Results and Discussion
Synthesis and structure of the imides (3aR,4R,5S,6aR)-octahydro-2-phenyl-1,3- dioxocyclopenta[c]pyrrole-4,5-dicarboxylic acid (16) and (3aR,4S,6R,6aS)-octahydro-2- phenyl-1,3-dioxocyclopenta[c]pyrrole-4,6-dicarboxylic acid (13)
The symmetric imide dicarboxylic acid (13) was prepared (Scheme 3) from the Diels-Alder adduct (12) of N-phenylmaleimide and cyclopentadiene by RuCl3/periodate oxidation across the 5,6-double bond following a method9 used for the synthesis of cis-cyclopentane-1,3-dicarboxylic acid from norbornene, the method applied also to the preparation of the 2-methylphenyl analogue of 13.10 The only previous preparation of 13 involved oxidation of the same adduct using ozone.11
RuCl3NaIO4NOOHbHO2CNOOHbHaHO2CCO2HHHHHHaHHCO2HNOOHHHHHH2' 3' 4' 123452' 3' 4' 123452' 3' 4' 312451213A13B
Scheme 3
The crystal structure of 13 as its monohydrate has two independent imide molecules in the asymmetric unit of the unit cell, each adopting an envelope conformation with the carboxylic acid groups in equatorial-like positions (cf 13A not 13B) as evident from one of the molecules depicted in Figure 1. The molecules differ slightly in the dihedral angle between the phenyl and imide rings (59.7 ° and 53.6 °), both values being close to the angle of 59.6 ° reported for N- phenylsuccinimide,12 but otherwise are very similar structurally. The dihedral angles between C- H bonds along the C1-C5 axis as numbered in 13A are referred to below in the context of coupling constants in the proton NMR spectrum of ionized 13 in D2O. The two independent imide molecules are linked into pairs by the two lattice water molecules, acting an a H-bond acceptor from one carboxylic OH and as H-bond donors to each of an imide and a carboxyl C=O.
Figure 1. Perspective view of one of the two imide molecules in the unit cell of 13 showing crystallographic numbering
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