Asymmetric version

Comparison witli tlie Hajos-Parrisb asymmetric version of tlie Robinson annulation [81] iSdieme 7.25iaj) shows tlie following distinct differences between tlie two metliods. Firstly, tlie cydoalkenone in tlie CuiOTf)2/ligand 18-catalyzed procedure is tlie Midiael acceptor, whereas tlie cydoalkanone is tlie Midiad donor in tlie proline-mediated annulation. Secondly, tlie asymmetric induction occurs in tlie 1,4-addition step in tlie new metliod, in contrast to tlie asymmetric aldol-cydization in tlie Hajos-Parrisb procedure. 

Nitronates show a similar reacdvity to that of nitrones, and nitrones are one of 1,3-thpoles that have been successfully developed to catalyzed asymmetric versions, as discussed in the secdon on nitrones fSecdon 8 3 1 However, asymmetric nitronate cycloadthdons catalyzed chiral metal catalysts have not been reported Kanemasa and coworkers have demonstrated that nitronate cycloadthdon is catalyzed by Lev/is acids fEq 8 93 This may open a new way to asymmetric nitronate cycloadthdon catalyzed by chiral catalysts... 

In general sulfur ylide-mediated epoxidation cannot be used to form an epoxide with an adjacent anion-stabilizing group such as an ester, as the requisite ylide is too stable and does not react with aldehydes [23], With the less strongly electron-withdrawing amide group, however, the sulfur ylide possesses sufficient reactivity for epoxidation. The first example of an asymmetric version of this reaction was by... 

Reactions between imines and a-diazo carboxylates afford aziridine-2-carboxylates [55]. An asymmetric version of this reaction using chiral nonracemic catalysts has been described [53, 56-58]. As an example, catalytic aziridination of inline 44 (Scheme 3.14) with ethyl diazoacetate in the presence of 10% catalyst generated... 

Asymmetric versions of the cyclopropanation reaction of electron-deficient olefins using chirally modified Fischer carbene complexes, prepared by exchange of CO ligands with chiral bisphosphites [21a] or phosphines [21b], have been tested. However, the asymmetric inductions are rather modest [21a] or not quantified (only the observation that the cyclopropane is optically active is reported) [21b]. Much better facial selectivities are reached in the cyclopropanation of enantiopure alkenyl oxazolines with aryl- or alkyl-substituted alkoxy-carbene complexes of chromium [22] (Scheme 5). 

The reaction of alkenylcarbene complexes and imines in the presence of a Lewis acid generates pyrroline derivatives as a result of a [3C+2S] cyclisation process [76]. This reaction has been extended to an asymmetric version by the use of chiral alkenylcarbene complexes derived from several chiral alcohols. However, the best results are found when (-)-8-phenylmenthol-derived complexes are used and catalytic amounts of Sn(OTf)2 are added to the reaction. In these conditions high levels of trans/cis selectivity are achieved and the hydrolysis of the major tram diastereoisomers allows the preparation of optically pure 2,5-disubstituted-3-pyrrolidinone derivatives (Scheme 29). 

An asymmetric version of this reaction was achieved by the use of complexes derived from chiral imidazolidinones. For example, the reaction of Danishefsky s diene with these chiral complexes occurs with both high exo endo selectivity and high facial selectivity at the dienophile [103] (Scheme 56). 

The thiol 48 (with R R, R = H) undergoes intramolecular cychsation in THF, in the presence of AIBN under UV irradiation, to give a 2-phosphonothiolane [36], a phosphorus and sulfur analogue of proline. More recently, an asymmetric version of the sequence, [2,3]-sigmatropic rearrangement and radical cychsation, has been carried out (see Sect. 5.1.1.) [41]. 

In the asymmetric version of the [1,2] -aWittig rearrangement (see Sect. 3.2), the deprotonation of S-methyl (ferf-butyl)arylphosphinothioate 103 followed by alkylation affords the corresponding (alkylthiomethyl)phosphine oxides 104 together with over-reacted products 105 (no diastereomeric excess is observed for this compound) and 106 [67] (Scheme 30). 

In 2008, Que and coworkers reported an asymmetric version of the dihydroxylation with a new type of ligands bearing bipyrrolidine as the chiral backbone [71]. The corresponding iron(II) complex showed general activity in the dihydroxylation of various olefins using H202- Satisfactory results are obtained with aliphatic as well as with aromatic olefins. For example, dihydroxylation of styrene gave styrene oxide and 1-phenylethane-1,2-diol in <1% and 65% yield, respectively (Scheme 10). 

Asymmetric versions of this transformation were also developed by using chiral imidazolium pro-ligands as NHC precursors, or silver transmetallation methodology with chiral NHC ligands (Fig. 2.23) [106]. Imidazolium salts with chiral A-substituents (132) or imidazolidinium salts with chirality at the backbone of the heterocycle (133) gave quantitative conversions at -78°C with good ee (58% and 70% respectively). 

The facile arylation of aldehydes with arylboronic acid has prompted the exploration of asymmetric versions of this reaction. However, this field has been scarcely explored and only few examples have been reported in the literature, with moderate results. The first diastereoselective example was described by Ftirstner and coworkers. By reacting the Gamer aldehyde 15 with phenylboronic acid under their set of experimental conditions (i.e. RhClj-SH O, IPr HCl) (Scheme 7.4) [21], the secondary alcohol was obtained in higher selectivity than that observed in the addition of phenylmagnesium bromide reported by Joullie (de = 94% versus 66%), with the anti isomer as the major compound [29]. 

In addition to the enhanced rate of hydroalumination reactions in the presence of metal catalysts, tuning of the metal catalyst by the choice of appropriate ligands offers the possibility to influence the regio- and stereochemical outcome of the overall reaction. In particular, the use of chiral ligands has the potential to control the absolute stereochemistry of newly formed stereogenic centers. While asymmetric versions of other hydrometaUation reactions, in particular hydroboration and hydrosi-lylation, are already weU established in organic synthesis, the scope and synthetic utiHty of enantioselective hydroalumination reactions are only just emerging [72]. 

On the other hand, the enantioselective 1,4-addition of carbanions such as enolates to linear enones is an interesting challenge, since relatively few efficient methods exist for these transformations. The Michael reaction of p-dicarbonyl compounds with a,p-unsaturated ketones can be catalysed by a number of transition-metal compounds. The asymmetric version of this reaction has been performed using chiral diol, diamine, and diphosphine ligands. In the past few years, bidentate and polydentate thioethers have begun to be considered as chiral ligands for this reaction. As an example, Christoffers et al. have developed the synthesis of several S/O-bidentate and S/O/S-tridentate thioether... 

The nitroaldol reaction or Henry reaction is a powerful and highly versatile carbon-carbon bond-forming reaction, allowing a plethora of key molecular frameworks, such as p-hydroxynitroalkanes, 1,2-amino alcohols or a-hydroxy carboxylic acids to be synthesised in a straightforward manner. Therefore, the development of practical catalytic asymmetric versions of this reaction is still largely desirable. The first catalytic asymmetric nitroaldol reaction was reported in 1992, " but despite its long history, relatively few chiral ligands have... 

Collaboration has been established with O Brien s group to develop a catalytic asymmetric version of the reaction [31]. O Brien has also used compound 26a to... 

In this chapter the development of the synthesis of taranabant will be presented. In Section 9.1, we focus on evaluation and optimization of the Medicinal Chemistry route, and development of an asymmetric version of this initial route. In Section 9.2 the discovery and implementation of a new asymmetric route will be discussed, and extensions of the utility of this chemistry will also be discussed. Finally, the factors involved in selecting a route as a potential manufacturing approach for taranabant are presented. 

Oxidizing enzymes use molecular oxygen as the oxidant, but epoxidation with synthetic metalloporphyrins needs a chemical oxidant, except for one example Groves and Quinn have reported that dioxo-ruthenium porphyrin (19) catalyzes epoxidation using molecular oxygen.69 An asymmetric version of this aerobic epoxidation has been achieved by using complex (7) as the catalyst, albeit with moderate enantioselectivity (Scheme 9).53... 

Later, Carreira et al. reported that nitrido-Mnv(salen) also underwent a nitrogen-transfer reaction under similar reaction conditions.142 An asymmetric version of this reaction has been realized by using the chiral nitrido-Mnv(salen) (44) (Scheme 32).143 When trifluoroacetic acid... 

Tin-based Lewis acids can be divided into two groups, Sn11 and SnIv. Although Sn11 is more cationic than SnIV, the covalent and ionic radii of Sn11 (1.02 A) are larger than those of SnIV (0.71 A).333 Tin Lewis acids have been applied to many kinds of reactions, including several asymmetric versions. 

Sn(OTf)2 can function as a catalyst for aldol reactions, allylations, and cyanations asymmetric versions of these reactions have also been reported. Diastereoselective and enantioselective aldol reactions of aldehydes with silyl enol ethers using Sn(OTf)2 and a chiral amine have been reported (Scheme SO) 338 33 5 A proposed active complex is shown in the scheme. Catalytic asymmetric aldol reactions using Sn(OTf)2, a chiral diamine, and tin(II) oxide have been developed.340 Tin(II) oxide is assumed to prevent achiral reaction pathway by weakening the Lewis acidity of Me3SiOTf, which is formed during the reaction. 

G-H functionalization at acetal C-H bonds generates protected forms of /3-ketoesters (Figure 4). /3-Ketoesters are often formed by Claisen condensation, but the asymmetric version is not a viable process, because the products would very likely racemize under the reaction conditions. Therefore, the C-H insertion equivalent to the Claisen condensation is very attractive, because the resulting /3-ketoester is protected, which allows for the enantioselective version to be feasible (Figure 4). 

This reaction has lent itself to the development of its asymmetric version (Scheme 88). The trick here is to remove the choride ligands from the coordination sphere of the platinum-chiral ligand complex. This makes the metal center more electrophilic, thus reactive reactions can be run at lower temperature. Interestingly, the best ligand was found to be the atropisomeric monophosphine (fJ)-Ph-BINEPINE.312 Enantiomeric excess up to 85% was observed. Very recently, enantioselectivity up to 94% ee has been achieved using [(AuCl)2(Tol-BINAP)] as pre-catalyst for the reaction of another enyne.313... 

An asymmetric version of such a cyclization has been developed, but the yield and the enantioselectivity are moderate.3 7 It is worth noting that these reactions can be performed in aqueous medium.358... 

Titanium-mediated pinacol coupling reactions have been reviewed until 2000.80 81 Since then, various intermole-cular pinacol couplings have been reported with aldehydes, - ketones, a-ketoesters, and imines, as well as asymmetric versions thereof.101-104 Scheme 29 shows one example of an asymmetric pinacol coupling of aromatic aldehydes, promoted and catalyzed by the new chiral titanium complex (A)-75, that has been developed by Riant and co-workers.101 Yields for pinacol products 76 are generally high. Under catalytic conditions, ee is moderate (up to 63%), while stoichiometric conditions allow to obtain up to 91% ee. 

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