Amides chelated, Diels-Alder reactions

The above-described structures are the main representatives of the family of nitrogen ligands, which cover a wide spectrum of activity and efficiency for catalytic C - C bond formations. To a lesser extent, amines or imines, associated with copper salts, and metalloporphyrins led to good catalysts for cyclo-propanation. Interestingly, sulfinylimine ligands, with the chirality provided solely by the sulfoxide moieties, have been also used as copper-chelates for the asymmetric Diels-Alder reaction. Amide derivatives (or pyridylamides) also proved their efficiency for the Tsuji-Trost reaction. 

Copper Lewis acids have also found utility in hetero-Diels-Alder reactions. In these transformations the Lewis acid can activate either the diene or the dienophile and both types of reaction have been reported. Evans et al. have evaluated unsaturated acyl phosphonates [95] and acyl esters (amides) [87] as dienes in hetero-Diels-Alder reactions. The reactions proceed with excellent chemical efficiency and high stereoselectivity with as little as 0.2 mol % of the catalyst (Sch. 50). The reaction tolerates a variety of substituents on the diene and the dienophile. A square-planar model 226 wherein the phosphonate and the carbonyl groups form a chelate with copper, and addition occurring from the less hindered face, accounts for the selectivity observed. It is interesting to note that the reaction of 185 with phosphonate 228 gives 230, an inverse-electron-demand product, in preference to the normal Diels-Alder adduct 229. This unusual reaction pathway has been attributed to the electron-withdrawing capacity of the phosphonate group. 

Evans exploited this concept of chelation in the transition state with the development of a new generation of chiral auxiliaries based on conversion of amino alcohols to oxazolidinone derivatives. As shown in Table 11.16,229 conversion of these auxiliaries (to give the corresponding conjugated amide 268) gave excellent selectivity in the intramolecular Diels-Alder reaction, which generated mixtures of 269 and 270. Coordination with the Lewis acid catalyst generates a complex in which facial selectivity is enhanced relative to the... 

Oppolzer et al. developed useful auxiliaries based on a camphor precursor,231 and one of the best was a sultam (277). When this sultam was converted to an acrylamide (278) and reacted with butadiene, cycloadduct 280 was produced (85% yield, 95 % ee).232 Selectivity in this Diels-Alder reaction is due to chelation of the amide oxygen and the sulfonyl group, as shown in 279. Oppolzer and co-workers applied this methodology to a chiral synthesis of the aglycone of loganin.233... 

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