Formals, macrocyclic

Although it has been known for many years that ethylene oxide and formaldehyde ean form statistical copolymers, it was not until the interest in crown ethers developed that the potential of macrocyclic formals as complexing agents was recognized. 

An important body of experimental evidence concerning the problem of the strain-energies of large rings has been presented by Yamashita et al. (1980), who reported an extensive investigation of ring-chain equilibria of the macrocyclic formals 1,3,6-trioxacyclooctane [30 m = 1], 1,3,6,9-tetraoxa-... 

Fig. 22 Macrocyclisation equilibrium constants for macrocyclic formals [30] in CH2CI2 in the presence of Et2O-BF3 as a catalyst (7) m = 1 (O) m = 2 (A) m = 3 ( ) m = 4. (Reproduced with permission from Yamashita ct al., 1980)...

Properties of Macrocyclic Formals. Macrocyclic formals are hygroscopic liquids and are soluble in ordinary organic solvents and water. Although they are easily hydrolyzed in acidic solution, they are fairly stable in basic solution, and can be used as accelerating agents in nucleophilic synthetic reactions. We compared the rate of an Sjj2 reaction between n-butyl bromide and alkali metal acetate accelerated by addition of equimolar amounts of macrocyclic formals. 

Figure 2. Rate enhancement by addition of macrocyclic formals. nBuBr, 0.01 mol alkali metal acetate, 0.01 mol benzene, 9 ml toluene, 1 ml chelating agent, 0.01 mol. Temperature, OO C (reflux).

Monomers Macrocyclic formals was synthesized from fractionally distilled polyethylene glycol and paraformaldehyde using p-toluenesulfonic acid as catalyst by the similar method in the literature (14). They were purified and dried by distillation over lithium aluminum hydride for four times. The purity was checked by gas chromatography. 

DXL and macrocyclic formals are the only cyclic acetals for which cyclization has been studied quantitatively.The formation of cyclic fraction may, however, be expected in the polymerization of any cyclic acetal. It has been indicated that cyclic oligomers are formed in the polymerization of TOX. Nonstabilized homopolymer always contains some amount (up to 25%) of a stable fraction, which, at least partially, is assumed to be cyclic (instability is related to the hemiacetal groups, which are absent in cyclic macromolecules). Cyclic oligomers are also believed to be responsible for the bimodal molecular weight distribution observed by GPC. ... 

The macrocyclic formals (77) have been prepared (from polyethylene glycols and paraformaldehyde) but are shown to be in general less effective PT catalysts than the corresponding crown ethers for the reaction of solid metal acetates with 1-bromobutane. Their ready decomposition with acid, and subsequent extractive removal, might conceivably be an advantage in some circumstances. 

The concept of expansion of the porphyrin macrocycle by formally inserting additional carbon atoms between the pyrrole rings was first considered and synthetically realized by LeGoff/ He also suggested a nomenclature for these macrocyclic systems using the word platyrin which... 

The name amethyrin refers to the fact that the dull-purple color of the protonated form of the macrocycle is that of amethyst stones, Formal oxidation or reduction products are the aromatic [22]hexaphyrin(l.0.0.1.0.0) or the [26]hexaphyrin(l.0.0,1.0.0), respectively. However, none of these products could be observed either as reaction products or as direct products of oxidation or reduction reactions. 

These topics are discussed in more detail in other chapters of this text. Formally, the pyrolytic elimination of sulphur dioxide from a sulphone, with the concomitant formation of a new carbon-carbon bond, constitutes a reduction at sulphur. These reductions have been valuable in the formation of new molecules, especially macrocycles and cyclophanes, and have been reviewed by Vogtle and Rossa205. Pyrolytic elimination of sulphur dioxide has been used by Julia and co workers in the formation of mixtures of isoprenoids206, and by Takayama and collaborators in the stereoselective synthesis of vitamin D, 19-alkanoic acids207. 

As will be discussed later, the novel pentacyclic antitumor alkaloid roseophilin continues to attract much synthetic effort and several approaches relied on the venerable Paal-Knorr condensation for construction of the pyrrole moiety. For instance, Trost utilized this reaction upon diketone 1 to afford the tricyclic core 2 of roseophilin in a strategy featuring an enyne metathesis as a key step <00JA3801>, while another formal synthesis of this alkaloid utilized a radical macrocyclization to produce the ketopyrrole core <00JCS(P1)3389>. 

According to this view the macrocyclic molecules found in the reaction product are formed by a back-biting reaction (3) in which the formal CH2 group adjacent to the tert.-oxonium ion reacts with an oxygen atom along the chain, generating a macrocyclic tert.-oxonium ion ... 

Table 11 Formal electrode potentials ( V vs. SCE) for the reduction processes of a few Co(III)-tetraaza macrocycles in MeCN solution...

Macrocyclization of esters of allylglycine with diols has been successfully used to prepare derivatives of 2,7-diaminosuberic acid [861,864]. The latter are surrogates of cystine, and therefore of interest for the preparation of peptide mimetics. For unknown reasons protected allylglycine derivatives can not be directly dimerized by self metathesis [864]. However, catechol [864], ethylene glycol [861], and 1,2- or 1,3-di(hydroxymethyl)benzene derivatives [860] of allylglycine are suitable templates for the formal self metathesis of this amino acid via RCM. 

A complete series of nickel(ii) complexes (136)—(139) has been produced wherein the macrocyclic ligands contain four N-donors and vary stepwise from containing one to four unsaturated N atoms. All the complexes are formally obtained by hydrogenation or oxidative-dehydrogenation of Ni(CR) (138). The complexes of (CR) and (CR -f- 4H) have been reported earlier. [(CR -h 2H)-... 

---