellent yields.

Recent advances in the application of Diels–Alder reactions involving o-quinodimethanes, aza-o-quinone methides and o-quinone methides in natural product total synthesis

This review summarizes recent advances in Diels–Alder reactions involving o-QDMs, o-QMs and aza-o-QMs. The power and potential of this strategy in organic synthesis and natural product total synthesis is highlighted.

Cnidimonins A-C, Three Types of Hybrid Dimer from Cnidium monnieri: Structural Elucidation and Semisynthesis

Three pairs of racemic dimers, (±)-cnidimonins A-C (1-3), were isolated from the fruits of Cnidium monnieri. They represent novel hybrid-dimerization patterns of coumarin skeleton with structurally diverse units (flavonol, benzofuran, and chromone) via an unprecedented terminal chiral carbon of prenyl. The absolute configurations of the enantiomers were determined by electronic circular dichroism (ECD). To investigate their bioactivities in depth, (±)-cnidimonins A-C (1-3) were synthesized. The racemic mixture (±)-1 exhibited stronger antiviral activity against HSV-1 (IC₅₀: 1.23 μM) than its corresponding optically pure enantiomers.

A cascade Claisen rearrangement/o-quinone methide formation/electrocyclization approach to 2H-chromenes

A new approach to 8-substituted 2H-chromenes is developed, featuring a novel cascade aromatic Claisen rearrangement/o-quinone methide formation/6π-electrocyclization.

Hyperjapones A-E, Terpenoid Polymethylated Acylphloroglucinols from Hypericum japonicum

Hyperjapones A-E (1-5), novel terpenoid polymethylated acylphloroglucinols (TPAPs) with unusual architectures, were characterized from Hypericum japonicum. Their structures and absolute configurations were determined by comprehensive spectroscopic data and X-ray diffractions. Compound 1 was obtained as a racemic mixture and was separated by a column coated with cellulose tris(4-methylbenzoate) after attempts with various chiral materials. Compounds 1, 2, and 4 exhibited moderate antitumor activities in vitro.

Phenyl trimethyl ammonium tribromide mediated robust one-pot synthesis of spiro-oxacycles – an economic route – stereoselective synthesis of oxaspirohexacyclodieneones

This paper entails the first recognition of Phenyl Trimethyl Ammonium Tribromide (PTAB) as an effective reagent for spiro-cyclizations proceeding via oxidative dearomatization.