Aldehydes conjugated esters

Other nucleophiles add to conjugated systems to give Michael-type products. Aniline derivatives add to conjugated aldehydes in the presence of a catalytic amount of DBU (p. 488). Amines add to conjugated esters in the presence of InCla, La(OTf)3, or YTb(OTf)3 at 3kbar, for example, to give P-amino esters. This reaction can be initiated photochemically. An intramolecular addition of an amine unit to a conjugated ketone in the presence of a palladium catalyst, or... 

D. Conjugate Reduction of Conjugated Aldehydes, Ketones, Acids, Esters and Nitriles... 

Tetraalkylammonium bromide was found to be a good catalyst for the conjugate addition of thiols to a,p-unsaturated nitriles, carboxylic esters, ketones, aldehydes, and nitro alkenes. The reactions proceeded rapidly and gave high yields of products (typically, 90%) (160). 

When a new carbon-carbon bond is produced by nucleophilic addition to conjugated systems, the process is called Michael addition. The generalised process involves an a, b-unsaturated compound and a compound containing an active hydrogen attached to a carbon atom (e.g., malonic ester, acetoacetic ester, nitrocompounds, aldehydes, ketones etc.) These are condensed in the presence of a base. The overall reaction and its mechanism can be represented as follows ... 

Identify the C=O absorptions in the spectra of these esters and note the effect of conjugation on the C=O stretch. You should also identify, and compare the wavenumbers of, the C—O absorptions of 2-methy propanal (Figure 3.14) and the conjugated aldehyde benzaldehyde (Figure 3.2). 

The conjugate addition of organocopper reagents to ot,(3-alkynic esters, ketones, aldehydes and acids is a useful method for the preparation of various tri- and tetra-substituted alkenes,87 although addition to aldehydes is less common.88 Use of a vinylcuprate results in the formation of a conjugated dienone, which further highlights the importance of this methodology (equation 43 ).87... 

Saito, S. Shiozawa, M. Yamamoto, H. Mixed crossed aldol condensation between conjugated esters and aldehydes using aluminum... 

If the Loupy-Seyden interpretation is correct, conjugated carbonyls, when used as dienophiles, should be able to react by the C=0 group when strongly complexed,46 There do not appear to be any cases of esters showing such reactivity (see Exercise 6 for a possible reason), but Danishefsky and Kerwin47 has used a Lewis acid-catalysed reaction to prepare dihydropyrans from conjugated aldehydes ... 

Conjugated aldehydes, ketones and esters may be modeled by acrolein, methylvinyl ketone and methyl acrylate, respectively. Their LUMOs are shown below ... 

Scheme 11.4 Catalytic reduction of conjugated aldehydes with ethyl Flantzsch ester [12].

Asymmetric Organocatalytic Reduction of Olefins 395 Table 11.1 Organocatalytic reduction of conjugated aldehydes by Hantzsch ester 4 [14]. 

Fig. 17.18. Mechanism of the Mn02 oxidation of conjugated aldehydes proceeding via cyanohydrin (B), which is generated in situ. Subseguently, the actual oxidation product (C) undergoes methanolysis (according to the Bflc2 mechanism, Figure 6.24) to the finally isolated methyl ester.

Conjugation with an oxygen atom has much the same effect—formate esters resonate at about 8 p.p.m.—but conjugation with 7t bonds does not. The simple conjugated aldehyde below and myrte-nal both have CHO protons in the normal region (9-10 p.p.m.). 

Conjugate reduction of enones. In the presence of I equiv. of aluminum chloride, 1 can effect conjugate reduction of open-chain n,p-enones. usually in 75-100% yield. Cyclic enones also are reduced, but in lower yicid (50-65%). This reagent is not useful for conjugate reduction of a,p-unsaturated esters or aldehydes. ... 

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. 

Regmselective conjugate reduction and reductive silylation o o.p-unsaturated ketones, esters, and aldehydes usmg a stable copper (I) hydride cluster ((Ph3P)CuH]e... 

C=0 stretch in unconjugated ketones, carbonyls and in ester groups (frequently of carbohydrate origin) conjugated aldehydes and carboxylic acids absorb around and below 1700 cm ... 

Note The frequencies given for the C=0 stretching vibrations for anhydrides, acid chlorides, esters, lactones, aldehydes, ketones, carboxylic acids and amides refer to the open chain or unstrained functional group in a nonconjugated system. If the carbonyl group is conjugated with a double bond or an aromatic ring, the frequency is 30 cm less. If it is... 

Oxidation of a cyanohydrin derived from a conjugated aldehyde (as the 0-TMS derivative) using py-ridinium dichromate (PE)C) in DMF gave an a,3-unsaturated lactone (5 -butenolide) as the major product (equation 12). Simple nonconjugated cyanohydrins are not satisfactory substrates for the synthesis of acyl cyanides using PDC, because they seem to add to the initially formed acyl cyanides, leading ultimately to cyanohydrin esters. Oxidation of cyanohydrin to acyl cyanides can be carried out either by means of manganese dioxide, ruthenium-catdyzed oxidation with t-butyl hydroperoxide or NBS. ... 

Tris(phenylthio)methyllithium (2c) adds in conjugate fashion to a number of enones, and the resulting adduct could then be hydrolyzed into a carboxylic acid ester or aldehyde (Scheme 20). 

---