Acetals reaction with Lewis acids

Study of the reactivity of aromatic C-H bonds in the presence of transition metal compounds began in the 1960s despite the quite early discovery of Friedel-Crafts alkylation and acylation reactions with Lewis acid catalysts. In 1967, we reported Pd(II)-mediated coupling of arenes with olefins in acetic acid under reflux [1], The reaction involves the electrophilic substitution of aromatic C-H bonds by a Pd(II) species, as shown in Scheme 2, and this is one of the earliest examples of aromatic C-H bond activation by transition metal compounds. Al-... 

The authors apphed this new concept to chemoselective functionalization of carbonyls rather than acetals [194], which is usually quite difficult to achieve because of the high reactivity of the acetal counterparts with Lewis acids. Reaction of a mixture of 1 equiv. each of acetophenone and its dimethyl acetal with ketene silyl acetal 191 under the influence of bidentate aluminum Lewis acid 188 in CH2CI2 at -78 °C for 3 h afforded aldol products 195 exclusively (88 % yield). It is worth noting that employment of dibutyltin bis(triflate) (DBTT) (10 mol%) as catalyst [195], which is quite useful for activation of aldehyde carbonyls rather than acetals, gave unsatisfactory results, producing the y3-methoxy ester preferentially (Sch. 147). 

A carbonyl group can be protected as a dithio acetal or ketal, 1,3-dithiane, or 1,3-dithiolane by reaction of the carbonyl compound in the presence of an acid catalyst with a thiol or dithiol. The derivatives are, in general, cleaved by reaction with Lewis acids or oxidation acidic hydrolysis is unsatisfactory. The acyclic derivatives are formed and hydrolyzed much more readily than their cyclic counterparts. Representative examples of formation and cleavage are shown below. 

Condensation reactions of simple carboxylic acids with imines are of intense interest because of their applications to 3-lactam synthesis. Activation of the carboxylic acid derivative is accomplished by preforming the enolate in situ or by using a silyl ketene acetal derivative with Lewis acid catalysis. The first example of an enolate-imine condensation of this type can be attributed to Gillman and Speeter, who in 1943 reported the synthesis of 3-lactams from Reformatsky reagents and Schiff bases. Subsequently, other workers have investigated the mechanism and syn-anti selectivity of this reaction. A review of these studies by Evans et al. covering work through 1980 has appeared in their review, Stereoselective Aldol Condensations . ... 

The first reactions forming tricyclic compounds were induced by oxonium ions (e.g., acetals treated with Lewis acids) to form trans-anti-trans six-six-six-ring systems. A mixture of diastereomers was observed due to nonspecific initiation and termination of the reaction61. 

Ether cleavage has also been observed in reactions with Lewis acids and acetic anhydride. Lewis acids which have been used in this context include FeCL, ... 

Notice in the list of Lewis bases just given that some compounds, such as carboxylic acids, esters, and amides, have more than one atom ivith a lone pair of electrons and can therefore react at more than one site. Acetic acid, for example, can be protonated either on the doubly bonded oxygen atom or on the singly bonded oxygen atom. Reaction normally occurs only once in such instances, and the more stable of the two possible protonation products is formed. For acetic add, protonation by reaction with sulfuric acid occurs on... 

As an alternative to lithium enolates. silyl enolates or ketene acetals may be used in a complementary route to pentanedioates. The reaction requires Lewis acid catalysis, for example aluminum trifluoromethanesulfonate (modest diastereoselectivity with unsaturated esters)72 74 antimony(V) chloride/tin(II) trifluoromethanesulfonate (predominant formation of anti-adducts with the more reactive a,/5-unsaturated thioesters)75 montmorillonite clay (modest to good yields but poor diastereoselectivity with unsaturated esters)76 or high pressure77. 

Ether cleavage can also be effected by reaction with acetic anhydride and Lewis acids such as BF3, FeCl3, and MgBr2.97 Mechanistic investigations point to acylium ions generated from the anhydride and Lewis acid as the reactive electrophile. 

Scheme 10.2 gives some examples of ene and carbonyl-ene reactions. Entries 1 and 2 are thermal ene reactions. Entries 3 to 7 are intermolecular ene and carbonyl-ene reactions involving Lewis acid catalysts. Entry 3 is interesting in that it exhibits a significant preference for the terminal double bond. Entry 4 demonstrates the reactivity of methyl propynoate as an enophile. Nonterminal alkenes tend to give cyclobutenes with this reagent combination. The reaction in Entry 5 uses an acetal as the reactant, with an oxonium ion being the electrophilic intermediate. 

The formation of the primary carbocation can be achieved by treatment of an alkene or an epoxide with a Bronsted or a Lewis add, by elimination of water from an alcohol or an alcohol from an acetal and by readion of enones and imines with Lewis acids. The two latter reactions may also be classified under anionic domino reactions depending on the following steps. 

Kusumoto and coworkers have found that the treatment of hemiacetal 1 with trifluoro- or trichloroacetic anhydride 94 (1 equiv) and trimethylsilyl perchlorate (0.2 equiv) selectively provides the corresponding anomeric ester intermediate 91 [152], Hemiacetal acylation occurs even in the presence of the alcohol acceptor. With Lewis acid assistance, the glycosyl ester intermediate is displaced to provide disaccharide products in good yields. This transformation allowed the synthesis of disaccharides 98 (81%) and 99 (91%). In some cases, acetic anhydride has been used as the electrophilic activator of hemiacetal donors and the reaction with thiol acceptors yields S-linked glycosides [153,154],... 

Alkoxycarbenium ions are important reactive intermediates in modem organic synthesis.28 It should be noted that other names such as oxonium ions, oxocarbenium ions, and carboxonium ions have also been used for carbocations stabilized by an adjacent oxygen atom and that we often draw structures having a carbon-oxygen double bond for this type of cations.2 Alkoxycarbenium ions are often generated from the corresponding acetals by treatment with Lewis acids in the presence of carbon nucleophiles. This type of reaction serves as efficient methods for carbon-carbon bond formation. 

The synthesis of the C20—C26 fragment started with a 4-alkylation of methyl aceto-acetate The first stereocentre was introduced by enantioselecuve catalytic hydrogenation with Noyort s (S)-binap rhodium complex (cf p 102f.) Stereoselective Frater-Seebach alkylation with allyl bromide introduced the second stereocentre in 90% yield (cf p 27) Stereospecifid introduction of the stereocentres C24 and C2 was achieved by a chelation controlled addition of an allylstannane to an aldehyde (see p 66f) After some experimentation with Lewis acid catalysts and reaction conditions a single diastereomer of the desired configuration was ob-... 

Regioselective debenzylation can be achieved by treatment with Lewis acids such as ferric chloride and S11CI4 or under acetolysis conditions with acetic anhydride and sulfuric acid, and several examples are depicted in Scheme 2.3.9 Acetolysis results in cleavage of the most acid-sensitive benzyl group. In general, primary benzyl ethers can be selectively acetolysed in the presence of secondary benzyl ethers. The regioselectivity of the reaction can be explained as follows sulfuric acid protonates acetic anhydride followed by the formation of an acetyl ion and acetic acid. The acetyl ion reacts with the sterically most accessible oxygen which is at... 

The Mukaiyama aldol reaction of carbonyl substrates with silyl enol ethers is the most widely accepted of Lewis acid-promoted reactions. Many Lewis acids for the reaction have been developed and used enantioselectively and diastereoselectively. In 1980, catalytic amounts of la were found by Noyori et al. to effect aldol-type condensation between acetals and a variety of silyl enol ethers with high stereoselectivity [2c,20]. Unfortunately, la has poor Lewis acidity for activation of aldehydes in Mukaiyama s original aldol reaction [21]. Hanaoka et al. showed the scope and limitation of 11-cat-alyzed Mukaiyama aldol reaction, by varying the alkyl groups on the silicon atom of silyl enol ethers [22]. Several efforts have been since been made to increase the reactivity and/or the Lewis acidity of silicon. One way to enhance the catalyst activity is to use an additional Lewis acid. 

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