Ene reaction, asymmetric

Enantiotopos discrimination, 93, 128, 142, 234, 235, 331 Ene reactions asymmetric, 223 binaphthol, 222 chiral metal complexes, 222 intramolecular, 226 methyl glyoxylate, 290 Enol silyl ether substrates, 128, 228, 230 Enol substrates, 28 Enolates ... 

Emulsion formation, in hydrophobic compounds 338 Ene reactions, asymmetric 693 Engelhard F-24 catalyst 650... 

The most recent, and probably most elegant, process for the asymmetric synthesis of (+)-estrone appHes a tandem Claisen rearrangement and intramolecular ene-reaction (Eig. 23). StereochemicaHy pure (185) is synthesized from (2R)-l,2-0-isopropyhdene-3-butanone in an overall yield of 86% in four chemical steps. Heating a toluene solution of (185), enol ether (187), and 2,6-dimethylphenol to 180°C in a sealed tube for 60 h produces (190) in 76% yield after purification. Ozonolysis of (190) followed by base-catalyzed epimerization of the C8a-hydrogen to a C8P-hydrogen (again similar to conversion of (175) to (176)) produces (184) in 46% yield from (190). Aldehyde (184) was converted to 9,11-dehydroestrone methyl ether (177) as discussed above. The overall yield of 9,11-dehydroestrone methyl ether (177) was 17% in five steps from 6-methoxy-l-tetralone (186) and (185) (201). 

Asymmetric Oiels-Alder reaction, ene reaction, hydrogenation, halogenatlon by means of chiral olelln, such as camphor derivative 1 or 2. 

Mikami K. Asymmetric Catalysis of Carbonyl-Ene Reactions and Related Carbon-Carbon Bond Forming Reactions Pure Appl. Chem. 1996 68 639 644 Keywords hefero-Diels-Alder reactions, asymmetric catalysis... 

The ene reaction of fuUerene (C o) with 3-methylene-2,3-dihydrofuran gives an easily isolated addition product in good yield <96JOC2559>. There is a continuous need for chiral acrylate esters for asymmetric Diels-Alder reactions with high diastereoselectivity. Lewis acid promoted Diels-Alder reactions of acrylate esters from monobenzylated isosorbide 28 (or isomannide) and cyclopentadiene provided exclusively e db-adducts with good yields and high diastereoselectivity <96TL7023>. 

ASYMMETRIC CATALYTIC GLYOXYLATE-ENE REACTION METHYL (2R)>2 HYDROXY-4-PHENYL-4 PENTENOATE (Benzenabutanoic acid, a-hydroxy-y-methylene, methyl ester, (R)-)... 

ASYMMETRIC CATALYTIC GLYOXYLATE-ENE REACTIONS WITH 1,1- PISUBSTITUTED OLEFINSa... 

Annual Volume 71 contains 30 checked and edited experimental procedures that illustrate important new synthetic methods or describe the preparation of particularly useful chemicals. This compilation begins with procedures exemplifying three important methods for preparing enantiomerically pure substances by asymmetric catalysis. The preparation of (R)-(-)-METHYL 3-HYDROXYBUTANOATE details the convenient preparation of a BINAP-ruthenium catalyst that is broadly useful for the asymmetric reduction of p-ketoesters. Catalysis of the carbonyl ene reaction by a chiral Lewis acid, in this case a binapthol-derived titanium catalyst, is illustrated in the preparation of METHYL (2R)-2-HYDROXY-4-PHENYL-4-PENTENOATE. The enantiomerically pure diamines, (1 R,2R)-(+)- AND (1S,2S)-(-)-1,2-DIPHENYL-1,2-ETHYLENEDIAMINE, are useful for a variety of asymmetric transformations hydrogenations, Michael additions, osmylations, epoxidations, allylations, aldol condensations and Diels-Alder reactions. Promotion of the Diels-Alder reaction with a diaminoalane derived from the (S,S)-diamine is demonstrated in the synthesis of (1S,endo)-3-(BICYCLO[2.2.1]HEPT-5-EN-2-YLCARBONYL)-2-OXAZOLIDINONE. 

ASYMMETRIC CATALYTIC GLYOXYLATE-ENE REACTION METHYL (2R)-2-HYDROXY-4-PHENYL-4-PENTEN0ATE... 

Yang12 has effected an intramolecular asymmetric carbonyl-ene reaction between an alkene and an a-keto ester. Reaction optimization studies were performed by changing the Lewis acid, solvent, and chiral ligand. Ligand-accelerated catalysis was observed for Sc(OTf)3, Cu(OTf)2, and Zn(OTf)2 (Equation (6)). The resulting optically active m-l-hydroxyl-2-allyl esters provide an entry into multiple natural products. 

Jacobsen and co-workers14 have shown that a tridentate Schiff base chromium complex 13 catalyzed an asymmetric carbonyl-ene reaction between a variety of aryl aldehydes (14, Equation (8)) and 2-methoxy propene 15 or 2-trimethylsiloxypropene. The highest yields were afforded when the aryl ring was substituted with an electron-withdrawing group however, the substituent did not seem to affect the enantioselectivity. 

Cationic cobalt(m) complexes have successfully been applied to the asymmetric carbonyl-ene reaction.15 The yield and enantioselectivity were dependent, to a large extent, upon the counterion, with SbF6- giving the best results (16, Equation (9)). The conditions were general for a variety of alkenes, but only glyoxaldehydes were used as the carbonyl component. 

Mikami and co-workers16-19 have done extensive work for developing catalysts for the asymmetric carbonyl-ene reaction. Excellent enantioselectivites are accessible with the binol-titanium catalyst 17 (Equation (10)) for the condensation of 2-methyl butadiene (R1 = vinyl) and glyoxalates (binol = l,T-binaphthalene-2,2 -diol).16 The products were further manipulated toward the total synthesis of (i )-(-)-ipsdienol. The oxo-titanium species 18 also provides excellent enantioselectivity in the coupling of a-methyl styrene with methyl glyoxalate.17 Reasonable yields and good enantioselectivites are also obtained when the catalyst 19 is formed in situ from titanium isopropoxide and the binol and biphenol derivatives.18... 

Enantio- and diastereoselective syntheses of a variety of heterocycles were accomplished by combining the ruthenium-catalyzed Alder-ene reaction with a palladium-catalyzed asymmetric allylic alkylation (AAA) (Scheme 7). For the AAA, y>-nitrophenol was found to function as a suitable leaving group and yet was stable to the Alder-ene conditions. Extensive solvent studies were performed to determine the best conditions for the one-pot procedure. 

While the transition metal-catalyzed Alder-ene reaction has been developed to offer excellent regio- and chemos-electivity, stereoselective variants have only recently begun to appear in the literature.58 The scope and limitations of many of these protocols have yet to be established. Nonetheless, several groups have published exciting examples of asymmetric Alder-ene cyclizations. 

The carbonyl group in a ketone or aldehyde is an extremely versatile vehicle for the introduction of functionality. Reaction can occur at the carbonyl carbon atom using the carbonyl group as an electrophile or through enolate formation upon removal of an acidic proton at the adjacent carbon atom. Although the carbonyl group is an integral part of the nucleophile, a carbonyl compound can also be considered as an enophile when involved in an asymmetric carbonyl-ene reaction or dienophile in an asymmetric hetero Diels-Alder reaction. These two types of reaction are discussed in the next three chapters. 

The self-assembly of a chiral Ti catalyst can be achieved by using the achiral precursor Ti(OPr )4 and two different chiral diol components, (R)-BINOL and (R,R)-TADDOL, in a molar ratio of 1 1 1. The components of less basic (R)-BINOL and the relatively more basic (R,R)-TADDOL assemble with Ti(OPr )4 in a molar ratio of 1 1 1, yielding chiral titanium catalyst 118 in the reaction system. In the asymmetric catalysis of the carbonyl-ene reaction, 118 is not only the most enantioselective catalyst but also the most stable and the exclusively formed species in the reaction system. 

Scheme 8-44. General scheme for Ti-catalyst asymmetric glyoxylate-ene reactions. Reprinted with permission by Wiley-VCH Verlag GmbH, Ref. 89.

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