Migration, acetyl benzoyl

The acetyl(benzoyl)rhenium complex, [Re(CO)4(COMe)(COPh)], shown in Scheme 5, has provided a useful way of comparing rates of migration and... 

Two disadvantages are associated with the use of S-acetyl or 5-benzoyl derivatives in peptide syntheses (a) base-catalyzed hydrolysis of 5-acetyl- and 5-benzoylcys-teine occurs with /S-elimination to give olefinic side products, CH2=C-(NHPG)CO—(b) the yields of peptides formed by coupling an unprotected amino group in an 5-acylcysteine are low because of prior S-N acyl migration. ... 

Acyl migrations in the acylbenzoquinone cycloadducts were also described elsewhere170. It was shown that the direction of acyl migrations in the cycloadducts obtained from dienes and 2-acetyl- as well as 2-benzoyl- 1,4-benzoquinones depends on the substituents in diene fragment. 

A number of 2,3-fused chromones have been prepared from ethyl chloroformate and acetylsalicylic acid, a source of the mixed anhydride of formic and salicylic acids, and piperidinocycloalkenes (69JCS(C)935). A plausible mechanism is outlined in Scheme 165. It has not proved possible to isolate the chromanone (460) but the formation of 3-acetyl-2-methylchromone from 2-pyrrolidinopropene lends support to the intermediacy of the chromanone. The migration of the acyl group from oxygen to carbon is supported by the synthesis of 3-benzoyl-2-methylchromone rather than 3-acetylflavone from benzoylsalicylic acid and the pyrrolidinopropene. 

Attempted sodium hydride mediated benzylation of methyl 3-0-benzoyl-4,6-0-benzylidene-P-D-galactopyranoside failed due to a benzoyl migration [65]. The acyl group migration and removal are also responsible for only 62 % yield of benzyl 2-acetamido-3,6-di-0-acetyl-4-0-benzyl-2-deoxy-a-D-glucopyranoside. Thallium eth-oxide instead of sodium hydride or alkoxide successfully restrained the acetyl group migration in this reaction [35]. 

Hi) Reduction of Glycosyl Halides with Radical Rearrangement. The reduction of acetylated or benzoylated halides or selenides with a low concentration of tributylstannane leads to 2-deoxy sugar derivatives (Scheme 11). The driving force of this radical reaction is the 1,2-cA-selective migration of an ester group because of the stabilization of the radical at C-2.46... 

SCHEME 4.5 1,2-Migration in the stereoselective synthesis of 1, 2-tram (amino)-glyco-sides and 2-deoxyglycosides as reported by (a) Danishefsky, (b) Yu, and (c) Lowary. Ac, acetyl All, allyl Bn, benzyl Bz, benzoyl DTBMP, 2,6-di-fert-butyl-4-mcthyl pyridine TBDPS, ferf-butyldiphenylsilyl THF, tetrahydrofuran TIPS, triisopropylsilyl Tol, 4-tolyl. 

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