Acyclic spacers

Imide-terminated telechelics are also synthesized by metathesis depolymerization, and it is found that phthalimide-substituted olefins allow for productive depolymerization when only one methylene spacer separates the nitrogen atom and the olefin (Fig. 8.21). This combination of steric hindrance around the nitrogen lone pair and decreased electron donation from resonance prevents the negative neighboring group effect. However, secondary acyclic amines are unable to produce telechelics through metathesis depolymerization because of unfavorable catalyst-amine interactions. 

Phosphoryl-containing podands are acyclic molecules with polyether spacer(s) linking two (in monopodands), three (in dipodands and tripodands), or four (in tripo-dands) terminal phosphine oxide groups (Figure 5.6c). Variations of the lengths of the spacers, of the molecular topology, and of the substituents at the phosphorus... 

Stoddart and coworkers have used imine condensation in two different contexts to form interlocked molecules. The authors have modified the crown ether component of their dialkylammonium / D B24C8 system to incorporate reversible imine subunits which allow for dynamic clipping around dialkylammonium cation 91 [60], Three diformyl spacers (92-94) have been investigated with the same acyclic diamine 95 for their thermodynamic stability and kinetics of macrocyclization (96). However, reduction with BH3 to the kinetically trapped macrocyclic secondary diamine has only been reported for reaction of the 2,6-diformylpyridine derivative (Scheme 10.19 97) [60a]. 

Another family of macrocycles consists from polycycles - compounds where two or more macrocycles are linked together through a spacer. Most of such compounds are synthesized directly from appropriate macrocycles or their derivatives (Section 14.11.6.3) however, there are some examples where the polycycles were obtained from acyclic precursors. 

Acyclic BFCs have also been used for inserting Ga " into peptides and antibodies. One example is Desferrioxamine-B (DFO), which forms stable complexes with Fe as well as with Ga. The stability constant of the Fe-DFO complex is log K = 30.4. DFO can be conjugated to a biomolecule via an activated primary amine group or derivatized with succinic acid anhydride as a spacer for coupling to the N-terminus (Macke et al. 1993 Smith-Jones et al. 1994 Stolz et al. 1994) (O Fig. 45.7). 

Podand An acyclic chain-like or branching host with a number of binding sites that are situated at intervals along the length of the molecule, or about a common spacer. 

A method for the synthesis of chiral binol-based [1 -1- l]-macrocyclic Schiff bases in high yields is described. Chiral dialdehydes afforded [1 + l]-macrocycles in higher yields with chiral diamines. The catalyst structure was tuned by incorporating lengthy spacers and bulkier groups, which displayed increased catalytic activity and enantioselectivity forming epoxides in good yields and over 94% ee. Cyclic olefins were found to be better substrates than acyclic olefins with NaOCl as the oxidant of choice. ... 

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