Rearrangement chiral, enantioselectivity

As an example of asymmetric intermolecular sigmatropic rearrangement, chiral Lewis acid promoted carbonyl-ene reactions have been utilized as the powerful tool for stereocontrolled carbon-carbon bond formation processes. In 2004, Mamoka et al. demonstrated enantioselective hetero-carbonyl-ene reaction of aldehydes and 2-methoxypropene catalyzed by chiral organoaluminum complex 64, giving enantiomerically enriched (3-hydroxymethylketones (Scheme 45) [78]. 

A DFT study of the origins of stereoselectivity in the aldol reaction of bicyclic amino ketones (20) with aromatic aldehydes has been reported (Scheme 18). ° Base-catalysed direct aldolization of a-alkyl-a-hydroxy trialkyl phosphonoacetates with aldehydes proceeds via a fully substituted glycolate enolate intermediate formed by a [l,2]-phosphonate-phosphate rearrangement. High enantioselectivity can be achieved by the application of chiral iminophosphorane catalysts. 

R)-Pantolactone is also prepared in a sequence involving Claisen rearrangement of the chiral glycolate (24), although with poor enantioselectivity... 

There are a number of powerful synthetic reactions which join two trigonal carbons to form a CC single bond in a stereocontrolled way under proper reaction conditions. Included in this group are the aldol, Michael, Claisen rearrangement, ene and metalloallyl-carbonyl addition reactions. The corresponding transforms are powerfully stereosimplifying, especially when rendered enantioselective as well as diastereoselective by the use of chiral controller groups. Some examples are listed in Chart 20. 

The Rh2(DOSP)4 catalysts (6b) of Davies have proven to be remarkably effective for highly enantioselective cydopropanation reactions of aryl- and vinyl-diazoacetates [2]. The discovery that enantiocontrol could be enhanced when reactions were performed in pentane [35] added advantages that could be attributed to the solvent-directed orientation of chiral attachments of the ligand carboxylates [59]. In addition to the synthesis of (+)-sertraline (1) [6], the uses of this methodology have been extended to the construction of cyclopropane amino acids (Eq. 3) [35], the synthesis of tricyclic systems such as 22 (Eq. 4) [60], and, as an example of tandem cyclopropanation-Cope rearrangement, an efficient asymmetric synthesis of epi-tremulane 23 (Eq. 5) [61]. 

The premier example of this process in an ylide transformation designed for [2,3]-sigmatropic rearrangement is reported in Eq. 15 [107]. The threo product 47 is dominant with the use of the chiral Rh2(MEOX)4 catalysts but is the minor product with Rh2(OAc)4. That this process occurs through the metal-stabilized ylide rather than a chiral free ylide was shown from asymmetric induction using allyl iodide and ethyl diazoacetate [107]. Somewhat lower enantioselectivities have been observed in other systems [108]. 

Because of the nature of the transition state in the pericyclic mechanism, optically active substrates with a chiral carbon at C-3 or C-4 transfer the chirality to the product, making this an enantioselective synthesis (see p. 1451 for an example in the mechanistically similar Claisen rearrangement). ... 

In 1997 the first asymmetric aza-Claisen rearrangement was reported by Overman et al. [55], which made use of diamines as bidentate ligands for Pd(II), allowing for moderate enantioselectivities. In the same year, Hollis and Overman described the application of the planar chiral ferrocenyl palladacycle 38 as a catalyst for the enantioselective aza-Claisen rearrangement of benzimidates 39 (Fig. 19) [56]. A related ferrocenyl imine palladacycle provided slightly inferior results, while a benzylamine palladacycle lacking the element of planar chirality was not able to provide any enantioselectivity [57]. 

Cohen F, Overman LE (1998) Planar-chiral cyclopalladated ferrocenyl amines and imines as enantioselective catalysts for allylic imidate rearrangements. Tetrahedron Asymmetry 9 3213-3222... 

Moyano A, Rosol M, Moreno RM, Lopez C, Maestro MA (2005) Oxazoline-mediated interannular cyclopalladation of ferrocene chiral palladium(II) catalysts for the enantioselective Azti-Claisen rearrangement. Angew Chem Int Ed 44 1865-1869... 

Ruble JC, Fu GC (1998) Enantioselective construction of quaternary stereocenters rearrangements of 0-acylated azlactones catalyzed by a planar-chiral derivative of 4-(pyrrolidino) pyridine. J Am Chem Soc 120 11532-11533... 

The Claisen rearrangement has attracted much attention as an attractive tool for the construction of new carbon-carbon bonds. Various catalytic systems have been developed to afford enantioselective versions of this process. On the other hand, relatively few chiral sulfur-containing ligands have been investigated for this type of reaction. As an example, Taguchi et al. have... 

The scope of this methodology was extended to the enantioselective rearrangement of difluorovinyl allyl ethers by these authors, furnishing a novel powerful tool for the synthesis of chiral p-substituted a,a -difluorocarbonyl compounds. As shown in Scheme 10.36, moderate to good enantioselectivities... 

Imidate esters can also be generated by reaction of imidoyl chlorides and allylic alcohols. The lithium anions of these imidates, prepared using lithium diethylamide, rearrange at around 0°C. When a chiral amine is used, this reaction can give rise to enantioselective formation of 7, 8-unsaturated amides. Good results were obtained with a chiral binaphthylamine.265 The methoxy substituent is believed to play a role as a Li+ ligand in the reactive enolate. 

Analogous rearrangement occurs under much milder conditions when the reactant is a zwitterion generated by deprotonation of an acylammonium ion. Substituted pyrrolidines were used as the chiral auxiliary, with the highest enantioselectivity being achieved with a 2-TBDMS derivative.267... 

An excellent enantioselective heterogeneous metal catalyst should be one in which the active metal sites are set in a chiral arrangement fulfilling the Ogston principles. Assuming no surface atom rearrangement, such a catalyst should furnish high enantioselectivities. 

Wipf and coworkers used a Claisen rearrangement of allyl phenyl ethers 4-309 followed by an enantioselective carboalumination using the chiral Zr-complex 4-310 and trimethyl aluminum (Scheme 4.67) [104]. After an oxidative work-up of the intermediate trialkylalane, the corresponding alcohols 4-311 were obtained with up to 80% ee and 78% yield. One can also transfer an ethyl group using triethyl aluminum with even better ee-values (up to 92%), but the yields were rather low (42%) due to a more sluggish oxidative cleavage of the Al-C bond. 

Finally, Katsuki and coworkers [271] described an enantioselective Ru-catalyzed domino reaction, which includes a sulfamidation of an aryl allyl sulfide 6/3-111 using the chiral Ru(salen)-complex 6/3-115, followed by a 2,3-sigmatropic rearrangement of the formed 6/3-112 to give N-allyl-N-arylthiotoluenesulfonamides 6/3-113. On hydrolysis, 6/3-113 yielded N-allyltoluenesulfonamides 6/3-114 (Scheme 6/3.33). The enantioselectivity ranged from 78 to 83% ee. 

The synthetic versatility and significance of the Zr-catalyzed kinetic resolution of exocyc-lic allylic ethers is demonstrated by the example provided in Scheme 6.9. The optically pure starting allylic ether, obtained by the aforementioned catalytic kinetic resolution, undergoes a facile Ru-catalyzed rearrangement to afford the desired chromene in >99% ee [20], Unlike the unsaturated pyrans discussed above, chiral 2-substituted chromenes are not readily resolved by the Zr-catalyzed protocol. Optically pure styrenyl ethers, such as that shown in Scheme 6.9, are obtained by means of the Zr-catalyzed kinetic resolution, allowing for the efficient and enantioselective preparation of these important chromene heterocycles by a sequential catalytic protocol. 

Oxidation of silyl enol ethers. Oxidation of silyl enol ethers to a-hydroxy aldehydes or ketones is usually effected with w-chloroperbenzoic acid (6, 112). This oxidation can also be effected by epoxidation with 2-(phenylsulfonyl)-3-( p-nitrophenyl) oxaziridine in CHC1, at 25-60° followed by rearrangement to a-silyloxy carbonyl compounds, which are hydrolyzed to the a-hydroxy carbonyl compound (BujNF or H,0 + ). Yields are moderate to high. Oxidation with a chiral 2-arene-sulfonyloxaziridine shows only modest enantioselectivity. 

Rearrangement of dienynols to vinylallene sulfoxides. A few years ago, Oka-mura et al. (11, 39) reported the rearrangement of a dienynol to an allenyldiene with transfer of chirality of the propargylic alcohol. This rearrangement has now been used for an enantioselective synthesis of a sesquiterpene, (+ )-sterpurene (3).Thus reaction of the optically active propargylic alcohol 1 with C6H,SC1 at 25° results in a vinylallene (a) that cyclizes to the optically active sulfoxide 2. Nickel-... 

Uemura and co-workers (91) demonstrated that copper catalysts effectively transfer nitrenoid groups to sulfides generating chiral sulfimides. A complex obtained from CuOTf and 55d catalyzes nitrenoid transfer to prochiral sulfides to afford products such as 139 in moderate to poor enantioselectivities (<71% ee, Eq. 78). Nitrenoid transfer occurs selectively to the sulfur atom of allylic sulfides generating allylic sulfenamide (140) in moderate selectivity, after [2,3] sigmatropic rearrangement of the initial sulfimide 141, Eq. 79. 

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