Secoorthosiphols A–C: three highly oxygenated secoisopimarane-type diterpenes from Orthosiphon stamineus

Three new highly oxygenated 2,3-secoisopimarane-type diterpenes, named secoorthosiphols A (1), B (2) and C (3), have been isolated as extremely minor constituents from the aerial parts of the Orthosiphon stamineus from Okinawa. A unique unprecedented structural feature of such a ring A opened system in newly isolated compounds was encountered for the first time in isopimarane-type diterpenes. Secoorthosiphol C (3) represents the first example of a biogenetically unique and unconventional secoisopimarane-type diterpene having a cyano group.