The Collaborative International Dictionary of English v.0.48:

Syntactic \Syn*tac"tic\, Syntactical \Syn*tac"tic*al\, a. [Cf.
   G. ? putting together. See Syntax.]
   Of or pertaining to syntax; according to the rules of syntax,
   or construction. -- Syn*tac"tic*al*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

syntactic
    adj 1: of or relating to or conforming to the rules of syntax;
           "the syntactic rules of a language" [syn: syntactic,
           syntactical]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "syntactic":
   adjectival, adverbial, attributive, conjunctive, copulative,
   correct, descriptive, formal, functional, glossematic,
   glottochronological, grammatic, graphemic, intransitive,
   lexicographic, lexicological, lexicostatistical, lingual,
   linguistic, linking, metalinguistic, morphological, morphophonemic,
   nominal, participial, philological, phonemic, phonetic,
   phonological, postpositional, prepositional, pronominal,
   psycholinguistic, semantic, structural, substantive, tagmemic,
   transitive, verbal