Calix arene asymmetric compounds

Butylcalix[n]arenes can be converted into monospirodienones 43 by mild oxidation (tetrabutylammonium tribromide) under alkaline conditions.89 The structure of two examples 43a,b has been established by single-crystal X-ray analysis.90 Their chirality is mainly due to the asymmetrically substituted spiro carbon atom, however, compound 43a has also some similarity with calix[4]arenes having three different phenolic units in the order AABC (see below). Under neutral conditions the oxidation of r-butylcalix[4]arene leads to the formation of 44 which has been also confirmed by single-crystal X-ray analysis.91... 

Calix[4]arene tetraethers can be converted into mono Cr(CO)3 complexes 72, which makes one of the phenolic units different from the others (AAAB).135 Thus, a single chiral compound is obtained from the 1,2-altemate conformer, while for the partial cone conformer only one of the three possible products is asymmetric (compare Figure 13). Mono derivatives fixed in the cone and 1,3-altemate conformation contain a symmetry plane. 

Most of the inherently chiral calix[5]arenes described up to now, owe their chirality, however, to the asymmetric substitution pattern at the narrow rim, and due to the lack of other general, selective derivatization reactions are derived from 1,2-or 1,3-crown ethers. Both compounds possess a symmetry plane and can be desymmetrized by a single O-alkyl or O-acyl residue in position 3 (=5) or 4 (=5).151,152,153 In practice, 1,2-crown ethers 79 were prepared from mono-O-alkyl derivatives by reaction with the appropriate ditosylates, while 80 was obtained from the 1,3-crown-ether by subsequent O-alkylation or O-acylation using a weak base to benefit from the fact, that the first deprotonation leads to a hydrogen-bonded (4/5)monoanion. The picolyl derivatives 79 (n = 2) were resolved by HPLC... 

The second example (95a), synthesized by Dalcanale and co-worker181 is asymmetric because of one substituted quinoxaline residue (similar to calix[4]arenes containing one m-substituted phenolic unit). Since the menthyl residue attached to the quinoxaline via an ester function is chiral itself a mixture of the two diastereom-ers was obtained which could be separated by chromatography on silica gel. The subsequent reduction of the ester function in 95a led to enantio-pure (+) and (-) alcohols 95b. The compound 95a was used in host-guest complexation studies182 with benzene, fluorobenzene, 2-fluorotoluene, and isobutane in the gas phase by means of Desorption Chemical Ionization Mass Spectrometry however, studies of chiral recognition have not yet been reported. 

Asymmetric calix[4]arenes also result from the incorporation of a single meta-substituted phenolic unit (9), an idea first realized by Vicens et al. [11] and subsequently picked up by Shinkai et al. [12]. Again, these compounds have been synthesized by 3+1 fragment condensation. 

With calix[4]arene-like macrocycles of type 25 having two different bridges X and Y [33-35] all mono- and triether derivatives are asymmetric. Their 1,3-diethers have C2 symmetry, while the two possible 1,2-diethers have a symmetry plane. The first examples of such compounds were mentioned by Nishimura [35]. 

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