occurs by attack of a nucleophile at sulfur if a leaving group is sufficiently stable. The pseudo-first-order rate constans for the reaction of 1a and methoxycarbonyl-substituted 9-ethylthiofluorenes have suggested that the methoxycarbonyl group at 1-position exerts through space interaction with sulfur atom(s). The rate constants for esterexchange in 1-methoxycarbonylfluorene and its derivatives have