Alkenes chiral ylides, asymmetric reactions

Asymmetric Wittig-Homer reaction chiral olefinations. 2 Reaction of the chiral ketone 1 with the ylide from (- )-8-phenylmenthyl phosphonoacetate (2) at -30 to -60° gives the (E)-olefin in a 90 10 ratio. The geometry of the alkene is determined mainly by the chiral auxiliary. Use of ent-2 results in 3 with the E/Z... 

Asymmetric ylide reactions such as epoxidation, cyclopropanation, aziridination, [2,3]-sigmatropic rearrangement and alkenation can be carried out with chiral ylide (reagent-controlled asymmetric induction) or a chiral C=X compound (substrate-controlled asymmetric epoxidations). Non-racemic epoxides are significant intermediates in the synthesis of, for instance, pharmaceuticals and agrochemicals. 

Imidazole and its derivatives continued to play an important role in asymmetric processes. Optically active pyrroloimidazoles 26 were prepared by the cycloaddition of homochiral imidazolium ylides with activated alkenes <96TL1707>. This reaction was used in the enantioselective preparation of pyrrolidines <96TL1711>. A review of the use of chiral imidazolidines in asymmetric synthesis was published <96PAC531> and the preparation and use of a new camphor-derived imidazolidinone-type auxiliary 27 was reported < 6TL4565> <96TL6931>. 

The use of chiral azomethine imines in asymmetric 1,3-dipolar cycloadditions with alkenes is limited. In the first example of this reaction, chiral azomethine imines were applied for the stereoselective synthesis of C-nucleosides (100-102). Recent work by Hus son and co-workers (103) showed the application of the chiral template 66 for the formation of a new enantiopure azomethine imine (Scheme 12.23). This template is very similar to the azomethine ylide precursor 52 described in Scheme 12.19. In the presence of benzaldehyde at elevated temperature, the azomethine imine 67 is formed. 1,3-Dipole 67 was subjected to reactions with a series of electron-deficient alkenes and alkynes and the reactions proceeded in several cases with very high selectivities. Most interestingly, it was also demonstrated that the azomethine imine underwent reaction with the electronically neutral 1-octene as shown in Scheme 12.23. Although a long reaction time was required, compound 68 was obtained as the only detectable regio- and diastereomer in 50% yield. This pioneering work demonstrates that there are several opportunities for the development of new highly selective reactions of azomethine imines (103). 

Chiral exocyclic alkenes such as 112, also having the chiral center two bonds away from the reacting alkene moiety, have been used in highly diastereoselective reactions with azomethine ylides, and have been used as the key reaction for the asymmetric synthesis of (5)-(—)-cucurbitine (Scheme 12.37) (169). The aryl sulfone 113 was used in a 1,3-dipolar cycloaddition reaction with acyclic nitrones. In 113, the chiral center is located four bonds apart from alkene, and as a result, only moderate diastereoselectivities of 36-56% de were obtained in these reactions (170). 

Some organic reactions can be accomplished by using two-layer systems in which phase-transfer catalysts play an important role (34). The phase-transfer reaction proceeds via ion pairs, and asymmetric induction is expected to emerge when chiral quaternary ammonium salts are used. The ion-pair interaction, however, is usually not strong enough to control the absolute stereochemistry of the reaction (35). Numerous trials have resulted in low or only moderate stereoselectivity, probably because of the loose orientation of the ion-paired intermediates or transition states. These reactions include, but are not limited to, carbene addition to alkenes, reaction of sulfur ylides and aldehydes, nucleophilic substitution of secondary alkyl halides, Darzens reaction, chlorination... 

Reactions.—Aldehydes. The condensation of optically active aldehydes with phos-phonium salts containing a chiral atom leads to alkenes with no racemization of the asymmetric centres [equation (1)]. The same ylide is obtained when either (12) or... 

Chiral epoxides frequently play a key role as intermediates in organic synthesis and the development of methods for the catalytic asymmetric synthesis of such compounds therefore remains an area of intensive research. Methods have focused principally on the asymmetric electrophilic oxidation of alkenes and good enantioselectivity has been achieved [1]. An alternative to oxidative processes for the synthesis of epoxides is the reaction of sulfur ylides with aldehydes and ketones [2,3,4,5,6]. Sulfur ylide epoxidation is a carbon-carbon bond forming reaction and is complementary to oxidative methods. The standard conditions for this reaction utilize the original Corey method treatment of a sulfonium salt with a strong base in the presence of or followed by the addition of an aldehyde... 

The 1,3-dipolar cycloaddition reaction of non-stabilized azomethine ylides, derived from A-alkyl-cf-amino acids, with 3-nitro-2-trifluoro(trichloro)methyl-2//-chromenes produced l-benzopyrano[3,4-c]pyrrolidines in good yields." AgOAc-catalysed asymmetric 3 + 2-cycloaddition reactions of azomethine ylides with e-deficient alkenes yielded enr/o-adducts with up to 99% ee. New chiral ferrocenyl P,N-ligands possessing a benzoxazole ring as the Af-donor (35) are effective asymmetric catalysts... 

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