SYNTHESIS, SPECTRAL PROPERTIES, AND CHEMICAL RNG OPENING OF TRICYCLO[3.3.0.02,8]OCTAN-3-ONE, A RIGID MODEL FOR UNSYMMETRICAL CYCLOPROPYL KETONE PARTICIPATION

Stereomodels reveal that the skeletal framework of tricyclo[3.3.0.02,8]octan-3-one (4) constitutes a rigid model for cyclopropyl ketone participation in an unsymmetrical conformation. Ketone 4 and three specifically deuterated derivatives (C2-d, C4-d2, and C3-d) were synthesized. These compounds were used, in conjunction with spin decoupling experiments, to interpret the nmr spectrum of ketone 4. Unambiguous chemical shift assignments for the C1, C2,C4, C5, and C8 protons were made. The major fragmentation pathway of this tricyclic ketone in the mass spectrum involves the loss of ketene to give a base peak at m/e 80. Reductive cleavage of ketone 4 with lithium in liquid ammonia yielded as products 95% cis-bicyclo[3.3.0]octan-3-one (12) and 5% bicyclo[3.2.1]-octan-3-one (13). Treatment of ketone 4 with hydrogen bromide in methylene chloride gave, after reductive removal of the bromine atom, a mixture of 80% 12 and 20% 13. Interpretatio of these results in terms of ground-state cyclopropyl ketone delocalization in an unsymmetrical conformation is discussed. The stereoselectivity of the dissolving metal cleavage is consistent with the known stereoelectronic requirements for reductive elimination of α-substituted ketones. The product mixture from the acid-catalyzed opening appears to reflect (in part) thermodynamic control. It is concluded that an evaluation of product composition data from either of these general chemical probes is not a valid method to assess ground-state cyclopropyl ketone delocalization. © 1969, American Chemical Society. All rights reserved.