Alcohols, carbonylation acids

Table 1.5. Alcohols, Carbonyls, Acids, and Esters in Milk.

Aroma compounds in cured vanilla beans from different countries, e.g. Madagascar, Tonga, Costa Rica, Java, Indonesia and Mexico, have been documented. Over 100 volatile compounds have been detected, including aromatic carbonyls, aromatic alcohols, aromatic acids, aromatic esters, phenols and phenol ethers, aliphatic alcohols, carbonyls, acids, esters and lactones, of which the aldehyde vanillin is the most abundant. The level of the aldehydes, e.g. vanillin and p-hydroxy-benzaldehyde and their respective acids (vanillic acid and p-hydroxybenzoic acid), in cured vanilla beans is used as an indicator of cured vanilla bean quality for commercial purposes (Klimes and Lamparsky, 1976 Adedeji et al., 1993 Ranadive, 1994). 

The point was made earlier (Section 5 9) that alcohols require acid catalysis in order to undergo dehydration to alkenes Thus it may seem strange that aldol addition products can be dehydrated in base This is another example of the way in which the enhanced acidity of protons at the a carbon atom affects the reactions of carbonyl com pounds Elimination may take place in a concerted E2 fashion or it may be stepwise and proceed through an enolate ion... 

Most ozonolysis reaction products are postulated to form by the reaction of the 1,3-zwitterion with the extmded carbonyl compound in a 1,3-dipolar cycloaddition reaction to produce stable 1,2,4-trioxanes (ozonides) (17) as shown with itself (dimerization) to form cycHc diperoxides (4) or with protic solvents, such as alcohols, carboxyUc acids, etc, to form a-substituted alkyl hydroperoxides. The latter can form other peroxidic products, depending on reactants, reaction conditions, and solvent. 

Conversion of Acid Chlorides into Alcohols Reduction Acid chlorides are reduced by LiAJH4 to yield primary alcohols. The reaction is of little practical value, however, because the parent carboxylic acids are generally more readily available and can themselves be reduced by L1AIH4 to yield alcohols. Reduction occurs via a typical nucleophilic acyl substitution mechanism in which a hydride ion (H -) adds to the carbonyl group, yielding a tetrahedral intermediate that expels Cl-. The net effect is a substitution of -Cl by -H to yield an aldehyde, which is then immediately reduced by UAIH4 in a second step to yield the primary alcohol. 

Carbonylation of alcohols to acids, table of examples, 46, 74 Carboxylation, by formic acid, 46, 74 of 2-methylcyclohexanol by formic acid-sulfuric acid to 1-methyl-cyclohexanecarboxylic acid, 46, 72... 

The general features of this elegant and efficient synthesis are illustrated, in retrosynthetic format, in Scheme 4. Asteltoxin s structure presents several options for retrosynthetic simplification. Disassembly of asteltoxin in the manner illustrated in Scheme 4 furnishes intermediates 2-4. In the synthetic direction, attack on the aldehyde carbonyl in 2 by anion 3 (or its synthetic equivalent) would be expected to afford a secondary alcohol. After acid-catalyzed skeletal reorganization, the aldehydic function that terminates the doubly unsaturated side chain could then serve as the electrophile for an intermolecular aldol condensation with a-pyrone 4. Subsequent dehydration of the aldol adduct would then afford asteltoxin (1). 

Other mechanisms for the synthesis of alkylformates, not via formic acid esterification, are possible. Hydrogenation of C02 to CO, followed by catalytic car-bonylation of alcohol, would produce alkyl formate. This mechanism seems more likely for the anionic metal carbonyls because they are known catalysts for alcohol carbonylation. However, Darensbourg and colleagues [64, 74, 85] showed... 

The conversion of 7 to 8 is a simple hydrolysis of an acetal. Acetals are functionally equivalent to alcohols + carbonyls and can be interconverted with them under acidic conditions. Several reasonable mechanisms can be drawn for this transformation, but all must proceed via S l substitutions. 

Oxidation of Oxygen-containing Compounds (Alcohols, Carbonyl Compounds, Carboxylic Acids)... 

The anode is an ideal reagent to oxidize organic substrates such as oxygen-containing compounds (alcohols, carbonyl compounds, and carboxylic acids). Thereby these substrates can be converted avoiding chemical reagents, which simplifies the reaction conditions and the work-up. Additionally, the electron transfer leads selectively to a variety of reactive species, which can find further use in organic synthesis. 

This article shows a variety of patterns of electrochemical oxidation of oxygen-containing compounds (alcohols, carbonyl compounds, and carboxylic acids), aiming to be helpful for both electroorganic and organic chemists to cover this field from a synthetic viewpoint. Since there have been excellent books [1-5] published on the subject, this article quotes only some typical and important papers from before 1990. 

The other acetogenins, shown in Table II, are a varied assortment of small molecule compounds which as a group contain alkene, alcohol, and carbonyl (acid, ester, aldehyde, and ketone) functionality. They are fairly typical substances with common structural features. Again, those compounds possessing an odd number of carbon atoms are less common, and biogenetlcally may be derived from a propionate starter unit. 

The hydrolysis of an ester to alcohol and acid (1) and the esterification of a carboxylic acid with an alcohol (2) are shown here as an example of the Sn2 mechanism. Both reactions are made easier by the marked polarity of the C=0 double bond. In the form of ester hydrolysis shown here, a proton is removed from a water molecule by the catalytic effect of the base B. The resulting strongly nucleophilic OH ion attacks the positively charged carbonyl C of the ester (la), and an unstable sp -hybridized transition state is produced. From this, either water is eliminated (2b) and the ester re-forms, or the alcohol ROH is eliminated (1b) and the free acid results. In esterification (2), the same steps take place in reverse. 

Mechanism. The carbonyl group of a carboxylic acid is not sufficiently electrophilic to be attacked by the alcohol. The acid catalyst protonates the carbonyl oxygen, and activates it towards nucleophilic attack. The alcohol attacks the protonated carbonyl carbon, and forms a tetrahedral intermediate. Intramolecular proton transfer converts the hydroxyl to a good leaving group as H2O. A simultaneous deprotonation and loss of H2O gives an ester. 

The Amadori compound may be degraded via either of two pathways, depending on pH, to a variety of active alcohol, carbonyl and dicarbonyl compounds and ultimately to brown-coloured polymers called melanoidins (Figure 2.31). Many of the intermediates are (off-) flavoured. The dicarbonyls can react with amino acids via the Strecker degradation pathway (Figure 2.32) to yield another family of highly flavoured compounds. 

Hydroperoxide formation is characteristic of alkenes possessing tertiary allylic hydrogen. Allylic rearrangement resulting in the formation of isomeric products is common. Secondary products (alcohols, carbonyl compounds, carboxylic acids) may arise from the decomposition of alkenyl hydroperoxide at higher temperature. 

---