Aldol chemoselective

The a, /3-epoxy ketone 119 and esters are hydrogenolyzed with triethylam-monium formate or H2 chemoselectively to aldols[116]. 

Chemoselective reduction of a,(3-epoxy carbonyl compounds to aldols and their analogs by organoseleniums and its application to natural product synthesis 98YGK736. 

Scheme 24) [38]. Chemoselective enolization of the less substituted enone moiety under hydrogenation conditions accompanied by subsequent aldol reaction provided the corresponding hydroxyl-enones, such as 87-89, which could be converted to various building blocks for polypropionate synthesis. p-Me2N styryl vinyl enone also was employed successfully as an enolate precursor, as demonstrated by the formation of hydroxy enone 90. 

Access to the corresponding enantiopure hydroxy esters 133 and 134 of smaller fragments 2 with R =Me employed a highly stereoselective (ds>95%) Evans aldol reaction of allenic aldehydes 113 and rac-114 with boron enolate 124 followed by silylation to arrive at the y-trimethylsilyloxy allene substrates 125 and 126, respectively, for the crucial oxymercuration/methoxycarbonylation process (Scheme 19). Again, this operation provided the desired tetrahydrofurans 127 and 128 with excellent diastereoselectivity (dr=95 5). Chemoselective hydrolytic cleavage of the chiral auxiliary, chemoselective carboxylic acid reduction, and subsequent diastereoselective chelation-controlled enoate reduction (133 dr of crude product=80 20, 134 dr of crude product=84 16) eventually provided the pure stereoisomers 133 and 134 after preparative HPLC. 

Aldol addition of aliphatic aldehydes to 105 [50] yielded the expected syn-aldol adducts 106, which spontaneously lactonized upon warming to 0 °C to give the y-butyrolactones 107 in good yields and excellent optical purities. The chemoselective deprotonation of 105 in a-position to the im-... 

A direct enantioselective cross-aldol-type reaction of acetonitrile with an aldehyde (RCHO) has been reported, giving /3-cyano alcohol product, R-CH (OH)-CH2-CN, (7e) in up to 77% ee.148 CH3CN, acting as an acetate surrogate, is chemoselectively activated and deprotonated using a soft metal alkoxide (CuO-Bu1) in a strong donor solvent (HMPA), with a bulky chiral diphosphine as auxiliary. 

An enantioselective intermolecular Michael addition of aldehydes (138) to enones (139), catalysed by imidazolidinones (140), has been reported. Chemoselectivity (Michael addition versus aldol) can be controlled through judicious choice of hydrogen bond-donating co-catalysts. The optimal imidazolidinone-hydrogen bond donor pair affords Michael addition products in <90% ee. Furthermore, the enamine intermediate was isolated and characterized and its efficacy as a nucleophile in the observed Michael addition reactions was demonstrated.172... 

A small amount of sodium hydroxide was enough for the aldol, and oxidation by Cr03 in pyridine was chemoselective. You might have supposed that the Wittig with a stabilised ylid... 

In the reaction of Figure 12.19, the alkoxide formed in this step deprotonates a carboxylic acid (cis-1 —> K), whereas in Figure 12.18 an iminium ion is deprotonated (B — C). Accordingly, different chemoselectivities are observed Figure 12.19 shows an enamine-mediated aldol addition, and Figure 12.18 presents an enamine-mediated aldol condensation. Hydrolysis of the iminium ion K in Figure 12.19 leads to the formation of the aldol addition products B and the amine which, together with the still unconsumed substrate A, forms the new enam-ine C, to start the catalytic cycle anew. 

Crossed aldol condensations occur with chemoselectivity only if some of the foregoing options cannot be realized. The following possibilities exist. 

Crossed aldol condensations between benzaldehyde or cinnamic aldehyde or their derivatives ketones pose no chemoselectivity problems. The least sterically hindered ketone, acetone, may condense with benzaldehyde, cinnamic aldehyde, and their derivatives with both enolizable positions if an excess of the aldehyde is employed. 

Chemoselective crossed aldol condensations between two different C,H-acidic aldehydes are impossible. There is only a single exception, and that is the intramolecular aldol condensation of an unsymmetrical dialdehyde. 

Exposure of 270 to the TBSOTf/DIPEA reagent couple once more triggered a cascade of reactions, namely chemoselective thiolactone enolization, intramolecular aldolization, and hydroxyl silylation, which ultimately gave rise to 3,4-trans and 3,4-cis bicyclic thiolactones 271 and 272. Final unmasking of the pseudoanomeric thiol function and terminal hydroxymethyl moiety was performed on the individual precursors 271 and 272 to deliver, as expected, the corresponding thiocarbasugars 273 and 274 in high isolated yields. 

Crossed aldol condensations between aliphatic aldehydes on the one hand and benzaldehyde or cinnamic aldehyde or their derivatives on the other also are possible. The reaction components can even be mixed together. The aldol adducts are formed without chemoselectivity, as a mixture of isomers, but their formations are reversible. The Elcb elimination to an a,/3-unsaturated carbonyl compound is fast only if the newly created C=C double bond is conjugated to an aromatic system or to another C=C double bond already present in the substrate. This effect is due to product-development control. All the starting materials thus react in this way via the most reactive aldol adduct. 

Bis(pentafluorophenyl) tin dibromide effects the Mukaiyama aldol reaction of ketene silyl acetal with ketones, but promotes no reaction with acetals under the same conditions. On the other hand, reaction of silyl enol ether derived from acetophenone leads to the opposite outcome, giving acetal aldolate exclusively. This protocol can be applied to a bifunctional substrate (Equation (105)). Keto acetal is exposed to a mixture of different types of enol silyl ethers, in which each nucleophile reacts chemoselectively to give a sole product.271... 

First, chemoselective (Chapter 24) conjugate addition of the silyl ketene acetal on the enone is preferred to direct aldol reaction with the aldehyde. Then an aldol reaction of the intermediate silyl enol ether on the benzaldehyde follows. The stereoselectivity results, firstly, from attack of benzalde-hyde on the less hindered face of the intermediate silyl enol ether, which sets the two side chains trans on the cyclohexanone, and, secondly, from the intrinsic diastereoselectivity of the aldol reaction (this is treated in some detail in Chapter 34). This is a summary mechanism. 

This NADPH reaction is typically stereo- and chemoselective, though the stereochemistry is rather wasted here as tile next step is a dehydration, typical of what is now an aldol product, and occurring by an enzyme-catalysed ElcB mechanism. 

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