wherein R1 is a hydrogen atom, an optionally substituted alkyl group or an optionally substituted alkenyl group; R2 is a hydrogen atom or a protective group of hydroxy group; R3 and R4 is a hydrogen atom, an optionally substituted alkyl group or an optionally substituted alkenyl group, alkoxyl group or an optionally substituted aryl group, or R3 and R4, taken together, may form --CH???CH--CH???CH-