Desymmetrization, alkenes

Stereoselective epoxidation of alkenes, desymmetrization of maso-TV-sulfonylaziri-dines, Baeyer-Villiger oxidation of cyclobutanones, Diels-Alder reactions of 1,2-dihydropyridines, and polymerization of lactides using metal complexes of chiral binaphthyl Schiff-base ligands 03CCR(242)97. 

Among recently described new Pd-catalysed enantioselective reactions, the ring opening of meso oxabicyclic alkenes with dialkyl zinc reagents in the presence of chiral P/P and P/N ligands reported by Tautens el al. constitutes a synthetically outstanding C-C bond-forming desymmetrization reaction. 

A hydroboration-oxidation sequence has been described for the desymmetrization of bicyclic hydrazino-alkenes. The use of BDPP as a chiral ligand on Rh provides the desired alcohol in 84% ee, following oxidation of the hydroborated... 

The catalytic desymmetrization shown in Scheme 5 involves a meso-tetraene substrate optically pure unsaturated siloxane 23 is obtained in >99% ee and 76% yield [16]. The unreacted siloxy ether moiety is removed to afford optically pure 24. Mo-alkylidenes derived from both enantiotopic terminal alkenes in 22 are likely involved. Since the initial metal-alkylidene generation is rapidly reversible, the major product arises from the rapid RCM of the matched segment of the tetraene. If any of the mismatched RCM takes place, a subsequent and more facile matched RCM leads to the formation of meso-bicycle. Such a byproduct is absent from the unpurified mixture containing 23, indicating the exceptionally high degree of stereodifferentiation induced by the chiral Mo com-... 

SE.3.1.2. Desymmetrization of gem-Dwarboxylates An equivalent of asymmetric carbonyl addition can be achieved by the alkylation of gem-dicarboxylates (Scheme 8E.17). The alkylation of gem-dicarboxylates, which are easily prepared by the Lewis acid-catalyzed addition of acid anhydrides to an aldehyde, converts the problem of differentiating the two enantiotopic 7t-faces of a carbonyl group into that of asymmetric substitution of either enantiotopic C-O bond of the gem-dicarboxylate. Although asymmetric induction may be derived from enantio-discrimination in the ionization step or in the alkene coordination step, the fast and reversible nature of alkene coordination suggests that the ionization step is more likely to be the source of enantio-discrimination. 

Trisubstituted cyclic alkenes have been kinetically resolved via a chiral dioxirane (4), generated in situ from the ketone and Oxone. A sequential desymmetrization and kinetic resolution of cyclohexa-1,4-dienes has also been achieved. The observed stereochemical results have been rationalized on the basis of a spiro-planar transition state model.93... 

Hoveyda and co-workers have developed chiral catalysts for asymmetric alkene metathesis. They have demonstrated that with their chiral molybdenum catalyst asymmetric syntheses of dihydrofurans through catalytic kinetic resolution by RCM and enantioselective desymmetrization by RCM are feasible processes (Scheme 40) <1998JA9720>. The use of Schrock s molybdenum catalysts for asymmetric alkene metathesis has been reviewed <2001CEJ945>. 

A Pd(ll) catalyst system with an oxazoline ligand 44 has been described that allows the desymmetrization of meso-Z-alkyl-2-propargylcyclohexane-l,3-diols in an asymmetric cyclization-carbonylation reaction. The products which contain a chiral quaternary carbon were obtained in excellent yields with high ee s (Scheme 56) <2006T9988>. 7-Hydroxy terminal <2005JOC3099> and internal <2006TL2793> alkenes can be converted to tetrahydrofurans by Pd(0)-catalyzed carboetherification reactions combined with a coupling of aryl or vinyl halides. 

A number of useful enantioselective syntheses can be performed by attaching a chiral auxiliary group to the selenium atom of an appropriate reagent. " Examples of such chiral auxiliaries include (49-53). Most of the asymmetric selenium reactions reported to date have involved Inter- or intramolecular electrophilic additions to alkenes (i.e. enantioselective variations of processes such as shown in equations (23) and (15), respectively) but others include the desymmetrization of epoxides by ringopening with chiral selenolates, asymmetric selenoxide eliminations to afford chiral allenes or cyclohexenes, and the enantioselective formation of allylic alcohols by [2,3]sigmatropic rearrangement of allylic selenoxides or related species. 

In addition to simple asymmetric hydroacylation of alkenes, highly efficient desymmetrization of alkenes has also been achieved by Wu and co-workers. Interestingly, the catalyst system (Rh(I)-BINAP) differentiated the enantiotopic faces of the olefins and thus the cyclopentanone products were obtained with excellent ee s. The authors also showed that neutral and cationic Rh(I)-BINAP complexes furnished dififerent products. The neutral catalyst system preferentially fiunished the cw-3,4-disubstituted cyclopentanone, and the cationic catalyst afforded the trans isomer. 

As mentioned above, the enantioselective desymmetrization of prochiral, as well as meso compounds, by using a Mizoroki-Heck coupling represents another pos-sibiUty to effect asymmetric transformations [81-84]. Therefore, the plane of symmetry in these precursors must be broken by differentiation of enantiotopic alkene-containing groups with a chiral palladium complex. 

Very recently, Shi and co-workers studied the asymmetric oxysulfenylation and oxyselenylation of unfunctionalized alkenes catalyzed by chiral Brpnsted acids [102]. An interesting process of desymmetrization of Z-alkenes was disclosed when using the V-triflyl phosphoramide 35 as catalyst and benzoic acid as the nucleophile (Scheme 14.36). 

Another desymmetrization reaction was introduced by the Tortosa group that relies on the high reactivity of cyclopropenes (Scheme 40) [93]. Strained alkenes... 

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