C-Glycosidic Ellagitannins and Their Influence on Wine Chemistry - Chemistry and Biology of Ellagitannins - page 335

favored from 1 , since this OH group is rather axially oriented on the six-
membered ring lactone defined by the double connection of the galloyl-
derived NHTP I-ring to the O-2 and C-1 positions of the open-chain
glucose core. In 2 , this OH-1 group adopts a less energetically
demanding equatorial orientation (Quideau et al. , 2005, Haslam, 1998),
thus also contributing to the lower overall strain energy of castalagin ( 2 )
versus vescalagin ( 1 ).

α -OH 1

"free"
β -OH 1

castalagin ( 2 , E rel = 0.0)

vescalagin ( 1 , E rel = + 2.9 kJ/mol)

H-bond
2.21 Å, 146°

III

1

III

1

2'

2'

1'

3'

I

1' 3'

I

II

II

Fig. 9.8 MM3* minimum-energy conformations of vescalagin ( 1 ) and castalagin ( 2 ).