A method of forming a compound of formula (IIIa): wherein: R1 in each occurrence is independently selected from H, halogen, amino, C1-5 alkyl, C1-5 alkenyl and C1-5 alkynyl; R2 in each occurrence is independently selected from H, halogen, amino, C1-5 alkyl, C1-5 alkenyl and C1-5 alkynyl; R4 is C1-5 alkyl; and R6 is a C1-5 fluoroalkyl; and PG is a protecting group, the method comprising the step of reacting a compound of formula (IIa) with a compound of formula (VIII):