A compound represented by formula (I), wherein R 1 represents a hydrogen atom, etc., R 2 and R 3 each independently represents a hydrogen atom, optionally oxidized C1-4 alkyl group or optionally protected hydroxyl group, or R 2 and R 3 taken together represent optionally oxidized C2-5 alkylene group, R 4 represents an optionally oxidized C1-6 alkyl group, etc., R 5 represents an optionally oxidized C1-6 alkyl group, etc., R 6 represents an optionally oxidized C1-6 alkyl group, etc., m represents 0 or an integer from 1 to 3, n represents 0 or an integer from 1 to 4, and i represents 0 or an integer from 1 to 7.
