\chemfig{
                   \mcfleft{\mcfatomno{16}}{O}% 16
             -[:60]\mcfatomno{13}% 13
           =_[:120]\mcfatomno{12}% 12
             -[:60]\mcfatomno{11}% 11
                 =_\mcfatomno{8}% 8
             -[:60]\mcfatomno{7}% 7
                  -\mcfatomno{10}% 10
            -[:300]\mcfatomno{1}% 1
                  -\mcfatomno{6}% 6
           =_[:300]\mcfatomno{5}% 5
            -[:240]\mcfatomno{4}% 4
                      (
                -[:300]\mcfright{O}{\mcfatomno{17}}% 17
                      )
            -[:180]\mcfatomno{3}% 3
            -[:120]\mcfatomno{2}% 2
                      (
                 -[:60]\phantom{1}% -> 1
                      )
                      (
                -[:180]\mcfatomno{9}% 9
                          (
                    -[:120]\phantom{8}% -> 8
                          )
               =_[:240]\mcfatomno{14}% 14
                          (
            -[:327.1,1.233]\mcfbelow{O}{\mcfatomno{15}}% 15
             -[:34.8,1.174]\phantom{3}% -> 3
                          )
                -[:180]\phantom{13}% -> 13
                      )
    -[:123.3,0.928]\mcfatomno{19}% 19
     -[:89.9,1.239]\mcfatomno{20}% 20
     -[:30.1,1.163]\mcfabove{N}{\mcfatomno{18}}% 18
                      (
                -[:240]\phantom{10}% -> 10
                      )
           -[:37.5]\mcfatomno{21}% 21
}
