The Collaborative International Dictionary of English v.0.48:

Whenever \When*ev"er\, adv. & conj.
   At whatever time. "Whenever that shall be." --Milton.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "whenever":
   anytime, at any time, at what time, at whatever time, at which,
   at which time, if ever, in what period, in which time,
   no matter when, on what occasion, on which occasion, once,
   upon which, what time, when, when as, whensoever, whereupon