  We introduce a frustrated spin 1/2 Hamiltonian which is an extension of the
two dimensional $J_1 - J_2$ Heisenberg model. The ground states of this model
are exactly obtained at a first order quantum phase transition between two
regions with different valence bond solid order parameters. At this point, the
low energy excitations are deconfined spinons and spin-charge separation occurs
under doping in the limit of low concentration of holes. In addition, this
point is characterized by the proliferation of topological defects that signal
the emergence of $Z_2$ gauge symmetry.
