Broadly effective enantioselective Diels-Alder reactions of 1-amino-substituted-1,3-butadienes

[reaction: see text] A broad range of substituted 1-amino-1,3-butadienes undergo enantioselective Diels-Alder reactions with methacrolein in the presence of 5 mol % of Cr(III)-salen complex 1. The reactions are carried out conveniently, at room temperature, and they afford the cycloadducts in high yields and excellent ee's.     

Diels-Alder reactivity of 4-aryl-1-phthalimido-2-siloxy-1,3-butadienes

The reaction of (1Z,3E) and (1E,3E) 4-aryl-1-phthalimido-2-trialkylsiloxy-1,3-butadienes with maleimides and quinones has been studied. The observed exo-stereospecifity can be attributed to the simultaneous presence of the phthalimido and aryl groups, which produce strong hindrance during the endo approach.     

Lewis Acid promoted tandem intermolecular Diels-Alder/intramolecular allylation reactions of silyl-substituted 1,3-butadienes leading to multisubstituted 7-norbornenones and related polycyclic compounds

7-Norbornenones of exo,exo-disubstituted patterns were formed highly selectively in good yields from Lewis acid-promoted tandem intermolecular Diels-Alder/intramolecular allylation reactions. The intermolecular Diels-Alder reaction between 1,4-bis(trimethylsilyl)-1,3-butadienes or 1-trimethylsilyl-1,3-butadienes with maleic anhydride in the presence of newly sublimed AlCl3 afforded their corresponding cycloaddition adducts, which underwent AlCl3-mediated intramolecular allylation of the carbonyl group by the in situ generated allylsilane moiety affording 7-norbornenones of exo,exo-disubstituted patterns.     

Diastereoselective Diels-Alder reactions of N-sulfonyl-1-aza-1,3-butadienes with optically active enol ethers: an asymmetric variant of the 1-azadiene Diels-Alder reaction

The first detailed study of a room-temperature asymmetric Diels-Alder reaction of N-sulfonyl-1-aza-1,3-butadienes is reported enlisting a series of 19 enol ethers bearing chiral auxiliaries, with many providing highly diastereoselective (endo and facial diastereoselection) reactions, largely the result of an exquisitely organized [4+2] cycloaddition transition state. Three new, readily accessible, and previously unexplored auxiliaries rationally emerged from the studies and provide remarkable selectivities (two of these give 49:1 endo:exo and 48:1 facial selectivity) that promise to be useful in systems beyond those detailed.     

Hetero Diels-Alder reactions of 1-amino-3-siloxy-1,3-butadienes under strictly thermal conditions

The hetero Diels-Alder reaction of 1-amino-3-siloxy-1,3-butadiene (1a) with a range of unactivated aldehydes proceeds readily under remarkably mild conditions: at room temperature and in the absence of Lewis acid catalysts. The cycloadducts are formed in good yields and can be converted directly to the corresponding dihydro-4-pyrones using acetyl chloride. Ketones and imines are also reactive in hetero Diels-Alder reactions with this diene.     

Acyclic isoprenoid 1,3-dieneamines in the Diels-Alder reaction

The dieneamines obtained from 3-methyl-2-butenal and citral (Ia) and (Ib, c) enter into [4 + 2]-cycloaddition with monoethyl citrylidenemalonate (II) and prenylidenemalonate (III), respectively, forming the esters of substituted 1,3-cyclohexadiene-1-carboxylic acids with side chains of the isoprenoid type. The same or analogous cyclohexadiene can be obtained from citral dieneamine (Ib, c) and typical dienophiles (methyl acrylate, diethyl fumarate, etc.) in a two-stage path, including the initial production of derivatives of 2-amino-3-cyclohexene-1-carboxylic acid in the Diels-Alder reaction and then elimination of the amino group from the cyclic adducts. The isomeric composition of the mixture of cyclic adducts formed in the reaction of (Ib + Ic) with diethyl fumarate under strictly aprotic conditions correlates with the ratio of the structural isomers with a ³ and ³(9) bond (Ib, c) in the citral dieneamine. In the reaction of the dieneamine with the less reactive methyl acrylate the obtained mixture of cyclic adducts contains a significantly larger fraction of the isomer corresponding to the minor ³⁽⁹⁾ isomer of the dieneamine.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1089–1100, May, 1991.     

Diels-Alder reaction of methyl coumalate with 1,3-dienes

Reaction of methyl coumalate (1b) with 1,3-butadienes at 100° afforded the tetrahydrocoumarins (6) and the 4-methoxycarbonyltricyclo[3.2.1.0^2,7]octenes (7). The similar reaction of 1b with cyclohexadiene gave exclusively tetrahydronaphthalene-2-carboxylate. With cyclopentadiene, 1b afforded a product (10) resulting from the addition reaction in which cyclopentadiene acted as a dienophile. The reaction giving 6 is the first example of the reaction of a 2-pyrone as a dienophile.     

Synthesis of 4-aryl- and 4-alkyl-2-silyl-1,3-butadienes and their Diels-Alder/cross-coupling reactions

An ene-yne cross methasis of silyl-substituted alkynes and alkenes has been developed as a route to 4-aryl- and 4-alkyl-2-silyl-substituted 1,3-dienes. The dienes prepared were used to affect highly diastereoselective Diels-Alder reactions and then the silicon-substituted Diels-Alder cycloadducts were used in Hiyama cross-coupling reactions. The cross-coupling reactions enable these silicon dienes to be used as synthons for a variety of other dienes one might prepare and need access to. Two of the silicon-substituted Diels-Alder cycloadducts and one of the Hiyama cross-coupling products were also characterized by X-ray crystallography.     

2-Phenylsulfinylmethyl-1,3-butadiene in the thermal Diels-Alder reaction

Russian Chemical Bulletin -     

1-Phthalimido-4-(3-indolyl)-2-siloxy-1,3-butadienes: synthesis and Diels-Alder reactivity

New 1-phthalimido-4-(3-indolyl)-2-trialkylsiloxy-1,3-butadienes were easily prepared from 1,3-dichloropropanone and their configurations were established from NMR data. Their Diels-Alder reactivity with different maleimides and quinones was studied, high yields of the exo cycloadducts being obtained, as confirmed by X-ray diffraction studies.     

Cationic polymerization of dimethyl-1,3-butadienes

The cationic polymerizations of dimethyl-1,3-butadienes with various catalysts in methylene chloride and toluene have been investigated. The activity of catalysts decreased in the order WCl₆ > AcClO₄ > SnCl₄·TCA > BF₃OEt₂. The homopolymerization rate of dimethyl-1,3-butadienes with WCl₆, AcClO₄, and SnCl₄·TCA decreased in the order 1,3-dimethyl-1,3-butadiene > 2,3-dimethyl-1,3-butadiene > 1,2-dimethyl-1,3-butadiene > 2,4-hexadiene. The polymers prepared with WCl₆, SnCl₄.TCA, and BF₃OEt₂ were rubberlike polymers or white powders, whereas those prepared with AcClO₄ were oily oligomers. The 1,4-propagation increased in the order 1,2-dimethyl-1,3-butadiene < 1,3-dimethyl-1,3-butadiene < 2,3-dimethyl-1,3-butadiene < 2,4-hexadiene. This order may indicate that the steric effect of methyl group determine primarily the microstructure of the polymer. The relative reactivity of dimethyl-1,3-butadienes toward a styryl cation decreased in the order 1,3-dimethyl-1,3-butadiene > 1,2-dimethyl-1,3-butadiene > 2,3-dimethyl-1,3-butadiene > 2,4-hexadiene. This order may be explained in terms of the stability of the resulting allylic cation.     

Racemic and asymmetric Diels-Alder reactions of 1-(2-oxazolidinon-3-yl)-3-siloxy-1,3-butadienes

Achiral and chiral 1-(2-oxazolidinon-3-yl)-3-siloxy-1,3-butadienes were prepared from readily available starting materials. Although more stable than the parent 1-amino-3-siloxy dienes, the 1-(2-oxazolidinon-3-yl)-3-siloxy-1,3-butadienes are still very reactive in Diels-Alder reactions, somewhat more than 1,3-dialkoxy-1, 3-butadienes (e.g., Danishefsky's diene). The cycloadditions of the achiral and chiral dienes with several different dienophiles were examined. The reactions proceeded in good yield, with modest to high endo selectivity. The chiral dienes exhibited excellent facial selectivity in cycloadditions with alpha-substituted acroleins, maleic anhydride and N-phenylmaleimide. Upon reduction and hydrolysis of the cycloadducts, substituted cyclohexenones were obtained with ee's ranging from 22% to >98%.     

Stereoselective intramolecular hetero Diels-Alder reactions of 1-oxa-1,3-butadienes: a novel approach for the synthesis of complex annulated uracils

The intramolecular hetero Diels-Alder reactions of 1-oxa-1,3-butadienes 4, obtained from salicylaldehyde 1 via O-allylation followed by Knoevenagel condensation with barbituric acids 3 in the presence of hydrochloric acid as catalyst, affords the tetracyclic uracil derivatives 5 and 6 in a stereoselective manner and high overall yields.     

Open analogues of arcyriaflavin A. Synthesis through Diels-Alder reaction between maleimides and 1-aryl-3-tert-butyldimethylsiloxy-1, 3-butadienes

The preparation of a range of open analogues of arcyriaflavin A is described. The synthetic approach is based on the use of perhydroisoindole-1,3,5-triones as key intermediates, which were obtained via Diels-Alder methodology using 1-aryl-3-siloxy-1, 3-butadienes as starting materials. Fischer indolization and aromatization processes afforded different methoxy-substituted arylpyrrolocarbazoles. The stereochemistry and conformation of the Diels-Alder products and the regiochemistry of the indolization reactions are supported by NMR and molecular modeling studies.     

Sequential ene, Diels-Alder reactions of AF4-yne with 1,3,5-cycloheptatriene

(4,5-Dehydro)-1,1,2,2,9,9,10,10-octafluoro [2.2]paracyclophane (AF4-yne) undergoes an ene reaction with 1,3,5-cycloheptatriene, the adduct of which subsequently undergoes a further Diels-Alder reaction with a second equivalent of AF4-yne to give two stereoisomeric 2:1 adducts. A very small amount of the classic 1:1 Diels-Alder adduct also can be isolated from the reaction. Structure assignments of all products were determined by NMR through a series of H1-H1, H1-C13 one bond, and H1-C13 two and three bond correlation experiments as well as H1-H1 NOE experiments.     

Carbon-phosphorus bond formation and transformation in the reaction of 1,2-diaza-1,3-butadienes with alkyl phenylphosphonites

The reaction of 1,2-diaza-1,3-butadienes with dialkyl phenylphosphonites under solvent-free conditions proceeds via zwitterionic intermediate and gives, by precipitation, the stable ylidic α-phosphanylidene-hydrazones that, in turn, can be transformed into the corresponding 3-phenyl-2H-1,2,3λ⁵-diazaphospholes. The latter compounds are converted by hydrolytic cleavage in methanol-water (95:5) into E-hydrazonophosphonates that are useful for the preparation of the corresponding β-ketophosphonates and 4-[alkoxy(phenyl)phosphoryl]-1,2-diaza-1,3-butadienes. These peculiar 1,2-diaza-1,3-butadienes, bearing an alkoxy(phenyl)phosphoryl group on the carbon atom in position 4 are also able to add different nucleophiles, such as methanol or thiourea, giving 2-[alkoxy(phenyl)phosphoryl]-2-methoxyhydrazones and 5-phosphinate-substituted thiazol-4-ones, respectively.