expanding a (g)vim snippet from menu

I have a snippet that expands properly from the vim insert mode:

#In vim insertmode, `prg will expand snippet prg in Ultisnips dir
:execute 'imap `prg prg' . g:UltiSnipsExpandTrigger . '<Esc>gg=G<C-j>'


But I tried to use this is vim's menu as well:
an Snippet<Tab>`prg :execute 'imap `prg prg' . :UltiSnipsExpandTrigger . '<Esc>gg=G<C-j>'

This is giving me error :
E115: Missing quote: '
E15: Invalid Expression: 'imap `prg prg' . g:UltiSnipsExpandTrigger . '

The RHS of the menu mode is the same function that is used in insert mode. Don't know why it is complaining, and how can I call it in menu.

I am using:
vi --version
VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Mar 2 2015 08:37:26)
Included patches: 1-207, 209-640
Modified by <bugzilla@redhat.com>
Compiled by <bugzilla@redhat.com>

and Ultisnips(https://github.com/SirVer/ultisnips).

Kindly help.

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