Stereoselective reactions acetate

Chiral acetals of ArCHO.1 The chiral acetal (1) of Cr(CO)3-complexed benz-aldehyde undergoes a highly stereoselective reaction with (CH3)3A1 and TiCl4 (1 equiv.) to provide (R,R,R)-2 in 72% yield and 97.5% de. A subsequent Ritter... 

Cossio and Linden et al. studied the reaction of aryl iminoester-derived azomethine ylides 65 with 2- and 1,2-disubstituted nitroalkenes 66 using silver acetate and triethylamine in acetonitrile (Scheme 2.18).35 The product 3-nitropyrrolidines were formed in yields of 24—69% with excellent regioselectivity. These findings mirrored earlier work by Toke et al.36 Unfortunately, diastereoselectivity was generally in the 1 1—4 region. Only in two cases where yields were low were the products formed in a 2 98 ratio. Lithium perchlorate was able to catalyze the reaction as well with similar yields. Remarkably, the diastereoselectivity with lithium perchlorate in some cases resembled that obtained with silver acetate and in others was reversed. Cossio also reported the stereoselective reaction of phenyl isocyanates with azomethine ylides 65 under the same conditions.37 These reactants combined to give the expected products as single stereoisomer in 30-50% yield. 

The mercury-induced cyclization of amidals in which a stereogenic center is positioned at the amidal center adjacent to the nucleophilic amide functionality, gave oxazolidines with high stereoselectivity. Reaction of the amidals 10 with mercury(ll) acetate/mercury(ll) trifluoroac-etate in acetonitrile in the presence of sodium hydrogen carbonate followed by reductive cleavage with sodium borohydride afforded the corresponding fused oxazolidines 11164. 

The matched double stereoselective reaction between the chiral C-phenyl A -( 1-phenylethyl) nitrone and the chiral silyl ketene acetals 558 in acetonitrile/dichloromethane at 20 °C under zinc iodide-catalyzed conditions resulted in only the single diastereomer 559 in 98% yield. Exposure of 559 to acid afforded the isoxazolidinone 560 in 74% yield, together with the chiral auxiliary (li ,2 S )-2-phenylcyclohexanol (Scheme 135) <1997JOC6672>. 

Palladium-catalyzed oxidation of dienamides 88 in acetone in the presence of acetic acid gives oxyamination products in stereoselective reactions (Scheme 8-33) [113]. Depending on the reaction conditions, it is possible to obtain trans- or cis-1,4-oxyamination by choice. As with the other palladium(II)-catalyzed 1,4-additions, CI (from LiCl) is used as a... 

Noyori s acetalization procedure and its variants have been used for the preparation of chiral cyclic acetals. The rigid 1,3-dioxolanes formed play an important role as temporary chiral auxiliaries in stereoselective reactions and as electrophilic reactants. ... 

Heterocyclic Synthesis. - The reactions of phosphorus ylides with phenan-threne-9,10-quinone (113) have been used to prepare phenanthrene [9,10-x]-fused compounds with four, five, and six membered heterocyclic rings. (E)-4-carbethoxymethylene-l,2,3,4-tetrahydro-2-quinolones 114 have been obtained from the stereoselective reaction of 3-hydroxy-1,2,3,4-tetrahydroquinoline-2,4-diones and ethyl(triphenylphosphoranylidene)acetate. A -trifluoroacetylanilines 115 react with Ph3P=C02Et producing enamine derivatives 116 as a mixture of (E)- and (Z)-isomers. Enamines 116 are useful precursors for the synthesis of indoles and quinolones. 

Acetals of a,j8-unsaturated aldehydes with 3-0-alkylated derivatives of 1,2-0-isopropylidene- -D-fructopyranose and l,2-0-isopropylidene-/l-D-psicopyranose, which are readily available from D-fructose, were cyclopropanated with diethylzinc/diiodomethane with good stereoselectivity. The acetals were hydrolyzed and the aldehydes reduced to give cyclopropyl-substituted alcohols e.g. cyclopropanation of 103 to give 104 and hydrolysis and reduction to R,2R)-2-phenylcyclopropylmethanol (105). Extensive studies were carried out, with both exo- and endo-acQta structures, to determine the effects of the structure of the acetals on the enantioselec-tivity. Among various isomeric compounds, the asymmetric cyclopropanation reaction provided good enantioselectivity (consistent attack on the same face) with high chemical yields especially with en sfo-acetals of l,2-0-isopropylidene-3-0-(4-phenylbenzyl)-iS-D-fructopyranose. 

P,P] The Michael addition of silyl ketene acetals to enoates requires more strongly Lewis acidic conditions. Both aluminum triflate (98) and clay montmorillonite (78) have been used to catalyze stereoselective reactions (Table 18, Scheme 49). In general, the selectivities obtained for ketene acetals... 

A selective reducing agent for double bonds (potassium azodicarboxylate-acetic acid) allowed the preparation of the epidioxide (268) from the A -compound/ A synthesis was reported for the 18,20-lactone (269) through the hypoiodite reaction of (205)-20-hydroxy-5a-cholestan-3/3-yl acetate/ A second synthesis of the 18,20-lactone (269) from the known lactone (270) involved the stereoselective reaction of the acetoxy-ketone (271) with iso-hexylmagnesium bromide to give the (205)-20-hydroxy-compound (272)/ These syntheses served as models for the synthesis of seychellogenin (273) and the 7,8,9,11-tetrahydro-derivative/ ... 

In summary, oxidative radical reactions by other metals are less common than single-electron transfer to cerium(IV), iron(III), copper(II), and especially manganese(III) complexes. However, even for manganese(III) acetate and cerium(IV) ammonium nitrate the synthetic potential is not completely utilized. During the next few years, many new applications should arise, which will focus on stereoselective reactions and the synthesis of complex organic molecules. 

Stereoselective reactions with acetals. Noyori et al. (10,438) have used this Lewis acid to promote an aldol-type reaction between enol silyl ethers and acetals and have noted high. syn-selectivity in this process. Molander and Haar report that reaction of acetals with cyanotrimethylsilane promoted by TMSOTf results in a-alkoxy cyanides and that this reaction can be diastereoselective when the acetal is substituted at the 4-position by an alkoxy group. The diastereoselectivity depends on the nature of the acetal and the 4-alkoxy group. Dimethoxy acetals show slight diastereoselectivity, but diisopropoxy and dibenzyl acetals can show diastereoselectivity of 5-10 1. The diastereoselectivity also depends on the type of 4-substituent. Acetoxy and t-butyldimethylsilyloxy groups have no effect on the diastereoselectivity, but methoxy, benzyloxy, and allyloxy groups promote anri-selectivity. Since a metal template is not involved, the diastereoselectivity... 

Nucleophilic substitution reactions of acetates 88 and 90 (Fig. 4.32) indicate that a single substituent at C2 is not the origin of stereoselectivity for acetal 83 (Fig. 4.32). Treatment of the trans acetate 88 with aUyltrimethylsilane in the presence of SnBr4 provided the 1,4-c/s product 89 with 67 33 stereoselectivity (Fig. 4.32). Use of BF30Et2 and Me3SiOTf as the Lewis acid provided similar selectivities. The reaction of the related cis acetate 90 also afforded a mixture of diastereoisomers. Low selectivity with C2 and C4-c/s-substituted five-membered ring oxocarbenium ions has been observed [108]. 

Intramolecular 1,4-Oxyamination and 1,4-Chloroamination Palladium-catalyzed oxidation of dienylcarboxamides 107 in acetone in the presence of acetic acid gives oxyamination products by stereoselective reactions (Scheme 11.39) [128]. 

Even if this explanation is correct, the specificity observed by Breslow in the cyclization of trans,trans-farnesyl acetate (392) remains remarkable (Scheme 145). Indeed, under the same conditions as above, 392 gives 395 in 20-30% yield. Considering the effect of the methyl substituent, as discussed above, and the trend to often observe a Cy6 rather than a Cy5 cyclization giving trans-decalin systems in bicycle formation (Section XI.2.A.a), the regioselectivity is not completely unexpected. More remarkable is the highly stereoselective reaction observed, since four asymmetric centers have been... 

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