Provided are a compound of formula (5) and a process for its preparationwherein: R1 represents an alkyl group R2 represents an aryl group R3 represents a protecting group or an alkyl group R4 represents a protecting group or a SO2R5 group where R5 is an alkyl group R6 represents (PR7R8)+X− or P(═O)R7R8 in which X is an anion and R7 and R8 each independently is an alkyl, aryl, alkoxy, or aryloxy group P1 represents hydrogen or a protecting group and W represents ═O or OP2, in which P2 represents hydrogen or a protecting group.