Asymmetric synthesis of lignans of the dibenzylbutanediol and tetrahydrodibenzocyclooctene series

Asymmetric synthesis, stereochemistry and rearrangement reactions of naturally occurring 7′-hydroxylignano-9,9′-lactones

“…The successful outcomes reported in the literature for certain related cases apparently all require the prior removal of the dithio attachment. 23, 24 It would appear an attractive possibility to first carry out a deketalization on the tandem product 14 and then perform a double stereoselective reduction (on the ketone and the ring-opened lactone aldehyde), but such a streamlined sequence is frustrated by the two competing relactonization modes, i.e., via either the primary or the secondary hydroxyl (see below). This is why in our experience also the silanyloxy-nitrile alternative (used in place of the dithiane moiety) 26 was not satisfactory due to the silanyloxy-nitrile hydrolysis which occurs in the menthyloxy removal step.…”

“…The crude compound was dissolved in CH 2 Cl 2 and allowed to cyclize overnight. After solvent removal, the crude compound was purified by flash chromatography (eluent CH 2 Cl 2 -MeOH 100 : 1) to obtain 17a as an amorphous solid (410 mg, 71%): [a] 24 D +6.6 • (c 1.0, CHCl 3 ); IR (thin film) m max 1780 cm −1 ; 1 H NMR (500 MHz, CDCl 3 ) d 0.18 (s, 6H), 1.00 (s, 9H), 1.87-1.98 (m, 2H), 2.41 (dd, J = 9.1, 17.9 Hz, 1H), 2.66-2.74 (m, 4H), 2.85 (dd, J = 9.2, 17.9 Hz, 1H), 3.00-3.05 (m, 1H), 3.82 (s, 3H), 4.18 (dd, J = 8.2, 9.5 Hz, 1H), 4.42 (dd, J = 8.1, 9.5 Hz, 1H), 6.87 (d, J = 8.4 Hz, 1H), 7.40 (dd, J = 2.4, 8.4 Hz, 1H), 7.47 (d, J = 2.4 Hz, 1H). 13…”

Hochselektive Achtringbildung durch oxidative Cyclisierung mit Molybdänpentachlorid – ein umweltfreundlicher und preiswerter Zugang zu 2,2′-Cyclolignanen Diese Arbeit wurde unterstützt von der Deutschen Forschungsgemeinschaft und der Firma H. C. Starck (Goslar) durch eine MoCl5-Spende. Wir danken besonders Prof. Dr. Dieter Hoppe für die fruchtbaren Diskussionen und seine Unterstützung in diesem Projekt.

“…Das zur Cyclisierung benötigte (−)‐Dimethylmatairesinol 2 wurde nach einer Vorschrift von Pelter und Ward hergestellt 7. Die Umsetzung mit Molybdänpentachlorid verlief problemlos und ergab selbst bei hoher Eduktkonzentration 50 % Ausbeute an 2,2′‐Cyclolignan 3 8.…”

Oxidative Aryl‐Coupling Reactions in Synthesis

“…In parallel studies to the ruthenium-based work, Ward and co-workers have also shown that PIFA (Scheme 44) [122,128] and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (Scheme 45) [122,129,130] are effective reagents for coupling the phenolic precursors 185a,b. Yields are generally lower than in the fully methylated case (spirodienone products 188 can be formed when a hydroxyl group is present at the para position of the 2-benzyl group of 185a) and unbiased mixtures of atropisomers 186/187 and 189/190 are obtained.…”