Annette M. Doherty

Natural Products Chemistry

Tricyclic compounds 756 and 757 have been found to exist in low concentration in lavender oil. Structure proof was realized by selenium dioxide oxidation of α-cedrene. Hydrogenation of 756 and subsequent copper-catalyzed oxidation of the saturated aldehyde furnished 757 [347].

Synthesis of Nonpolycyclopentanoid Natural Products by Way of Diquinane Intermediates

Di- and triquinanes have increasingly found service as building blocks for construction of natural products lacking formal polycyclopentanoid frameworks [374]. In this segment of the review, the most recent applications of successful strategems of this type are surveyed.

Synthesis of Triquinane Natural Products

Hirsutene (1124) is the simplest member of a group of fungal metabolites possessing the linearly fused cis,anti,cis-tricyclo[6.3.0.02,6]undecane carbon skeleton [503]. As a result of the antibiotic and antitumor properties shown by some of these triquinanes, synthetic chemists have strived to devise new annulation procedures for their construction. Several new syntheses of hirsutene itself have recently been reported where a wide range of different strategies to provide access to the tricyclopentanoid ring system are described.

Synthesis of Diquinane Natural Products

In recent years, the tricyclic cedranoid sesquiterpenes have continued to be the subject of numerous synthetic studies. The novel carbocyclic framework of this group of natural products has resulted in a wide variety of synthetic approaches.

Physical Data for, and Theoretical Analysis of, Polyquinanes

The two 3,7-disubstituted diquinanes 510 and 511 have been investigated by single crystal X-ray diffraction techniques and shown to possess the illustrated W conformation [224]. Nonbonded H----H distances of 1.754(4) and 1.713(3) A have been measured by low-temperature neutron diffraction experiments for 512 and 513 [225]. At 2.921(4) A, the distance between the pairs of double bonds in 514 is also quite short [226].

Molecules of Theoretical Interest

The effects of electron-donating substituents on the bond alternations in pentalene have been evaluated by using the Pariser-Parr-Pople-type SCF MO CI method in conjunction with a variable bond-length technique [257]. A valence bond treatment of aromaticity, first introduced by Mulder and Oosterhoff, has now been applied to pentalene [258]. MNDO, UMDDO, and MNDO/CI calculations have also been reported for pentalene [259].

New Synthetic Developments

Lee and Richardson have disclosed that it is entirely feasible to cyclopentannulate silyl enol ethers in a one-pot two-step process. For example, condensation of 1 with the readily available stannane 2 in the presence of trimethylsilyl triflate leads to an adduct that cyclizes to 3 on subsequent exposure to titanium tetrachloride [5]. This new strategy is notable in that a doubly functionalized product results. 1,2-Bis(trimethylsilyloxy)cyclopentene (4) condenses with chloride 5 in the presence of anhydrous zinc chloride to give a 1:1 mixture of 6 and 7 [6]. The first relevant point is that alkylation proceeds without annihilation of the allylsilane moiety. Consequently, this functionality is available for subsequent ring closure as illustrated by the independent conversion of 6 and 7 to 8 and 9, respectively, when treated with ethylaluminum dichloride.

Functional Group Manipulation Within Polyquinanes

An attempt to prepare 392 by intramolecular C—H insertion within the carbene generated from 391 led instead to 393 [179]. The conversion to anhydride 394 was successful, and this propellane anhydride was also prepared from two other precursors.