WordNet (r) 3.0 (2006):

too soon
    adv 1: before the usual time or the time expected; "she
           graduated early"; "the house was completed ahead of time"
           [syn: early, ahead of time, too soon] [ant:
           belatedly, late, tardily]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "too soon":
   a bit previous, advanced, far ahead, forward, half-baked,
   half-cocked, hasty, ill-considered, ill-seasoned, ill-timed,
   improper, impulsive, inappropriate, inauspicious, inconvenient,
   inexpedient, infelicitous, inopportune, intempestive, intrusive,
   irrelevant, late, mal a propos, malapropos, mistimed, not firm,
   off base, out of line, out of phase, out of time, overhasty,
   oversoon, precipitate, precocious, premature, previous, rushed,
   too early, too late, unbefitting, uncrystallized, unfavorable,
   unfit, unfitting, unfortunate, unhandy, unhappy, unjelled, unlucky,
   unmatured, unmeditated, unpremeditated, unprepared, unpropitious,
   unready, unripe, unseasonable, unsuitable, untimely, untoward,
   wrong