# Parenthesis inside chemfig environment

How to display "(" or ")" inside chemfig environment. For example I what to write following chemical formula:

My try, I used \text{(} and \text{)} to get "(" and ")" inside the chemfig environment. But I now can't able to get the double bond 'O' properly.

\documentclass[12pt,a4paper]{report}
\usepackage[top=1in,bottom=1in,left=1.25in,right=1in]{geometry}
\usepackage[utf8]{inputenc}
\usepackage{times,chemfig,amsmath}
\begin{document}
\chemfig{3 \text{(} ^{-}OC=OCH_2 \text{)}_2C \text{(} OH \text{)} C=OO^{-}}
\end{document}


I get following output:

When I try to make the double bond vertical by adding "[:90]" in between '=' and 'O':

\chemfig{3 \text{(} ^{-}OC=[:90]OCH_2 \text{)}_2C \text{(} OH \text{)} C=[:90]OO^{-}}


I get weird output like:

Any suggestion will be of great help.!

Here is a way. Basically I first write the horizontal part of the molecule and at the end I add the oxo parts. Usually parentheses denote branches forchemfig. In order to print them they must be put in braces. The real branches at the end can be placed at the right position with chemfig's departure atom option (the number at the third position in the option of the bonds).
\documentclass{article}