Diels-Alder reaction promoted

Annual Volume 71 contains 30 checked and edited experimental procedures that illustrate important new synthetic methods or describe the preparation of particularly useful chemicals. This compilation begins with procedures exemplifying three important methods for preparing enantiomerically pure substances by asymmetric catalysis. The preparation of (R)-(-)-METHYL 3-HYDROXYBUTANOATE details the convenient preparation of a BINAP-ruthenium catalyst that is broadly useful for the asymmetric reduction of p-ketoesters. Catalysis of the carbonyl ene reaction by a chiral Lewis acid, in this case a binapthol-derived titanium catalyst, is illustrated in the preparation of METHYL (2R)-2-HYDROXY-4-PHENYL-4-PENTENOATE. The enantiomerically pure diamines, (1 R,2R)-(+)- AND (1S,2S)-(-)-1,2-DIPHENYL-1,2-ETHYLENEDIAMINE, are useful for a variety of asymmetric transformations hydrogenations, Michael additions, osmylations, epoxidations, allylations, aldol condensations and Diels-Alder reactions. Promotion of the Diels-Alder reaction with a diaminoalane derived from the (S,S)-diamine is demonstrated in the synthesis of (1S,endo)-3-(BICYCLO[2.2.1]HEPT-5-EN-2-YLCARBONYL)-2-OXAZOLIDINONE. 

Chiral a,/ -unsaturated A-acyloxazolidinones have been regarded as a complement for type I dienophile reagents. Evans et al.4 reported a Diels-Alder reaction promoted by dialkyl aluminum chloride. In this reaction, chiral a,/ -unsaturated A-acyloxazolidinones were used as highly reactive and diaster-eoselective dienophiles. The stereogenic outcome of the Diels-Alder adducts... 

K Endo, T. Koike, T. Sawaki, O. Hayashida, H. Masuda, Y. Aoyama, "Catalysis by organic solids. Stereoselective Diels-Alder reactions promoted by microporous molecular crystals having an extensive hydrogen-bonded network , J. Am Chem Soc. 1997,119, 4117-4122. 

Recent progress in this area has led to the development of various, highly selective, asymmetric reactions by employing chiral dienes and dienophiles in the presence of Lewis acids.(12) On the other hand, little work has been done on asymmetric Diels-Alder reactions promoted by chiral Lewis acids.(13)... 

Figure 40. Catalytic, asymmetric Diels-Alder reaction promoted by La-Li-BINOL derivative complexes.

A kinetic resolution with respect to the racemic diene 66 could be achieved in the Diels-Alder reaction promoted by a chiral Lewis acid prepared from BHj in the presence of (S)-3,3 -diphenyl-l,r-binaphthalene-2,2 -diol and juglone (47) in the synthesis of (+)-emycin A and (+)-ochromycinone (35) [66] (compare previous investigations by Kelly [67]). 

There are a few reports of hetero-Diels-Alder Reactions promoted by LPDE. Intriguing stereoselectivity is observed for the [4 + 2] cyclization between Danishefsky s diene 77 and a-heteroatom-substituted aldehydes. For example, reaction of 77 with N-Boc-protected a-aminoaldehyde with 76 gave the threo isomer selectively, a result in keeping with a chelation-controUed process. In contrast, the threo diastereoselectivity observed could be reversed by changing the amino protecting group from A-Boc to A,A(-dibenzyl. In this instance, the erythro isomer was generated exclusively via a non-chelation-controlled transition state (Sch. 38) [89]. 

The Diels-Alder reaction of a relatively unreactive dienophile such as cyclopente-none can be effected with TiCU at a low temperature, although the exocyclic double bond of the product migrates to the more stable endo position (Eq. 145) [357]. A weaker Lewis acid, Ti(0-r-Pr)2Cl2, resulted in no reaction even under forcing conditions. Acceleration of reaction and the improvement of diastereoselectivity were achieved in a titanium Lewis acid-mediated intramolecular reaction as shown in Eq. (146) [358]. Other relevant Diels-Alder reactions promoted by titanium Lewis acids are summarized in Table 12. 

Narasaka has demonstrated that TADDOL-Ti dichloride prepared from TADDOL and Cl2Ti(OPr )2 in the presence of MS 4A acts as an efficient catalyst in asymmetric catalytic Diels-Alder reactions with oxazolidinone derivatives of acrylates, a results in extremely high enantioselectivity (Sch. 45) [112]. Narasaka reported an intramolecular version of the Diels-Alder reaction, the product of which can be transformed into key intermediates for the syntheses of dihydrocompactin and dihydromevinolin (Sch. 46) [113]. Seebach and Chapuis/Jurczak [114] independently reported asymmetric Diels-Alder reactions promoted by chiral TADDOL- and 3,3 -diphenyl BINOL-derived titanium alkoxides. Other types of chiral diol ligands were also explored by Hermann [115] and Oh [116]. 

Catalytic Asymmetric Diels-AIder Reactions and Hetero Diels-Alder Reactions Promoted by Chiral Lanthanide Catalysts... 

Titanium rivals boron for the amount of attention it has received in the development of catalytic enantioselective Diels-Alder reactions (for enantioselective Diels-Alder reactions promoted by stoichiometric amounts of chiral titanium... 

Scheme 76. A three-component tandem metathesis/Diels-Alder reaction promoted by second generation Grubbs catalyst to form functionabzed tricycles.

Scheme 19.59 Highly enantioselective aza-Diels-Alder reaction promoted by a primary amine-thiourea.

Several groups exploited the power of the intermolecular Diels-Alder reaction early in their syntheses for the formation of substituted cyclohexenes. In his synthesis of platencin, Nicolaou used a Danishefsky-like diene in an asymmetric Diels-Alder reaction for the synthesis of a chiral cylohexenone. Kanai and Shibasaki developed a catalytic asymmetric Diels-Alder reaction promoted by barium isopropoxide for the first step in their synthesis of Tamiflu. Danishefsky constructed the cyclohexene ring in paecilomycine A by employing a highly e t/o-selective Diels-Alder reaction of siloxydiene 115 and enyne dienophile 116 to yield target 117. ... 

Scheme 13.19 Catalytic asymmetric hetero Diels-Alder reactions promoted by Eu(hfc)3.

Table 13.20 Catalytic asymmetric quinone Diels-Alder reaction promoted by Gd and Sm PyBOX complexes...

Table 13.21 Catalytic Asymmetric Aza Diels-Alder reactions promoted by chiral Yb catalyst...

Scheme 24.6 Asymmetric inverse-electron demand Diels-Alder reaction promoted by bifunctional catalyst 128.

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