Carbohydrate dienophiles, Diels-Alder reactions

Carbohydrates have found widespread use as chiral auxiliaries in asymmetric Diels-Al-der reactions156. A recent example is a study conducted by Ferreira and colleagues157 who used carbohydrate based chiral auxiliaries in the Lewis acid catalyzed Diels-Alder reactions of their acrylate esters 235 with cyclopentadiene (equation 66). Some representative results of their findings, including the ratios of products 236 and 237, have been summarized in Table 9. The formation of 236 as the main product when diethylaluminum chloride was used in dichloromethane (entry 3) was considered to be the result of an equilibrium between a bidentate and monodentate catalyst-dienophile complex. The bidentate complex would, upon attack by the diene, lead to 236, whereas the monodentate complex would afford 236 and 237 in approximately equal amounts. The reversal of selectivity on changing the solvent from dichloromethane to toluene (entry 2 vs 3) remained unexplained by the authors. 

Diels-Alder reactions can be applied to the synthesis of cyclohexane rings by use of carbohydrate-derived dienophiles or dienes, and intramolecular processes allow the controlled elaboration of bicyclic systems. 

An attractive entry to the carbohydrate synthesis is provided by the cycloaddition reaction. Hetero-Diels-Alder reaction, either between an oxa-diene (a,[3-unsaturated aldehyde) and an nucleophilic dienophile, or between activated diene and carbonyl compound (usually an aldehyde), leads to dihydropyrans, which can be subsequently functionalized to sugars in the desired manner (Scheme 3). 

Diels-Alder reactions carbohydrates.2 This 3,y-unsaturated a-keto ester undergoes Diels-Alder reactions with electron-rich dienophiles to provide protected sugars. The thermal and pressure-promoted reaction of 1 with ethyl vinyl ketone provides the endo-adduct 2, which can be converted in two steps into the ethyl (3-mannopyranoside 3. The same reaction of 1 with (Z)-l-acetoxy-2-benzy-loxyethylene is even more endo-selective, and gives the endo-adduct 4, which can be converted into a derivative of benzyl DL-mannopyranoside. 

The dihydropyrans resulting from an oxa Diels-Alder reaction represent valuable intermediates for the synthesis of numerous natural compounds. In particular, they exhibit many structural elements of carbohydrates. It is therefore not surprising that both the normal electron demand cycloaddition of dienes to carbonyl dienophiles as well as the reaction of 1-oxa-l,3-butadienes with electron-rich alkenes have extensively been used for the synthesis of sugar derivatives. Nevertheless, various approaches to other natural products have been worked out by means of these powerful tools. 

The inverse electron demand hetero Diels-Alder reaction of 1-oxa-l,3-butadienes and electron-rich dienophiles is an extremely versatile tool in natural product synthesis. This cycloaddition represents the key step of numerous approaches not only to carbohydrates, but also to terpenes, alkaloids, polyethers, steroid derivatives and various biologically active metabolites. 

A very recent study presented by Dujardin et al. describes the complementary use of chiral enol ethers as dienophiles in oxa Diels-Alder reactions. This approach has yielded promising results with regard to the synthesis of enantio-merically pure carbohydrates [481]. Further noteworthy studies directed to the preparation of biologically active amino sugars from enaminoketones have been carried out in our group [110]. 

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). 

Horton et al. were interested in the synthesis of tetra-C-substituted carbocycles, and extensively studied asymmetric Diels-Alder reactions employing acyclic unsaturated carbohydrate derivatives. Thus, the thermal cycloaddition of the D-arabinose-derived c/i-dienophile (Z)-100 with cyclopentadiene gave endo-adduct 101 in excellent optical purity. The high diastereo-facial differentiation in this reaction arises from a highly favored conformation at the allylic center [76,77] (Scheme 10.33). Therefore, conformer 102 seems to be more favored than 103, where the latter suffers from severe allylic strain between the methoxycarbonyl and the C4-acetoxy group... 

SCHEME 10.46 Carbohydrate-directed hetero Diels-Alder reactions with aza-dienophiles. 

Cycloadditions are widely used methods for carbocycle construction [352], In this context, the Diels-Alder reaction has been studied in carbohydrate chemistry. Using this approach, the sugar can be used as the dienophile or as the diene with a large preference for the former. 

There are also special procedures which allow the preparation of sugars with only endo- or only exo-cyclic double bonds. The first group of compounds with endo-cyclic double bond may be prepared by total synthesis from non-carbohydrate precursors. The particularly useful hetero Diels-Alder reaction (O Fig. 3) allows one to obtain the dihydropyran skeleton either by reaction of a diene with a heterodienophile [3,17] or by reaction of a heterodiene with a normal dienophile [18]. 

An important development in the hetero Diels-Alder reaction came when Jurczak used high pressure to help improve the reactivity of dienes with aldehydes. 1-Methoxybutadiene, for example, was shown to react at 15-25 kbar (I bar = l(X) kPa) with a variety of unactivated aldehydes. - These reactions give predominantly endo adducts and allow for the use of simple alkyl and aromatic aldehydes as dienophiles. Jurczak also realized the importance of this reaction in the synthesis of carbohydrates and applied it to the construction of simple monosaccharide derivatives (see Section 2.5.2. ). " ... 

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