Within this naphthoquinone category, there are two basic types of vitamin K.    From Wordnik.com.  [The World's Healthiest Foods] Reference

The anti-vitamin K activity was thought to be associated with the hydroxyl group attached to the naphthoquinone ring ofthe compound.    From Wordnik.com.  [Find Me A Cure] Reference

In the very same year Doisy reported that, together with his associates, he had succeeded not only in determining the chemical structure of vitamin K - it was found to be a naphthoquinone derivate - but also in synthetically preparing in his laboratory a K vitamin identical with the natural one.    From Wordnik.com.  [Physiology or Medicine 1943 - Presentation Speech] Reference