Total Synthesis of Acortatarin A Using a Pd(II)-Catalyzed Spiroketalization Strategy

Stereoselective Synthesis of Spirolactone Analogues of Pyrrolomorpholine Alkaloids

The synthesis of a spirolactone analogue of xylapyrraside B1, a potent antioxidant agent, is described. The key step is the stereoselective formation of the spirolactone skeleton via the formal [3+2] annulation of the isopropylidene-protected glyceraldehyde and δ-methylene lactone, mediated by trifluoroborane etherate. This study addresses the stereoselective synthesis of pyrrolomorpholine spiroketal alkaloids, enabling the production of these bioactive compounds and their analogues.

Organocatalytic Asymmetric Construction of Spirooxazines via Chemoselective Cascade Addition of N-Substituted Hydroxylamine with Keto-bis-enone

Spirooxazines represent a privileged heterocyclic scaffold having pronounced biological importance. Herein, we introduce a chiral bifunctional squaramide catalyzed highly chemoselective cascade reaction involving aza-Michael/1,2-addition/oxa-Michael addition of N-substituted hydroxylamine with keto-bis-enones. This strategy enables the synthesis of highly enantioenriched oxa-spirooxazines with a broad substrate tolerance. Scalability and synthetic transformation have demonstrated the feasibility of the protocol. Furthermore, control experiments provided insights into the reaction mechanism.

Pyrrole-2-carboxaldehydes: Origins and Physiological Activities

Pyrrole-2-carboxaldehyde (Py-2-C) derivatives have been isolated from many natural sources, including fungi, plants (roots, leaves, and seeds), and microorganisms. The well-known diabetes molecular marker, pyrraline, which is produced after sequential reactions in vivo, has a Py-2-C skeleton. Py-2-Cs can be chemically produced by the strong acid-catalyzed condensation of glucose and amino acid derivatives in vitro. These observations indicate the importance of the Py-2-C skeleton in vivo and suggest that molecules containing this skeleton have various biological functions. In this review, we have summarized Py-2-C derivatives based on their origins. We also discuss the structural characteristics, natural sources, and physiological activities of isolated compounds containing the Py-2-C group.

Synthesis of a Constitutional Isomer of Armeniaspirol A, Pseudoarmeniaspirol A, via Lewis Acid-Mediated Rearrangement

The natural product armeniaspirol possesses a unique spirocyclic N,O-ketal in an α,β-dichloro-α,β-unsaturated lactam scaffold that has proved challenging to synthesize. Herein, we characterize the oxidative chlorination of pyrrole-2-carboxylate derivatives that rapidly generates this scaffold. The scope of this oxidation was extended to a series of esters and amides. Pyrrole-2-ketones could not be converted into the lactam due to an oxidative fragmentation. This result was unexpected since chloro-armeniaspirol has been synthesized via oxidative chlorination of a pyrrole-2-ketone. Examination of this successful oxidation showed that the desired scaffold was accessed due to intramolecular trapping from the neighboring free phenol, preventing fragmentation. Using the product of methyl N-methyl pyrrole-2-carboxylate oxidation 7b, we attempted to access the natural product armeniaspirol 2; however, an unanticipated Lewis acid-mediated rearrangement led to formation of a constitutional isomer, pseudoarmeniaspirol A 1. A small panel of pseudoarmeniaspirol analogues was synthesized and evaluated for antibiotic activity, inhibition of the targets of armeniaspirol, ClpXP and ClpYQ, and protonophore activity. While pseudoarmeniaspirol shows antibiotic activity, it does not target ClpXP or ClpYQ and has less protonophore activity than the natural product.

TFA-Catalyzed [3+2] Spiroannulation of Cyclobutanols: A Route to Spiro[cyclobuta[a]indene-7,1'-cyclobutane] Skeletons

A straightforward method for the synthesis of spiro[cyclobuta[a]indene-7,1'-cyclobutane] derivatives from cyclobutanols has been developed via one-pot [3+2] spiroannulation. A series of new spiro[cyclobuta[a]indene-7,1'-cyclobutane] derivatives are facilely synthesized in good yields under mild reaction conditions.

2-Formylpyrrole natural products: origin, structural diversity, bioactivity and synthesis

2-Formylpyrroles constitute a large and growing family of bioactive Maillard reaction products found in food, traditional medicine and throughout nature.

Catalytic Dehydrative Lactonization of Allylic Alcohols

A convenient strategy for the synthesis of phthalides and γ-butyrolactones is reported. The method utilizes readily prepared allylic alcohols in formal Au(I)- and Pd(II)-catalyzed S_N2' reactions. Using these catalysts, exclusive formation of the desired five-membered lactones is observed, completely avoiding the competing direct lactonization pathway that forms the undesired seven-membered ring with protic acids and alternative metal salts. This mild and operationally simple method notably tolerates exomethylene groups and should find use in both phthalide and terpene syntheses.

Stereocontrolled Synthesis of Spiroketals: An Engine for Chemical and Biological Discovery

Spiroketals are key structural motifs found in diverse natural products with compelling biological activities. However, stereocontrolled synthetic access to spiroketals, independent of their inherent thermodynamic preferences, is a classical challenge in organic synthesis that has limited in-depth biological exploration of this intriguing class. Herein, we review our laboratory's efforts to advance the glycal epoxide approach to the stereocontrolled synthesis of spiroketals via kinetically controlled spirocyclization reactions. This work has provided new synthetic methodologies with applications in both diversity- and target-oriented synthesis, fundamental insights into structure and reactivity, and efficient access to spiroketal libraries and natural products for biological evaluation.

Family-level stereoselective synthesis and biological evaluation of pyrrolomorpholine spiroketal natural product antioxidants

The pyranose spiroketal natural products pollenopyrroside A and shensongine A (also known as xylapyrroside A, ent-capparisine B) have been synthesized by stereoselective spirocyclizations of a common C1-functionalized glycal precursor. In conjunction with our previously reported syntheses of the corresponding furanose isomers, this provides a versatile family-level synthesis of the pyrrolomorpholine spiroketal natural products and analogues. In rat mesangial cells, hyperglycemia-induced production of reactive oxygen species, which is implicated in diabetic nephropathy, was inhibited by pollenopyrroside A and shensongine A with mid-μM IC50 values, while unnatural C2-hydroxy analogues exhibited more potent, sub-μM activity.

Taste-Active Maillard Reaction Products in Roasted Garlic (Allium sativum)

In order to gain first insight into candidate Maillard reaction products formed upon thermal processing of garlic, mixtures of glucose and S-allyl-l-cysteine, the major sulfur-containing amino acid in garlic, were low-moisture heated, and nine major reaction products were isolated. LC-TOF-MS, 1D/2D NMR, and CD spectroscopy led to their identification as acortatarin A (1), pollenopyrroside A (2), epi-acortatarin A (3), xylapyrroside A (4), 5-hydroxymethyl-1-[(5-hydroxymethyl-2-furanyl)methyl]-1H-pyrrole-2-carbalde-hyde (5), 3-(allylthio)-2-(2-formyl-5-hydroxymethyl-1H-pyrrol-1-yl)propanoic acid (6), (4S)-4-(allylthiomethyl)-3,4-dihydro-3-oxo-1H-pyrrolo[2,1-c][1,4]oxazine-6-carbaldehyde (7), (2R)-3-(allylthio)-2-[(4R)-4-(allylthiomethyl)-6-formyl-3-oxo-3,4-dihydropyrrolo-[1,2-a]pyrazin-2(1H)-yl]propanoic acid (8), and (2R)-3-(allylthio)-2-((4S)-4-(allylthiomethyl)-6-formyl-3-oxo-3,4-dihydropyrrolo-[1,2-a]pyrazin-2(1H)-yl)propanoic acid (9). Among the Maillard reaction products identified, compounds 5-9 have not previously been published. The thermal generation of the literature known spiroalkaloids 1-4 is reported for the first time. Sensory analysis revealed a bitter taste with thresholds between 0.5 and 785 μmol/kg for 1-5 and 7-9. Compound 6 did not show any intrinsic taste (water) but exhibited a strong mouthfullness (kokumi) enhancing activity above 186 μmol/kg. LC-MS/MS analysis showed 1-9 to be generated upon pan-frying of garlic with the highest concentration of 793.7 μmol/kg found for 6, thus exceeding its kokumi threshold by a factor of 4 and giving evidence for its potential taste modulation activity in processed garlic preparations.

Synthesis of the 2-formylpyrrole spiroketal pollenopyrroside A and structural elucidation of xylapyrroside A, shensongine A and capparisine B

A convergent synthesis enabled structural elucidation of the 2-formyl pyrrole spiroketals pollenopyrroside A and shensongine A/xylapyrroside A. The key step involves a Maillard-type condensation to furnish the 2-formylpyrrole ring system.

Metabolites from the mushroom Ganoderma lingzhi as stimulators of neural stem cell proliferation

Ganoderma lingzhi is a valuable, edible and medicinal fungus that has been widely used for the prevention and treatment of a broad range of diseases. In this study, spirolingzhines A-D, four meroterpenoids with a spiro[benzofuran-2,10-cyclopentane] motif, lingzhines A-F, six meroterpenoids with diverse ring systems, along with two known compounds were isolated from the fruiting bodies of this fungus. The structures and stereochemistry of these substances were determined by using spectroscopic, X-ray crystallographic and computational methods. Chiral HPLC was used to separate (-)- and (+)-antipodes of seven of ten meroterpenoids, which were isolated from the fungus as racemic mixtures. Several of the metabolites were found to promote proliferation of neural stem cells (NSCs) and, as such, they constitute a class of NSC stimulators. The most potent member of this series, (-)-spirolingzhine A, was shown to affect NSC cell cycle progression using the 5-bromo-2-deoxyuridine (BrdU) incorporation assay.

Cascade N-Alkylation/Hemiacetalization for Facile Construction of the Spiroketal Skeleton of Acortatarin Alkaloids with Therapeutic Potentiality in Diabetic Nephropathy

The concise building of the spiroketal core of acortatarin-type alkaloids as potential therapeutic agents in diabetic nephropathy was established in four steps, through a tandem N-alkylation/hemiacetalization between pyrrole units and the corresponding halo alcohols generated by convenient halomethylation of chiral lactones from natural aldoses.

Chorley D, Brimble MA. Recent developments in transition metal-catalysed spiroketalisation

This perspective updates recent developments (since 2012) in the synthesis of spiroketals using transition metal catalysis.