Captain reported loss of the Left Air Data Computer in cruise. This caused unreliable airspeed indications. The flight crew elected to divert and make a precautionary landing.

Narrative

Approaching 140W enroute to ZZZ-ZZZ1. Loss of Air Data Computer Left and Primary Altimeter Source in RVSM Airspace. Complied with QRH. After selection of reversionary mode of Right ADC crew received multiple Auto Pilot Caution messages; loss of VNAV vertical navigation mode; IRUs operating in split operational mode; Stall Warn Sys Status MSG; Rudder Ratio Status MSG; BTB 3 Status MSG; Erroneous speed tape maneuver margins displayed. Coordinated with Dispatch via SATCOM after reviewing RVSM contingency procedures. Joint crew decision in coordination with Maintenance; Dispatch; and Duty Pilot to divert to ZZZ2. Coordinated revised clearance with [Center] to ZZZ2 and complied with RVSM Contingency procedures. Normal Arrival; Approach; Landing in ZZZ2. All defects noted in aircraft maintenance log.Loss of ADC L and AOA Fault causing erroneous information and loss of primary Altimeter in RVSM airspace. RVSM Contingency procedures used. Using all available resources decided best course of action to divert to ZZZ2 in the event ATC could not accommodate our RVSM degraded capabilities.Unknown origin of mechanical defects inflight. Followed procedures in QRH; FCOM; FOM. Joint coordination with Dispatch; Maintenance; ATC.