Formal Diels Research Articles

Radical‐Cation Cascade to Aryltetralin Cyclic Ether Lignans Under Visible‐Light Photoredox Catalysis

AbstractThe development of concise, sustainable, and cost‐effective synthesis of aryltetralin lignans, bearing either a fused lactone or cyclic ether, is of significant medicinal importance. Reported is that in the presence of Fukuzumi's acridinium salt under blue LED irradiation, functionalized dicinnamyl ether derivatives are converted into aryltetralin cyclic ether lignans with concurrent generation of three stereocenters in good to high yields with up to 20:1 diastereoselectivity. Oxidation of an alkene to the radical cation is key to the success of this formal Diels–Alder reaction of electronically mismatched diene and dienophile. Applying this methodology, six natural products, aglacin B, aglacin C, sulabiroin A, sulabiroin B, gaultherin C, and isoshonanin, are synthesized in only two to three steps from readily available biomass‐derived monolignols. A revised structure is proposed for gaultherin C.

Diels–Alder and Formal Diels–Alder Cycloaddition Reactions of Ynamines and Ynamides

Nitrogen‐substituted alkynes such as ynamines and ynamides are competent partners in [4+2] cycloaddition reactions. This microreview aims to discuss in a historical context the main achievements in the work on this class of reactions that leads to valuable nitrogen‐containing heterocycles in a single‐step fashion. In the last section, our own work in the field of inverse‐electron‐demand Diels–Alder reactions between ynamides and pyrimidines is discussed.

Quantifying Possible Routes for SpnF-Catalyzed Formal Diels-Alder Cycloaddition.

The Diels-Alder reaction is a cornerstone of modern organic synthesis. Despite this, it remains essentially inaccessible to biosynthetic approaches. Only a few natural enzymes catalyze even a formal [4 + 2] cycloaddition, and it remains uncertain if any of them proceed via the Diels-Alder mechanism. In this study, we focus on the [4 + 2] cycloaddition step in the biosynthesis of spinosyn A, a reaction catalyzed by SpnF enzyme, one of the most promising "true Diels-Alderase" candidates. The four currently proposed mechanisms (including the Diels-Alder one) for this reaction in water (as a first-order approximation of the enzymatic reaction) are evaluated by an exhaustive quantum mechanical search for possible transition states (728 were found in total). We find that the line between the recently proposed bis-pericyclic [J. Am. Chem. Soc. 2016, 138 (11), 3631] and Diels-Alder routes is blurred, and favorable transition states of both types may coexist. Application of the Curtin-Hammett principle, however, reveals that the bis-pericyclic mechanism accounts for ∼83% of the reaction flow in water, while the classical Diels-Alder mechanism contributes only ∼17%. The current findings provide a route for modeling this reaction inside the SpnF active site and inferring the catalytic architecture of possible Diels-Alderases.

Expeditious and Divergent Total Syntheses of Aspidosperma Alkaloids Exploiting Iridium(I)‐Catalyzed Generation of Reactive Enamine Intermediates

AbstractA new approach for the divergent total syntheses of (±)‐vincaminorine, (±)‐N‐methylquebrachamine, (±)‐quebrachamine, (±)‐minovine and (±)‐vincadifformine, each in less than 10 linear steps starting from a single δ‐lactam building block, is reported. Key to our route design is the late‐stage generation of reactive enamine functionality from stable indole‐linked δ‐lactams via a highly chemoselective iridium(I)‐catalyzed reduction. The efficiently formed secodine intermediates subsequently undergo either a formal Diels–Alder cycloaddition or a competitive Michael addition/reduction to access aspidosperma‐type alkaloids in excellent diastereoselectivities. Product selectivity could be controlled by changing the indole N‐protecting group in the reductive cyclization precursors. An asymmetric variant of this synthetic strategy for the synthesis of (+)‐20‐epi‐ibophyllidine is also described.

ChemInform Abstract: Organocatalytic Atroposelective Formal Diels—Alder Desymmetrization of N‐Arylmaleimides.

AbstractA newly developed organocatalytic atroposelective formal Diels—Alder desymmetrization reaction is applied for the synthesis of new succinimides bearing a chiral axis and three stereogenic centers.

Total Synthesis of Kingianins A, D, and F

A synthesis fit for a king: The total synthesis of (±)-kingianins A, D, and F has been achieved in ten steps. Key features include the gram-scale synthesis and partial reduction of a conjugated tetrayne to a (Z,Z,Z,Z)-tetraene, the domino 8π–6π electrocyclic ring closure of a (Z,Z,Z,Z)-tetraene, and the radical-cation-catalyzed formal Diels–Alder dimerization of functionalized bicyclo[4.2.0]octadiene precursors.

Howard E. Zimmerman (1926-2012)

Howard E. Zimmerman (1926-2012)

ChemInform Abstract: Formal Diels—Alder Reactions of Chalcones and Formylcyclopropanes Catalyzed by Chiral N‐Heterocyclic Carbenes.

AbstractThe reaction proceeds with complete enantioselectivity.

Formal Diels–Alder Reactions of Chalcones and Formylcyclopropanes Catalyzed by Chiral N-Heterocyclic Carbenes

Highly enantioselective (formal) hetero-Diels-Alder reactions between chalcones and formylcyclopropanes are disclosed. The challenging N-heterocyclic carbene (NHC)-bounded enolate intermediates from formylcyclopropanes were captured for new C-C bond forming reactions. The reaction products were obtained with high diastereo- and enantioselectivities and could be easily transformed to optically pure multisubstituted cyclohexane derivatives.

Cover Picture: Total Synthesis Confirms Laetirobin as a Formal Diels–Alder Adduct (Chem. Asian J. 2/2010)

Cover Picture: Total Synthesis Confirms Laetirobin as a Formal Diels–Alder Adduct (Chem. Asian J. 2/2010)

Photochemical C2−C6 Cyclization of Enyne−Allenes: Detection of a Fulvene Triplet Diradical in the Laser Flash Photolysis

A series of enyne-allenes, with and without benzannulation at the ene moiety and equipped with aromatic and carbonyl groups as internal triplet sensitizer units at the allene terminus, was synthesized. Both sets, the cyclohexenyne-allenes and benzenyne-allenes, underwent thermal C(2)-C(6) cyclization exclusively to formal ene products. In contrast, the photochemical C(2)-C(6) cyclization of enyne-allenes provided formal Diels-Alder and/or ene products, with higher yields for the benzannulated systems. A raise of the temperature in the photochemical cyclization of enyne-allene 1b' led to increasing amounts of the ene product in relation to that of the formal Diels-Alder product. Laser flash photolysis at 266 and 355 nm as well as triplet quenching studies for 1b,b' indicated that the C(2)-C(6) cyclization proceeds via the triplet manifold. On the basis of a density functional theory (DFT) study, a short-lived transient (tau = 30 ns) was assigned as a triplet allene, while a long-lived transient (tau = 33 micros) insensitive to oxygen was assigned as fulvene triplet diradical. An elucidation of the reaction mechanism using extensive DFT computations allowed rationalization of the experimental product ratio and its temperature dependence.

Evidence for nonstatistical dynamics in the Wolff rearrangement of a carbene.

Two 13C-labeled isomers of the formal Diels−Alder adduct of acetylmethyloxirene to tetramethyl 1,2,4,5-benzenetetracarboxylate have been synthesized. Flash vacuum thermolysis of these adducts leads to various isotopic isomers of acetylmethylketene, the ratios of which have been determined by NMR. The surprising finding that the principal product comes from methylpyruvoyl carbene rather than its more stable isomer diacetylcarbene is explained by MPWB1K density functional calculations, which show that the reactant probably undergoes a unimolecular rearrangement to a norcaradiene derivative prior to its fragmentation. Coupled-cluster calculations on the methylpyruvoyl carbene show that it is capable of undergoing three unimolecular isomerizations. The fastest is 1,2-acetyl migration to give acetylmethylketene directly. The next is rearrangement via acetylmethyloxirene to diacetylcarbene and thence by Wolff rearrangement to acetylmethylketene. The least-favorable reaction is degenerate rearrangement via 1,3-dimethyl-2-oxabicyclo[1.1.0]butan-4-one (the epoxide of dimethylcyclopropenone). The combined experimental and computational results indicate that Wolff rearrangement of the diacetylcarbene occurs with a 2.5:1 ratio of the methyl groups despite the fact that they are related by a twofold axis of symmetry in the carbene. Preliminary molecular dynamics simulations are consistent with this conclusion. Taken together, the results suggest that the Wolff rearrangement is subject to the same kind of nonstatistical dynamical effects detected for other kinds of thermally generated reactive intermediates.

Two Manifolds for Metal‐Catalyzed Intramolecular Diels–Alder Reactions of Unactivated Alkynes

Being unconventional: Although conventional metal-catalyzed [4+2] cycloadditions of inactivated dienynes are known to proceed by an oxidative cyclization/insertion mechanism, two entirely different pathways also result in formal Diels–Alder reactions (see scheme). These alternative mechanisms open new vistas for further functionalization of the products, as exemplified by an unprecedented cycloaddition/alkylation cascade.

Thermal C2−C6 Cyclization of Enyne−Allenes. Experimental Evidence for a Stepwise Mechanism and for an Unusual Thermal Silyl Shift

Enyne-allenes 4a-c bearing various cyclopropyl systems as radical clock reporter groups at the allene terminus have been synthesized and subjected to thermal C2-C6 cyclization. The ratio of ene versus formal Diels-Alder products could be rationalized on the basis of steric effects. Only the thermolysis of 4c, equipped with the fast diphenylcyclopropylcarbinyl radical clock, afforded a 1,3-butadienyl benzofulvene clearly formed via cyclopropyl ring opening. This finding provides unambiguous evidence for a stepwise mechanism of the C2-C6 cyclization making it possible to suggest a lifetime for the intermediate diradical of >1x10(-10) s (at 170 degrees C). An interesting corollary was the isolation of an unexpected silyl shift product in the thermolysis of all three enyne-allenes that allows explanation of the loss of the TIPS group in some of the Diels-Alder products. For a full understanding of the mechanism, silyl and hydrogen shift processes were interrogated using DFT.

Total Synthesis of (±)-Hedychenone: Trimethyldecalin Terpene Systems via Stepwise Allenoate Diene Cycloaddition

The total synthesis of hedychenone 1 is described. The cycloaddition of the hindered diene 2 and the allenecarboxylate 3 has been shown conclusively to proceed via the [2+2] cycloadduct 5 to give a 2:1 mixture of the desired formal Diels-Alder adducts, the exo and endo isomers 4xn and is thus a stepwise [4+2] cycloaddition. The exo isomer 4x was converted in four steps (reduction, oxidation, olefination, and desilylation) into hedychenone 1. [reaction: see text]

Enantioselective Total Synthesis of Avrainvillamide and the Stephacidins

In this article, full details regarding our total synthesis of avrainvillamide and the stephacidins are presented. After an introduction and summary of prior synthetic studies in this family of structurally complex anticancer natural products, the evolution of a final synthetic approach is described. Thus, a thorough description of three separate model studies is provided for construction of the characteristic bicyclo[2.2.2]diazaoctane ring system common to these alkaloids. The first and second approaches sought to build the core using formal Diels-Alder and vinyl radical pathways, respectively. Although these strategies failed in their primary objective, they fostered the development of a new and mechanistically intriguing method for the synthesis of indolic enamides such as those found in numerous bioactive natural products. The scope and generality of this simple method for the direct dehydrogenation of tryptophan derivatives is described. Finally, details of a third and successful route to the core of these alkaloids are described which features oxidative C-C bond formation. Specifically, the first heterocoupling of two different types of carbonyl species (ester and amide) is accomplished in good yield, on a preparative scale, and with complete stereocontrol. The information gained in these model studies enabled an enantioselective total synthesis of stephacidin A. The absolute configuration of these alkaloids was firmly established in collaboration with Professor William Fenical. A full account of our successful efforts to convert stephacidin A into stephacidin B via avrainvillamide is presented. Finally, the first analogues of these natural products have been prepared and evaluated for anticancer activity.

Lewis Acid Catalyzed Addition of Allylsilane to o‐Quinonediimides: Formal Diels—Alder Reaction versus Allylation.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Dynamic Kinetic Asymmetric Allylic Alkylations of Allenes.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Polycyclic Aromatic Compounds via Radical Cyclizations of Benzannulated Enyne‐Allenes Derived from Ireland—Claisen Rearrangement.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Reactions between Triazolinediones and Equilibrating Forms of Cycloheptatriene Derivatives Featuring 7,7-Spiro and 1,7-Fused Heterocyclic Rings

Nitrile oxide−azaheptafulvene adducts consist of rapidly equilibrating mixtures of fused (1) and spiro (2) isomers, the relative stabilities of which are nicely reproduced by B3LYP/6-31G* calculations. The reaction between these compounds and MTAD affords only two diastereomeric adducts [9 (dominant) and 10], both deriving from the reaction of MTAD with 1 even in cases in which that isomer could not be detected by NMR spectroscopy. These adducts are formal Diels−Alder adducts deriving from attacks on the two diastereotopic faces of the triene moiety of 1 and involving only the diene system adjacent to the amino substituent (N4−C4a=C5−C6=C7). The structures of the adducts are firmly supported by spectroscopic data and X-ray analysis, and so previous incorrect assignments are revised. The mechanism of the reaction between MTAD and 1 is briefly discussed.