Conformational furan rings

An illustrative example for the potency of catalyst C is Paquette s highly efficient total synthesis of the natural products teubrevin G 180 and teubrevin H 181, which feature a cyclooctane core fused and spiroannulated to smaller oxygen containing rings. In the retrosynthetic analysis, the viability of an RCM step for annulation of a cyclooct-enone ring to the furan played a central role. Despite the presence of a conformational constraint by the furan ring in... 

In thioamide vinylogues (96) the conjugation was proved in a qualitative sense from the enhanced values of dipole moments223. The derivatives of furane (97) were investigated by several experimental methods224 while the conformation on C—N followed mainly from IR spectra, dipole moments served to determine conformation of the furan ring on the Car—C bond but the result was not convincing (due also to some misprints). 

An intramolecular [4+4] photocycloaddition of a 2-pyridone with a furan ring yields the complex 1,5-cyclooctadiene 231 <20060L3367>. The proposed transition state conformation leading to the realized (and desired) f vsy -product is shown (Equation 110). The isopropyl group on the cyclopentane of the pyridone demonstrated stereocontrol and the... 

Multilayered cyclophanes have been prepared that show much the same properties regarding strain or rotation of furan units examples include 165278 and 166.279 New features emerge from studies of compounds like the [2.2] (l,4)naphthaleno(2,5)-furanophane (167) in which rotation of the furan units is possible while there is yet a strongly preferred conformation in the case of 167 the anti form shown. As a result, the naphthalene proton Hb suffers rather strong shielding by the furan ring and its resonance moves upheld by about 0.9 ppm, whereas the furan proton Ha is not much affected.271 The NMR results have been confirmed by an X-ray diffraction... 

Octaethylcalix[4]furan (6) in its X-ray structure (94JA10775) showed a double saddle-shaped conformation with two cavities where both C = C and C H bonds are exposed and accessible to appropriate metal ion for interaction. The octa-methylcalix[4]furan (5), in its X-ray structure reveals a square ring conformation with furan rings alternately up and down (89AX(C)137). 

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