Special reactions of α,β-unsaturated ketones:: Part II.: Structure and stereochemistry of perhydro-5a,11a-epoxydibenz[b,f]oxepin-4a-ol, a bridged Diels-alder dimer of 2-methylenecyclohexanone

The paper deals with the structure elucidation of (4aRS,5aRS,9aRS,11aRS)-perhydro-5a,11a-epoxy-dibenz[b,f]oxepin-4a-ol (6), which is unexpectedly formed from 2-dimethylaminomethylcyclohexanone hydrochloride (1.HCl) in the presence of phenylguanidine in boiling ethanol. In detail, two molecules of 2-methylenecyclohexanone (generated from 1 by elimination of dimethylamine) afforded via Diels-Alder reaction, subsequent addition of water and ring closure a product C14H22O3, which finally proved to be the title compound. The structure of the tetracyclic hemiacetal 6 was deduced from one- and two-dimensional NMR experiments (1D TOCSY, DEPT-135, H-1,H-1-COSY, gs-HSQC, gs-HMBC, gs-HSQC-TOCSY). In particular, the one-dimensional selective TOCSY and the gradient-selected HSQC-TOCSY, performed as a pseudo-3D experiment without the evolution period in the TOCSY part, proved to be very helpful for the unambiguous assignment of signals in overcrowded regions. NOE data confirmed the stereochemistry of epoxydibenzoxepinol 6. Copyright (C) 2001 John Wiley & Sons, Ltd.