Pentalenene synthesis

Morimoto, T., Horiguchi, T., Yamada, K., Tsutsumi, K., Kurosawa, H., and Kakiuchi, K. (2004) Acid-catalyzed rearrangement of an allene-cydohexenone photoaddud and its application in the synthesis of ( )-Pentalenene. Synthesis, 753—756. 

In Paquette s total synthesis of ( )-pentalenene, 56) the key step involved the ring expansion of (162) to (163) by treatment with CH2N2 56). 

Recently, the oxygen-free neutral precursor to the pentalenolactone family of metabolites was isolated, identified as 759, and named pentalenene Annis and Paquette have since devised a synthesis of 759 which efficiently elaborates its ring junction quaternary center and three angularly fused cyclopentane rings Condensation... 

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). 

The formation of /3-hydroxyselenides through the reaction of a selenium-stabilized carbanion with carbonyl compounds has been extensively used also in the context of natural product synthesis. The phenylselenoalkyllithium compound 115 was reacted with aldehyde 116 to afford /3-hydroxyselenide 117. In a radical cyclization cascade the tricyclic molecule 118 was generated in good yields and subsequent transformations led to the synthesis of pentalenene 119 (Scheme 28).1 9 Also other natural products like zizaene and khusimone have been synthesized via a similar route.200... 

This annelation provides a key step in a synthesis of the sesquiterpene pentalenene (2, equation I). ... 

Paquette, L. A. Feng Geng, F. A highly abbreviated synthesis of pentalenene by means of the squarate ester cascade, Org. Lett. 2002, 4, 4547-4549. 

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. ... 

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). 

A pair of reductive eliminations on chlorohydrin intermediates was used to achieve regioselective introduction of alkene functionality in a synthesis of (18), a precursor of the sesquiterpenoid pentalenene (Scheme 7). Key steps in Tochtermann s synthesis of [6]-paracyclophane derivative (21 Scheme 8) were bromination of the bridged oxepine (19) followed by reductive elimination of the resultant dibro-mooxirane (20). 

The sequence of intramolecular photocycloaddition-cyclobutane fragmentation has been used by Crimmins in an excellent manner for the synthesis of pentalenenes and the even more sophisticated lauren-l-ene (27). The latter synthesis was accomplished in 27 steps from cyclopentenone U66). One of the key steps is the reductive cleavage of the tetracyclic cyclobutane (167), followed by hydrogenation of the resulting P,7-unsaturated ester to give keto ester (168 Scheme 57). 

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. ... 

---