Synthesis of chiral functional polyhydroxylated arenes via Mukaiyama aldol reaction from Perlin aldehydes

Functionalized aryl polyhydroxylated compounds could be of great synthetic value for natural product synthesis. However, the synthesis of such compounds usually requires multi-step synthesis or the usage of sensitive reagents. We present here a practically simple route for the synthesis of such functionalized arylpolyols from glycal derived α,β-unsaturated 2,3-dideoxy aldehyde as well as α,β-saturated 2,3-dideoxy aldehyde (Perlin aldehydes) via Mukaiyama cross aldol condensation in the presence of silyl enol ether and TiCl4. It was observed that the nature of the electronic substitution of the silyl enol ether does not play any role in the yield of the desired products. Further functionalization of the products has also been shown.

Bidentate Lewis Acid Catalyzed Domino Diels-Alder Reaction of Phthalazine for the Synthesis of Bridged Oligocyclic Tetrahydronaphthalenes

A domino process consisting of an inverse and a normal electron-demand Diels-Alder reaction is presented for the formation of bridged tri- and tetracyclic 1,2,3,4-tetrahydronaphthalenes catalyzed by a bidentate Lewis acid. The products were synthesized in a one-pot reaction from commercially available starting materials and contain up to six stereogenic centers. The tetrahydronaphthalenes were isolated as single diastereomers and are derivatives of phenylethylamine, which is well-known as a scaffold of amphetamine or dopamine.

Domino intramolecular Diels-Alder reactions to construct a 6/6/5/5 fused tetracyclic framework

Domino intramolecular Diels-Alder (IMDA) reactions towards the 6/6/5/5 fused tetracyclic natural products were developed in satisfactory yield and high stereoselectivity. Four rings, six contiguous stereocenters and four C-C bonds were formed in a single operation. 4-epi-Hydromitchellene B was also synthesised efficiently via this strategy.

Synthetic Strategies Toward the Decalin Motif of Maklamicin and Related Spirotetronates

Herein we describe a scalable approach to the decalin moiety of maklamicin. Key to the synthesis is an intramolecular Diels-Alder (IMDA) reaction that proceeds via an endo-axial transition state to generate the desired stereochemistry. We explored the diastereoselectivity of the IMDA reaction as a function of both chiral catalysis and acyclic precursor stereochemistry.

Asymmetric synthesis of 3,3,5,5-tetrasubstituted 1,2-dioxolanes: total synthesis of epiplakinic acid F

The first enantioselective total synthesis of epiplakinic acid F (1) was achieved through a pivotal step involving a radical-mediated asymmetric peroxidation of vinylcyclopropanes with molecular oxygen to construct highly substituted 1,2-dioxolanes.

A hybrid approach to new molecular scaffolds

Our various efforts toward the synthesis of a set of novel sugar hybrid scaffolds of several biologically active natural products such as taxol, steroids, β-lactams, and otteliones are presented. We have shown the application of the hybrid approach to design and rapidly generate a library of novel natural product-like compounds, which may have interesting biological features, using metathesis and/or cycloaddition reactions as key steps.

Highly selective domino multicyclizations for forming polycyclic fused acridines and azaheterocyclic skeletons

Highly selective four-component domino multicyclizations for the synthesis of new fused acridines and azaheterocyclic skeletons have been established by mixing common reactants in isobutyric acid under microwave irradiation. The reactions proceeded at fast rates and were conducted to completion within 20-30 min. Up to seven new chemical bonds, four rings, and four stereocenters were assembled in a convenient one-pot operation. The resulting hexacyclic and pentacyclic fused acridines and their stereochemistry have been fully characterized and determined by X-ray structural analysis.

Efficient multicomponent strategy to pentacyclic pyrazole-fused naphtho[1,8-fg]isoquinolines through cleavage of two carbon-carbon bonds

Multicomponent reactions of acenaphthylene-1,2-dione with diaroylmethanes and electron-rich pyrazol-5-amines have been discovered. A series of new and polyfunctionalized pentacyclic pyrazole-fused naphtho[1,8-fg]isoquinolines were regioselectively synthesized. The reactions were easy to perform simply by mixing three common reactants in AcOH. During these reaction processes, the insertion of active methylene of diaroylmethane into the sp(2)-sp(2) C-C bond of the cyclohexa-2,5-dienone ring was readily achieved and two C-C bonds were cleaved under transition-metal-free conditions.

Nature-inspired total synthesis of (-)-fusarisetin A

A concise, protecting group-free total synthesis of (-)-fusarisetin A (1) was efficiently achieved in nine steps from commercially available (S)-(-)-citronellal. The synthetic approach was inspired by our proposed biosynthesis of 1. Key transformations of our strategy include a facile construction of the decalin moiety that is produced via a stereoselective IMDA reaction and a one-pot TEMPO-induced radical cyclization/aminolysis that forms the C ring of 1. Our route is amenable to analogue synthesis for biological evaluation.

Fusarisetin A: Scalable Total Synthesis and Related Studies

Fusarisetin A (1) is a recently isolated natural product that displays an unprecedented chemical motif and remarkable bioactivities as a potent cancer migration inhibitor. We describe here our studies leading to an efficient and scalable total synthesis of 1. Essential to the strategy was the development of a new route for the formation of a trans-decalin moiety of this compound and the application of an oxidative radical cyclization (ORC) reaction that produces fusarisetin A (1) from equisetin (2) via a bio-inspired process. TEMPO-induced and metal/O(2)-promoted ORC reactions were evaluated. Biological screening in vitro confirms the reported potency of (+)-1. Importantly, ex vivo studies show that this compound is able to inhibit different types of cell migration. Moreover, the C(5) epimer of (+)-1 was also identified as a potent cancer migration inhibitor, while (-)-1 and 2 were found to be significantly less potent. The optimized synthesis is applicable on gram scale and provides a solid platform for analogue synthesis and methodical biological study.

On the effect of tether composition on cis/trans selectivity in intramolecular Diels-Alder reactions

Intramolecular Diels-Alder (IMDA) transition structures (TSs) and energies have been computed at the B3LYP/6-31+G(d) and CBS-QB3 levels of theory for a series of 1,3,8-nonatrienes, H(2)C=CH-CH=CH-CH(2)-X-Z-CH=CH(2) [-X-Z- = -CH(2)-CH(2)- (1); -O-C(=O)- (2); -CH(2)-C(=O)- (3); -O-CH(2)- (4); -NH-C(=O)- (5); -S-C(=O)- (6); -O-C(=S)- (7); -NH-C(=S)- (8); -S-C(=S)- (9)]. For each system studied (1-9), cis- and trans-TS isomers, corresponding, respectively, to endo- and exo-positioning of the -C-X-Z- tether with respect to the diene, have been located and their relative energies (E(rel) (TS)) employed to predict the cis/trans IMDA product ratio. Although the E(rel) (TS) values are modest (typically <3 kJ mol(-1)), they follow a clear and systematic trend. Specifically, as the electronegativity of the tether group X is reduced (X=O --> NH or S), the IMDA cis stereoselectivity diminishes. The predicted stereochemical reaction preferences are explained in terms of two opposing effects operating in the cis-TS, namely (1) unfavorable torsional (eclipsing) strain about the C4-C5 bond, that is caused by the -C-X-C(=Y)- group's strong tendency to maintain local planarity; and (2) attractive electrostatic and secondary orbital interactions between the endo-(thio)carbonyl group, C=Y, and the diene. The former interaction predominates when X is weakly electronegative (X=N, S), while the latter is dominant when X is more strongly electronegative (X=O), or a methylene group (X=CH(2)) which increases tether flexibility. These predictions hold up to experimental scrutiny, with synthetic IMDA reactions of 1, 2, 3, and 4 (published work) and 5, 6, and 8 (this work) delivering ratios close to those calculated. The reactions of thiolacrylate 5 and thioamide 8 represent the first examples of IMDA reactions with tethers of these types. Our results point to strategies for designing tethers, which lead to improved cis/trans-selectivities in IMDAs that are normally only weakly selective. Experimental verification of the validity of this claim comes in the form of fumaramide 14, which undergoes a more trans-selective IMDA reaction than the corresponding ester tethered precursor 13.

A New, Efficient and Stereoselective Synthesis of Tricyclic and Tetracyclic Compounds by Samarium Diiodide Induced Cyclisations of Naphthyl-Substituted Arylketones—An Easy Access to Steroid-Like Skeletons

In this report, we present the application of samarium diiodide induced cyclisations of naphthyl-substituted ketones towards an easy and stereoselective access to tri- and tetracyclic-functionalised compounds. Typical naphthalene derivatives were studied to investigate the scope and limitations of this novel cyclisation process. The model substrates studied demonstrate that the samarium ketyl cyclisations are essentially restricted to the formation of six-membered rings. The diastereoselectivity of these reactions is strongly influenced by the connection of the alkyl side chain to the naphthalene core. Gamma-naphth-1-yl-substituted ketones furnished cyclisation products, such as 17 or 22-26, as single diastereomers, whereas gamma-naphth-2-yl-substituted precursors gave mixtures of diastereomers--as demonstrated by the conversion of model compound 10 into tricyclic products 18 a/18 b, or that of cyclohexanone derivative 33 into tetracyclic diastereomers 34 a/34 b. Cyclic ketones as ketyl precursors furnished steroid-like tetracyclic skeletons; however, due to the cis/cis fusion of rings B/C and C/D these products have an "unnatural" bowl-like shape. Several of the cyclisation products have been identified by X-ray analyses, which not only proved the constitutions, but also the relative configurations and the preferred conformations. Steroid analogue 23 was subjected to subsequent transformations, which demonstrate that the styrene-like double bond of such compounds can be used for further structural diversification. First attempts to synthesise related azasteroids by incorporating nitrogen atoms into the ketone moiety are also reported. Thus, pyrrolidine derivatives 44 and 47 as well as piperidine derivatives 50 and 52 were subjected to samarium diiodide induced cyclisations. The expected tetracyclic products 48, 49 a/49 b, 51 and 53 a/53 b were obtained in moderate to good yields. The stereoselectivities observed follow the rules already established for the all-carbon precursors. The resulting products, bearing a nitrogen atom in ring D, are interesting azasteroid analogues with "unnatural" configuration.

Recent advances in oxasteroids chemistry

The continuous interest in the field of oxasteroids mainly arises from the fascinating aspects associated with their peculiar stereochemical features and their promising and sometimes unexpected biological activities. The recent development in the partial and total syntheses of oxasteroids are herein described.

An efficient synthesis of 2,3-dideoxy-α,β-unsaturated carbohydrate enals by mixed Lewis acid (HfCl4 and ZnI2) catalyzed hydration of glycals

A new, efficient method has been developed for converting acyl-, arylalkyl- and alkyl-protected glycals into corresponding 2,3-dideoxy-alpha,beta-unsaturated carbohydrate enals utilizing the in situ generated push-pull effect resulting from the synergistic combination of HfCl4 and ZnI2 in catalytic amounts. This new procedure eliminates the use of highly toxic Hg2+ ions and acidic conditions (0.01-0.02 N H2SO4), besides radically shortening the reaction time.

Design and synthesis of novel sugar-oxasteroid-quinone hybrids

A new class of sugar-oxasteroid-quinone hybrid molecules has been designed and synthesized involving an efficient enyne metathesis/Diels-Alder reaction strategy.