This is a context from an English translation of "Crime And Punishment" by Dostoevsky
"I have considerable reason to believe that Marfa Petrovna, who was so unfortunate as to fall in love with him and to pay his debts eight years ago, was of service to him also in another way."
Research: so as in British English (takes an infinitive) in order (to) to slim so as to lose weight
The sentence in bold would make sense to me if it went "so unfortunate to fall in love.." Is this just a dated way of saying the same thing?