Asymmetric synthesis hetero-Diels-Alder reaction

Zamojski, A. De novo synthesis of enantiopure carbohydrates preparation of ethyl (i-D- and 3-L-mannopyranosides by an asymmetrically induced hetero Diels-Alder reaction. Chemtracts Org. Chem. 1995, 8, 69-72. 

Stolz and co-workers and Funk and co-workers successfully performed model studies toward the related indole natural products communesin and perophoramidine featuring biogenetically inspired Diels-Alder reactions with ortho quinone methide imines. Based on these important preliminary discoveries. Quin and co-workers recently achieved the enantioselective total synthesis of (-l-)-pero-phoramidine via an asymmetric intermolecular hetero Diels-Alder reaction. ... 

Waldmann H. Asymmetric Hetero Diels-Alder Reactions Synthesis 1994 535-551... 

The use of chiral bis(oxazoline) copper catalysts has also been often reported as an efficient and economic way to perform asymmetric hetero-Diels-Alder reactions of carbonyl compounds and imines with conjugated dienes [81], with the main focus on the application of this methodology towards the preparation of biologically valuable synthons [82]. Only some representative examples are listed below. For example, the copper complex 54 (Scheme 26) has been successfully involved in the catalytic hetero Diels-Alder reaction of a substituted cyclohexadiene with ethyl glyoxylate [83], a key step in the total synthesis of (i )-dihydroactinidiolide (Scheme 30). 

The copper complex of these bis(oxazoline) compounds can also be used for hetero Diels-Alder reactions of acyl phosphonates with enol ethers.43 5 A favorable acyl phosphonate-catalyst association is achieved via complexation between the vicinal C=0 and P=0 functional groups. The acyl phosphonates are activated, leading to facile cycloaddition with electron-rich alkenes such as enol ethers. The product cyclic enol phosphonates can be used as building blocks in the asymmetric synthesis of complicated molecules. Scheme 5-36 shows the results of such reactions. 

Hetero Diels-Alder reactions are very useful for constructing heterocyclic compounds, and many important chiral molecules have thus been synthesized. Although the retro Diels-Alder reaction does not itself involve the asymmetric formation of chiral centers, this reaction can still be used as an important tool in organic synthesis, especially in the synthesis of some thermodynamically less stable compounds. The temporarily formed Diels-Alder adduct can be considered as a protected active olefin moiety. Cyclopentadiene dimer was initially used, but it proved difficult to carry out the pyrrolytic process. Pentamethyl cyclopentadiene was then used, and it was found that a retro Diels-Alder reaction could easily be carried out under mild conditions. 

The second asymmetric synthesis deals with the preparation of a C-difluoromethyl iminosugar. The key step of this synthesis is a hetero-Diels-Alder reaction with a chiral imine of difluoroacetaldehyde (Figure 6.32). ... 

A similar chiral bis(oxazoline)/Cu(II) catalyst is useful for the asymmetric hetero Diels-Alder reaction of Danishefsky s diene and glyoxylates [63] (Eq. 8A.39). Other bis(oxa-zoline)/M(OTf)2 (M = Sn, Mg) complexes are not effective. This method provides new routes to asymmetric aldol synthesis upon hydrolysis of the resulting adducts. 

The stereocontrolled synthesis of a-hydroxyakylated piperidines, a motif frequently encountered in natural products, represents a difficult synthetic challenge that was recently tackled by Hall and co-workers using the aza-variant of the Vaultier-Lallemand three-component reaction described in Scheme 12.14 [62]. One interesting feature of this reaction is the use of hydrazines, as masked amines, which allows the hetero-Diels-Alder reaction to operate on a normal electron demand manifold. Toure and Hall recently applied this powerful MCR to the asymmetric synthesis of (—)-methyl dihydropalustramate 192 [91], a degradation product and postulated biosynthetic precursor of (+)-palustrine (Scheme 12.27) [92]. 

A hetero-Diels-Alder reaction has been used to prepare racemic 2-ethoxycarbonyl-3,6-dihydro-2H-pyran (9). This ester 9 was resolved by Bacillus lentus protease to provide the R-isomer. Reduction, protection, and ozonolysis provided the bis-mesylate 10 (Scheme 26.9), a key intermediate in the synthesis of the PKC (protein kinase C) inhibitor LY333531 (ll).278 This resolution approach was used because it was more efficient than an asymmetric Diels-Alder reaction. 

H. Waldmann, Asymmetric hetero-Diels-Alder reactions, Synthesis 1994, 635-651. 

A short asymmetric synthesis of the 2-ketocarbacepham (27) involving as the initial step for the preparation of the starting piperidone, a hetero Diels-Alder reaction of the benzylimine derived from the enantiomer of Gamer s aldehyde with Danishesky s diene, has been described <02JOC598>. The key cyclization step to form the bicyclic P-lactam system was achieved from a P-amino acid precursor using the Mukaiyama reagent, 2-chloro-A-methylpyridinium iodide. 

N-Acylimines which may react as l-oxa-3-aza-l,3-butadienes represent a class of heterodienes which exhibit a close relationship to l-thia-3-aza-l,3-butadienes [13]. A very impressive application of such an l-oxa-3-aza-l,3-butadiene has been worked out by Swindell et al.[445]. The asymmetric hetero Diels-Alder reaction described therein opens a very elegant approach to the A-ring side chain of taxol. This synthesis takes advantage of the bulky chiral auxiliary attached to the dienophile 6-5 which upon cycloaddition with the l-oxa-3-aza-1,3-butadiene 6-4 yielded the 1,3-oxazine derivative 6-6. Subsequent hydrolysis, hydrogenolysis and transesterification gave the methyl ester of the taxol A-ring side chain 6-7 in good endo and excellent zr-facial selectivity (Fig. 6-2). 

This methodology which bases on Oppolzer s sultams has also been employed in the synthesis of 2,6-N,N-diacetyl-D-purpurosamidine C [475,476]. Hetero Diels-Alder reactions of carbonyl dienophiles are furthermore involved in the preparation of ( )-ketoheptulosic acid [477] and in the already mentioned asymmetric approach to the C-26-C-32 tetrahydropyran subunit of swinholide A [92]. 

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). 

The remarkable affinity of the silver ion for hahdes can be conveniently applied to accelerate the chiral palladium-catalyzed Heck reaction and other reactions. Enantioselectivity of these reactions is generally increased by addition of silver salts, and hence silver(I) compounds in combination with chiral ligands hold much promise as chiral Lewis acid catalysts for asymmetric synthesis. Employing the BINAP-silver(I) complex (8) as a chiral catalyst, the enantioselective aldol addition of tributyltin enolates (9) to aldehydes (10) has been developed." This catalyst is also effective in the promotion of enantioselective allylation, Mannich, ene, and hetero Diels-Alder reactions. 

Hetero Diels-Alder Reaction. The hetero-Diels-Alder reaction involving glyoxylate as the dienophile provides an efficient access to the asymmetric synthesis of monosaccharides. The hetero Diels-Alder reaction with methoxydienes proceeds smoothly with catalysis by BINOL-TiCl2 to give the cis product in high enantiomeric excess (eq 14). The dibromide affords a higher cis selectivity, however, with a lower enantioselectivity, particularly in the trans adduct. The product thus obtained can be readily converted to the lactone portion of HMG-CoA inhibitors such as mevinolin or compactin. ... 

Tietze, L F, Schneider, C, Montenbiuck, A, Asymmetric 1,6-induction in hetero-Diels-Alder reactions of chiral oxabutadienes for a de novo synthesis of enantiomerically pure carbohydrates Lewis acid dependent reversal of facial selectivity, Angew. Chem. Int. Ed. Engl., 33,980-982, 1994. 

The domino Knoevenagel condensation hetero-Diels-Alder reaction was used for the enantioselective total synthesis of the active anti-influenza A virus indole alkaloid hirsutine and related compounds by L.F. Tietze and co-workers. The Knoevenagel condensation was carried out between an enantiopure aldehyde and Meldrum s acid in the presence of ethyienediamine diacetate. The resulting highly reactive 1-oxa-1,3-butadiene underwent a hetero-Diels-Alder reaction with 4-methoxybenzyi butenyl ether ElZ = 1 1) in situ. The product exhibited a 1,3-asymmetric induction greater than 20 1. 

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