Naphthopyranquinones. Confirmation of the structures of the ventiloquinones E, G and J by synthesis

A history of the Chemistry Department, University of Cape Town

“…Research flourished on several fronts during the existence of the Department of Organic Chemistry, the main areas being carbohydrate chemistry (Stephen, 1983(Stephen, , 1995(Stephen, , 2004Stephen et al, 1990); Cragg's steroid research and interest in borane chemistry (Cragg, 1975); Giles' synthetic group (Giles & Roos, 1976;Giles et al, 1979Giles et al, , 1984Giles et al, , 1994De Koning et al, 1991); Modro's active school of phosphorus chemistry (Modro, 1981;Hendrickse & Modro, 1984;Symes & Modro, 1986;Modro et al, 1987;Niven et al, 1988); Elsworth's work on the chemistry of marine organisms (Elsworth & Naude, 1986),and his participation in synthesis (Elsworth et al, 1988), and Campbell's burgeoning involvement in the chemistry and potential uses of fynbos constituents and medicinal plants (Campbell et al, 1980(Campbell et al, , 1986. The CSIR Fynbos Biome Research Unit was located on the top floor, otherwise occupied by the Department, and cooperation was maintained through its chemical director, a graduate of the Chemistry Department, Dr J.C. Patterson-Jones.…”

The Synthesis of Ventiloquinone F and Isoventiloquinone F as Racemates

“…However and more specifically, Brassard et al [7] described some early protocols developed for the synthesis of ventilagone 1 and ventiloquinone H 2 employing DielsϪAlder chemistry. This was followed by the work of Giles et al [8] who described their work on the syntheses of ventiloquinones E (3), G (4) and the regioisomer 5 of ventiloquinone J (6) ( Figure 1). We have recently developed unambiguous syntheses for ventiloquinone J and isoventiloquinone J as their racemates, thus confirming the structure of ventiloquinone J to be 6.…”

“…Assignment of structure 23 to the first naphthopyran-6,9-quinone to elute is based inter alia on the following one-proton signals for the pyran ring in the 1 H NMR spectrum; a ddd at δ ϭ 2.42 ppm with J ϭ 17.6 Hz for geminal coupling between the diastereotopic 4Ј-H's, J ϭ 11.0 Hz for coupling between 4-H ax and 3-H ax , and J ϭ 1.8 Hz representing coupling between 4-H ax and 1-H ax and is assigned to pseudoaxial 4-H; a ddd at δ ϭ 2.90 ppm with a similar J ϭ 17.6 Hz representing coupling between 4-H eq and 4-H ax , J ϭ 3.0 Hz for coupling between 4-H eq and 3-H ax , J ϭ 1.0 Hz representing coupling between 4-H eq and 1-H ax and is assigned to pseudoequa-Scheme 2. Reagents and conditions: (i) MeMgBr/diethyl ether; (ii) Hg(OAc) 2 /THF/H 2 O/NaOH/NaBH 4 ; (iii) cerium() ammonium nitrate/H 2 O/CH 3 CN torial 4-H; a multiplet at δ ϭ 3.68 ppm assigned to 3-H ax ; a ddq at δ ϭ 4.85 ppm with J ϭ 6.6 Hz representing coupling of 1-H ax to the C1ϪCH 3 , J ϭ 1.8 Hz representing coupling between 1-H ax and 4-H ax , J ϭ 1.0 Hz representing coupling between 1-H ax and 4-H eq and is assigned to the pseudoaxial 1-H. [8,24] The second naphthopyran-6,9-quinone to elute was assigned the structure 25 based on obvious similarities in the 1 H NMR spectrum to quinone 23 with a major difference being in the position of the signal due to 3-H ax which appeared as a multiplet at δ ϭ 3.95 ppm that allowed assignment of a trans-1,3-dimethyl structure. [8,24] Fraction B afforded the two individual naphthopyran-6,7-quinones 24 (18%) and 26 (24%).…”

“…Reagents and conditions: (i) MeMgBr/diethyl ether; (ii) Hg(OAc) 2 /THF/H 2 O/NaOH/NaBH 4 ; (iii) cerium() ammonium nitrate/H 2 O/CH 3 CN torial 4-H; a multiplet at δ ϭ 3.68 ppm assigned to 3-H ax ; a ddq at δ ϭ 4.85 ppm with J ϭ 6.6 Hz representing coupling of 1-H ax to the C1ϪCH 3 , J ϭ 1.8 Hz representing coupling between 1-H ax and 4-H ax , J ϭ 1.0 Hz representing coupling between 1-H ax and 4-H eq and is assigned to the pseudoaxial 1-H. [8,24] The second naphthopyran-6,9-quinone to elute was assigned the structure 25 based on obvious similarities in the 1 H NMR spectrum to quinone 23 with a major difference being in the position of the signal due to 3-H ax which appeared as a multiplet at δ ϭ 3.95 ppm that allowed assignment of a trans-1,3-dimethyl structure. [8,24] Fraction B afforded the two individual naphthopyran-6,7-quinones 24 (18%) and 26 (24%). Assignment of the cis structure 24 to the first 6,7-quinone to elute was based on the comparative one-proton signals of its 1 H NMR spectrum to quinone 23.…”

“…[2] As anticipated, the 1 H NMR spectra for quinones 7 and 8 were similar and the signals for the proton 3-H ax proved to be diagnostic viz., for quinone 7 it appeared as a ddq at δ ϭ 3.75 ppm while for quinone 8 it appeared as an multiplet at δ ϭ 4.05 ppm. [8,25] Catalytic hydrogenolysis of the 6,7-quinone 26 only occurred when a trace amount of concentrated aqueous hydrogen chloride was added to the mixture in ethyl acetate. Again after two mol equivalents of H 2 had been absorbed, the reaction was stopped to initially afford an unstable amorphous yellow clay from which a yellow crystalline molecule was isolated with difficulty and to which structure 29 (Figure 2) has been assigned based inter alia on a high resolution mass spectrum.…”