Intramolecular Diels-Alder reactions of 4-vinylimidazoles

[reaction: see text] The synthesis of a variety of unsaturated ethers, esters, and amides derived from urocanic acid and their Diels-Alder reactions are reported. Propargylic ethers and esters cyclize with reasonable efficiencies, but the related mono- and unactivated olefins did not cyclize. On the other hand, the corresponding amines and amides, along with the doubly activated ester/amide derivatives participate in the cycloaddition reaction.     

Intramolecular Diels-Alder reactions of N-substituted oxazolones

The first intramolecular Diels-Alder reactions of simple trienes featuring an N-substituted oxazolone as the dienophilic component have been investigated and are reported herein.     

Intramolecular Diels-Alder reactions of ester-linked 1,3,8-nonatrienes

[structures: see text] Penta-1,3-dienyl acrylates undergo kinetically controlled intramolecular Diels-Alder (IMDA) reactions and DFT calculations (B3LYP/6-31+G(d)) predict stereoselectivities that are in very good agreement with the experimental values. The nature of the diene C1 substituent has virtually no influence upon reactivity or trans/cis-stereoselectivity whereas terminal C9 dienophile substituents have a substantial effect on both the reactivity and stereoselectivity of these IMDA reactions. The TSs highlight contributions from strain in the developing tether-containing ring, and steric and electronic effects between tether and dienophile substituents, thus providing insight into the origins of IMDA reactivity and stereoselectivity.     

Intramolecular Diels-Alder reactions of unsaturated acylcyclopentadienes and cyclopentadienecarboxylates

The dimers of unsaturated acylcyclopentadienes and cyclopentadienecarboxylates have been synthesized. Pyrolyses provide monomers which undergo the intramolecular cycloadditions. Three types of adducts are obtained.     

Intramolecular ionic Diels-Alder reactions of alpha-acetylenic acetals

The intramolecular ionic Diels-Alder reaction of alpha-acetylenic acetals as a precursor of the propargyl cation has been investigated in the presence of Lewis acids and in protic acids. The reaction of diene-tethered alpha-acetylenic acetals (1-2) with formic acid yielded the regioselective intramolecular ionic Diels-Alder reaction products, bicyclodienal (9) and bicyclodienone (11) derivatives, in good yields.     

Intramolecular Diels-Alder reactions of siloxacyclopentene constrained trienes

The synthesis of siloxacyclopentene-constrained trienes 7 and 10 and studies of their IMDA cycloadditions are described. The use of appropriately chosen thermal or Lewis acid conditions allows for cycloadducts 3-6 to be obtained with high levels of diastereoselectivity. These adducts possess trans-relationships between the hydroxyl group and the adjacent ring fusion proton, a stereochemical relationship not previously attainable in IMDA reactions.     

Intramolecular Diels-Alder reactions of 5-vinyl-1,3-cyclohexadienes

[reaction: see text] The intramolecular Diels-Alder reaction of 5-vinyl-1,3-cyclohexadienes affording substituted tricyclo[3.2.1.0(2,7)]oct-3-enes is studied.     

Intramolecular hetero Diels-Alder reactions of vinyl allenes and aldehydes

Compounds containing a vinyl allene moiety linked to an aldehyde by a tether have been synthesised and shown to undergo the intramolecular hetero Diels-Alder reaction. Only one compound with stereochemistry consistent with an exo approach of the dienophile to the dienic portion of the vinyl allene was obtained in each case. Depending on the length of the tether and on the substitution pattern on the allene the reactions were spontaneous at room temperature or required heating or the presence of a Lewis acid to proceed.     

Intramolecular Diels-Alder reactions with furan: The gem-dialkyl effect revisited

Remote substituents that influence ground state state rotamer populations were found to be important controlling factors in the intramolecular Diels-Alder reaction with furan.     

Intramolecular Diels-Alder reactions using alpha-methylene lactones as dienophile

Several dienals were prepared and reacted in the presence of zinc metal with ethyl 2-bromomethylacrylate to provide in a Reformatsky-like reaction alpha-methylene lactones carrying a dienyl side chain. Thermolysis of these compounds (11, 21, 29, and 37) gave in an intramolecular Diels-Alder reaction the corresponding tricyclic cycloadducts (51, 52, 53, and 54). The cycloadditions took place with good diastereoselectivity and yields. The stereochemistry of the major isomer is in accordance with an endo transition state for cycloadducts 51, 52, and 54. In one instance (compound 58), the structure was supported by X-ray crystallography. In contrast to the other substrates, the nonatriene 29 cyclized to the exo product 53exo. The stereochemical situation could also be proven by NOESY NMR. However, the intramolecular Diels-Alder reaction did not work with furan as dienophile (compound 41) and substrate 50 featuring a densely functionalized tether connecting diene and alpha-methylene lactone.     

Intramolecular Diels-Alder and tandem intramolecular Diels-Alder/1,3-dipolar cycloaddition reactions of 1,3,4-oxadiazoles

The scope of intramolecular Diels-Alder and a novel tandem Diels-Alder/1,3-dipolar cycloaddition cascade of 1,3,4-oxadiazoles is disclosed. In the cases examined, the tandem cycloadditions construct three new rings with formation of four new C-C bonds and set all six stereocenters about a central six-membered ring in a single step including three contiguous and four total quaternary centers without a trace of a second diastereomer.     

Intramolecular Diels-Alder reactions of ethyl 2,4,9-decatrienoate and 2,4,10-undecatrienoate

The title compounds undergo intramolecular Diels-Alder reactions at 180°C to give ester-substituted hydrindane and decalin ring systems, with $\text{trans}$/$\text{cis}$ ratios of 43:57 and 50:50, respectively.     

Intramolecular Diels-Alder cycloadditions of fulvenes. Application to the kigelinol, neoamphilectane, and kempane skeletons

A variety of polycyclic ring skeletons (e.g., kigelinol, neoamphilectane, and kempene systems) can be prepared rapidly via intramolecular Diels-Alder cycloadditions (IMDA) of fulvenes. The length of the tethers and the diversity of the substituents on the fulvene core dictate the nature of the IMDA pathway. [reaction: see text]     

Intramolecular Diels-Alder reactions of oxazole-olefins: synthesis of the Rauwolfia alkaloids suaveoline and norsuaveoline

A full account of the highly stereoselective total synthesis of two indole alkaloids, suaveoline (4) and norsuaveoline (5), is presented. Central features of the synthetic strategy include the conversion of l-tryptophan methyl ester (12) into the oxazole derivative 11 and the intramolecular Diels-Alder reaction of the oxazole-olefin 19 leading to the pentacyclic pyridine derivative 21.     

Intramolecular Diels-Alder reaction of selenoaldehydes

Intramolecular Dieis-Alder reaction of selenoaldehydes which were generated from bis(trimethylsilyl) selenide and dienals gave the corresponding bicyclic adducts.     

Intramolecular Diels-Alder Reaction of Thioaldehydes

Intramolecular Diels-Alder reaction of thioaldehydes which were generated from bis(trimethylsilyl) sulfide and dienals gave the corresponding bicyclic adducts.