Four pairs of tautomers from rhizomes of Acorus calamus and fruits of Cornus officinalis.

A pair of new guaiane-type sesquiterpene tautomers (1) was isolated from rhizomes of Acorus calamus. Meanwhile, three pairs of known compounds, including a pair of dihydroflavone glycoside tautomers (2), a pair of 4-hydroxybenzoic acid ester glycoside tautomers (3), as well as a pair of secoiridoid glycoside tautomers (4) were isolated from fruits of Cornus officinalis. Their structures were elucidated by extensive spectroscopic and computational methods. Furthermore, the tautomeric mechanisms were discussed. [ABSTRACT FROM AUTHOR]