Cyclization of polymers

Cationic cyclization of unsaturated elastomers such as poly(c -l,4-isoprene), poly(3,4-isoprene), poly(l,2-butadiene), and poly( 1,4-butadiene) usually leads to the formation of cyclized resinous products of no commercial value. An extensive review on the subject has been published by Schults et al. (1983). Cyclization of unsaturated elastomers, such as polyisoprene, can be carried 

---

Cox and co-workers reported the cyclization of polymer-supported aminopyrimidines formed by the reduction of nitropyrimidines 73 to produce cyclized pyrimidines 74 after cleavage from the resin <00TL8177>. 

Note 2 Examples of cyclization of polymer molecules are (a) cyclization of... 

Often these internal cyclizations are incomplete giving products with mixed moieties. Even so, such internal cyclization is the source of a number of interesting and important polymers. A number of ladderlike structures have been synthesized from the internal cyclization of polymers. Following are several examples that illustrate this. The most important commercial products are those utilized to form the so-called carbon fibers. 

Solid-phase synthesis by the cyclization of polymer-bound hydrazones of [1-keto esters. Jung and co-workers (99JOC1362) (Scheme 24) have reported the synthesis of pyrazol-3-one 102 from polymer-bound /1-keto ester 100 and phenyl hydrazine. In the first step, polymer-bound 3-hydroxy-2-methylidenepropionic acid 99 is derived from a Baylis-Hillman reaction between polymer-bound allylic alcohol... 

Furthermore, it was believed that prior oxidation of polymer reduced the cyclization rate and prior cyclization of polymer reduced the oxidation rate (rate of oxygen diffusion). 

A general method has been reported for the parallel solid-phase synthesis of 1,2,4-triazole derivatives based on the cyclization of polymer-bound dithiocarbazate 42 (by a two-step sequence from Fmoc-hydrazine) with electrophiles using a similar route to the formation of pyrazole 7." The linker 42 served as a nucleophile for the cyclization reactions with electrophiles such as cyanocarboimidates 46 (R diversity elements) (Scheme 10.11). 

A reaction of type (II) has indeed been demonstrated by Fridkin et al. (1965) who synthesized various diketopiperazines and cyclic tetrapeptides by cyclization of polymer bound active esters of appropriate peptides. 

A related study of the Dieckmann cyclization of polymer-bound triethyl-carbinyl pimelate half-esters (73) led to essentially exclusive formation of the /S-ketoester (74), only trace amounts of the isomeric (75) being observed this result is comparable to that obtained by self-condensation of benzyl tri-ethylcarbinyl pimelate. 

Three types of experiments have been applied to the study of diffusion controlled cyclization of polymers. These are excimer formation in polymers containing pyrene groups at both chain ends [Py-polymer-Py ], exciplex formation in polymers containing a pyrene at one end and a dimethylaminophenyl group at the other [DMA polymer-Py i ], and triplet-triplet [TT] annihilation between anthracene groups at both ends of a polymer [A -polymer-A 3]. A fourth obvious approach, as yet to be reported, is intramolecular phosphorescence quenching in a polymer... 

Aryl-alkenyl and aryl-aryl cydizations via Stille coupling are far less developed. In the case of alkenyl-aryl couplings, the olefmic moiety can be extracyclic or intracyclic with Z or E stereochemistry. An example is the cyclization of polymer-bound alkenylstannane with aryl iodide moiety in the total synthesis of (S)-zeara-lenone [127] (Scheme 6.16). 

---