Alternatively, the methanesulphonate of formula (XII) wherein n is 1 or 2 (prepared similarly from a compound of the formula (IB) wherein n is 2) may be reacted with a compound of formula R3A2M, wherein A2 is O or S and M is a metal, such as sodium, to produce (after deprotection) the corresponding compound of formula (I) wherein A is O or S. The sulphide (i.e. wherein A???S) may then be oxidised