Acetals are formed from aldehydes or

Acetals are formed from aldehydes or ketones plus alcohols in the presence of acid 

We said that a solution of acetaldehyde in methanol contains anew compound a hemiacetal. We ve also said that the rate of formation of hemiacetals is increased by adding an acid (or a base) catalyst to an alcohol plus aldehyde mixture. But, if we add catalytic acid to our acetaldehyde-methanol 

2 Loss of water by elimination. This elimination leads to an unstable and highly reactive oxonium ion 

3 Addition of methanol to the oxonium ion (breaking the n bond and not the a bond, of course) 

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ACETALS ARE FORMED FROM ALDEHYDES OR KETONES PLUS ALCOHOLS IN THE PRESENCE OF ACID... 

The most popular lands of the diols for asymmetric synthesis are bis-secondary diols that have a C2 axis of symmetry [212]. The presence of the symmetry axis avoids the formation of diastereoisomeric esters or acetals [213], (1R, 27 )-Cyclohexanediol 1.34 (n = 1) has been used as an auxiliary in asymmetric cyclopropanation [214] and (IS, 2S)-cycloheptanediol 1.34 (n = 2) in 1,4-addition of cuprates[157], Dioxolane derivatives of 1.34 have been used for asymmetric P-ketoester alkylations [215] and cuprate 1,4-additions [216]. Linear 1,2-diols 1.35 (R = Me, i-Pr, c-CgH j, Ph) and functionalized 1,2-diols 1.36 (Y = COOalkyl, CONR 2, CH2OR ) are readily available from optically active tartaric acids 1.36 (Y = COOH). Acetals derived from these diols are valuable reagents m asymmetric synthesis [173, 213, 217], as the related 1,3-diols 1.37. Acetals of 1,3-butanediol 137 (R = Me, R = H) have also been used. When these acetals are formed from aldehydes under thermodynamic conditions, one 1,3-di-oxane stereoisomer often predominates. In this favored isomer, the substituent from the aldehyde and the methyl group from 1.37 are both in equatorial orientar... 

Oxathiolanes (11) are formed from aldehydes and ketones by reaction with 2-mercaptoethanol (HS-CH2-CH2OH) in the presence of, for example, zinc chloride-sodium acetate in dioxane solution at room temperature,139 or boron trifluoride-etherate.140 They are more stable to an acidic medium than the 1,3-dithianes, and therefore may be the protective group of choice in certain instances. 

Oxazolidines (53) are readily formed from aldehydes or ketones and ethanolamines they can be hydrolyzed with ease and show reactions that might be expected of the imino alcohol intermediate (54). Among these are the addition of Grignard reagentsand catalytic hydrogenolysis of the C—O Ixjnd (equation 28).This reaction is exothermic over Adam s catalyst in methanol but slower in acetic acid. Nickel and copper chromite are also effective but at higher temperatures and pressures,as is the case with palladium. The same cleavage occurs with LAH (unassisted)and with the borane-THF complex. ... 

Silyl enol ethers are other ketone or aldehyde enolate equivalents and react with allyl carbonate to give allyl ketones or aldehydes 13,300. The transme-tallation of the 7r-allylpalladium methoxide, formed from allyl alkyl carbonate, with the silyl enol ether 464 forms the palladium enolate 465, which undergoes reductive elimination to afford the allyl ketone or aldehyde 466. For this reaction, neither fluoride anion nor a Lewis acid is necessary for the activation of silyl enol ethers. The reaction also proceed.s with metallic Pd supported on silica by a special method[301j. The ketene silyl acetal 467 derived from esters or lactones also reacts with allyl carbonates, affording allylated esters or lactones by using dppe as a ligand[302]... 

Reactions. Heating an aqueous solution of malonic acid above 70°C results in its decomposition to acetic acid and carbon dioxide. Malonic acid is a useful tool for synthesizing a-unsaturated carboxyUc acids because of its abiUty to undergo decarboxylation and condensation with aldehydes or ketones at the methylene group. Cinnamic acids are formed from the reaction of malonic acid and benzaldehyde derivatives (1). If aUphatic aldehydes are used acryhc acids result (2). Similarly this facile decarboxylation combined with the condensation with an activated double bond yields a-substituted acetic acid derivatives. For example, 4-thiazohdine acetic acids (2) are readily prepared from 2,5-dihydro-l,3-thiazoles (3). A further feature of malonic acid is that it does not form an anhydride when heated with phosphorous pentoxide [1314-56-3] but rather carbon suboxide [504-64-3] [0=C=C=0], a toxic gas that reacts with water to reform malonic acid. 

For instance, 2-methylpropene reacted with acetic acid at 18°C in the presence of Al-bentonite to form the ester product (75). Ion-exchanged bentonites are also efficient catalysts for formation of ketals from aldehydes or ketones. Cyclohexanone reacted with methanol in the presence of Al-bentonite at room temperature to give 33% yield of dimethyl ketal after 30 min of reaction time. On addition of the same clay to the mixture of cyclohexanone and trimethyl orthoformate at room-temperature, the exothermic reaction caused the liquid to boil and resulted in an almost quantitative yield of the dimethyl ketal in 5 min. When Na- instead of Al-bentonite is used, the same reaction did not take place (75). Solomon and Hawthorne (37) suggest that elimination reactions may have been involved in the geochemical transformation of lipid and other organic sediments into petroleum deposits. 

A new route to bromopyrroles was developed. It depends on addition of HBr to A-protected y-aminoynones. When applied to alkynyl ketones, 2-aryl or 2-alkyl 4-bromopyrroles are formed. 2-Alkyl or 2-aryl 3-bromopyrroles can be obtained from acetals of V-aminoynals. The ketones are made from A -protected propargylamines by ( -acylation. The acetals are made from 3,3-diethoxypropyne by addition to an aldehyde followed by introduction of the amino group by reaction with phthalimide under Mitsunobu conditions. <95S276>... 

Enolate ions formed from, ketones or aldehydes are extremely important in the synthesis of more complex organic molecules. The ease with which an enolate ion is formed is related to the acidity of the a proton. The pKa of propane (acetone) is 19.3 that means that it is a stronger acid compared to ethane (pKa 60) and a much weaker acid than acetic acid (pKa 4.7), i.e. strong bases like sodium hydride, sodium amide, and lithium diisopropylamide LiN(i-C3H7)2 are needed to form an enolate ion. 

Just as protonated carbonyl groups are much more electrophilic than unprotonated ones, these oxonium ions are powerful electrophiles. They can react rapidly with a second molecule of alcohol to form new, stable compounds known as acetals. An oxonium ion was also an intermediate in the formation of hemiacetals in acid solution. Before reading any further, it would be worthwhile to write out the whole mechanism of acetal formation from aldehyde or ketone plus alcohol through the hemiacetal to the acetal, preferably without looking at the fragments of mechanism above, or the answer below. 

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