X
O
H
A
R
N
HN
O
N
N
N
N
O
CH2
NH
R=
N
R
H
B
O
6,7-dimethyl-8-ribityllumazine
6,7-dimethyl-8-ribityllumazine
= unknown nucleophile
H
C
OH
H
C
OH
H
C
OH
CH2OH
O
R
X
N
O
NH
N
N
H
N
N
HN
O
X
CH2
N
R
O
H
A
BH
A
O
R
H
O
X
N
H
N
R
X
H
N
O
HN
N
N
N
N
H
H
N
O
HN
NH
O
A
N
N
H
R
NH
N
H
H
O
H
N
H
O
R
O
B
B
O
O
R
N
N
N
N
N
H
H
R
H
N
O
B
N
H
N
N
H
H
N
O
N
H
R
O
A
H
H
N
NH
O
O
H
O
N
HN
NH
B
R
X
HN
O
H
O
A
R
N
R
N
HN
H
N
HN
H
N
HN
O
N
N
NH
O
N
N
R
H2N
O
O
R
NH
H2N
O
O
riboflavin
5-amino-6-ribitylamino-2,6dihydroxypyrimidinedione