Acetylation regioselective

The fact that the isomeric structure of azolides is thermodynamically controlled has been used by Olofson and Kendall to prepare 1-alkylazoles regioselectively (70JOC2246). An asymmetric pyrazole yields two alkylated derivatives (Scheme 21 see Section 4.04.2.1.3 (viii)), but the alkylation with a powerful alkylating agent of the acetylated derivative leads to the less abundant isomer via the salt (249), which is too unstable to be isolated. 

With an activated C—C triple bond two successive additions can occur if the intermediate alkene is reactive enough. DMAD and 3,5-dimethylpyrazole give an initiaj fumarate (255) which reacts further at the other end to form regioselectively the succinates (256). On the other hand, methyl ethynyl ketone reacts twice at the same carbon atom with pyrazole to form 1,1-pyrazolylbutanone (258) (68ZC458). The probable intermediate, a pyrazolide vinylogue (257), can be prepared from methyl chlorovinyl ketone and pyrazole, in a reaction which is similar to acetylation (Section 4.04.2.1.3(x)). 

Because of the extremely low solubility of the cyclized products, only the 8-acetyl-9-X oxo-6,9-dihydrothiazolo[5,4-g]quinoline was isolated. On the other hand, 6-substituted derivatives having the same possibility to produce linearly or angularly annelated thiazoloquinolines cyclize regioselectively. 

The primary OH group can be selectively blocked by the bulky triphenyl-methyl (trityl) moiety, followed by esterification at the secondary OH groups and removal of the protecting trityl group. Thus 2,3-di-O-acetyl cellulose has been obtained by this procedure. Moreover, regioselectively substituted mixed cellulose esters, acetate/propionate, were prepared by subsequent acy-... 

Acetylchloride is a trapping agent that allows the reaction to go completion, transforming the product into a less oxidizable compound.The results of other reactions between indole (57) and substituted cyclohexa-1,3-dienes show that the photo-induced Diels-Alder reaction is almost completely regioselective. In the absence of 59 the cycloaddition did not occur the presence of [2+2] adducts was never detected. Experimental data support the mechanism illustrated in Scheme 4.14. The intermediate 57a, originated from bond formation between the indole cation radical and 58, undergoes a back-electron transfer to form the adduct 60 trapped by acetyl chloride. 

In a modified version of a previously published procedure <83JOC3214>, the thallium induced transposition of 3-acetyl-1-tosylpyrrole to the corresponding 2-(3-pyrrolyl)acetic acid has been reported employing thallium(III) nitrate supported on acidic montmorillonite K-10 <96SC1289>. In this sequence, Af-tosylpyrrole (39) undergoes regioselective C-3 acylation to afford 40 which is rearranged with T1(III)/K-10 in methanol to yield the ester 41. 

Chloro-3-(5-chloropyrazol-3-yl)quinoxaline (137, R = H) gave mainly 2-(2-acetyl-5-chloropyrazol-3-yl)-3-chloroquinoxaline (137, R = Ac) [neat Ac20, reflux, 7 min, then 20°C, 30 min 86% (after separation from a little of the 1-acetyl isomer) structure confirmed by X-ray analysis] or mainly 2-(l-acetyl-5-chloropyrazol-3-yl)-3-chloroquinoxaline [neat Ac20, 80°C, 2 min ( ), then 20°C, 12 h 70% (after separation from the 2-acetyl isomer)] an extraordinary pair of regioselective reactions.495... 

For this reason the regioselective methylation of various 4-substituted 1-acetyl-imidazoles was studied.1[2] While reaction with l-acetyl-4-phenylimidazole furnished the isomers l-methyl-5-phenylimidazole and l-methyl-4-phenylimidazole in a ratio of 93 7, reaction with l-acetyl-4-methylimidazole produced an isomer ratio of 86 14. 

M. Adinolfi, G. Barone, A. Iadonisi, and M. Schiattarella, An easy approach for the acetylation of saccharidic alcohols. Applicability for regioselective protections, Tetrahedron Lett., 44 (2003) 4661 -663. 

A useful method for the preparation of functionalised thiazoles has been described. Palladium catalysed cross coupling reactions between 4-thiazolyl-5-acetyl triflates 36 and alkynes afforded 4-alkynyl-5-acetylthiazoles 37 in good yields (56-82%). If 37 is then treated with ammonia in methanol, thiazolo[5,4-c]pyridines 39 are formed, probably via the intermediate imine 38 which then undergos a regioselective 6-endo dig cyclisation <99EJOC3117>. 

The reaction of adenosine with 2-acetoxy-2-methylpropanoyl chloride or the corresponding bromide is less regioselective than that of uridine,400 and gives 9-(2-0-acetyl-3-deoxy-3-halo-/3-D-xylofuranosyl)-and9-(3-0 - acetyl - 2 - deoxy - 2 - halo - /3 - D - arabinofuranosyl) - adenine,... 

Prior to his work with internal alkynes, Larock found that o-thallated acetanilide undergoes Pd-catalyzed reactions with vinyl bromide and allyl chloride to give (V-acetylindole and N-acetyl-2-methylindole each in 45% yield [409]. In an extension to reactions of internal alkynes with imines of o-iodoaniline, Larock reported a concise synthesis of isoindolo[2,l-a]indoles 313 and 314 [410]. The regioselectivity was excellent with unsymmetrical alkynes. 

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