Use async pipe on method returning Observable

How can I use async pipe when calling service method from template?

This code works:

    <div *ngFor="let filename of filenames | async">
        <div>{{filename}}</div>
    </div>

    //code behind
    constructor(public importService: ImportService)
    {
        this.filenames = this.importService.list();  //returns Observable - from http.get
    }

but I would like to avoid assigning value to filenames (this.filenames = this.importService.list();) and call method directly from html:

<div *ngFor="let filename of importService.list() | async">
    <div>{{filename}}</div>
</div>

But this execute unlimited request to server. (As I would call importService.list().subscribe() in endless loop).

I also tried this (source for ideas):

<div *ngFor="let filename of (o = (importService.list() | async))">
    <div>{{filename}}</div>
</div>


<div *ngFor="let filename of (o = (importService.list()) | async)">
    <div>{{filename}}</div>
</div>


<div *ngIf="importService.list(); let o">
    <div *ngFor="let filename of o | async">
        <div>{{filename}}</div>
    </div>
</div>

But none works.
This Github issue described exactly the same thing. I hope something has changed in year and half and this could be done.

Edited:
Desired effect can be achieved with several approaches.
First:

<div *ngFor="let filename of filenames">
    <div>{{filename}}</div>
</div>

public filenames: string[];

constructor(public importService: ImportService)
{
    this.importService.list().subscribe(res => this.filenames = res);
}

Second, more effective (shorter):

<div *ngFor="let filename of filenames | async">
    <div>{{filename}}</div>
</div>

public filenames: Observable<string[]>;

constructor(public importService: ImportService)
{
    this.filenames = this.importService.list();
}

and third even shorter (what I want to achieve):

constructor(public importService: ImportService) { }

<div *ngFor="let filename of importService.list() | async">
    <div>{{filename}}</div>
</div>