Aldehydes Constitution

In general, aldehydes constitute the more efficient radical acceptors. Surprisingly, when enones were employed as radical acceptors 1,2-addition to the carbonyl group was in some cases preferred over the conjugate 1,4-addition [116]. 

Aldehydes constitute useful electrophilic partners in such nickel-catalyzed reactions because the condensation between alkynes, aldehydes and diorganozinc compounds can afford stereodefined cyclic or acyclic ally lie alcohols67-69, as illustrated by the stereoselective cyclization of 111 to the corresponding 3-hydroxypyrrolidine (equation 46). Allenes or 1,3-dienes instead of alkynes also lead to similar reactivity70. 

Peptide aldehydes constitute a rather general example of protease inhibitors. The electrophilic carbonyl group is attacked reversibly by the cleaving nucleophile, forming a covalent acetal or thioacetal intermediate. With cysteine proteases the preferred inhibitors are strong electrophiles, for example ketones, chloromethyl ketones, epoxides, or vinyl sulfones. Many cysteine protease inhibitors form an enzyme-inhibitor complex irreversibly these are therefore denoted suicide-inhibitors . 

The hydroformylation of alkenes to give linear aldehydes constitutes the most important homogeneously catalyzed process in industry today [51]. The hydroformylation of propene is especially important for the production of n-bu-tyraldehyde, which is used as a starting material for the manufacture of butanol and 2-ethylhexanol. Catalysts based on cobalt and rhodium have been the most intensively studied for the hydroformylation of alkenes, because they are industrially important catalysts. While ruthenium complexes have also been reported to be active catalysts, ruthenium offers few advantages over cobalt or... 

Such substitutions using lithium amides, secondary and in some cases tertiary amines as nucleophiles, have been introduced in early sixties as the first expedient method for this unique class of compounds. It is relevant that / ,/ -difluoro- and chlorofluorole-fins readily available through modified Wittig reaction from aldehydes constitute also good ynamine precursors. In the past decade, however, the more versatile lithium aminoacetylide method has gained more prominence. Substitution reactions are still used, among others, for phenyl, tert.-butyl, cinnamyl and cyclopropyl ynamines. 

Table VIII shows the dramatic differences between fresh and dried leaves. In this case, as oj sed to peppermint, rosemary and thyme, trans-2-hexenal is more in the fresh than in the dried and aged. The same is true for phenyl ethyl alcohol. Interestingly, trans-cinnamic aldehyde constitutes 50% of the total living headspace volatiles, but it is still less than in the aged leaf and commercial oil. However, cinnaniyl alcohol represents 20% of the fresh volatiles but is only a trace conpcanent of the aged leaf and oil. 4-Methoxy cinnamic aldehyde, identified as a cassia constituent for the first time, also increases 3-fold on drying but has disappeared completely in the comtnercial oil. 2-Methoxy cinnamic aldehyde, sometimes called the character impact component of cassia oil, is present in the headspace of the leaves to only a minor extent but it is the second most abundant component of the oil.

The borolanyl triflate (eq 10) has also been employed to form the chiral boron enolates of methyl ketones which have additional chiral centers present in their carbon framework. Reaction of these enolates with chiral aldehydes constitutes a triple asymmetric synthesis , in which the approximate multiplicativity of the three diastereofacial selectivities appears to be valid. ... 

The protic acid and Lewis acid-catalyzed [4 + 2] cycloaddition reactions of electron-rich alkenes with imines derived from anilines and aryl aldehydes constitute an extensively explored class of 2-azadienes capable of providing the products of a formaJ Diels-Alder reaction (equation 9).i5.27.i77 a useful extension of these studies and in efforts to increase the rate of the Att participation of simple 2-aza-1,3-buta-dienes in [4 + 2] cycloaddition reactions, Mariano and coworkers have examined the Lewis acid-catalyzed intermolecular reactions of (l ,3 )-l-phenyl-2-aza-l,3-pentadiene with electron-rich dienophiles, including enol ethers. Reductive work-up of the cycloaddition reactions provided the pro-... 

Such a lactone would be the anhydride of a compound containing two hydroxyl groups linked to one carbon atom. A compound of this character would also be able to lose water in another way as discussed in connection with aldehyde (p. 115) yielding a product of the aldehyde constitution as follows ... 

The chromium-mediated reaction of allylic phosphates with aldehydes constitutes an important advance in stereoselection [143]. The reaction of y-disubsti-tuted and y-disubstituted allylic phosphates with aldehydes proceeds with good to excellent diastereoselectivity. All of the reactions examined were stereodiver-gent, with the stereochemical outcome dependent upon the configuration of the starting allyl phosphate (Table 10-25). The dilference in behavior of the allylic phosphates is attributed to a very slow isomerization of the intermediate y-disub-... 

Merritt et al. (1963) estimated that aldehydes constitute 50.7 % of the coffee aroma (see section 5.B), half of this being acetaldehyde. 

Single-stage vapor-phase air oxidation of cyclohexane in presence of solid catalysts results in complete combustion, whereas without catalysts aldehydes constitute the major products. ... 

As base. For vicarious subsum under photochemical conditions. TB. with a-amino aldehydes constitutes 1,3-diamino-2-alkanols.- ... 

Because of its widespread use it occurs in the environment — in air and water. After formaldehyde it is the second most abundant aldehyde, constituting 5% of total aldehydes in air. Acrolein is one of the toxic gases produced in a wood or building fire or when polyethylene or other polymer substances bum (Morikawa 1988 Morikawa and Yanai... 

The cross-aldol reaction is actively studied with the aim of improved control of the stereoselectivity.58 The use of silyl enol ethers for a condensation with aldehydes constitutes important progress, but catalysis by Lewis acids can be unsatisfactory for acid labile substrates, and the predominant anti stereoselectivity is not always optimal. An attempt was made to solve this problem by running the reaction sonochemically in the presence of alumina, without any solvent.59 products are absent, and the anti-isomer forms predominantly (Eq. 16). 

Oxygen Transfer- Evidence for metallaoxetane involvement in metalloporphyrins is contained in a review, ) ) as are approaches to fine chemicals by oxidation.) ) ) The combination of a metal acac complex and an aldehyde constitutes a new and powerful method for alkene... 

Modified benzoin condensations in which the acyl acceptor is not an aldehyde constitute a variation of the classical cross-benzoin condensation. Aldehydes and ketones can be coupled in an intramolecular annulation reaction to give tertiary a-hydroxyketones. The selective cross-coupling of... 

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