Metacyclophane skeleton

Products with a [5]metacyclophane skeleton are obtained when dihalocarbene adducts of certain cycloheptene derivatives are treated with silver(I)perchlorate in the presence of 2,4,6-collidine in tetrahydrofuran [177, 178]. 

Resorcinol (and some 2-substituted derivatives such as 2-methyl resorcinol and phloroglucinol) react with aldehydes different from formaldehyde to form cyclic tetramers II, having the same [l4]metacyclophane skeleton.5 Among four possible stereoisomers, the rccc, rctt, and less frequently the rcct isomers are usually available. Various names have been used for these tetramers, which will be called resorcarenes6 in this review. 

It has been suggested to distinguish the single phenolic units by letters A, B,C,. .. instead of 1,2,3,... since numbers are also used for the single carbon atoms of the metacyclophane skeleton. 

Whereas caiixarenes (I) are based on a [l ]metacyclophane skeleton, a//-homocalix-arenes (H) contain an expanded (c.f Sessier "Expanded Porphyrins" [5]) cavity, which is achieved by synthesis of [2 ]metacyclophanes, and yieids one additional CH2-group compared to calixarenes in all Aeir bridges (Fig. 1). In addition, homocalixarenes allow use of pyridines as aromatic units in their ring skeleton Homocalixpyridines (HI). 

We have chosen the metacyclophane skeleton as a versatile and stable platform for functional units. A critical appraisal of the efficienty of the syntheses of cyclo-phanes incorporating more than two aromatic units is given below. The specific focus is upon those metacyclophanes which can be considered, in the broadest sense, a/Z-homocalixarenes. 

Even the distorted boat-like deck in [2.2]metacyclophanes can be constructed by an intramolecular version of the benzannulation. A suitable precursor bears a chromium vinylcarbene and an allcyne moiety linked to a meta-phenylene core by two-atom bridges, as shown for complexes 80. Benzannulation under the typical conditions affords hydro-quinonophanes 81 in fair yields (Scheme 31) [73]. Interestingly, the intramolecular benzannulation approach even tolerates heteroatom bridges, which impose both additional strain and helicity on the cyclophane skeleton [73b]. 

The calix[ ]arene ( indicates the number of aromatic units in the metacyclophane) building block provides a three-dimensional skeleton that is needed to fix different donor ligands in the optimal spatial positions for binding one ion selectively [94,95]. Calix[n]arenes are easily accessible from the base-eatalyzed condensation of p-tert-butylphenol and formaldehyde [96]. Gutsche proposed to define the two faces of a calix[4]arene molecule as the lower (phenolic groups) and the upper rim. The calix[4]arene with free hydroxyl groups is conformationally flexible and the... 

The basic skeleton of these compounds, calixarenes in the original (narrow) sense, is that of a [l ]metacyclophane and it is appropriate to include into the family of calixarenes also those cyclic oligomers, in which the phenolic hydroxy groups are situated in exo-position at the wide rim . Here especially cyclic compounds II (nearly exclusively tetramers) built up by resorcinol units (eventually substituted in the 2-position) are important and will be called resorcarenes within this article. Alternatively calixresorcinols is used for n, in distinction to calixphenols for I. 

For a long time, [2.2]para- and [2.2]metacyclophanes with two C2 bridges were the ones with the shortest bridges in any known [m.n]para- and [m.n]metapa-racyclophanes. Apparently, however, [2.1]paracyclo-(3,5)-naphthalinophane-1-ene (111) with one C2 and one C, bridge is not extremely strained, as it was obtained as a stable coumpound in 54 % yield upon treating bis([2.2]paracyclo-phane-l,9-dienyl)methanol 110 with zinc chloride in dichloromethane. The unusual skeleton of 111 was established by an X-ray crystal structure analysis [70]. 

The interaction of [2.2]metacyclophane-l,2-dione 22a (n = 0) with the 1,2-DAB in ethanol for 24 h at room temperature resulted in the desired [2.2]metacyclophane 23a in an almost quantitative yield having a quinoxaline skeleton (2000NJC221), whereas the tetracarbonyl derivative 22b (n = 1) easily available by the Albright-Goldman oxidation of5,12,20-tetra-tert-butyl-l,2,16,17-tetrahydroxyl-8,15,23,30-tetramethoxyl-[2.1.2.1] metacyclophane (20060L1995) yielded [2.1.2.1]metacyclophane 23b with... 

Whereas calixarenes contain a [l ]metacycIophane skeleton, the a//-homocali-xarenes are [2 ]metacyclophanes in which an additional CH2 group is present in all their bridges (Figure 1). 

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