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The two amide groups contain acid-cleavable phenolic benzyl groups allowing temporary connection via transesterification to template 6.
The terminal alkene and azides/alkynes allow robust RCM and Cu AAC reactions towards the macrocyclic spiro geometry 1 and quasicatenane 2 or stopper 11 attachment via cross-metathesis to give quasirotaxane 3 and its unwound conformer 4.
for the second and final ring closure or introduction of the quasi-rotaxane stopper elements.
For synthetic reasons and to ensure the perpendicular mutual arrangement of the two pairs of ring-precursor chains at precursor 5, we have chosen a spiro-linkage via the tetrahedral carbon atom of a 9H-fluorene moiety (Fig. The design of the temporal covalent scaffolding tether at the central precursor 5 is based on amide-N-benzylic moieties containing phenolic hydroxyl groups that will be esterified to template 6 and may be removed from the final spiro or rotaxane compounds by consecutive transesterification and protolytic cleavage of the amide benzylic linkages.
Here we disclose the synthesis of this molecule class that we have coined quasicatenanes.
Next to their fascinating and aesthetic shape, the higher compactness as compared to regular spiro bicycles is noteworthy.
These ring sizes are significantly larger than the smallest accessible rotaxane ring fragment of 21 atoms and in the same range as the natural peptide rotaxanes.
For mechanically locking the conformation of the 31-membered ring of the quasirotaxane by cross-metathesis, we selected tris(4-(tert-butyl)phenyl)methane-functionalized acrylamide 11 as the stopper element Pivotal to the design of central precursor 5 is a tricyclic fluorene moiety containing the tetrahedral carbon atom that ensures a perfect perpendicular arrangement of the acyclic fragments.
Alternatively, by installation of large stoppers at the end of the two linear chain fragments, next to the regular closure to give 4, cyclization via backfolding gives access to the mechanically locked quasirotaxane 3 conformation.Besides the regular geometry (as for structure 1, Fig.1), an alternative connectivity may be envisioned by backfolding of the two rings at the central tetrahedral carbon atom to give an inverted spiro configuration (that is, 2, Fig. An intermediate in the landmark catenane synthesis by the Schill group already displayed a similar inverted spiro geometry.Capping of 8 by bulky stoppers 11 followed by template cleavage provides quasirotaxane 3 (route E).Capping of monocycle 10 gives the unwound conformer 4 (route F).