I found out that you can use this kind of syntax using square brackets:
<li *ngFor="#course of courses; #i = index" [id]="'someselector-'+i">{{course}}</li>
For more information, please have a look to this interesting article from Pascal Precht: http://blog.thoughtram.io/angular/2015/08/11/angular-2-template-syntax-demystified-part-1.html