Acyloxy shift

Examples of the acyloxy shift that proceed with less than 100% inversion of the carboxyl oxygens [120-123] are now best interpreted in terms of looser ion pairs, resulting from more highly stabilized and/or substituted alkene... 

In a computational investigation of substituent and solvent effects on acyloxy shifts in radicals, SM2/AM1 and SCIPCM models were exploited with explicit consideration of water molecules. All solvent models utilized agreed in predicting the [3,2] acyloxy shift to proceed more readily in aqueous solution than in gas phase, but in spite of the qualitative agreement with experimental data the calculated substituent and solvent effects were smaller than those observed experimentally [129]. 

Taking into account this lack of stereoselectivity and equilibration between allenic stereoisomers, Benn proposed a mechanism in which silver ion facilitates the acyloxy shift on formation of n complex (or a bridged ionic silver intermediate).52 The thus-formed acetoxyallenes would remain coordinated to silver, allowing the formation of an organosilver allyl cation. Free rotation at this stage would provide both acetoxy-allene isomers after reversible loss of silver (Scheme 3.32). 

In the carbohydrate and nucleoside series the acyloxy shift has been coupled with radical allylation using allyltributylstannane to give rearranged allylated products (Scheme 17) [36, 37]. 

Carbonyl stretch frequencies, carbonyl 13C and amide 15N chemical shifts for a wide range of A-acyloxy-A-alkoxyamides are listed in Table 2 together with those of the precursor hydroxamic esters. Spectroscopically, mutagens can be categorised into six types ... 

C NMR resonances for both carbonyls of Af-acyloxy-Af-alkoxyamides and that of the parent hydroxamic ester are given for most compounds in Table 5. With the exception of para-substituted benzyloxy-V-acyloxy-V-alkoxyamides, amide carbonyl "C NMR shifts differ from those of their precursor hydroxamic esters by on average -1-8.0 ( 0.6) ppm. Steric and electronic effects influence hydroxamic esters and V-acyloxy-Af-alkoxyamides similarly. This includes substrates with branching alpha to the amide carbonyl. 

Limited carbonyl NMR data are available for these anomeric amides. However, carbonyl shifts for hydrazines 217 and 218 were on average 3 ppm higher than their hydroxamic ester precursors. This reflects a higher degree of residual amide resonance in the hydrazines relative to A-acyloxy-iV-alkoxyamides where the difference was closer to 8.0 ppm. As reported for A-acyloxy-A-alkoxyamides (see Section IV.B.2), analysis of variance in the hydrazine and hydroxamic ester shifts indicates that substituents affect the hydroxamic ester carbonyl shifts ( 2.6) more than those of the hydrazines ( 1.3 ppm). 

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