Lewis acid catalyzed intramolecular Diels-Alder reactions of acyclic (Z)-substituted 1,3-dienes

Synthesis of Highly Functionalized Hydrindanes via Sequential Organocatalytic Michael/Mukaiyama Aldol Addition and Telescoped Hydrozirconation/Cross-Coupling as Key Steps: En Route to the AB System of Clifednamides

The AB ring systems of the clifednamide family, polycyclic tetramate macrolactames (PoTeMs), were prepared by a new, convergent approach employing an intramolecular Diels-Alder (IMDA) reaction. Key steps comprise an organocatalytic Michael addition (>90% enantiomeric excess (ee)), a Mukaiyama aldol reaction for the convergent installation of a diene moiety, and a telescoped hydrozirconation/cross-coupling grafting an enone. The following IMDA furnished a highly functionalized hydrindane (diastereomeric ratio (dr) = 91:1) with the same configuration as the clifednamide scaffold. Advantages of this route are only one required protecting group, 13% overall yield over 9 steps (reduced from previously 17 steps/1.3% overall), and the potential access to the key intermediates in the clifednamide biosynthesis.

Saegusa Oxidation of Enol Ethers at Extremely Low Pd-Catalyst Loadings under Ligand-free and Aqueous Conditions: Insight into the Pd(II)/Cu(II)-Catalyst System

A highly efficient and practical Pd(II)/Cu(OAc)₂-catalyst system of Saegusa oxidation, which converts enol ethers to the corresponding enals with a number of diverse substrates at extremely low catalyst loadings (500 mol ppm) under ligand-free and aqueous conditions, is described. Its synthetic utility was demonstrated by large-scale applications of the catalyst system to important nature molecules. This work allows Saegusa oxidation to become a highly practical approach to preparing enals and also suggests new insight into the Pd(II)/Cu(II)-catalyst system for dehydrogenation of carbonyl compounds and decreasing Pd-catalyst loadings.

One-step preparation of novel 1-(N-indolyl)-1,3-butadienes by base-catalysed isomerization of alkynes as an access to 5-(N-indolyl)-naphthoquinones

A series of novel 1-(N-indolyl)-1,3-butadienes, as (1 : 1) mixtures of the (E) and (Z) dienes, was prepared in one step by base-catalysed isomerization of N-alkylindoles with a terminal butyne chain. The reaction conditions are mild, and in all cases the yields were very high (>90%). The (E) and (Z) dienes were separable by preparative TLC and could be fully characterized. This isomerization proceeded readily in the case of a butynyl chain, but didn't take place with a pentynyl chain. A mechanism was proposed for this reaction, based on previous studies on the isomerization of alkynes in basic media, and a key intermediate that supports the proposed mechanism could be isolated and fully characterized. A theoretical study of the proposed mechanism was performed by computational methods and the results validated the proposal. The reactivity of the synthesized dienes was studied in Diels-Alder reactions with p-benzoquinone, to obtain a small library of new 5-(N-indolyl)-1,4-naphthoquinones.The lack of reactivity in the case of the (Z) isomers was explained by calculation of the rotational curves of the central bond of the (Z) and (E) dienes. Finally, the cytotoxicity of the new 5-(N-indolyl)-1,4-naphthoquinones was tested against a panel of three cell lines.

Synthesis of the octahydronaphthalene core of nahuoic acid A via a B(C6F5)3-catalyzed intramolecular Diels-Alder (IMDA) reaction

Model tetraenal 9b underwent intramolecular Diels-Alder cycloaddition in CH₂Cl₂ at -10 °C under catalysis by the bulky Lewis acid B(C₆F₅)₃ to deliver as major components the cis-fused angularly-methylated octahydronaphthalene products, which are formed through the alternative exo orientations of the reacting moieties. One of these diastereomers features the relative and absolute configuration present in the core of nahuoic acid A, a natural product that acts as a cofactor-competitive inhibitor of the lysine methyl transferase SETD8. By contrast, catalysis of the reaction by Me₂AlCl at -40 °C selectively afforded the trans-fused isomer resulting from the Re-endo orientation.

Enantioselective first total synthesis of eujavanoic acid B through organocatalyzed IMDA reaction

The first total synthesis of the polyketide eujavanoic acid B has been accomplished using 1,3-propane diol as the starting material and involving Maruoka asymmetric allylation, Julia olefination, HWE olefination and organocatalyzed IMDA reaction as the key steps.

Total synthesis of myceliothermophins C, D, and E

The total synthesis of cytotoxic polyketides myceliothermophins E (1), C (2), and D (3) through a cascade-based cyclization to form the trans-fused decalin system is described. The convergent synthesis delivered all three natural products through late-stage divergence and facilitated unambiguous C21 structural assignments for 2 and 3 through X-ray crystallographic analysis, which revealed an interesting dimeric structure between its enantiomeric forms.

Synthesis of the decalin core of codinaeopsin via an intramolecular Diels-Alder reaction

We describe the synthesis of the decalin core of codinaeopsin (1), a tryptophan-polyketide hybrid natural product with promising antimalarial activity (IC50 4.7 μM, against Plasmodium falciparum), via an intramolecular Diels-Alder (IMDA) reaction. A convergent synthesis was developed to prepare the precursors for the IMDA reaction in 10 steps. The exo cycloadducts were derived from thermal, IMDA reactions of the substrates containing a Weinreb amide or ester conjugated dienophile, and the endo adducts were from Lewis acid promoted reactions of the substrates with a formyl group. Both exo and endo products of the IMDA were exclusively isolated and characterized by NMR spectroscopy. One endo cycloadduct was further confirmed with X-ray crystallography. Theoretical calculations reveal the influence of the substituents of the decalin core on the IMDA process.

Intramolecular Diels-Alder reactions of cycloalkenones: stereoselectivity, Lewis acid acceleration, and halogen substituent effects

The intramolecular Diels-Alder reactions of cycloalkenones and terminal dienes occur with high endo stereoselectivity, both thermally and under Lewis-acidic conditions. Through computations, we show that steric repulsion and tether conformation govern the selectivity of the reaction, and incorporation of either BF3 or α-halogenation increases the rate of cycloaddition. With a longer tether, isomerization from a terminal diene to the more stable internal diene results in a more facile cycloaddition.

Probing the Lewis Acid-Catalyzed Intramolecular Diels−Alder Cyclizations of Allylic Alkoxy-Substituted (Z)-1,3-Dienes

[reaction: see text] The Lewis acid-promoted Diels-Alder reaction of (E,Z,E)-trienal 1 provides not only the expected cis-fused cycloadduct 16 but also the trans-fused products 17 and 18. Trans-fused cycloadducts 17 and 18 are also products of the Lewis acid-promoted cyclization of (E,Z,E)-trienyl acetal 2. These products presumably derive from a stepwise cyclization pathway.