Apache proxy virtual host on backend server

I have 2 web Debian servers, a frontend accepting request from port 80 and a backend via proxy from the frontend.

What I want to do is have a many different URLs going the backend server with virtual hosts.

Here is the frontend Apache setting

<VirtualHost *:80>
    ServerName dev.example.com
    ProxyPass / http://192.168.144.100:80/
    ProxyPassReverse / http://192.168.144.100:80/
</VirtualHost>

<VirtualHost *:80>
    ServerName sandbox.example.com
    ProxyPass / http://192.168.144.100:80/
    ProxyPassReverse / http://192.168.144.100:80/
</VirtualHost>

And here is the backend Apache setting

 <VirtualHost *:80>
                ServerAdmin webmaster@localhost

                DocumentRoot /var/www/
                <Directory />
                        Options FollowSymLinks
                        AllowOverride None
                </Directory>
                <Directory /var/www/>
                        Options Indexes FollowSymLinks MultiViews
                        AllowOverride None
                        Order allow,deny
                        allow from all
                </Directory>
    </VirtualHost>
    <VirtualHost *:80>
            ServerAdmin webmaster@localhost

            ServerName dev.example.com

            DocumentRoot /var/www/example.com/dev/
            <Directory />
                    Options FollowSymLinks
                    AllowOverride None
            </Directory>
            <Directory /var/www/example.com/dev/>
                    Options Indexes FollowSymLinks MultiViews
                    AllowOverride None
                    Order allow,deny
                    allow from all
            </Directory>
    </VirtualHost>

   <VirtualHost *:80>
            ServerAdmin webmaster@localhost

            ServerName sandbox.example.com

            DocumentRoot /var/www/example.com/sandbox/
            <Directory />
                    Options FollowSymLinks
                    AllowOverride None
            </Directory>
            <Directory /var/www/example.com/sandbox/>
                    Options Indexes FollowSymLinks MultiViews
                    AllowOverride None
                    Order allow,deny
                    allow from all
            </Directory>
    </VirtualHost>

When I go to dev.example.com or sandbox.example.com, it takes me to the default /var/www files.

Basically, I want the ability to run multiple virtual hosts on the backend server.

Thanks for your help.