Acetals as reagents

Two 2-hydroxyaldehydes protected as acetals (63) have been resolved by lipase-catalyzed acylation with vinyl acetate as reagent and solvent (Scheme 4.24) [84]. For the vinyl derivative (n = 0) Chirazyme L2 (CALB) gives the best E, whereas Pseudomonas fluorescens lipase (PFL) provides the best E for n = 1 [84]. 

SoUadid has introduced a-sulfinyl acetates as reagents for asymmetric aldol reactions.Compound (211) is prepared in good optical purity from the menthyl ester of p-tolylsulfinic acid. The magnesium enolate of (211), prepared by reaction of the sulfrnyl ester with r-butylmagnesium bromide, reacts with aldehydes and ketones to give diastereomeric mixtures of a-sulfinyl-3-hydroxy esters (Scheme 13). No... 

Scheme 14 Ethyl acetate as reagent and solvent of an enantioselective transesterification.

Certain features of the addition of acetyl nitrate to olefins in acetic anhydride may be relevant to the mechanism of aromatic nitration by this reagent. The rapid reaction results in predominantly cw-addition to yield a mixture of the y -nitro-acetate and y5-nitro-nitrate. The reaction was facilitated by the addition of sulphuric acid, in which case the 3rield of / -nitro-nitrate was reduced, whereas the addition of sodium nitrate favoured the formation of this compound over that of the acetate. As already mentioned ( 5.3. i), a solution of nitric acid (c. i 6 mol 1 ) in acetic anhydride prepared at — 10 °C would yield 95-97 % of the nitric acid by precipitation with urea, whereas from a similar solution prepared at 20-25 °C and cooled rapidly to —10 °C only 30% of the acid could be recovered. The difference between these values was attributed to the formation of acetyl nitrate. A solution prepared at room... 

Diethyl 3-oxoheptanedioate, for example, is clearly derived from giutaryl and acetic acid synthons (e.g. acetoacetic ester M. Guha, 1973 disconnection 1). Disconnection 2 leads to acrylic and acetoacetic esters as reagents. The dianion of acetoacetic ester could, in prin-ciple,be used as described for acetylacetone (p. 9f.), but the reaction with acrylic ester would inevitably yield by-products from aldol-type side-reactions. 

The use of ynamines, ketenimines and mercuric acetate as electrophiles in DMSO oxidations has also been reported but, as yet, appear to offer no advantages over the reagents described above. 

Oxymercuration (Section 7.4) A method for double-bond hydration using aqueous mercuric acetate as the reagent. 

Ammonia solutions are significantly more efficient than ammonium acetate as washing reagents. The process of fluorine content reduction is related to subsequent hydrolysis of fluoride and oxyfluoride contaminants by ammonia. 

Molecular ion Chemical ionization using ammonia as reagent gas establishes the molecular weights of sugar acetates. 

The product obtained from this distillation usually contains small amounts of acetone cyanohydrin acetate, as evidenced by an ester carbonyl band at 1740 cm.-1 in its infrared spectrum. This material does not interfere with the nitration reactions of the reagent. It may be removed by fractionation through a more efficient column. 

The hydration of triple bonds is generally carried out with mercuric ion salts (often the sulfate or acetate) as catalysts. Mercuric oxide in the presence of an acid is also a common reagent. Since the addition follows Markovnikov s rule, only acetylene gives an aldehyde. All other triple-bond compounds give ketones (for a method of reversing the orientation for terminal alkynes, see 15-16). With allqmes of the form RC=CH methyl ketones are formed almost exclusively, but with RC=CR both possible products are usually obtained. The reaction can be conveniently carried out with a catalyst prepared by impregnating mercuric oxide onto Nafion-H (a superacidic perfluorinated resinsulfonic acid). ... 

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. 

In a more recent study, Westman and Lundin have described solid-phase syntheses of aminopropenones and aminopropenoates en route to heterocycles [32], Two different three-step methods for the preparation of these heterocycles were developed. The first method involved the formation of the respective ester from N-pro-tected glycine derivatives and Merrifield resin (Scheme 7.12 a), while the second method involved the use of aqueous methylamine solution for functionalization of the solid support (Scheme 7.12 b). The desired heterocycles were obtained by treatment of the generated polymer-bound benzylamine with the requisite acetophenones under similar conditions to those shown in Scheme 7.12 a, utilizing 5 equivalents of N,N-dimethylformamide diethyl acetal (DMFDEA) as reagent. The final... 

Both the palladium- and the nickel-catalysis enables the use of allylic acetate as starting reagent. In the two approaches a zinc compound is evoked as key intermediate, though its formation has been demonstrated only indirectly. In the two methods allylic transposition is observed. The authors have then concluded that these electrochemical allylation reactions closely parallel the chemical allylation reactions involving allylzinc intermediates. 

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