use AppleScript version "2.4" -- Yosemite (10.10) or later
use framework "Foundation"
use scripting additions

set shell to "tell application 'System Events' to keystroke 'r' using {option down, command down}"
set shellQuote to quoted form of shell
log {"shell is:", shell}
log {"shellQuote is:", shellQuote}
--tell application "System Events" to keystroke "r" using {option down, command down}

** Notice that I had to use single quotes instead of double to create the shell string

When I run this the properties are:

Shell:    

tell application 'System Events' to keystroke 'r' using {option down, command down}    

shellQuote:    

'tell application '\''System Events'\'' to keystroke '\''r'\'' using {option down, command down}'

From the Script Dictionary for System Events on the Text Class:

quoted form A rendering of the text string suitable for handing to the shell as an argument to a command. The text string is wrapped in single quotation marks and internal quotation marks are escaped.

You also may want to try the System Events Command:

key code key code (verb)cause the target process to behave as if key codes were entered (from Processes Suite) COMMAND SYNTAX key code integer or list of integer ¬ using using or list of using PARAMETERS Parameter Required Type Description direct parameter required integer or list of integer The key code(s) to be sent. May be a list. using optional using or list of using modifiers with which the key codes are to be entered