A compound represented by formula (I),whereinR1 represents a hydrogen atom, etc.,R2 and R3 each independently represents a hydrogen atom, optionally oxidized C1-4 alkyl group or optionally protected hydroxyl group, or R2 and R3 taken together represent optionally oxidized C2-5 alkylene group,R4 represents an optionally oxidized C1-6 alkyl group, etc.,R5 represents an optionally oxidized C1-6 alkyl group, etc.,R6 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, andi represents 0 or an integer from 1 to 7.
