We study by numerical integration the network of nuclear reactions that are expected to occur when the helium zones of a star are instantaneously heated and allowed to expand adiabatically. The dependence of the results on nuclear cross-sections, on the peak temperature, and on the time scale of the hydrodynamic expansion are discussed. Of particular interest is the result that the nucleus `4N, constituting about 2 percent by mass following the earlier operation of the CNO cycle, is substantially converted into 150, 15F, 15Ne, and 21Ne in ratios close to the solar abundances of 55N, 80, 55F, and 21Ne. It is very likely that these nuclei are synthesized in such zones. The nucleus 22Ne, on the other hand, is apparently not produced in the explosion in sufficient amounts relative to 21Ne, which indicates that 22Ne is probably synthesized in hydrostatic helium burning and survives the ejection.