Reaction with conjugated aldehydes

The combination of thionation by Lawesson s reagent [98] of oxoenamino-ketones 96 with normal electron-demand Diels-Alder reaction of conjugated aldehydes allows a variety of thiopyrans 97 to be synthesized by a regio-selective and chemoselective one-pot methodology [99] (Equation 2.28). Thionation occurred at the more electrophilic ketonic carbonyl group. O O... 

Pentadienyltrimethylstannanes undergo regioselective conjugate additions to aldehydes, catalysed by Lewis acids. The dominant product obtained depends on the catalyst used, as shown in reaction 46. In the case of titanium tetrachloride catalysis the reaction is also stereoselective and only one diasteroisomer is obtained297. Reaction with chiral aldehydes leads to asymmetric induction with similar organotin compounds298. 

The first reaction involves a ketone reaction with an aldehyde under basic conditions, so enolate anion chemistry is likely. This is a mixed aldol reaction the acetone has acidic a-hydrogens to form an enolate anion, and the aldehyde is the more reactive electrophile. The reaction is then driven by the ability of the intermediate alcohol to dehydrate to a conjugated ketone. 

Aromatic or conjugated aldehydes react in excellent yields, whereas the reaction with aliphatic aldehydes requires longer times and leads to A-hydroxyamides in lower, althongh satisfactory, yield. When both aldehyde and ketone gronps are present on the same... 

In another study Feringa et al. [20] reported a catalytic enantioselective three-component tandem conjugate addition-aldol reaction of dialkyl zincs. Here, zinc enolates were generated in situ via catalytic enantioselective Michael addition of dialkylzinc compounds to cydohexenone in the presence of a chiral Cu catalyst. Their diastereoselective reaction with an aldehyde then gave trans-2,3-disubstituted cyclohexanones in up to 92% yields and up to >99% ees (Scheme 9.11). 

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. 

Reactions with Achiral Aldehydes. The reaction of tartrate allylboronates with achiral aldehydes proceeds with moderate to excellent enantioselectivity (60-92% ee) and high yield (80-90%). Simple aliphatic aldehydes give good enantioselectiv-ities (decanal 86% ee, CyCHO 87% ee, eq 2), while p-alkoxy and conjugated aldehydes give diminished selectivities (60-80% ee) (eq 3). The enantioselectivity is highly temperature and solvent dependent. Best results for reactions with the vast majority of aldehydes are obtained in toluene at —78 °C. 4°A molecular sieves are included to ensure that the reaction is anhydrous. Other tartrate esters (e.g. diethyl tartrate) may also be used without loss of enantioselectivity. 

The reaction of a phosphonate ester, DBU, Nal, and HMPA with an aldehyde leads to a conjugated ester with excellent (Z)-selectivity. A (Z)-selective reaction was reported using a trifluoroethyl phosphonate in a reaction with an aldehyde and potassium tert-butoxide. ... 

The general representation of the classic Wittig reaction is presented in equation (21). The ( )- and (Z)-selectivity may be controlled by the choice of the type of ylide (95), the carbonyl derivative (94), the solvent and the counterion for ylide formation. As a general rule, the use of a nonstabilized ylide (95 X and Y are H or alkyl substituents and is phenyl) and salt-free conditions in a nonprotic, polar solvent favors the formation of the (Z)-alkene isomer (96) in reactions with an aldehyde. A stabilized ylide with strongly conjugating substituents such as an ester, nitrile or sulfone forms predominantly the (f -alkene. 

Corey s method20 relies on metal exchange with the bromocyclopropane 69 prepared by carbene addition. The extra stabilisation of cyclopropyl anions (chapter 8) makes both this lithium derivative and the ylid 63 more easily handled. Addition to aldehydes or ketones gives mixtures of adducts 70 [it turns out that none of the stereochemistry of 69 or 70 matters] which fragment under Lewis acid catalysis to give the thioacetal 71. Careful hydrolysis releases the 3,4-enal -72, the product of a homoaldol reaction with an aldehyde homoenolate and RCHO and a difficult compound to make as the double bond moves into conjugation very easily. 

Alkenes combine with conjugated aldehydes and ketones in the Michael reaction style when a silyl triflate is present to polarize the acceptors/ (Note styrenes are activated at the P-carbon.)... 

The Morita-Bayhs-Hillman (MBH) reaction involves the conversion of an a,p-imsaturated carbonyl compound into an aldol-like adduct. The reaction is catalysed by tertiary amines (7.155), which form an intermediate enolate (7.157) by conjugate addition (rather than by direct deprotonation of the a-proton). The enolate imdergoes an aldol reaction with an aldehyde (7.01), followed by loss of the amine catalyst to provide the Baylis-Hillman adduct (7.158), as shown in Figure 7.7. 

An important synthetic route to conjugated esters involves a Wittig reaction with an aldehyde or ketone. Phenylacetaldehyde was converted to 3.6 in one example, allowing the synthesis of J. 7.4 This reaction involves conjugate addition of ammonia... 

Since free and conjugated dihydroxy cholanic acids cannot be separated completely by paper and thin-layer chromatography they must be differentiated by specific reactions. The following procedures are suitable for quantitative determination of free bile acids reaction with concentrated sulfuric acid for determination of cholic acid (Hammarsten 1925), reaction with salicylic aldehyde and sulfuric acid for determination of deoxycholic acid (Szalkowski and Mader 1952), reaction with ethyl acetate, concentrated sulfuric acid and acetic acid anhydride for chenodeoxycholic acid (Isaksson 1954). 

A mechanistic proposal includes addition of MesSiSeLi to aldehydes, followed by elimination of MesSiOLi to generate selenoaldehydes, which undergo Diels-Alder reaction with dienes. Aldehydes bearing a conjugate diene unit provide intramolecular Diels-Alder adducts (eq 7). ... 

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