Acylals, simple partial

The acid-catalyzed hydrolysis of simple partial acylals has been studied in condensed phase, " " and a study of the behavior of methoxymethyl formate and methoxymethyl acetate in chemical ionization mass spectrometry was undertaken " to provide a comparison of the gaseous and solution phase chemistry of these simple partial acylals. Methoxymethyl formate and methoxymethyl acetate undergo hydrolysis in aqueous acid by an A lI mechanism,... 

Olivier and Berger335, who measured the first-order rate coefficients for the aluminium chloride-catalysed reaction of 4-nitroben2yl chloride with excess aromatic (solvent) at 30 °C and obtained the rate coefficients (lO5/ ) PhCI, 1.40 PhH, 7.50 PhMe, 17.5. These results demonstrated the electrophilic nature of the reaction and also the unselective nature of the electrophile which has been confirmed many times since. That the electrophile in these reactions is not the simple and intuitively expected free carbonium ion was indicated by the observation by Calloway that the reactivity of alkyl halides was in the order RF > RC1 > RBr > RI, which is the reverse of that for acylation by acyl halides336. The low selectivity (and high steric hindrance) of the reaction was further demonstrated by Condon337 who measured the relative rates at 40 °C, by the competition method, of isopropylation of toluene and isopropylbenzene with propene catalyzed by boron trifluoride etherate (or aluminium chloride) these were as follows PhMe, 2.09 (1.10) PhEt, 1.73 (1.81) Ph-iPr, (1.69) Ph-tBu, 1.23 (1.40). The isomer distribution in the reactions337,338 yielded partial rate factors of 2.37 /mMe, 1.80 /pMe, 4.72 /, 0.35 / , 2.2 / Pr, 2.55337 339. 

Compound 51 was found to be unstable and difficult to purify, as described in the literature [93—95]. Therefore, 51 was not isolated, but was instead converted to the stable pinacol 1-acetamido-l-hexylboronate derivative 52. However, the acylated derivative 52 could not be purified by column chromatography as it was destroyed on silica gel and partially decomposed on alumina. Fortunately, we found that it dissolves in basic aqueous solution (pH > 11) and can then be extracted into diethyl ether when the pH of the aqueous layer is 5—6. Finally, pure 52 was obtained by repeated washing with weak acids and bases. It should be mentioned here that exposure to a strongly acidic solution, which also dissolves compound 51, results in its decomposition. Compared with other routes, the present two-step method involves mild reaction conditions (THF, ambient temperature) and a simple work-up procedure. It should prove very useful in providing an alternative access to a-aminoboronic esters, an important class of inhibitors of serine proteases. 

The mechanism of CDI-mediated acylation of amines is well understood. The first step involves a partial protonation of the basic imidazole-nitrogen, protonated A-acetylimidazole has a p a of 3.6,f l leading to an activated species which is then attacked by the carboxylate. The resulting mixed anhydride extrudes carbon dioxide giving rise to A-acylimidazole which on treatment with an amine compound leads to the desired anoide (Scheme 1). An important advantage of this method over the carbodiimide method is that the byproducts carbon dioxide and imidazole are readily and quantitatively separated from the reaction product by simple washing procedures. 

Pozsgay, V, A simple method for avoiding alkylthio group migration during the synthesis of thioglycoside 2,3-orthoesters. An improved synthesis of partially acylated 1-thio-a-L-rhamnopyrano-sides, Carbohydr. Res., 235, 295-302, 1992. 

BP extended this Ir carbonylation chemistry with the discovery of proprietary promoters to achieve commercially viable high reaction rates at low reaction water conditions with essentially no dependence of CO partial pressure on the reaction rate [20]. These promoters can be categorized in two groups simple iodide complexes of Zn, Cd, Hg, Ga, and In or carbonyl complexes of Re, Ru, Os, or W. It is believed these promoters participate in the rate-determining step to abstract iodide from [Ir(CH3)(CO)2(I)3] , thus facilitating methyl migration to form the corresponding acyl complex, [Ir(CH3)(CO)2 (1)3]-. 

Despite their enormous structural diversity, polyketide metabolites are related by their common derivation from highly functionalised carbon chains whose assemblies are controlled by multifunctional enzyme complexes, the polyketide synthases (PKSs) which, like the closely related fatty acid synthases, catalyse repetitious sequences of decarboxylative condensation reactions between simple acyl thioesters and malonate, as shown in Fig. 3 [7]. Each condensation is followed by a cycle of modifying reactions ketoreduction, dehydration and enoyl reduction. In contrast to fatty acid biosynthesis where the full cycle of essentially reductive modifications normally follow each condensation reduction, the PKSs can use this sequence in a highly selective and controlled manner to assemble polyketide intermediates with an enormous number of permutations of functionality along the chain. As shown in Fig. 3, the reduction sequence can be largely or entirely omitted to produce the classical polyketide intermediate which bears a carbonyl on every alternate carbon and which normally cyclises to aromatic polyketide metabolites. On the other hand, the reductive sequence can be used fully or partially after each condensation to produce highly functionalised intermediates such as the Reduced polyketide in Fig. 3. Basic questions to be answered are (i) what is the actual polyketide intermediate... 

The cyclodextrin ring does not have a well-defined depth, so we adopted a simple procedure to turn it from an open tube into a cup. We attached some flexible groups on the primary carbons of the cyclodextrin that could partially invade the cavity and produce a flexible floor.Later we studied acylation reactions with cyclodextrins that had been modified to include a floor on the bottom of the cavity - and also with improved substrates and a somewhat better solvent - that led to a relative rate of 750,000 compared with the uncatalysed hydrolysis rate at the same pH. When we looked at related compounds in which we had frozen out one of the degrees of freedom of the substrate the improvement came up to a 3,200,000-fold acceleration compared with the hydrolysis rate in water, and with some selectivity among the geometries of the complex. ... 

Natural abundance C n.m.r. in H20 has been used to examine the possibility of acyl migration during the synthesis of di- and tri-saccharides in which a-D-galactopyranose becomes attached to 0-4 in partially acylated galactose units. Expected structures were confirmed, showing that no 6 - 4 acyl migration had occurred. As models for cellobiose and maltose, twenty-one simple derivatives of cyclohexyloxytetrahydropyran have been studied by C and n.m.r. ... 

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