Dipolarophiles diastereofacial selectivity

Saito and coworkers have used C2-symmetrical alkenes derived from a variety of tartaric acid derivatives, for controller in discriminating 71 faces of dipolarophile in nitrone cycloaddition. Excellent endolexo and diastereofacial selectivity (de) are obtained. Endo transition state assembly shown in Eq. 8.50 could be responsible for the formation of preferred distereoisom-... 

There have been two main approaches to the development of dipolarophile facial selectivity (1) the use of chiral substrates, templates, and auxiharies and (2) the use of chiral rhodium catalysts [35]. In one of the earhest examples of chiral substrate selectivity, Pirmng and Lee reported a selective hydroxy-directed cycloaddition with chiral hydroxy-substituted vinyl ethers [95]. This effort was followed by a number of chiral template approaches to diastereocontrol, including the use of (R)- or (S)-phenylglycinol to form a cycHc phenyloxazinone for the facially selective cycloaddition of isomtinchnones [96, 97]. Padwa and Prein demonstrated acycHc diastereofacial control in the cycloaddi-... 

Similarly, variously substituted 3-imidazoline nitrone reagents and the related nitroxides (363 R = O) are popular targets and have demonstrated their reactivity toward various dipolarophiles, (e.g., styrene. Scheme 1.80) (420-431). However, only one group reports the use of a 2-imidazoline nitrone (432 34), which has demonstrated complete diastereofacial selectivity in its reactions with electron-... 

The high diastereofacial selectivity of the reaction is believed to be due to the highly ordered endo transition state in which the Sm(ll) cation is coordinated to the dipole and dipolarophile, a situation not possible in the exo transition state (Fig. 3.22). 

Poor (<4% de) to modest (56% de) amounts of diastereofacial selection is observed in the cycloaddition of nitrile oxides to optically active acrylates. The plan in each case, of course, was to use a chiral auxiliary which would preferentially shield one of the two ir-faces of the dipolarophile. Of the auxiliaries used, the sulfonamide esters derived from (+)-camphorsulfonyl chloride worked best, the menthyl esters derived from (-)-menthol the poorest (<4% de). As illustrated in Table 19, changes in both temperature and solvent had either no or little affect on the product ratios. Unlike Diels-Alder reactions, the addition of Lewis acids, specifically Et2AlCl, EtAlCh and TiCL, resulted in significant decreases in both the rate of cycloaddition and isolated yield, without an appreciable change in diastereomer ratio. ... 

Independent of the nature of the dipolarophile, the N-chiral nitrones cannot simultaneously attain good levels of diastereofacial selection on the nitrone and enantiofacial selection on the dipolarophile. These cycloadditions produce stereoisomeric mixtures. 

Diastereoselective 1,3-Dipolar Cycloadditions. Several examples of high diastereofacial selectivity with homochiral dipolarophiles have been reported. Cycloaddition of (1) with the cyclic dipolarophile (3) occurs with complete r-facial selectivity as a result of addition from the side opposite the bulky sUyloxymethyl group (eq 4). The key step in an asymmetric synthesis of (S)-(—)-cucurbitine involves cycloaddition of (1) with the a,p dehydrolactone (4) to give the pyrrolidine (5) as a single diastere-omer (eq 5). ... 

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