Ring cyclic ethers

The orbital assignments of the first ionization potential, as well as of the higher bands which are broader and less intense, have been confirmed by ab initio MO calculations and by comparison with PE spectra of other small-ring cyclic ethers, amines, sulfides, silanes and cycloalkanes (77JA3226). 

Crown ethers are large-ring cyclic ethers with several O atoms. A typical example is 18-crown-6 ther, Fig. 14-l(a). The first number in the name is the total number of atoms in the ring, the second number is the number of O atoms. Crown ethers are excellent solvaters of cations of salts through formation of ion-dipole bonds. 18-Crown-6 ether strongly complexes and traps K, [from, e.g., KF, as shown in Fig. 14-1(6)]. 

Medium-ring cyclic ethers. Unsaturated eight- and nine-membered cyclic ethers can be obtained by cyclization of unsaturated acetals with SnCl4 (2 equiv.) in CH2C12. 

The formation of the six-membered ether ring via epoxy ester-ortho ester cyclic ether rearrangement supports the hypothesis that epoxy ester-ortho ester cyclic ether rearrangement may be involved in the biosynthesis of ladder-type marine polyether toxins. This reaction represents a biomimetic preparation of medium ring cyclic ethers. 

Alkenes undergo reaction with peroxycarboxylic acids (RCO3H) to give three-membered-ring cyclic ethers called epoxides. For example, 4-octene reacts with a peroxyacid to yield 4,5-epoxyoctane ... 

In addition to step and chain polymerizations, another mode of polymerization is of importance. This is the ring-opening polymerization of cyclic monomers such as cyclic ethers, esters (lactones), amides (lactams), and siloxanes. Examples of commercially important types are given in Table 10.1. Of those listed, only the polyalkenes are composed solely of carbon chains. Those that have enjoyed the longest history of commercial exploitation are polyethers prepared from three-membered ring cyclic ethers (epoxides), polyamides from cyclic amides (lactams), and polysiloxanes from cyclic siloxanes. 

Ring- Cyclic ethers Cyclic acetals Cyclic sulfides Cyclic amines... 

C-0 stretch of ethers Ether bridge between rings Cyclic ethers containing COCOC groups Alcohols 1000-1300 1230-1250 1025-1141 1049-1276 3200-3640... 

Epoxides are three-membered ring, cyclic ethers. 

Substitution of the higher cyclic ethers generally leads to deactivation. Thus oxacyclopentane (tetrahydrofuran) will form only oligomers in case of methyl substitution at the 2-position and low polymers from B-methyltetrahydrofuran. However, stereochemical results have not been rapported and studies concerning other substituted larger ring cyclic ethers have not been carried out. 

The use of heteroatom nucleophiles is a very popular variation of the Nicholas reaction. Alcohols and amines are most prevalent, while examples of azides, thiols,carboxylic acids, epoxides, hydrides, and fluorides are known. Isobe has made extensive use of alcohol nucleophiles in the preparation of various sized cyclic ethers. In studies directed toward the synthesis of ciguatoxin, Isobe converted alcohol 15 into eight-membered ring cyclic ether 16 using the Nicholas reaction. 

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