Reverse coupe

Figure 2-47 after Cinquini et al. [75] who painstakingly documented the analogy with the pomaceous model. Only examples of the reverse coupe du roi had been known prior to their work. Thus Anet et al. [76] reported the synthesis of chiral 4-(bromomethyl)-6-(mercaptomethyl)[2.2]metacyclophane. They then showed that two homochiral molecules can be combined to form an achiral dimer as shown in and illustrated by Figure 2-48. 

Figure 2-58. Reverse coupe du roi and the formation of a dimer from two homochiral 4-(bromo-methyl)-6-(mercaptomethyl)[2.2]metacyclophane molecules. After Anet eiat. [2-57], Used by permission. Copyright (1983) American Chemical Society.

An ingenious experimental example for the reverse of the socalled la coupe de Roi (i.e. the dividing of finite geometric objects into isometric segments) — namely the assembly of two homochiral compounds into an achiral one — was provided by making use of appropriate [2.2]metacyclophanes. Whereas self-coupling of (+)-4-(bromomethyl)-6-(mercaptomethyl)[2.2]metacyclophane (cf. formulas 55 to 60 for... 

Methoxycarbonyl-functionalized polydimethylacrylamide resin was chosen as the solid support. It was converted into a primary amine by treating it with ethylenediamine. An internal reference-spacer amino acid was incorporated by coup-ing with A -Fmoc-norleucine anhydride and the cleavage of the A -Fmoc group with 20% piperidine, followed by the addition of the reversible-linkage agent [2,4,5-trichlorophenyl 4-(hydroxymethyl)benzoate] in the presence of HOBt. 

The. caesium catalyst is essential. The self-coupling of the homo-chiral metacyc lophane to yield the achiral dimer (64) is a chemical analogue of the reverse of "la coupe du roi"/a term adopted from a French parlour trick, to define the process when an achiral molecule is bisected into two isometric homochiral halves. X-Ray studies on... 

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