Asymmetric Wittig-type reaction

Recent Progress in Asymmetric Wittig-Type Reactions... 

Different types of the reagents (see Fig. 8-4) have been applied in asymmetric Wittig-type reactions. Because no new sp3 stereocenter is formed in a Witting-type reaction, a substrate containing a stereogenic or pro-stereogenic unit apart from the carbonyl group is usually required to induce an asymmetric process. 

T. Rein, T. M. Pedersen, Asymmetric Wittig Type Reactions, Synthesis 2002, 579-594. 

Asymmetric Wittig-type reaction The possibility of carrying out asymmetric version of the Wittig-type reaction is not readily apparent as there is no sp stereocentre formed. However, asymmetric Wittig-type alkenation reactions have been carried out by placing stereogenic centres adjacent to or near the reaction centre. Three types of asymmetric Wittig-type... 

Rein, T., Reiser, O. Recent advances in asymmetric Wittig-type reactions. Acta Chem. Scand. 1996, 50, 369-379. 

The first attempt to use a chiral catalyst in an asymmetric Wittig-type reaction was reported in 1970 [22]. A combination of the stabilized ylide 9 and a chiral organic acid as an external catalyst was evaluated in the olefination of 4-substituted cyclohexanone derivatives 1. In this study, (S)-(+)-mandelic acid was found to be... 

Z Further Application of Asymmetric Wittig-Type Reactions in Enantioselective Synthesis 325... 

Methods for the formation of carbon-carbon double bonds in an asymmetric manner through non-Wittig-type reactions [12,13] have also been reported in recent years, including asymmetric induction by reactions with chiral sulfoxides [13], sul-fones (Julia olefination) [14], sulfoximides [12a, 15], or selenides [16] Pd-catalyzed allylic nucleophilic substitutions [17], as well as asymmetric deprotonation [18]. In the context of the topic of asymmetric carbonyl olefination, some of these asymmetric transformations are beyond the scope of this chapter, although a few of the transformations closely related to the Wittig-type reactions will be discussed in a later part of this chapter. 

As discussed below, the strategies for asymmetric induction through olefination by Wittig-type reactions can be broadly classified into four groups. According to the type of these approaches to optically active olefinic compounds, asymmetric olefi-nations based on Wittig and related reactions will be reviewed in this chapter. 

Generally, arsonium ylides [62] are more reactive but less accessible than phos-phonium ylides. Recently, the chiral arsonium reagent 30 has appeared, and has been applied in asymmetric Wittig-type carbonyl olefinations. This first chiral arsonium reagent also bears 8-phenylmenthyl as a chiral auxiliary on its carboalkoxy portion [63], and gave moderate chemical yields and diastereoselectivities in the conversion of 4-substituted cyclohexanone derivatives to axially chiral non-racemic alkylidene cyclohexanes under the same reaction conditions as used for the related reactions with phosphorus reagents (Scheme 7.15). On the other hand, the corre-... 

Recently, asymmetric induction mediated by external chiral ligands that are not covalently bonded to the reagent has attracted much attention, and it is believed that the information obtained from these studies will prove useful in developing a novel system for efficient catalytic asymmetric transformation. In order to explore the possibilities, a variety of reaction systems capable of effective asymmetric induction at a specific site in the course of a reaction have been devised, and several investigations have also been directed towards Wittig-type olefination. An early study using an optically active organic acid with stabilized ylides [22] was unfruitful, as discussed in the introductory section. 

Camphor sultam derivatives have proved to be effective chiral auxiliaries in many different types of asymmetric reactions. As shown in Scheme 5-44, chiral camphor sulfam can be applied in the synthesis of (—)-pulo upone precursor 151 using an intramolecular Diels-Alder reaction. A Wittig reaction of 148 with 147 connects the chiral auxiliary to the substrate, and subsequent intramolecular Diels-Alder reaction via transition state 150 affords product 151. Compound 151 already has the stereochemistry of (—)-pulo upone 153.72... 

The essence of asymmetric synthesis is producing a new stereogenic center in such a manner that the product consists of stereoisomers in unequal amount. In most cases, this can be achieved by the formation of a new sp3 stereocenter. There is also another type of asymmetric reaction in which the employed substrates contain either a stereogenic unit or a pro-stereogenic unit apart from the functional group, and asymmetric synthesis occurs even though the nature of the reaction is not directly related to the newly formed sp3 stereocenter. The Wittig reaction is invoked for the asymmetric synthesis of such molecules.47... 

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