"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 Crick’s theories, whose influence continues to be felt in laboratories around the world.

See also here and here and here.