Triquinane -pentalenene synthesis

The last example depicted in Scheme 6.11 illustrates the exclusive ris-product formation observed with cyclohexenones and allenes in conjunction with a high HH-preference [38]. In addition, substrate 29 is chiral, and perfect facial diastereoselectivity was observed due to cyclic stereocontrol. Product 30 served as intermediate in a formal total synthesis of the triquinane ( )-pentalenene (31). 

Similar methodology was used in a total synthesis of the angularly fused triquinane pentalenene (53) [65]. The known phenol 271 was used as the starting material. In this and the previous instance (viz. with 269) constant-current electrolyses were performed, this time converting 271 to a mixture of the desired adduct 272 (64%) as well as a material epimeric at the CH20Ac-bearing carbon (16%). The major adduct corresponds to one wherein the original olefin stereochemistry has been maintained. Conversion of 272 to the natural product followed chemistry similar to that used in the synthesis of silphinene (56). 

Since our direct route to angularly fused triquinanes from cycloaddition of l-(4-pentynyl)cyclopen-tenes is limited to trisubstituted alkenes and simple terminal alkynes, bisnorisocomene, but not iso-comene itself, could be prepared (Scheme 22). However, this limitation is not a factor for most other compounds in this class of natural products, and the steric interactions described earlier worked to our advantage in a diastereocontrolled synthesis of pentalenene (see structure. Scheme 20). The natural product was obtained by subjecting the product of Scheme 14 to the sequence i, Li, NH3, MeOH ii, MeLi, Et20 iii, p-TsOH, benzene, reflux. ... 

Transannulation of the mesylate (33), derived from 4,8-dimethylcycloocta-4-en-l-ol, using sodium carbonate in aqueous dioxane has provided the bicyclo[3.3.0]octanol (34), a central precursor to the monoterpene iridomyrmecin (35 equation 13). In studies of the biomimetic synthesis of the natural tri-quinanes capnellene (37 equation 14) and pentalenene (40 equation IS) Pattenden et al. have shown that both molecules can be produced from appropriate cycloocta-l,S-diene precursors, i.e. (36) and (39) [or indeed their corresponding positional isomers (38) and (41), respectively] by treatment with boron trifluoride. Mehta et al. have described an alternative transannulation approach to the triquinane unit found in pentalenene, i.e. (42) - (43 equation 16), and also to the ring system (44 equation 17) found... 

Pauson-Khand reaction. In the presence of tetramethylthiourea the Pauson-Khand reaction suceeds with enynes in which the double bond is present in a silylcyclopropene unit. It enables a synthesis of the angular triquinane sesquiterpene (-)-pentalenene. ... 

The intramolecular PKR was used by Fox and Pallerla (150) to obtain a key intermediate in the enantioselective synthesis of (—)-pentalenene 176, the unnatural enantiomer of the angular triquinane natural product. Using Co2(CO)8 (60) mol%) as catalyst in tolnene and 1 atm of CO, enantiomerically enriched (91% ee) cyclopropene 174 gave the PK addnct 175 (4 1 dr) in 64% isolated yield. The use of the known PKR promoter tetramethylthiourea (TMTU) was found to be critical in obtaining good yield of the desired tricyclic product (Scheme 80). 

In the synthesis of ( )-pentalenene, the PKR was used to construct the angularly fused triquinane ring system 77 from cyclopentene 76. 

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