R2 is selected from the group consisting of an alkyl group having 1 to 6 carbon atoms such as methyl group, ethyl group, propyl group and butyl group; an alkenyl group having 1 to 6 carbon atoms such as 1-propenyl group, 2-propenyl group, 2-methyl-1-propenyl group, 2-methyl-2-propenyl group, 1-butenyl group, 2-butenyl group and 3-butenyl group; an aryl group such as phenyl group, benzyl group, flu