Regioselective palladium-catalyzed alkylation of allylic halides with benzylic grignard reagents. Two-step synthesis of abietane terpenes and tetracyclic polyprenoid compounds

A highly regioselective palladium-catalyzed alpha-alkylation of allylic bromides 1a,c-e and chloride 1b with substituted and unsubstituted benzylic Grignard reagents is reported. The resulting all-trans polyenehomobenzene derivatives were obtained in excellent yields and regioselectivity. These products were easily converted to abietane-type diterpenes (10-12) and tetracyclic polyprenoid compounds (13, 14) through a Lewis acid-promoted cascade polyene cyclization reaction.     

Novel Variant of the Tricyclooctanone Approach to Cyclopentanoid Natural Products. Synthesis of (±)-Coriolin. Preliminary Communication

A total synthesis of the antitumor sesquiterpene coriolin ( 9 ; racemic) in 11 steps from 3,3,6-trimethylbicyclo[2.2.2]oct-7-ene-2,5-dione ( 2a / 2b ) is described (yield 2a / 2b → 8: 28%). The sequence is unprecedentedly short and avoids difficult separation problems. The key step in the scheme is a novel facet of oxadi-π-methane photochemistry, i.e., the steering by subtle steric effects of the β,γ-unsaturated ?-diketone to undergo a regioselective photorearrangement involving one β,γ-enone partial chromophore. Furthermore, the overall phototransformation, which can be carried out at unusually high concentrations (≥20% solutions), involves also a Norrish type I process equilibrating the two epimeric starting enediones 2a and 2b in favour of the desired stereoisomer.     

The Tricyclooctanone Approach to the Total Synthesis of Steroids. The Cyclization of the A-CD Unit

The first steps of a novel approach to the total synthesis of 9, 11-dehydroestrone via tricyclo[3.3.0.0^2,8]octan-3-one (2) are described. One route involves a tandem-type transformation of the key intermediate 3 (A-CD unit) consisting of cyclopropane cleavage and ring B closure to afford C, 18-bisnor-13 α, 17 α-estradiol derivatives. E.g. the 3-methoxy-9 ζ-hydroxy-17 α-methanesulfonyloxy derivative (-)- 6 has been synthesized in 8 steps and 10% overall yield from 1,3-cyclohexadiene. As an alternative, the A-CD type intermediate 4b has been prepared and could be used for a ring C enlargement prior to cyclization.     

Experimental study of the vibrations of acetylene chemisorbed on Ni(111)

High resolution electron energy-loss measurements of normal and deuterated acetylene chemisorbed on Ni(111) have been obtained. Observed vibrational modes are identified using the frequency shifts for the deuterated species and comparisons to the free molecule and a di-cobalt compound of acetylene. These vibrational frequencies indicate that chemisorbed acetylene is strongly rehybridized having a state of hybridization between ~sp^2.5 and sp³. Consideration of the types of modes observed, their assignments and the surface selection rule suggests a molecular orientation with the C-C bond axis slightly skewed relative to the surface and with the plane of the distorted molecule normal to the surface. A bonding geometry is proposed which has the carbon atoms residing above two adjacent 3 fold hollow sites of the Ni surface. This molecular geometry differs from that deduced previously by electron energy-loss spectroscopy for molecularly adsorbed acetylene on Pt(111).