Cyclic hindered sulfamidates exhibited an outstanding performance in their ring-opening reactions with alcohols and in the absence of any external activator. The mechanism of this unprecedented transformation was thoroughly studied both experimentally and theoretically. As a result, a nontrivial stepwise pathway involving solvent-induced conversion of the sulfamidates to activated aziridinium and then to oxazolinium cations, which are finally opened at their 5-position with inversion of configuration, is proposed. The presence of the SO(3) moiety in the sulfamidate was revealed as a "built-in activator". In fact, the spontaneous SO(3) cleavage takes place under the reaction conditions and avoids the subsequent step of hydrolysis after the ring opening of the sulfamidates. This is another important improvement of this methodology with respect to the standard basic conditions, allowing a greater compatibility with other functional groups. Furthermore, the carbamate group plays a key role in this mechanism. Briefly, a highly chemoselective and stereoespecific formal solvolysis of hindered sulfamidates with alcohols without further activation is described. This reaction takes place exclusively at the quaternary center with inversion of configuration, providing a new straightforward synthetic route to O-substituted alpha-methylisoserines.