Eos, the all-business-class airline launched in 2005, filed for Chapter 11 bankruptcy Saturday and flew its last flight yesterday. The Purchase, N.Y.-based airline operated an upscale, twice-daily service between New York JFK and London Stansted Airport, charging round-trip prices ranging from $3,500 to $9,000. The airline, in a letter to its customers, did not comment on the reasons for the filing except to say that last-minute efforts to find additional financing had failed. Maxjet, which had been operating a similar all-business-class service, filed for bankruptcy and ceased operations in December. Following the Eos announcement, Silverjet, the only remaining stand-alone, all-business-class airline on the New York-London route, offered Eos ticket-holders access to a limited number of seats on its flights between Newark Liberty International in northern New Jersey and London Luton Airport.