  It is shown that the endpoint of the first order transition line which merges
into a crossover regime in the phase diagram of the Nambu--Jona-Lasinio model,
extended to include the six-quark 't Hooft and eight-quark interaction
Lagrangians, is pushed towards vanishing chemical potential and higher
temperatures with increasing strength of the OZI-violating eight-quark
interactions. We clarify a connection between the location of the endpoint in
the phase diagram and the mechanism of chiral symmetry breaking at the quark
level. Constraints on the coupling strengths based on groundstate stability and
physical considerations are explained.
