HYDROBORATION OF TERPENES .V. ISOMERIZATION OF(+)-SABINENE TO (-)-ALPHA-THUJENE - HYDROBORATION OF (+)-SABINENE AND (+)-ALPHA-THUJENE WITH CONFIGURATIONAL ASSIGNMENTS FOR THUJANOLS

A revised nomenclature for various configurational thujanols is proposed. (+)-Sabinene readily isomerizes to an equilibrium mixture of 91% (-)-α-thujene (2) and 9% (+)-sabinene (1) under the influence of potassium t-butoxide in dimethyl sulfoxide, and this provides an improved synthetic route to (-)-α-thujene. Catalytic hydrogenation of (+)-α-thujene over platinum preferentially takes place away from the side of the cyclopropane ring to give chiefly (-)-trans-thujane (4) whereas catalytic hydrogenation of (+)-sabinene (1) takes place preferentially from the opposite side to give chiefly (-)-cis-thujane. Hydroboration of (+)-α-thujene takes place exclusively from the same direction as hydrogenation, i.e., the side away from the cyclopropane ring. Hydroboration of sabinene also takes place from the side away from the cyclopropane ring, from the direction which is opposite to that taken in hydrogenation. The organoborane formed preferentially in the hydroboration of sabinene appears to be the thermodynamically more stable isomer, since it did not epimerize under isomerization conditions. A similar thermodynamic stability favoring the isomer with the mobile substituent (10-C) located on theside toward the cyclopropane ring is presentin 10-thujaldehydes and 3-thujones. This unusual stability of the epimers which contain the substituents cis to the cyclopropane ring is interpreted in terms of a preferred existence of thujane derivatives in a boat conformation, rather than in the more usual planar or chair forms. The nmr spectra are subjected to a detailed analysis and provide support for these conformational assignments. © 1969, American Chemical Society. All rights reserved.