There's 6 identified R-groups, (note that I modified the R in the picture such that the ketone is now part of "R").
The following papers are probably of interest to you:
MOIR, M. (2000) J. Amer. Soc. Brew. Chem., 58, 131. (of particular interest)
STEVENS, R. (1967) Chem. Rev., 67, 19.
WINDISCH, W., KOLBACH, P. and SCHLEICHER R. (1927) Wochschr. Brau., 44, 453.
The Moir paper in the JASBC is a very good one, as it provides a millenium-review of what we know of hop chemistry. It was quite a good read, in fact. In it, it references the following paper which proposed the specific mechanisms of the isomerization:
De Keukeleire, D., and Verzele, M. The absolute configuration of
the isohumulones and the humulinic acids. Tetrahedron 27:4939-
Unfortuanately, I don't have access to Tetrahedron like I do the JASBC, but I'll let you know what I find when the article comes in.
So, the mechanism I am unsure of, but it doesn't occur on a stereo center of the R-group:
Wish I had more to say about it, but between not having the paper handy and risking copyright infringement of what I've got, this is what I can tell you.
Remember, it's all just chemistry.
EDIT: Wow that image it huge...