Masayuki Kitadani

Interconversion between hibaene and kaurene

Abstract Hibaene-α-epoxide, upon treatment with BF 3 -etherate in benzene, undergoes skeletal rearrangement into the kaurane ring system, giving an unsaturated alcohol or a glycol, respectively. The unsaturated alcohol, after conversion to the corresponding unsaturated ketone, affords kaurane, isokaurene, and kaurane by Huang-Minlon reduction. It has also been found that an equilibrium mixture of kaurene, isokaurene and hibaene, in the ratio of 2:3:5, is produced when either kaurane or hibaene is heated in boiling xylene in the presence of iodine; thus the interconversion between hibaene and kaurene has been achieved. The stereochemistry of the intermediates in the transformation of hibaene to kaurene has been established.

A new approach of aromatic solvent-induced shifts (ASIS) in 13C n.m.r. spectroscopy for solving stereochemical problems in some carbonyl compounds

A simple method for the estimation of aromatic solvent-induced shifts (ASIS) in 13C n.m.r. spectroscopy is reported. The 13C ASIS values produced by perdeuteriobenzene (C6D6) are obtained for a variety of structurally different compounds containing carbonyl groups. The 13C ASIS values observed vary from compound to compound, depending upon conformational variations involving the carbonyl groups. A useful relationship between the magnitude of the 13C ASIS values and the geometrical factors estimated on the basis of an assumed model for a 1 : 1 solute–C6D6 association is presented as a new approach for the prediction of preferential conformations of cyclohexanones in solution at room temperature.

The photodecomposition of cyclic N-bromo imides: evidence for stereoelectronic control in intramolecular hydrogen transfer in imidyl radicals

In order to examine the reactivities and configurations of imidyl radicals, several cyclic N-bromo imides possessing five- or six-membered imide rings have been photolytically decomposed in the presence of 1,1-dichloroethene in methylene chloride in order to scavenge bromine atoms and bromine. The five-membered N-bromo imides and (±)-N-bromocamphorimide undergo ring opening to afford products derived from C-bromo acyl isocyanates. While the corresponding imidyl radicals abstract a hydrogen atom from methylene chloride, none of them undergo intramolecular H-atom abstraction to effect an intramolecular hydrogen–bromine exchange reaction, even when there are suitably located C–H bonds at a fifth (or sixth) position that can overlap the π-orbital of the imidyl radicals. This phenomenon clearly reveals the presence of Stereoelectronic controls on the intramolecular H-abstraction of imidyl radicals, and indicates that the reactive state of imidyl radicals does not possess the Π-electronic configuration. We conclude that imidyl radicals possess the Iµ-electronic configuration in their reactive state (which is probably the ground state). The intramolecular addition of a C-radical centre to an isocyanate group is established; this indicates that the reversibility of the ring-opening reaction of imidyl radicals is controlled by steric strains among other factors. Because of the presence of reactive allylic hydrogens, the N-bromo imide (10) upon photolysis forms the C-bromo compounds (11) efficiently by a bromine atom chain process which is retarded by the presence of a better Br˙ scavenging alkene.

SYNTHESIS OF AZAPOLYCYCLIC COMPOUNDS BY THE AMINIUM RADICAL ROUTE: REARRANGEMENTS OF C-NITROSO COMPOUNDS AND THEIR STEREOELECTRONIC REQUIREMENTS

The aminium radicals generated from the photolysis of N-nitroso-endo-2-methylamino-methylbicyclo[2.2.1]hept-5-en and N-nitroso-endo-2-methylaminomethylbicyclo[2.2.2]oct-5-ene cyclize at the C-6 position, since the relevant reacting centers can achieve excellent orbital overlap with minimum molecular motion in comparison to the alternative cyclization path at the C-5 position. The photolysis of the former compound gave a C-nitroso compound which underwent a series of unusual intramolecular cleavages, deoximations, and redox reactions during the work-up. Stereoelectronic requirements for these intramolecular reactions are discussed. Thermolysis of N-chloro-endo-2-methylaminomethylbicyclo[2.2.2]oct-5-en in methanol also gave the product with azaisotwistane skeleton.

Syntheses sf azapolycyclic compounds by aminium radical routes: trapping of the radical intermediates1

Photolysis of three alkenyl nitrosamines in the presence of oxygen or bromotrichloromethane resulted in the interception of the intermediate C-radicals by these radical trapping agents and the reaction pathways were cleanly diverted leading to the formation of the nitrate esters or halides with pyrrolidine rings as the primary products. The exo-nitrates in the oxidative photolyses decomposed by secondary ionic pathways; these reactions were hydrolysis, nitrous acid elimination and a cleavage reaction (promoted by a β-amino group), among others. The efficiency of the cleavage reaction is controlled by a stereoelectronic factor that requires the participating bonds and the lone-pair nitrogen orbital be oriented in an antiperiplanar conformation. When such a conformation exists in a rigid or semiflexible framework, cleavage occurs extensively. However, in freely rotating acyclic systems, cleavage does not occur even when the required conformation can be attained. Only halides resistant to intramolecular nucl...