sudo apt-get install xbindkeys xvkbd

~/.xbindkeysrc

"/usr/X11R6/bin/xvkbd -xsendevent -text "\[Alt_L]\[Left]""
  m:0x0 + b:6
"/usr/X11R6/bin/xvkbd -xsendevent -text "\[Alt_L]\[Right]""
  m:0x0 + b:7


/etc/X11/xorg.conf

Section "InputDevice"
        Identifier      "Configured Mouse"
        Driver          "mouse"
        Option          "CorePointer"
        Option          "Device"        "/dev/input/mice"
        Option          "Protocol"      "ExplorerPS/2"
        Option          "Buttons" "7"
        Option          "ButtonMapping" "1 2 3 6 7"
EndSection

add "xbindkeys" to your sessions