Selective sulphonation of the primary alcohol of a diol containing both primary and secondary alcohols

In the process for preparing a compound of formula I:

• wherein
• R1 is H;
• and
• R2 is C1-C12 alkyl, C2-C12 alkenyl, C3-C8cycloalkyl, C1-C10 alkoxy, aryl, aryl-(C1-C6 alkyl), aryl-Z—(C1-C6 alkyl), heteroaryl, heteroaryl-(C1-C6 alkyl), or heteroaryl-Z—(C1-C6 alkyl), each of which may contain zero to three R5 substituents;
• or
• R1 and R2 together complete a 5-6-membered ring that may have up to three R5 substituents;
• R5 is C1-C12 alkyl, halo, hydroxy or C1-C3 alkoxy;
• N is 0, 1, or 2;
• Z is 0 or S;
• and
• Rp is C1-C? alkylsulfonyl or arylsulfonyl comprising reacting
• a) a compound of formula II
• with
• b) a compound of formula RpSO2X or (RpSO3)2O wherein X is halo or imidazolyl;
• and
• c) a tertiary amine,
• the improvement comprising adding
• d) a catalytic quantity of a compound of the formula (Ra)2Sn(Xa)m
• wherein Ra is C1-C12 alkyl;
• Xa is O, Cl, Br, OAc or ORb;
• m is 1 or 2;
• and
• Rb is C1-C6 alkyl or aryl.