Dodecahedrane synthesis

Scheme 4.17 presents a couple of other strange-looking cationic species which were discovered in studies in a related field. In connection with the problem of dodecahedrane synthesis via the isomerization of pagodane 34 (cf. data in Schemes 4.10 and 4.11), Olah s and Prinzbach s groups engaged in studies of the behavior of pagodane derivatives under superacid conditions. Their hope was to force the cationic isomerization of 34 to 3. Despite all attempts, this route was unworkable. As a reward for these apparently futile efforts they were able to observe the unexpected formation of a very stable cationic species, the pagodane dication 55 (Scheme 4.17). The pattern of its NMR spectra combined with the nature of its quenching adduct 56, and the theoretical analysis of possible alternatives, enabled the authors to ascribe to this dication the unprecedented four-center/two-electron delocalized bis-homoaromatic structure.

Stereoselectivity. See Asymmetric induction Axial/equatorial-, Cis/trans-, Enantio-, Endo/exo- or Erythro/threo-Selectivity Inversion Retention definition (e.e.), 107 footnote Steric hindrance, overcoming of in acylations, 145 in aldol type reactions, 55-56 in corrin synthesis, 261-262 in Diels-Alder cyclizations, 86 in Michael type additions, 90 in oiefinations Barton olefination, 34-35 McMurry olefination, 41 Peterson olefination, 33 in syntheses of ce-hydrdoxy ketones, 52 Steric strain, due to bridges (Bredt s rule) effect on enolization, 276, 277, 296, 299 effect on f3-lactam stability, 311-315 —, due to crowding, release of in chlorophyll synthesis, 258-259 in metc-cyclophane rearrangement, 38, 338 in dodecahedrane synthesis, 336-337 in prismane synthesis, 330 in tetrahedrane synthesis, 330 —, due to small angles, release of, 79-80, 330-333, 337... 

A different kind of dehydrogenation was used in the final step of Paquette s synthesis of dodecahedrane. ... 

If no y-H atom is available, or if for sterie reasons abstraction of a 6-H atom is facilitated, this latter reaction occurs with formation of a cyclopentanol. A series of such sequences has been used in the synthesis of dodecahedrane 409a,b>. 

The availability of cyclopentenones from butanolides allows the lactone annulation to facilitate the synthesis of cyclopentyl natural and unnatural products. An example that highlights the latter is dodecahedrane (178) for which 179 constitutes a critical synthetic intermediate 136,137). Lateral fusion of cyclopentenones as present in 179 can arise by acid induced reorganization and dehydration of 180. While a variety of routes can be envisioned to convert a ketone such as 182 into 180, none worked satisfactorily137 On the other hand, the cyclobutanone spiro-annulation approach via 181 proceeds in 64 % overall yield. Thus, the total carbon cource of dodecahedrane derives from two building blocks — cyclopentadiene and the cyclopropyl sulfonium ylide. 

Strategies based on known, highly elaborated, but nevertheless readily accesible, starting materials with a "complexity index" as near as possible to the "complexity index" of the target molecule. This strategy has also been applied to non-natural compounds as, for instance, in the synthesis of triquinacene by Woodward [37] and in the syntheses of dodecahedrane by Paquette (Domino Diels-Alder adduct) [38] and Prinzbach ("pagodane") and their associates [39]. 

The product diacid has served as starting material for the synthesis of tetracycio[7.2.1.0 . 0 ]dodeca-2,7-diene-5,12-dione, C g-hexa-quinacene, (Cj)-Ci7-heptaquinane derivatives, the parent dodecahedrane mol ecu e, and a number of substituted dodecahedranes. ... 

In a third approach substituents were removed from a preformed dodecahedrane cage. In this way the synthesis of [20-fJfullerene was possible in the gas phase [154]. However, extensions of this approach to syntheses of fullerenes consisting of 60 or more C-atoms are conceptually very difEcult. 

The hollow interior of dodecahedrane and other organic cage compounds described in section 4.9 is much too small to envelop atoms, ions, or molecules. Tight closed-shell macromolecules have been obtained from vesicles by several research groups, by polymerization of amphiphiles possessing double or triple bonds within the membrane or at the head groups. Smaller, but well-defined, closed-shell containers have been obtained by two other methods described below, namely by directed synthesis and by formation of closed-shell all-carbon molecules in graphite vapor. 

Paquette [5] applied this chemistry very elegantly to the synthesis of 1,16-dimethyl-dodecahedrane (IS), as shown in Scheme 8.3. 

In 1982, the synthesis and properties of a new polycyclic C2oH2o hydrocarbon, dodecahedrane, was reported [21], The twenty carbon atoms of this molecule are arranged like the vertices of a regular dodecahedron. When, in the early 1960s, H. P. Schultz discussed the topology of the polyhedrane and prismane molecules (vide infra) [22], at that time it was in terms of a geometrical diversion rather than true-life chemistry. Since then it has become real chemistry. 

We do not want, however, to leave the reader with the impression that cycloadditions are some sort of golden key that unlock the pathway to the creation of almost any cyclic system. Even reactions from this powerful arsenal can misfire and an otherwise brilliant retrosynthetic idea remains just that if it cannot be translated into a real synthesis. An instructive example is the exquisite proposal suggested by Woodward for the synthesis of dodecahedrane 115. The idea of this synthesis corresponds to a symmetrical dissection of poly-... 

Woodward s projected synthesis of dodecahedrane was actually based upon the recognition of the symmetry present in 115. While this approach failed in this particular instance, considerations of this type might turn out to be extremely useful in the search for a short synthetic route even in cases involving rather complicated structures. The synthesis of tropinone by Robinson is probably the earliest example that illustrates the effectiveness of such an approach. This impressive accomplishment was actually achieved by utilizing symmetrical bifunctional reagents to secure the formation of a symmetrical bicyclic structure as a result of a single chemical operation (see Section 3.2.1). 

While the story relating the design and synthesis of dodecahedrane sounds truly exciting, Paquette s synthesis of this compound required so much time and effort (23 steps starting from the cyclopentadienyl anion) that it seemed inconceivable to expect that any in-depth studies would be possible in that field. Fortunately, these concerns turned out to be not as serious as... 

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