Intramolecular Hetero-Michael Addition of β-Hydroxyenones for the Preparation of Highly Substituted Tetrahydropyranones

Structure-Unit-Based Total Synthesis of (-)-Sinulochmodin C

Herein we report a structure-unit-based asymmetric total synthesis of sinulochmodin C, a norcembranoid diterpenoid bearing a transannular strained ether bridge β-keto tetrahydrofuran moiety. Our synthetic route features an intramolecular double Michael addition to construct stereospecifically the [7,6,5,5] tetracyclic skeleton, a vinylogous hydroxylation/oxidation procedure or a stereospecific epoxide opening/oxidation sequence to establish the γ-keto enone intermediate, a Lewis acid/Brønsted acid mediated transannular oxa-Michael addition to fuse the β-keto tetrahydrofuran moiety, a Mukaiyama hydration/Pd-C hydrogenation to reverse the C1-configuration of the isopropenyl unit, and a bioinspired transformation of sinulochmodin C into scabrolide A.

Studies directed toward the synthesis of hedycoropyrans: total synthesis of des-hydroxy (-)-hedycoropyran B (ent-rhoiptelol B)

A full account of our efforts directed towards the synthesis of the diarylheptanoid-derived natural products hedycoropyrans that led to the total synthesis of ent-rhoiptelol B is described. In this endeavor, we have attempted two distinct synthetic strategies to access hedycoropyrans A and B, which led us to establish a facile synthetic route for des-hydroxy (-)-hedycoropyran B (ent-rhoiptelol B) from simple and readily accessible building blocks of 4-allylanisole and vanillin, employing Sharpless asymmetric epoxidation, CBS reduction, and an intramolecular AgOTf-catalyzed oxa-Michael reaction of suitably functionalized hydroxy-ynone as key transformations. The investigations disclosed herein will provide insights into designing novel synthetic routes for THP-DAH-derived natural products.

Stereoselective Synthesis of Polysubstituted Tetrahydropyranones via Acid-Promoted Cyclization of β-Silyl-γ-ethylidene-γ-butyrolactones with Aldehydes and Ketones

β-Silyl-γ-ethylidene-γ-butyrolactone upon one-pot treatment with aldehydes and ketones in the presence of Lewis acids underwent a tandem Hosomi-Sakurai/Prins cyclization to give polysubstituted tetrahydropyranones stereoselectively. Various aldehydes and ketones can be used in this reaction to produce the corresponding tetrahydropyranones. The optical purity of the starting γ-butyrolactone was substantially retained in the resulting tetrahydropyranones.

Modular Total Synthesis of iso-Archazolids and Archazologs

Full details on the design, development, and successful implementation of suitable synthetic strategies directed toward the total synthesis of iso-archazolids and archazologs are reported. Both a biomimetic and a multistep total synthesis of iso-archazolid B, the most potent and least abundant archazolid, are described. The bioinspired conversion from archazolid B was realized by a high-yielding 1,8-Diazabicyclo[5.4.0]undec-7-ene catalyzed one-step double-bond shift. A highly stereoselective total synthesis was accomplished in 25 steps, involving a sequence of highly stereoselective aldol reactions, an efficient aldol condensation to forge two elaborate fragments, and a challenging ring-closing metathesis macrocyclization with an unusual Stewart-Grubbs catalyst. These strategies proved to be generally useful and could be successfully implemented for the preparation of three novel iso-archazolids as well as five novel archazologs, lacking the thiazole side chain. A wide variety of further archazolids and archazologs may now be targeted for exploration of the promising anticancer potential of these polyketide macrolides.

Synthesis of a 3-Amino-2,3-dihydropyrid-4-one and Related Heterocyclic Analogues as Mechanism-Based Inhibitors of BioA, a Pyridoxal Phosphate-Dependent Enzyme

Amiclenomycin (ACM) is a chemically unstable antibiotic with selective activity against Mycobacterium tuberculosis (Mtb) due to mechanism-based inhibition of BioA, a pyridoxal 5'-phosphate (PLP)-dependent aminotransferase. The first-generation ACM analogue dihydro-2-pyridone 1 maintains a similar bioactivation mechanism concluding with covalent labeling of the PLP cofactor. To improve on 1, we report the synthesis of dihydro-4-pyranone 2, dihydro-4-pyridone 3, and dihydro-4-thiopyranone 13, which were rationally designed to boost the rate of enzyme inactivation by lowering the pK_a of their α-protons. We employed a unified synthetic strategy for construction of the desired heterocycles featuring α-amino ynone generation followed by 6-endo-dig cyclization. However, competitive 5-exo-dig cyclization, β-elimination of the ynone, and dimerization of the resultant α-amino carbonyls all complicated the syntheses of the dihydro-4-pyranone and dihydro-4-pyridone scaffolds. These obstacles were overcome by Teoc protection of the β-amino group in the assembly of 3 and Boc-MOM protection of the α-amino group in the synthesis of 2, enabling the efficient construction of 2 and 3 in seven steps from commercially available starting materials. Dihydro-4-pyridone 3 possessed improved enzyme inhibition as measured by its k_inact value against BioA.

Contemporary Strategies for the Synthesis of Tetrahydropyran Derivatives: Application to Total Synthesis of Neopeltolide, a Marine Macrolide Natural Product

Tetrahydropyrans are structural motifs that are abundantly present in a range of biologically important marine natural products. As such, significant efforts have been paid to the development of efficient and versatile methods for the synthesis of tetrahydropyran derivatives. Neopeltolide, a potent antiproliferative marine natural product, has been an attractive target compound for synthetic chemists because of its complex structure comprised of a 14-membered macrolactone embedded with a tetrahydropyran ring, and twenty total and formal syntheses of this natural product have been reported so far. This review summarizes the total and formal syntheses of neopeltolide and its analogues, highlighting the synthetic strategies exploited for constructing the tetrahydropyran ring.

Cationic palladium(ii)-catalyzed dehydrative nucleophilic substitutions of benzhydryl alcohols with electron-deficient benzenethiols in water

An efficient direct nucleophilic substitution of benzhydryl alcohols with electron-deficient benzenethiols using cationic Pd(ii) catalysts as Lewis acids in water is reported.

Formation of highly substituted tetrahydropyranones: application to the total synthesis of cyanolide A

A new tetrahydropyranone synthesis has been developed that leads to cis-2,6-disubstituted 3,3-dimethyltetrahydropyran-4-one rings by condensation of an aldehyde and a hydroxy silyl enol ether. The reaction works with a variety of aldehydes to produce the tetrahydropyranone products in moderate to high yields. This new method was applied to the enantioselective synthesis of cyanolide A and its aglycone.

Gold (III) Chloride-Catalyzed 6-endo-trig Oxa-Michael Addition Reactions for Diastereoselective Synthesis of Fused Tetrahydropyranones

Alkynones were treated with boron trifluoride diethyl etherate to generate β-iodoallenolates, which underwent intramolecular aldol reactions to produce cycloalkenyl alcohols. Diastereoselective oxa-Michael ring closure could then be induced by treatment with a catalytic amount of gold(III) chloride, affording highly functionalized tetrahydropyran-containing ring systems.

Biotransformation of turmerones by Aspergillus niger

Biotransformation studies conducted on (+)-(S)-ar-turmerone (1) and (+)-(S)-dihydro-ar-turmerone (2) by the fungus Aspergillus niger have revealed that 1 was metabolized to give four oxidized metabolites, (+)-(7S)-hydroxydehydro-ar-todomatuic acid (3), (+)-(7S,10E)-12-hydroxydehydro-ar-todomatuic acid (4), (+)-(7S,10E)-7,12-dihydroxydehydro-ar-todomatuic acid (5), and (+)-(7S)-15-carboxy-9,13-epoxy-7-hydroxy-9,13-dehydro-ar-curcumene (6), and (+)-(S)-dihydro-ar-turmerone (2) was metabolized to (+)-7,11-dihydroxy-ar-todomatuic acid (7). Metabolites 3-7 were characterized using spectroscopic techniques. Metabolites 3-7 inhibited acetylcholinesterase (AChE) although less so than the parent substrates.

Bismuth-triflate-catalyzed Prins–Ritter reaction — An efficient synthesis of 4-amidotetrahydropyrans

Bismuth(III) triflate is found to be an effective Lewis acid in catalyzing a three-component reaction of homoallylic alcohol, carbonyl compound, and nitrile at ambient temperature via the Prins–Ritter sequence to furnish 4-amidotetrahydropyrans in high yields, all with cis selectivity. Spirocyclic 4-amidotetrahydropyrans are obtained in case of cyclic ketones.Key words: Prins cyclization, bismuth(III) triflate, homoallyl alcohol, 4-amidotetrahydropyrans.

Activation of 1,1-Difluoro-1-alkenes with a Transition-Metal Complex: Palladium(II)-Catalyzed Friedel–Crafts-Type Cyclization of 4,4-(Difluorohomoallyl)arenes

Cationic palladium(II) ([Pd(MeCN)4](BF4)2) provides the first transition-metal-catalyzed method for electrophilic activation of electron-deficient 1,1-difluoro-1-alkenes, which allows their Friedel-Crafts-type cyclization with an intramolecular aryl group via a Wacker-type process. By using BF3.OEt2, the cyclization was effected by a catalytic amount of the palladium without its reoxidation.