Claisen rearrangements vinylogous anomeric effect

D. P. Curran and Y. Suh, Selective mono-Claisen rearrangement of carbohydrate glycals. A chemical consequence of the vinylogous anomeric effect, Carbohydr. Res. 777 161 (1987). 

A C(6) donor substituent exerts a similar enhancing efiect on the rate of the Claisen rearrangement, though somehow less pronounced than the rate-accelerating effect of the 2-silyloxy substituent (approx. 1.4 kcal mol" ). As Curran and coworkers reported, " this vinylogous anomeric effect of the C(6) donor substituent is especially effective in glycal systems. The energy of the TS is decreased by assist-... 

A substituent-controlled "mono-Clatsen" rearrangement was also employed as a key reaction in the construction of chiral intermediates for the synthesis of pseudomonic acids from diacetyl-L-arabinal418. The acetate groups of the starting material are chemically differentiated by virtue of the accelerated rate of the first Claisen rearrangement (ascribed to a vinylogous anomeric effect). 

Figure 9.21 Connection between the vinylogous anomeric effect and substituent effects in the Claisen rearrangement.

Curran proposed a vinylogous anomeric effect concept to explain the observed rate difference of the Ireland-Claisen rearrangements of dihydropyran 152 and cyclohexene system 155. Oxygen attached to Ce is proposed to stabilise the polar Ireland-Claisen intermediates 160-161 which contribute to a faster rate of reaction. ... 

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