Wittig reactions asymmetric reaction

Enantioselective Intramolecular Wittig Reaction. Asymmetric induction has been reported to accompany intramolecular Wittig cyclization of a phosphonium ylide performed in the presence of (/J,5)-CAMPHOS catalyst. However, this reaction proceeds to afford the corresponding cyclization product with only 10% ee (eq 4) ... 

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... 

A challenging goal is the development of catalytic asymmetric induction processes. Denmark et al.48 have reported an asymmetric Wittig reaction using... 

Scheme 6.109 Typical N-Boc-protected P-amino-a-methylene ethyl esters obtained from the 106-catalyzed asymmetric Mannich reaction and subsequent Wittig reaction with...

The Wittig and related reactions have been reviewed in the context of natural product synthesis 120 mechanistic studies of the Wittig reaction have also been reviewed with particular reference to asymmetric induction.121... 

Reviews have featured epoxidation, cyclopropanation, aziridination, olefination, and rearrangement reactions of asymmetric ylides 66 non-phosphorus stabilized carbanions in alkene synthesis 67 phosphorus ylides and related compounds 68 the Wittig reaction 69,70 and [2,3]-Wittig rearrangement of a-phosphonylated sulfonium and ammonium ylides.71 Reactions of carbanions with electrophilic reagents, including alkylation and Wittig-Homer olefination reactions, have been discussed with reference to Hammett per correlations.72... 

Asymmetric intramolecular Wittig reaction. Trost and Curran2 have examined eight readily available optically active phosphines in a stereoselective synthesis of the ilikctone (2), a useful intermediate to several natural products. Of these, CAMP is dearly the most efficient phosphine for this purpose (equation I). 

Intermolecular aza-Wittig reaction has been described for the one-step synthesis of pyrazolofl, 5-a Ipyrimidinc and imidazo[l,2-fc]pyrazole derivatives.37 The asymmetric synthesis of ft-quaternary azacycles has been accomplished by aza-Wittig reaction mediated by chiral phosphorus(ni) reagents.38... 

The Wittig reaction currently occupies a central position in the arsenal of the synthetic organic chemist due to its versatility and generality. Ongoing advances in understanding its reaction mechanism and the ensuing catalytic, asymmetric and solid phase applications will permit its continued application to new and exciting synthetic problems. 

The aminocyclitol moiety was synthesized in a stereocontrolled manner from cis-2-butene-l,4-diol (Scheme 40)112 by conversion into epoxide 321 via Sharpless asymmetric epoxidation in 88% yield.111 Oxidation of 321 with IBX, followed by a Wittig reaction with methyl-triphenylphosphonium bromide and KHMDS, produced alkene 322. Dihydroxylation of the double bond of 322 with OSO4 gave the diol 323, which underwent protection of the primary hydroxyl group as the TBDMS ether to furnish 324. The secondary alcohol of 324 was oxidized with Dess-Martin periodinane to... 

The asymmetric total synthesis of the natural enantiomer (—)-nakadomarin A was completed by Nishida et al. in 2004 (Scheme 8.12) [82]. Diels-Alder reaction between siloxydiene 173 and chiral dienophile 172 (prepared from L-serine in 10 steps [83]) gave the highly functionalized key intermediate hydroisoquinoline 174, which was subjected to Luche reduction, cyclization, and HCl treatment to furnish the tricyclic intermediate 175. Compound 175 was converted to 177 via ozonolysis cleavage of ring B followed by recyclization of the unstable bisaldehyde to a five-membered ring by aldol condensation. The Z-olefin 178 was obtained from Wittig reaction of 177, and was further converted to furan 180 via peroxide 179. The... 

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... 

---