Dienes kinetic resolution

Cu(i(-catalyzed kinetic resolutions of racemic, cyclic 1,3-diene monoepoxides through the use of dialkylzinc [123] or trialkylaluminium reagents [124] have re-... 

Winterfeldt E. Enantiomerically Pure Cyclopentadienes Chem. Rev. 199393 827 843 Keywords synthesis of steroid cyclopentadienes, hydrindan dienes and their kinetic resolution... 

As the representative examples in Scheme 6.11 illustrate, similar stragies may be applied to the corresponding alkenyl ethers (vs. styrenyl ethers) [26], The Zr-catalyzed kinetic resolution/Ru-catalyzed metathesis protocol thus delivers optically pure 2-substituted di-hydrofurans that cannot be accessed by resolution of the five-membered ring heterocycles (see Scheme 6.8). It should be noted, however, that the efficiency of the Zr-catalyzed resolution is strongly dependent, and not in a predictable manner, not only on the presence but the substitution of the acyclic alkene site of the diene substrate. The examples shown in Scheme 6.11 clearly illustrate this issue. 

A wide variety of chiral sulfinyl substituents have been employed as chiral auxiliaries on both dienes162 and dienophiles163 in asymmetric Diels-Alder reactions. Carreno and colleagues164, for example, used Diels-Alder reactions of (Ss)-2-(p-tolylsulfinyl)-1,4-naphthoquinone (249) to separate racemic mixtures of a wide variety of diene ena-tiomers 250a and 250b via kinetic resolution and to obtain enantiomerically enriched... 

Kinetic resolution of chiral dienes has been realized by RCM with enantiomeri-cally pure molybdenum carbene complexes [888-890]. High enantiomeric excesses of recovered diene (> 99% ee) and cycloalkene (93% ee) could be obtained. 

Catalysts lacking phosphorus ligands have also been used as catalysts for allylic substitutions. [lr(COD)Cl]2 itself, which contains a 7i-accepting diolefin ligand, catalyzes the alkylation of allylic acetates, but the formation of branched products was only favored when the substitution reaction was performed with branched allylic esters. Takemoto and coworkers later reported the etherification of branched allylic acetates and carbonates with oximes catalyzed by [lr(COD)Cl]2 without added ligand [47]. Finally, as discussed in Sect. 6, Carreira reported kinetic resolutions of branched allylic carbonates from reactions of phenol catalyzed by the combination of [lr(COE)2Cl]2 and a chiral diene ligand [48]. 

Grubbs synthesized molybdenum catalyst 74 and reported the first example of asymmetric induction by kinetic resolution of diene using RCM with complex 74." After 90% conversion of the reaction of diene with 74, an enantiomeric excess (ee) of the recovered staring material 78 showed 84% and an absolute configuration is determined to be 5 ... 

The preparation and catalytic activity of chiral complex 2, based on the original Mo-alkylidene l,has been reported by Grubbs and Fujimura [8]. These workers report on the kinetic resolution of various dienes [9]. As illustrated by the resolution of 3, however, levels of enantiodifferentiation were typically low (krei<3). 

The catalytic kinetic resolution of various dienes through ARCM can be carried out in an efficient manner at 22 °C in the presence of 5 mol %4a [10]. As the data in Scheme 1 illustrate, 1,6-dienes 5-7 are resolved with excellent levels of enan-tiocontrol (krei>20) [11]. Chiral complex 4a readily promotes the resolution of allylic ethers 8-10 as well [12]. 

Scheme 1. Mo-catalyzed kinetic resolution of 1,6-dienes through ARCM...

One especially interesting kinetic resolution/ asymmetric epoxidation substrate is (R,S)-2,4-hexadien-3-ol (80) [77]. The racemic diene has eight different olefinic faces at which epoxidation can occur and thereby presents an interesting challenge to the selectivity of the epoxidation catalyst. The selectivity can be tested by using slightly less than 0.5 equiv. of oxidant (because the substrate is a racemate, the maximum yield of any one product is 50%). When the reaction was run under these conditions, the only product that was formed was the (l/f,2/f,3/f)-epoxy alcohol 81. 

A 1,3-substituted allene, which has axial chirality instead of carbon central chirality, has been prepared by a palladium-catalyzed cross-coupling of 4,4-dimethylpenta-l,2-dienylzinc chloride (83) with phenyl iodide (5c) or by that of l-bromo-4,4-dimethylpenta- 1,2-diene (84) with phenylzinc chloride [60] (Scheme 8F.20). The highest enantiomeric purity (25% ee) of the allene (S)-85 was obtained in the former combination with (f ,/ )-diop (1) as chiral ligand. It is interesting that the enantiomeric purity was independent of the ratio of the reagents though the reaction seems to involve a kinetic resolution of the racemic 83. 

Enantioselective RCM is achieved using the chiral Mo complex 79 [30]. Kinetic resolution occurred in the reaction of the racemic diene 80 catalysed by 79, and the cyclized product 81 with 93% ee was obtained, and the unreacted diene 80 (19%) of 99% ee was recovered. Also the optically active dihydrofuran 83 with 93% ee was obtained in 85% yield by enantioselective desymmetrization through RCM of triene 82 using the Mo complex 79 [30a]. 

Complexes of unsymmetrically substituted conjugated dienes are chiral. Racemic planar chiral complexes are separated into their enantiomers 84 and 85 by chiral HPLC on commercially available /f-cyclodextrin columns and used for enantioseletive synthesis [25]. Kinetic resolution was observed during the reaction of the meso-type complex 86 with the optically pure allylboronate 87 [26], The (2R) isomer reacted much faster with 87 to give the diastereomer 88 with 98% ee. The complex 88 was converted to 89 by the reaction of meldrum acid. Stereoselective Michael addition of vinylmagnesium bromide to 89 from the opposite side of the coordinated Fe afforded 90, which was converted to 91 by acetylation of the 8-OH group and displacement with EtjAl. Finally, asymmetric synthesis of the partial structure 92 of ikarugamycin was achieved [27],... 

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... 

Kinetic resolution of racemic dienes, such as 31, can be accomplished by ARCM using a chiral catalyst (Figure 28.2).51-54 60... 

One does not immediately associate a reaction which generates sp1 carbon centres with asymmetric inductive capability, however the development of non-racemic catalysts such as 40, 41 and 42 (Fig. 6) has allowed the efficient synthesis of optically active alkenes via the kinetic resolution (KR) of dienes and the desymmetrisation of meso-alkenes via either RCM or ROM-CM. For a short review of asymmetric metathesis see Ref. [85]. 

Kinetic resolution of racemic dienal iron tricarbonyl complexes by reduction using baker s yeast has been developed. Porcine pancreatic lipase-catalyzed transesterification of hydroxymethyl-substituted complexes have also been used to kinetically resolve diene complexes (Scheme 127). 

A kinetic resolution with respect to the racemic diene 66 could be achieved in the Diels-Alder reaction promoted by a chiral Lewis acid prepared from BHj in the presence of (S)-3,3 -diphenyl-l,r-binaphthalene-2,2 -diol and juglone (47) in the synthesis of (+)-emycin A and (+)-ochromycinone (35) [66] (compare previous investigations by Kelly [67]). 

A second example of the allylboration of a metal carbonyl containing substrate is a highly group- and face-selective allylboration of a meso iron-diene dialdehyde complex (eq 6). Efficient kinetic resolutions of racemic diene aldehyde-Fe(CO)3 complexes have also been demonstrated. ... 

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