Strategies and approaches for constructing 1-oxadecalins

Oxa-[3+3] annulation of MBH-carbonates of propiolaldehydes with α-nitro/bromo ketones to access 2H-pyrans

A novel alkyne-assisted annulation reaction of MBH-carbonates of propiolaldehydes with α-nitro/bromo ketones is reported, providing a facile synthesis of substituted 2H-pyrans in good yields. This reaction divulges the inimitable reactivity of the MBH-carbonates of propiolaldehydes as C3-synthons wherein the alkyne functionality promoted the reaction without participating in the oxa-[3+3] annulation. The obtained products, having alkyne and ester functionalities, allowed further annulations to generate diverse pyrano[3,4-c]pyran-1-ones.

Organobase catalyzed straightforward synthesis of phosphinyl functionalized 2H-pyran cores from allenylphosphine oxides and 1,3-diones

A cost-effective system is revealed here to construct polyfunctionalized 2H-pyran cores containing phosphinyl groups with an organobase as a catalyst.

Achmatowicz Reaction and its Application in the Syntheses of Bioactive Molecules

Substituted pyranones and tetrahydropyrans are structural subunits of many bioactive natural products. Considerable efforts are devoted toward the chemical synthesis of these natural products due to their therapeutic potential as well as low natural abundance. These embedded pyranones and tetrahydropyran structural motifs have been the subject of synthetic interest over the years. While there are methods available for the syntheses of these subunits, there are issues related to regio and stereochemical outcomes, as well as versatility and compatibility of reaction conditions and functional group tolerance. The Achmatowicz reaction, an oxidative ring enlargement of furyl alcohol, was developed in the 1970s. The reaction provides a unique entry to a variety of pyranone derivatives from functionalized furanyl alcohols. These pyranones provide convenient access to substituted tetrahydropyran derivatives. This review outlines general approaches to the synthesis of tetrahydropyrans, covering general mechanistic aspects of the Achmatowicz reaction or rearrangement with an overview of the reagents utilized for the Achmatowicz reaction. The review then focuses on the synthesis of functionalized tetrahydropyrans and pyranones and their applications in the synthesis of natural products and medicinal agents.

Synthesis and biological evaluation of new pyranopyridine derivatives catalyzed by guanidinium chloride-functionalized γ-Fe2O3/HAp magnetic nanoparticles

Herein, we described the synthesis of γ-Fe₂O₃@HAP-GndCl MNPs as an efficient and recyclable nanocatalyst for synthesis of new pyranopyridine derivatives under solvent-free condition and then evaluated for theirs antioxidant and antifungal activities.

Diastereoselective and regiodivergent oxa-[3 + 2] cycloaddition of Achmatowicz products and cyclic 1,3-dicarbonyl compounds

Development of two novel oxa-[3 + 2] cycloaddition reactions of Achmatowicz products with 1,3-dicarbonyl compounds for rapid and highly efficient assembly of polycyclic furopyranones is described. Plausible mechanisms were proposed.

Highly torquoselective electrocyclizations and competing 1,7-hydrogen shifts of 1-azatrienes with silyl substitution at the allylic carbon

Highly torquoselective electrocyclizations of chiral 1-azatrienes are described. These 1-azatrienes contain an allylic stereocenter that is substituted with a silyl group and are derived in situ from condensation of γ-silyl-substituted enals with vinylogous amides. The ensuing stereoselective ring closures are part of a tandem sequence that constitutes an aza-[3 + 3] annulation method for constructing 1,2-dihydropyridines. Several mechanisms for the formal 1,7-hydrogen shift of these 1-azatrienes were evaluated computationally.

Efficient catalyst for tandem solvent free enantioselective Knoevenagel-formal [3+3] cycloaddition and Knoevenagel-hetero-Diels–Alder reactions

Tandem Knoevenagel-cycloaddition reactions forming bicyclic tetrahydro-2H-chromen-5(6H)-ones and tricyclic octahydro-2H-benzo[c]-chromen-1(6H)-ones are achieved with up to 98.8% ee in the presence of chiral LBA and titanium-isopropoxy-(S)-BINOLate.

Cu-catalyzed intramolecular aryl-etherification reactions of alkoxyl alkynes with diaryliodonium salts via cleavage of a stable C–O bond

Cu-catalyzed aryl-etherification reactions of alkoxyl alkynes with diaryliodonium salts are realized to produce oxo-heterocyclesviacleavage of a stable C–O bond.

Lipase-catalyzed regioselective domino reaction for the synthesis of chromenone derivatives

2H-Chromenones and 2-hydroxyl-2H-chromenones were synthesized under BPL- and PFL-catalyzed domino reactions from the same substrates respectively.

No acid required: 4π and 6π electrocyclization reactions of dienyl diketones for the synthesis of cyclopentenones and 2H-Pyrans

The 1,6-conjugate addition of nucleophiles to dienyl diketones produces either cyclopentenone or 2H-pyran products with high selectivity through either Nazarov (4π) or 6π electrocyclization, respectively. The outcome of the reaction is dependent upon the nature of the nucleophile used. Nucleophiles that are anionic or easily deprotonated exclusively produce cyclopentenones via Nazarov cyclization, whereas the neutral nucleophile DABCO promotes 6π cyclization to afford 2H-pyrans. Experimental evidence is presented for both retro-4π and -6π electrocyclization in these systems, lending support to the bifurcated mechanistic hypothesis proposed for these cyclizations.

Terminal olefins to chromans, isochromans, and pyrans via allylic C-H oxidation

The synthesis of chroman, isochroman, and pyran motifs has been accomplished via a combination of Pd(II)/bis-sulfoxide C-H activation and Lewis acid co-catalysis. A wide range of alcohols are found to be competent nucleophiles for the transformation under uniform conditions (catalyst, solvent, temperature). Mechanistic studies suggest that the reaction proceeds via initial C-H activation followed by a novel inner-sphere functionalization pathway. Consistent with this, the reaction shows reactivity trends orthogonal to those of traditional Pd(0)-catalyzed allylic substitutions.

The [3 + 3]-cycloaddition alternative for heterocycle syntheses: catalytically generated metalloenolcarbenes as dipolar adducts

The combination of two or more unsaturated structural units to form cyclic organic compounds is commonly referred to as cycloaddition, and the combination of two unsaturated structural units that forms a six-membered ring is formally either a [5 + 1]-, [4 + 2]-, [2 + 2 + 2]-, or [3 + 3]-cycloaddition. Occurring as concerted or stepwise processes, cycloaddition reactions are among the most useful synthetic constructions in organic chemistry. Of these transformations, the concerted [4 + 2]-cycloaddition, the Diels-Alder reaction, is by far the best known and most widely applied. However, although symmetry disallowed as a concerted process and lacking certifiable examples until recently, stepwise [3 + 3]-cycloadditions offer advantages for the synthesis of a substantial variety of heterocyclic compounds, and they are receiving considerable attention. In this Account, we present the development of stepwise [3 + 3]-cycloaddition reactions from virtual invisibility in the 1990s to a rapidly growing synthetic methodology today, involving organocatalysis or transition metal catalysis. With origins in organometallic or vinyliminium ion chemistry, this area has blossomed into a viable synthetic transformation for the construction of six-membered heterocyclic compounds containing one or more heteroatoms. The development of [3 + 3]-cycloaddition transformations has been achieved through identification of suitable and compatible reactive dipolar adducts and stable dipoles. The reactive dipolar species is an energetic dipolar intermediate that is optimally formed catalytically in the reaction. The stepwise process occurs with the reactive dipolar adduct reacting as an electrophile or as a nucleophile to form the first covalent bond, and this association provides entropic assistance for the construction of the second covalent bond and the overall formal [3 + 3]-cycloaddition. Organocatalysis is well developed for both inter- and intramolecular synthetic transformations, but the potential of transition metal catalysis for [3 + 3]-cycloaddition has only recently emerged. The key to the rapid development of the transition metal-based methodology has been recognition that certain catalytically generated vinylcarbenes are effective dipolar adducts for reactions with stable dipolar compounds, including aryl and vinyl ylides. In particular, metallo-enolcarbenes that are generated catalytically from conveniently prepared stable enoldiazoacetates or from donor-acceptor cyclopropenes are highly effective dipolar adducts for [3 + 3]-cycloaddition. The electron-donating oxygen of the silyl ether enhances electrophilic ring closure to the metal-bound carbon of the initial adduct from vinylogous addition, and this enhancement inhibits the alternative [3 + 2]-cycloaddition across the carbon-carbon double bond of the vinylcarbene. Catalytically generated metallo-enolcarbenes react under mild conditions with a broad spectrum of compatible stable dipoles, including nitrones, azomethine imines, ylides, and certain covalent precursors of stable dipoles, to form [3 + 3]-cycloaddition products having the β-ketoester functionality (in dihydrooxazines, tetrahydropyridazines, pyrazolidinone and pyraxole derivatives, dihydroquinolines, and quinolizidines, for example) in high yield. Two ways to access these metallo-enolcarbenes, either by dinitrogen extrusion from enoldiazoacetate esters or by rearrangement of donor-acceptor cyclopropenes, enhance the versatility of the process. The [3 + 3]-cycloaddition methodology is a complementary strategy to [4 + 2]-cycloaddition for the synthesis of heterocyclic compounds having six-membered rings. High levels of enantioselectivity are obtained with the use of chiral ligands on transition metal catalysts that include those on dirhodium(II) and silver(I).

Diastereoselective synthesis of highly functionalized cis-1-oxadecalines via 6-endo-tet-cyclizations of 2-C-branched sugars

Diastereoselective synthesis of highly functionalized cis-1-oxadecalines via 6-endo-tet-cyclizations of 2-C-branched sugars.

Facile access to cis-2,6-disubstituted tetrahydropyrans by palladium-catalyzed decarboxylative allylation: total syntheses of (±)-centrolobine and (+)-decytospolides A and B

cis-2,6-Tetrahydropyran is an important structural skeleton of bioactive natural products. A facile synthesis of cis-2,6-disubstituted-3,6-dihydropyrans as cis-2,6-tetrahydropyran precursors has been achieved in high regio- and stereoselectivity with high yields. This reaction involves a palladium-catalyzed decarboxylative allylation of various 3,4-dihydro-2H-pyran substrates. Extending this reaction to 1,2-unsaturated carbohydrates allowed the achievement of challenging β-C-glycosylation. Based on this methodology, the total syntheses of (±)-centrolobine and (+)-decytospolides A and B were achieved in concise steps and overall high yields.

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.

Establishing the concept of aza-[3 + 3] annulations using enones as a key expansion of this unified strategy in alkaloid synthesis

A successful enone version of an intramolecular aza-[3 + 3] annulation reaction is described here. Use of piperidinium trifluoroacetate salt as the catalyst and toluene as the solvent appears to be critical for a successful annulation. We also demonstrated for the first time that microwave irradiation can accelerate aza-[3 + 3] annulation reactions. An attempt to expand the scope of the enone aza-[3 + 3] annulation was made in the form of propyleine synthesis as a proof of concept. While synthesis of the enone annulation precursor was successfully accomplished, the annulation proved to be challenging and was only modestly successful.

Facile construction of structurally diverse thiazolidinedione-derived compounds via divergent stereoselective cascade organocatalysis and their biological exploratory studies

In this article, we present a new approach by merging two powerful synthetic tactics, divergent synthesis and cascade organocatalysis, to create a divergent cascade organocatalysis strategy for the facile construction of new "privileged" substructure-based DOS (pDOS) library. As demonstrated, notably 5 distinct molecular architectures are produced facilely from readily available simple synthons thiazolidinedione and its analogues and α,β-unsaturated aldehydes in 1-3 steps with the powerful strategy. The beauty of the chemistry is highlighted by the efficient formation of structurally new and diverse products from structurally close reactants under the similar reaction conditions. Notably, structurally diverse spiro-thiazolidinediones and -rhodanines are produced from organocatalytic enantioselective 3-component Michael-Michael-aldol cascade reactions of respective thiazolidinediones and rhodanines with enals. Nevertheless, under the similar reaction conditions, reactions of isorhodanine via a Michael-cyclization cascade lead to structurally different fused thiopyranoid scaffolds. This strategy significantly minimizes time- and cost-consuming synthetic works. Furthermore, these molecules possess high structural complexity and functional, stereochemical, and skeletal diversity with similarity to natural scaffolds. In the preliminary biological studies of these molecules, compounds 4f, 8a, and 10a exhibit inhibitory activity against the human breast cancer cells, while compounds 8a, 9a, and 9b display good antifungal activities against Candida albicans and Cryptococcus neoformans. Notably, their structures are different from clinically used triazole antifungal drugs. Therefore, they could serve as good lead compounds for the development of new generation of antifungal agents.

Synthetic studies toward (+)-cortistatin A

We describe herein the synthesis of a late-stage intermediate en route to cortistatin A. Key transformations included a Snieckus-like cascade sequence culminating in a 6π-electrocyclization, an alkylative dearomatization, and the stereoselective functionalization of the cortistatin A-ring. While the total synthesis we sought was not accomplished, the work sets the stage for several approaches to the preparation of novel analogs via diverted total synthesis.

Constructing the Architecturally Distinctive ABD-Tricycle of Phomactin A through an Intramolecular Oxa-[3 + 3] Annulation Strategy

Our efforts in constructing the ABD-ring of phomactin A through an intramolecular oxa-[3 + 3] annulation strategy is described. This struggle entailed finding a practical and efficient preparation of annulation precursor, and a realization of the unexpected competing regioisomeric pathway. The success entailed accessing the A-ring through Diels-Alder cycloaddition of Rawal's diene. Furthermore, the discovery that the regioisomers from the annulation existed as atropisomers with respect to the D-ring olefin and that they could be equilibrated to the desired ABD-tricycle, allowing large quantities of tricycle to be accessed.

Synthesis of chromans via Pd-catalyzed alkene carboetherification reactions

A new method for the construction of 2-substituted and 2,2-disubstituted chromans via Pd-catalyzed carboetherification reactions between aryl/alkenyl halides and 2-(but-3-en-1-yl)phenols is described. These reactions effect formation of a C-O bond and a C-C bond to afford the chroman products in good yield, and also provide stereoselective access to tricyclic chroman derivatives.