Anulenes

With regard to the stability of the skeletons of bridged anulenes, the possibility of a bridge inversion is of considerable interest. The availability of optically active bridged [10]- and [14]anulenes offered the chance to study these phenomena by race-mization experiments  

Both the bisoxido- and bisimino-(cis)bridged dibromo[14]anulenes 115 and 116 with C2-symmetry could be quantitatively separated into their enantiomers by chromatography on triacetylcellulose in ethanol at 1.7 bar 124). The dextrorotatory enantiomers with [a]546 values of 1700° 115) and 1500° 116) in ethanol were eluted first. They exhibit very similar CD-curves with strong Cotton effects around 260 

The dibromo-oxido[10]anulene 104 (for its partial resolution see the previous Section 3.2) on the other hand is optically stable up to 250 °C (for 2 h)124) with a calculated AG -value of ca. 255 kJ mol-1 125). Obviously the ten-membered ring is more rigid than the fourteen-membered one — as one might expect. 

Nonbridged and nonaromatic anulenes such as derivatives of cyclooctatetraene ([8]anulene) (cf. 117 to 121) are chiral owing to the nonplanarity of the eight-mem bered 

9 Added in proof for ( + )-/16 the chirality (S) — as shown in the formula — has been established only recently by the Zh/voeMnethod (J. Lex, unpublished). 

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Fig. 1. Plane of Chirality Enantio-morphous figures and corresponding torsional angles A—X—Y—Z 7). Molecular examples ([n]paracyclophane, [2.2]meta-cyclophane, bridged[10]anulene and ( )-cyclooctene) with descriptors (/ , S)...

On the basis of this definition (Fig. 1) the following classes of chiral compounds will be treated in this article cyclophanes, bridged anulenes and [8]anulenes, and ( )-cyclooctene and related structures. As mentioned above, metallocenes will be excluded. 

Bridged [10]- and [14]anulenes8 are of considerable interest not only because of their aromaticity (complying with HiickeFs rule) but also because of their (rigid) molecular geometry. The most simple representative l,6-methano[10]anulene (95) first obtained in 1964 U5), as well as others with oxido or imino bridges (cf. 103), have C2v-symmetry, but become chiral (C,) by substitution in any position. Such chiral anulenes can be classified as planar chiral under the assumption that the ten-... 

Anulene is derived from the latin word anulus (small ring) and should therefore be spelled, in contrast to the general custom, with one n. 

Spiniferin-1 is a naturally occurring furanosequiterpene with the skeleton of 1,6-methano[10]anulene, isolated from a sponge found in the Bay of Naples. The total synthesis of the racemate was recently reported 118e). 

Apart from a footnote in a review article U8b) the first optically active bridged [10]-anulene was described in 1971 thus experimentally proving their chirality 119) The carboxylic acid 96 was resolved through i s salts with (+ )- and (—)-l-phenylethyl-amine ([a]D +250° in ethanol via the (—)-amine 117, U9)). The enantiomeric purity... 

Introduction of nitrogen into the anulene ring (e.g. of 95) leads to a methano-azaanulene 107 121) with Q-symmetry which is therefore chiral (like its mono- or disubstituted derivatives)118). The low basicity of 107 (pKa 3.20) prevented its optical resolution by conventional methods (e.g. through salts with optically active acids). Excellent results were obtained, however, (as also in the case of the two isomeric carbocyclic methylesters 97 and 101 and of several derivatives of azaanulene) by chromatography on microcrystalline triacetyl cellulose in ethanol at 7 bar 1221 (see also Section 2.7.1). In many cases base line separations were accomplished to give both (optically pure) enantiomers. Enantiomeric relations were confirmed in all cases by recording the CD-spectra of both fractions. Some results of these separations are shown in Fig. 4 together with the optical rotations ([a]D in ethanol) of the enantiomers. 

Fig. 4. Separation of enantiomeric methano[10]anulenes (97) and [10]azaanulenes (107, 109, 111 and 113) on triacetylcellulose in ethanol at 7 bar and 40 °C. Optical rotations [ot]u° in ethanol1221...

The chirality of methano[10]anulene-2-carboxylic acid (96) was derived from results of a kinetic resolution as outlined for cyclophanes in Section 2.9.3. The reaction of the anhydride of 96 with (—)-l-phenylethylamine as well as the kinetic resolution of 102 (unambiguously correlated with 96 via the acetyl derivative 99) with (+)-2-phenylbutanoic anhydride (Horeau s method) — affording an excess of (+)-carbinol 102 — led to the assignment of the descriptor (S) to (+ )-96 and all its derivatives U7). 

Comparison of the CD-spectra of (—)(R)-2-methyl-methano[10]anulene 100)117) and of (—)-8-methyl-methano[10]azaanulene 108) 1221 allowed a rather tentative assignment of the descriptor R) to -)-108 and — on the basis of the above-mentioned correlation — also to (—)-107 and to other levorotatory [10]azaanulenes, as shown in the following ... 

From the several other chiral [10]anulenes described so far (mainly by E. Vogel and his group)118) (for a tetracyclic derivative 114 with a steroid skeleton, cf. Ref.123)),... 

SchlogI, K., Widhalm, M., Vogel, E., Schwamborn, M. Stereochemistry of planar chiral compounds, IX. Chromatographic separation into enantiomers, chiroptic properties and relative configurations of 1,6-methano-[10]anulenes and 2,7-methano[10]azaanulenes. 

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