Olefins chiral dipolarophiles

The azomethine ylide was generated by treatment of A -benzyl-Af-(methoxy-methyl)-trimethylsilylmethylamine (155) with TFA and underwent the required cycloaddition step with chiral dipolarophile 156, stereocontrol being induced by Evan s auxiliary. The ot, p-unsaturated acid dipolarophile was tethered to a chiral oxazoladine in two easy, high-yielding steps. The auxiliary served three purposes to give asymmetric control to the reaction, to allow for separation of the reaction products by generating column separable diastereoisomers, and hnally to activate the olefin in the cycloaddition step (Scheme 3.45). 

The relative rate constants (fe ) do not account for the fact that approach of the nitrile oxide to the 7i-bond can occur from both olefinic diastereofaces with two regioisomeric modes of reaction (Scheme 6.14). In the case of achiral 1-alkenes, only one regioisomer is formed. With chiral dipolarophiles, preference for one of the two is usually found (diastereodifferentiation). The relative diastereofacial reactivity (fejH) is used to evaluate this effect (121). With ethylene, there are four possibilities of attack (two for each face corresponding to the different regio-isomers), and the of each is set as 0.25. In diastereodifferentiating cycloadditions, such as those with a-chiral alkenes, the major isomer generally results... 

The reactions of diazoalkanes 9.21 with alkenes lead to pyrazolines 9.22, which are thermally transformed into cyclopropanes. Similar transformations occur during thermal reactions of diazoesters. The use of diazoesters of chiral alcohols did not give useful results, so chiral residues have been introduced on the olefin dipolarophile. Meyers and coworkers [327] carried out the reaction of diazomethane 9.21 (R = R = H) and diazopropane 9.21 (R = R = Me) with chiral lactams 1.92 (R = i-Pr or ferf-Bu, R = Me). These 1,3-dipolar cycloadditions are regioselective, but only CH2N2 leads to an interesting stereoselectivity (Figure 9.9). Morever, when the RM substituent of lactam 1.92 is H, the reaction is no longer stereoselective. 

Most of the efforts towards the stereocontrolled synthesis of 4,5-dihydroisoxazoles were based on the use of chiral alkenes as dipolarophiles. Among these, the reactions of chiral acyclic olefins featuring an allylic stereocenter are of interest from both the practical and theoretical points of view 1 25,1 37 - 157. (More recent examples can be found in references 356-387.)... 

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