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-epoxydibenzb,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, ¹H,¹H-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 © 2001 John Wiley & Sons, Ltd.