Hetero Diels-Alder reaction catalysed

Scheme 24.2 Asymmetric hetero-Diels-Alder reaction catalysed by TADDOLs 2.

Scheme 19.47 Asymmetric hetero-Diels-Alder reactions catalysed by BINOL-AlMe.

Scheme 5.9 Cu-catalysed hetero-Diels-Alder reactions with C2-symmetric bis(sul-foximines) ligand.

Bis(trifluoromethanesulfonylamides) ligand was also used by Mikami et al., in 1995, for the lanthanide-catalysed hetero-Diels-Alder reaction of Danishefsky s diene with butyl glyoxylate. This reaction produced the corresponding... 

SCHEME 13. A Lewis acid catalysed hetero-Diels-Alder reaction in aqueous solution... 

An extensive review of the hetero-Diels-Alder reactions of 1-oxabuta-1,3-dienes has been published. Ab initio calculations of the Diels-Alder reactions of prop-2-enethial with a number of dienophiles show that the transition states of all the reactions are similar and synchronous.Thio- and seleno-carbonyl compounds behave as superdienophiles in Diels-Alder reactions with cyclic and aryl-, methyl-, or methoxy-substituted open-chain buta-1,3-dienes.The intramolecular hetero-Diels-Alder reactions of 4-benzylidine-3-oxo[l,3]oxathiolan-5-ones (100) produce cycloadducts (101) and (102) in high yield and excellent endo/exo-selectivity (Scheme 39). A density functional theoretical study of the hetero-Diels-Alder reaction between butadiene and acrolein indicates that the endo s-cis is the most stable transition structure in both catalysed and uncatalysed reactions.The formation and use of amino acid-derived chiral acylnitroso hetero-Diels-Alder reactions in organic synthesis has been reviewed. The 4 + 2-cycloadditions of A-acylthioformamides as dienophiles have been reviewed. ... 

Extensive reviews of Diels-Alder reactions and hetero-Diels-Alder reactions in aqueous media have been presented. " " " " Micelles in the presence of catalytically active transition-metal ions catalyse the Diels-Alder reaction between 3-(/ -substituted phenyl)-l-(2-pyridyl)prop-2-en-l-ones with cyclopentadiene by a factor of 1.8 x 10 compared with the uncatalysed reaction in MeCN. " Diels-Alder reactions have been shown to be accelerated by encapsulation of both reactants by pseudospherical capsules assembled from self-complementary molecules (103). " ... 

In hetero-Diels-Alder reactions, the effect of ligand structure and acidity on the catalytic activity of lanthanide catalysts has been reviewed.191 The effect of different C(2)-symmetric bisoxazolines on the zinc(II)-catalysed hetero-Diels-Alder reaction of ethyl glyoxylate with conjugated 1,3-dienes has been investigated.192 The hetero-Diels-Alder reaction 4-dimethylamino-2-phenyl-l-thiabuta-1,3 -diene with methyl acrylate and /V-cnoyloxazolidinone produces cw-3,4-disubstituted 3,4-dihydro-2//-... 

Silyloxy-2-aldehydes can undergo hetero-Diels-Alder reaction with aldehydes to give useful heterocycles.350 A model reaction, H2C=C(OSiH3)-N=CH2 with formaldehyde, has been explored theoretically. Lewis acids such as boron trifluoride catalyse the reaction by coordinating to the aldehyde oxygen, making the aldehyde more electrophilic. Concerted and stepwise mechanisms for this process are considered. 

Asymmetric intramolecular hetero-Diels-Alder reactions mediated by a sulfinyl group not directly bonded to n systems involved in the cycloadditions have also been reported [177]. Thus, Lewis acid catalysed intramolecular cyclization of tt -sulfinyl-a,/Lunsaturated ketone, shown in Scheme 108, gave mixtures of... 

Further calculations were performed on 2,3-diaza-1,3-butadiene with different heterodienophiles such as ethene, formaldehyde and formaldimine showing the same exo oxygen or nitrogen lone pair preference [49] as well as on the nitro-soethene/ethene system [50]. Recently, ab initio studies have also been performed for the Lewis acid catalysed hetero Diels-Alder reaction of isoprene and sulfur dioxide by Sordo [51]. 

Chiral dienophiles, e.g. enol ethers and enamines, allow to conduct these hetero Diels-Alder reactions in a highly stereoselective manner. In an exemplary transformation described by Backvall et al. the nitrone 4-50 was formed as a single diastereomer upon treatment of the chiral enamine 4-48 with the nitroalkene 4-49 which was generated in situ from the seleno compound 4-47 [387]. Interestingly, the enamine 4-48 did not only act as dienophile, it also catalysed the initial base induced elimination of PhSeH from 4-47 (Fig. 4-11). 

Recent work from Ireland s group directed to the synthesis of monensin subunits takes as well advantage of the suitability of hetero Diels-Alder reactions for generating spiroketals [495]. In situ generation of the highly labile dienophile 7-19 from 7-18, subsequent cycloaddition to acrolein which acted as 1-oxa-1,3-butadiene and reduction yielded the spiroketal 7-21 accompanied by small amounts of diastereomeric byproducts. A mild acid catalysed rearrangement is the next key transformation to the spiroketal subunit 7-20 of monensin 7-22 (Fig. 7-5). 

Hetero Diels-Alder reactions with imino dienophiles have been employed as key step in several syntheses of naturally occuring alkaloids. With regard to stereoselective transformations, the approach to (S)-anabasin worked out by Kunz et al. impressively illustrates the high utility of natural carbohydrates as source of chirality in asymmetric synthesis [505]. The N-galactosyl imine 7-28 underwent a Lewis acid catalysed aza Diels-Alder reaction with Danishefsky s diene which proceeded with excellent induced diastereoselectivity to yield the adduct 7-29. A short sequence then afforded the desired alkaloid 7-30. This work also deals with the suitability of several other dienes and imino dienophiles for such transformations (Fig. 7-7). 

Diels-Alder type [4 -1- 2] cycloadditions of nonactivated coupling partners have been effected by various transition-metal catalyses [1]. Interestingly, the cationic ruthenium complex 68 catalyzed the intramolecular [4 -1- 2] cycloaddition between alkyne and enone moieties of 97 leading to 98 (Scheme 4.38) [83]. Such a formal hetero Diels-Alder reaction might proceed via a mthenacyclopentene 99 and an oxam-thenacycloheptadiene 100, which is an mthenium enolate species. 

Jurczak has reported an interesting approach to optically active 6-substituted-2-methoxy-5,6-dihydropyrans. The approach is based on a high-pressure catalysed hetero-Diels-Alder reaction of activated diene 49 with non-activated heterodienophile 52 (Scheme 7.13). These authors studied the eifects of catalyst... 

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