Allylic Stereocontrol of the Intramolecular Diels-Alder Reaction

Recent advances in the Overman rearrangement: synthesis of natural products and valuable compounds

This review documents the reports since 2005 on the Overman rearrangement, an important C–N bond forming reaction that has been profoundly used in the synthesis of natural products, synthetic intermediates, building blocks and valuable compounds.

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.

Origins of regio- and stereochemistry in type 2 intramolecular N-acylnitroso Diels-Alder reactions: a computational study of tether length and substituent effects

Quantum mechanical calculations have been used to investigate type 2 intramolecular N-acylnitroso Diels-Alder reactions. Experimentally observed regioselectivities and diastereoselectivities of these reactions have been reproduced using B3LYP/6-31+G(d) DFT calculations. The factors that govern selectivity (i.e., tether length, tether substitution and diene substitution) were systematically investigated. Tethers less than 6 carbon atoms lead to 1,3-regioisomers due to conformational restrictions. Substituents on the tether lead to diastereoselective outcomes dictated by transannular interactions in the transition states. The modest diastereoselectivity of diene-substituted substrates is rationalized as arising from reduction of eclipsing interactions in the flattened diene transition states. This method should prove valuable for planning syntheses involving type 2 intramolecular Diels-Alder reactions.

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.

Intramolecular Diels−Alder Reactions of Ester Linked 1,3,9-Decatrienes: Cis/Trans Selectivity in Thermal and Lewis Acid Promoted Reactions of Ethylene-Tethered and Benzo-Tethered Systems

High cis (i.e., endo) diastereoselectivities are witnessed in heat-promoted intramolecular Diels-Alder (IMDA) reactions of ethylene-tethered hexadienyl acrylates. The cis stereoselectivity is improved by promotion with Et2AlCl. The first examples of Et2AlCl catalyzed intramolecular Diels-Alder reactions of ester-activated dienophiles are reported. In contrast, the corresponding benzo-tethered hexadienyl acrylates undergo moderately trans (i.e., exo) selective IMDA reactions. Very high trans stereoselection is obtained upon promotion with ATPH. The outcomes of these reactions are essentially insensitive to dienophile (C10) geometry and substitution. DFT (B3LYP/6-31+G(d)) computed cis/trans product distributions-based on Boltzmann transition structure populations-are in good agreement with the experimental results. These computational investigations provide useful insights into the origins of stereoselection in these systems. The stereoselectivity exhibited by the ethylene-tethered hexadienyl acrylates is ascribed to stabilizing secondary orbital interactions at play in the cis-transition structures (TSs). In the benzo-tethered series, this effect is overridden by stabilizing pi-conjugative interactions, between the benzo moiety and the 1,3-diene component, which are stronger in trans TSs, compared to the cis TSs. The computed TS geometries generally exhibit advanced peripheral bond forming asynchronicity, with the tether carbonyl group in conjugation with the dienophile. Such TS features significantly weaken the stereodirecting influence of terminal dienophile substituents.

A Racemic Synthesis of an AB-Ring System of Hexacyclinic Acid

[structure: see text] An AB-ring system of the polyketide natural product hexacyclinic acid has been synthesized in racemic form. The key steps were an intramolecular Diels-Alder cyclization of an ester tethered 1,3-nonadiene-8-yne, which generated the B-ring, and a samarium diiodide mediated reductive annulation, which was used to form the A-ring.

Regio- and stereocontrolled Diels–Alder reaction tethered by Asp-Thr dipeptide

The Asp-Thr tethered Diels-Alder reaction of 1a was accomplished to provide 2a exclusively in a regio- and stereoselective manner.