Acid-Catalyzed Reactions of Sarcophytoxide, a Marine Cembranoid: An Apparently Enantio-Directive Reaction, Unusual Products and Stereochemical Reconsideration of Epoxide–Ketone Rearrangement

Abstract

Perchloric acid treatment of sarcophytoxide, a marine cembranoid possessing an epoxide, brought about epoxideketone rearrangement affording ketones. When the reaction time was long (22 h), a minor ketone that was antipodal to the ketone obtained in a short-time (10 min) reaction was formed. These puzzling findings, considering that the starting epoxide had three asymmetric carbons, were interpreted by surveying the structures of other ketonic products. The stereochemistry of a major ketone, which had been wrongly assigned, was corrected by extensive analyses of NMR spectra. The correct stereochemistry indicated that the epoxide-ketone rearrangement took a course via a cationic intermediate.

Total Synthesis of (±)-Maoecrystal V

“…28 In any case, the epoxide isomers were readily separated and, in the final step of the synthesis, olefin 49 was treated with BF 3 •OEt 2 to deliver maoecrystal V ( 1 ) as a single isomer in 85% isolated yield. 29 The 1 H and 13 C NMR data were in full accord with those reported for the natural product itself. The mission of synthesizing the racemate of maoecrystal V had been accomplished.…”

Bioactive Hydroperoxyl Cembranoids from the Red Sea Soft Coral Sarcophyton glaucum

“…The absolute stereochemistry of 1 at C-2 was determined via circular dichroism (CD) analysis ( Figure 3 ). The observed positive Cotton effect {[θ] 248 +0.7} followed by a negative value {[θ] 225 −3.23} observed in the CD spectrum for the electronic transitions of the 2(5 H )-furanone moiety, indicated a left hand ( M ) helix configuration for the five-membered α,β-unsaturated-γ-lactone ring [ 24 ]. Supporting CD data for 1 , CD spectral comparison between 1 and ent -sarcophine ( 4 ) indicated the same R absolute configuration for the two compounds at C-2 [ 18 , 19 , 21 , 22 ].…”

“…Spectral data suggested that 3 was similar to the sarcophinone previously isolated from the soft coral Sarchophyton molle Tix [ 23 ], except for an up-field shift for H 3 -19 ( δ H 1.06) and an increase in its coupling constant (7.5 Hz) in comparison with sarcophinone H 3 -19 ( δ H 1.13, J = 6.4 Hz). This up-field shift for such a methyl attached to a methine carbon can be explained by an alternative stereochemistry since the β-configuration methyl group is down-field relative to the α-stereochemistry [ 23 , 24 ].…”

Total Synthesis of (−)‐Rotundone and (−)‐epi‐Rotundone from Monoterpene Precursors

“…The first challenge was an enantio-and diastereoselective access to precursor 8c, which had been separated by distillation and with good purity from nitrile 9 (Scheme 3). Larger amounts of cis-dihydrocarvone 7b were therefore prepared from commercially available (+)-(R)-limonene oxide 14 according to Kusumi, using slightly modified conditions (e. g., at lower temperature to avoid cis/trans-epimerization of the α-methyl group in 7b) [29,30] (Scheme 6).…”