Type casting within a template in Angular 2

I'm working on an Angular project (Angular 4.0.0) and I'm having trouble binding a property of an abstract class to ngModel because I first need to cast it as the concrete class it actually is in order to access the property.

i.e. I have an AbstractEvent class this has a a concrete implementation Event which has a boolean property 'acknowledged' which I need a two way binding via ngModel to set with a checkbox.

I currently have this element in my DOM:

<input type="checkbox" *ngIf="event.end" [(ngModel)]="(event as Event).acknowledged"
[disabled]="(event as Event).acknowledged">


Unfortunately this is throwing the following error:

Uncaught Error: Template parse errors: Parser Error: Missing expected ) at column 8 in [(event as Event).acknowledged]
​

Googling around seemed to suggest this might be because using 'as' is not supported when using it inside a template? Although I'm not certain about this.

I also can't work out how to just write a function for it in my typescript file driving the template because this would break the two way binding on ngModel that I require.

If anyone has any way to get around this or perform type casting in angular templates correctly I would be very appreciative!

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