Seven-membered oxepanes

This has been shown for cationic polymerization of five-membered and seven-membered oxepane. The... 

In contrast, in the cationic polymerization of cyclic ethers (e.g. THF or seven-membered oxepane), the dissociation equilibrium constants have considerably higher values, depending on the solvent polarity Ro-lO molh (CH2CI2) and - 10 molT (CH3NO2, QH5NO2) with SbClg as a counterion (see Table 8 in Ref [3a]). 

Fully saturated seven-membered heterocycles with one or two heteroatoms are normally in mobile twist-chair conformations (Section 5.17.1.1, Chapter 5.18) (b-77SH(2)123). Annelation and the introduction of exocyclic double bonds can have profound effects oxepan-2-one, for example, is in a near chair conformation (67JA5646). 

Having developed effective synthetic methodology for the construction of seven-membered cyclic ethers, we were confident that the problem of the frans-fused bis(oxepane) system could now be addressed on a solid foundation. It was our hope that the breve-toxin-type bis(oxepane) system could be assembled by a stepwise strategy utilizing both photochemical dithioester and reductive hydroxy ketone cyclization methods. 

D. Sabatino and M. J. Damha, Oxepane nucleic acids Synthesis, characterization, and properties of oligonucleotides bearing a seven-membered carbohydrate ring, J. Am. Chem. Soc., 129 (2007) 8259-8270. 

Oxepane (seven-membered) rings (1,6-anhydrohexitols)52 have been prepared from the 3,4-isopropylidene acetals of D-mannitol, D-glucitol, and L-iditol, by way of alkaline hydrolysis of the corresponding 1,2 5,6-dianhydrides. The ring structures of the products were established through periodate oxidation and lead tetraacetate oxidation the requisite amount of formic acid was produced, and 3 equivalents of lead tetraacetate were consumed. No inversions at any of the asymmetric centers were involved in the reactions conducted, so the oxepanes had retained the configurations of the starting hexitols. 

Tetrahydrofuran, a five-member cyclic ether, polymerizes cationically to yield an elastomeric polymer [28]. Oxepane, a seven-member analog, polymerizes to a crystalline polymer. 

Similar methods can also be used for seven-membered and larger rings however, high dilution techniques must often be utilized. Oxepane, for example, is obtained by the dehydration of hexane 1,6-diol (80BCJ3031). 

Yamamoto has investigated the intra-molecular condensation of y-(alkoxy)allylstaimanes with aldehydes as a general approach for the stereocontrolled synthesis of polycyclic ethers (Scheme 5.2.26). The strategy has been iteratively applied to the total synthesis of hemibrevetoxin B. i In this fashion, the Lewis acid-promoted reaction of 122 leads to formation of seven-membered ether 123, which is transformed into 124 for intra-molecular allylation to the fused-oxepane 125. 

Processes that do not terminate were described for the polymerization of five- and seven-membered cyclic ethers THF (4, 91) and oxepane (93). Under proper working conditions both cyclic acetals, namely 1,3-dioxolane and 1,3-dioxepane (94. 95). as well as bicyclic acetals or 1,6-anhydro-l,3,4-tri-0-benzyl-a-D-glucopyranose (96) can polymerize under living conditions. 

Intramolecular Ulknann coupHng has been used to constmct seven-membered rings by formation of an ether linkage. In a synthesis of bulbo-phylol-B, phenol 27 was treated with CuBr-DMS in the presence of NaH to form the oxepane core of the molecule (Scheme 15) (2008JNP1938). 

Monosaccharides occur as substances with a free carbonyl group (acyclic compounds) and as cyclic hemiacetals, also called lac-tols. Trioses are exclusively acyclic substances, tetroses and higher monosaccharides exist predominantly in five- and six-membered and, exceptionally, also in seven-membered cyclic structures. They can therefore be regarded as substances derived from oxolane (tetrahydrofuran), oxane (tetrahydropyran) or oxepane, and are thus actually heterocyclic compounds. Acyclic forms, which exist in constitutional equilibrium with cyclic forms, occur in zigzag conformers, as well as alditols (see Section 4.3.1.1.1). 

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