Enol ethers catalyzed

Michael addition of various a,fi-unsaturated carbonyl compounds to silyl enol ether catalyzed by Ti-beta and TS-1... 

Diels-Alder reactions. Adducts of high ee are obtainable from the condensation of a-pyrone derivatives with enol ethers catalyzed by Ti-BINOLate. ... 

The enantioselective hetero-Diels-Alder reactions of a,p-unsaturated acylphosphonates with enol ethers catalyzed by Cu(II)bzT(oxazoline) complexes have been investigated in depth. It was found that Cu(II)bA(oxazoline) complexes activate a,p-unsaturated acylphosphonates to the extent that they undergo facile cycloaddition reactions at low temperature with electron-rich alkenes. For example, dimethyl ( )-l-oxo-2-butenylphosphonate reacts with ethyl vinyl ether in the presence of Cu[(S,5)-t-Bu-box] (OTf)2 complex (10 mol% catalyst) to generate the cycloadduct in 89% yield (Scheme 7.86) with exceptional stereoselectivity (endo/exo = 99/1, 99% ee). Cyclic enol ethers also undergo stereoselective reactions with dimethyl ( )-l-oxo-2-butenylphosphonate. It specifically reacts with 2,3-dihydrofuran in the presence of Cu[(5,5)-t-Bu-box] (OTf)2 to deliver the bicyclic enolphosphonate. Of particular merit is the fact that a large variety of p.y-unsaturated acylphosphonates may be tolerated with no loss in selectivity for the derived cycloadducts. ... 

Stereoselective aldol condensation of aldehydes with silyl enol ethers catalyzed by Lewis acids (Ti (IV), Sn (ll),Yb (OTf)3l lnCI3, chiral Cu-oxazolines) (see 1st edition). 

Double asymmetric aldol reaction The asymmetric aldol reaction of aldehydes with silyl enol ethers catalyzed by tin(Il) triflatc and a chiral diamine such as (S)-l, l-methyl-2-[(N-naphthylamino)mcthyl]pyrrolidinc, (16,221-222), has been extended to aldol reactions of a silyl cnol ether with chiral aldehydes. 

Wei, S. Du, H. A Highly Enantioselective Hydrogenation of Silyl Enol Ethers Catalyzed by Chiral Frustrated Lewis Pairs. /. Am. Chem. Soc. 2014, 136, 12261-12264. 

Nakajima M, Yokota T, Saito M, Hashimoto S (2004) Enantioselective aldol reactions of trichlorosilyl enol ethers catalyzed by chiral iVA/-dioxides and monodentate V-oxides. Tetrahedron Lett 45 61-64... 

Yasu Y, Koike T, Akita M (2012) Sunlight-driven synthesis of y-diketones via oxidative coupling of enamines with silyl enol ethers catalyzed by [Ru(bpy)3]. Chem Commun 48 5355-5357... 

The Pd-catalyzed elimination of the mesylate 909 at an anomeric center, although it is a saturated pseudo-halide, under mild conditions is explained by the facile oxidative addition to the mesylate C—O bond, followed by elimination of /3-hydrogen to give the enol ether 910[767],... 

Pd(II)-catalyzed cyclization of the siloxyhexatriene 34 offers a cyclohexe-none annulation method. The Pd enolate 35, formed by transraetallation of the silyl enol ether with Pd(II), is an intermediate which undergoes intramolecular eWo-alkene insertion. Then Pd(II) is regenerated to give 36, and finally cyclohexenone is formed[38]. 

The chemoselective desilylation of one of the two different silyi enoi ethers in 10 to give the monosilyl enol ether II is realized by the Pd-catalyzed reaction of Bu3SnF. The chemoselectivity is controlled by steric congestion and the relative amount of the reagent[7,8]. An interesting transformation of the 6-alkoxy-2,3-dihydro-6//-pyran-3-one 12 into the cyclopentenone derivative 13 proceeds smoothly with catalysis by Pd(OAc)2 (10 mol%)[9]. 

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). 

Symmetrical labile ethers such as cycloalkenyl ethers (15) or mixed acetals (16) can also be prepared from the 3-hydroxyl group by acid-catalyzed exchange etherification or by acid-catalyzed addition to cyclohexanone methyl enol ether. 

Diacetato(l, 10-phenanthroline)palladium(II)-catalyzed transetheriflcation of polyfluorinated alcohols with enol ethers leads to the corresponding fluormated vinylic ethers [14] (equation 13)... 

For some condensations with silylated substrates as starting compounds, trimethylsilyl inflate can be used as a catalyst [103, 104, 105] Atypical example of such a reaction is the aldol type condensation of silyl enol ethers and acetals catalyzed by 1-5 mol% of trimethylsilyl inflate [103] (equation 53)... 

The rhodium-catalyzed isomerization can also be carried out with the chiral catalyst, Ru2Cl4(diop)3 (H2, 20-80°, 1-6 h, 47-90% yield). In this case, optically enriched enol ethers are obtained. ... 

The first asymmetric Mn(salen)-catalyzed epoxidation of silyl enol ethers was carried out by Reddy and Thornton in 1992. Results from the epoxidation of various silyl enol ethers gave the corresponding keto-alcohols in up to 62% ee Subsequently, Adam and Katsuki " independently optimized the protocol for these substrates yielding products in excellent enantioselectivity. 

The chiral BOX-copper(ll) complexes, (S)-21a and (l )-21b (X=OTf, SbFg), were found by Evans et al. to catalyze the enantioselective cycloaddition reactions of the a,/ -unsaturated acyl phosphonates 49 with ethyl vinyl ether 46a and the cyclic enol ethers 50 giving the cycloaddition products 51 and 52, respectively, in very high yields and ee as outlined in Scheme 4.33 [38b]. It is notable that the acyclic and cyclic enol ethers react highly stereoselectively and that the same enantiomer is formed using (S)-21a and (J )-21b as the catalyst. It is, furthermore, of practical importance that the cycloaddition reaction can proceed in the presence of only 0.2 mol% (J )-21a (X=SbF6) with minimal reduction in the yield of the cycloaddition product and no loss of enantioselectivity (93% ee). 

More recently, further developments have shown that the reaction outlined in Scheme 4.33 can also proceed for other alkenes, such as silyl-enol ethers of acetophenone [48 b], which gives the endo diastereomer in up to 99% ee. It was also shown that / -ethyl-/ -methyl-substituted acyl phosphonate also can undergo a dia-stereo- and enantioselective cycloaddition reaction with ethyl vinyl ether catalyzed by the chiral Ph-BOX-copper(ll) catalyst. The preparative use of the cycloaddition reaction was demonstrated by performing reactions on the gram scale and showing that no special measures are required for the reaction and that the dihydro-pyrans can be obtained in high yield and with very high diastereo- and enantioselective excess. 

For example in the so-called Mukaiyama aldol reaction of an aldehyde R -CHO and a trimethylsilyl enol ether 8, which is catalyzed by Lewis acids, the required asymmetric environment in the carbon-carbon bond forming step can be created by employing an asymmetric Lewis acid L in catalytic amounts. 

The acid-catalyzed reaction of enol ethers 2 (X = OR) and enamines 2 (X = NR2) to form y-lactol derivatives proceeds with great ease even on silica gel chromatography. Vinyl sulfides 2 (X = SR) or vinyl chlorides 2 (X = Cl) are difficult to hydrolyze. 

An interesting example from carbohydrate chemistry is the boron trifluoride-diethyl ether complex catalyzed nucleophilic addition of silyl enol ethers to chiral imines (from n-glyceralde-hyde or D-serinal)22. This reaction yields unsaturated y-butyrolactones with predominantly the D-arabino configuration (and almost complete Cram-type erythro selectivity). 

The diastereomeric ratio of the trimethylsilyl triflate catalyzed amidoalkylation of a number of silyl enol ethers at — 40 CC appears to be dependent on the substituents in the substrate87. At — 40 °C the diastereomeric ratio is shown to be kinetically controlled. On allowing the reaction mixture to warm to 20 "C slow epimerization, increasing the amount of the minor isomer, is observed. In the case of the naphthalene derivative, sodium methoxide catalyzed epimerization of the kinetic mixture [(antijsyn) 88 12] produces the thermodynamic mixture [(antijsyn) 9 91]. 

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