Enantioselective reactions eliminations

This reaction is nicely applied to total synthesis of Clavularin A26b as shown in Scheme 7.3. The key reactions involve a high enantioselective addition-elimination process261 and stereoselective synthesis of ra-2,3-disubstituted cycloheptanones.27... 

Zirconium-Catalyzed Enantioselective Carbomagnesation-Elimination Tandem Reaction of Allylic Derivatives... 

Review. Reetz1 has reviewed newer published reactions of these reagents as well as numerous unpublished results from his own research. The review emphasizes the chemo-, diastereo-, and enantioselective reactions that can be achieved. Drawbacks are that secondary and tertiary alkyltitanium compounds generally are not available, owing to /1-hydride elimination, and that ready ligand exchange makes the preparation of derivatives chiral at titanium difficult. The review includes useful suggestions for experimental techniques. 

Other Enantioselective Reactions. Enantioselective epoxide elimination by chiral bases has been demonstrated. More recently, the enantioselective [2,3]-Wittig rearrangement of a 13-membered propargylic ally lie ether has been performed using the lithium amide of (f ,f )-(l) as the base for deprotonation (eq 15). For this particular substrate, THF is a better solvent than ether, although pentane produces better results in a related transformation (eq 16). In fact, a change in solvent in this type of reaction has been shown to lead to a reversal of the stereoselectivity of the transformation. ... 

In view of their importance in enantioselective reactions, many bornane sultams have been synthesized and characterized by MS. Their El mass spectra show molecular ions with low abundance or are absent. N-Substituted bornane sultams generally show the typical fragmentations of the substituents <2000THS(4)405>. 1-Alkyl-7-nitro-2,1-benzisothiazole 2,2-dioxides show elimination of an acyl radical from their molecular ions <1997EJS55>. 

The aza-Morita-Baylis-HiUman reaction is known to be a useful and atom-economical C-C bond-forming reaction of electron-deficient alkenes with imines usually catalyzed by Lewis bases [202]. It formally involves a sequence of reactions including a Michael addition, a Mannich reaction, a proton transfer, and a retro-Michael reaction ( -elimination). Although there are many reports in the field of the enantioselective aza-Morita-Baylis-Hilhnan reaction, only rare examples of asymmetric domino reactions initiated by this reaction have been reported. In 2010, Sasai et al. [203] developed the first organocatalyzed asymmetric domino aza-Morita-Baylis-Hillman/aza-Michael reaction of a,p-unsaturated carbonyl compounds with N-tosylimines, allowing an easy access to chiral cis-1,3-disubstituted isoindolines as single diastereomers. The process was induced by a Hg-BINOL-derived catalyst and provided these products in high yields and enantioselectivities, as shown in Scheme 10.18. 

Enantioselective Reactions Due to syn- - or P"-hydride hydride elimination, stereogenic centers may be created in the presence of chiral ligands [3n-s, 17, 18]. The asymmetric intramolecular version (Scheme 19.14c) (Shibasaki, Overman) [17] is more developed than the asymmetric inter-molecular version (Scheme 19.14d) (Hayashi) [18]. 

Zireonitun-Catalyzed Enantioselective Carboalomination-Elimination Ihiidem Reaction Allylic Derivatives and Its Application to Kinetic Resolution... 

Diels-Alder reaction of 2-bromoacrolein and 5-[(ben2yloxy)meth5i]cyclopentadiene in the presence of 5 mol % of the catalyst (35) afforded the adduct (36) in 83—85% yield, 95 5 exo/endo ratio, and greater than 96 4 enantioselectivity. Treatment of the aldehyde (36) with aqueous hydroxylamine, led to oxime formation and bromide solvolysis. Tosylation and elimination to the cyanohydrin followed by basic hydrolysis gave (24). 

The condensation of nitro compounds and imines, the so-called aza-Henry or nitro-Mannich reaction, has recently emerged as a powerful tool for the enantioselective synthesis of 1,2-diamines through the intermediate /3-amino nitro compounds. The method is based on the addition of a nitronate ion (a-nitro carbanion), generated from nitroalkanes, to an imine. The addition of a nitronate ion to an imine is thermodynamically disfavored, so that the presence of a protic species or a Lewis acid is required, to activate the imine and/or to quench the adduct. The acidic medium is compatible with the existence of the nitronate anion, as acetic acid and nitromethane have comparable acidities. Moreover, the products are often unstable, either for the reversibility of the addition or for the possible /3-elimination of the nitro group, and the crude products are generally reduced, avoiding purification to give the desired 1,2-diamines. Hence, the nitronate ion is an equivalent of an a-amino carbanion. 

Reaction of allylic silanes with enantiomerically pure 1,3-dioxanes has been found to proceed with moderate enantioselectivity.104 The homoallylic alcohol can be liberated by oxidation followed by base-catalyzed (3-elimination. The alcohols obtained in this way are formed in 70 5% e.e. 

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