No sé si te entendí:
Código PHP:
<form autocomplete="off">
<select name="pp" size="5" multiple="multiple" id="pp">
<option value="z">a</option>
<option value="y">b</option>
<option value="x">c</option>
<option value="w">d</option>
<option value="v">e</option>
</select>
<input onclick="pp.form.reset();e=pp.options;e[0].selected=e[2].selected=e[4].selected=1" type="button" value="seleccionar 0,2,4" />
<input onclick="pp.form.reset();e=pp.options;e[1].selected=e[3].selected=e[4].selected=1" type="button" value="seleccionar 1,3,4" />
</form>