"Whichs" isn't a word. You would traditionally have had to turn it around and write "the syntax of which". However, modern usage permits "whose" as the genitive of "which" as well as "who/whom". See e.g. the
American Heritage Book of English Usage:
You can use whose as a possessive to refer to both animate and inanimate nouns. Thus you can say Crick, whose theories still influence work in laboratories around the world or Crickâs theories, whose influence continues to be felt in laboratories around the world.
See also
here and
here and
here.
