A novel type of intramolecular Diels-Alder reaction involving dienol ethers: an unusual preference for a boat transition state in the incipient ring formation (original) (raw)

1984, The Journal of Organic Chemistry

protons7 (Table I) on the linked carbons (C-8,9,12 and 13 in Figure 1). No couplings was observed between H-11 and H-12 as the dihedral angle is close to 90°. The NOE between the proton at 1-H and 3-Me(16-H) and one of the gem-dimethyls (19-H) indicated the locations of three methyls on the transoid diene system. 2D-NOE8 of 1 (3-Me/l-H, 19-H(Me)/lH, 2-H/13-H, 13'-H/20-H(Me), 13'-H/7-Me) revealed the gross conformation which is similar to that of its oxidation product 2.9 The location of the sec-methyl was suggested from the NOE between the 20-Me and 11-H of 2. 13C NMR (25 MHz) data (C&) also indicated a bicyclic structure with three trisubstituted double bonds (Table I); the assignments are based on the (1NEPT)'O method, selective decoupling, and the comparison with the data of 2. Compound 2, C2oH3zO (m/z 288.2452, calcd 288.2498) was isolated from the stored (6 years a t-20 OC) hexane extract of C. ugandensis soldier heads by column chromatography using 1O:l hexane/EtOAc over Si02. The tricyclic nature of compound 2 was disclosed from the 13C NMR data, which showed the presence of one trisubstituted double bond, an exocyclic disubstituted double bond, and a strongly shielded triplet carbon (19.0 ppm, C-13; shielded by the 7,17-ene) (Table I). The 13C NMR assignments are based on heteronuclear chemical shift correlation spectroscopy (CSCM).l' IH NMR (360 or 300 MHz) with COSY: 2D-NOE,8 sequential additions of lanthanide shift reagent [ E~(f o d)~-d ,~] , extensive homonuclear difference decoupling, and NOE measurements allowed for complete proton assignments (Table I) and also established the relative configurations 2. A possible biogenesis12 of cubugene 1 together with the co-occurring irregular cubitene 32 is shown in Figure 1.