I don't know how you might directly get what you want, but here's one 
way to get an equivalent result.

class element_aux (c : [`Data of string | `Element of element list]) = 
object end

type content = Data of string | Element of element list;;

let content_of_variant = function Data d -> `Data d | Element e -> 
`Element e

class element (c : content) = element_aux (content_of_variant c)