The predominance of axial conformers fortrans-4-substituted cyclohexene oxides

Abstract

A 'H N M R conformational study of cis-and trans-4-substituted cyclohexene oxides revealed an increased predominance, as compared with the parent 4-substituted cyclohexenes, of the equatorial conformer for cisisomers and a preference of the axial conformer for trans-isomers. These conformational shifts can be rationalized in terms of intramolecular dipole-dipole and/or steric interactions. However, molecular mechanics calculations failed to reproduce the relative stability of the axial conformer in trans-4-substituted cyclohexene oxides.

Trans-2-aminocyclohexanols as pH-triggered molecular switches

“…The latter fact also points out that a relative flexibility of cyclohexane ring sets a natural limit to the effective power of conformational tools (levers, locks, counterbalances) in such systems. If the power applied to both ends of the system exceeds the energy difference between the chair and twist-forms of cyclohexane (23-26 kJ/mol 48 ), then the ring may be screwed (for the relevant discussion see 13,17,27,42,44,49 ).…”

“…Conformational control via introduction of various substituent(s) into a trans-fused six-membered cycle was proposed by us as a new principle for modification of the complexing ability of (cyclohexano)crown compounds and non-macrocyclic ionophores (podands). [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] Similar ideas were suggested for cyclohexanebased podands by Raban et al [24][25][26][27] In these structures, a substituent plays a role of 'conformational lever', or 'counterbalance', and the cyclohexane moiety serves as a mechanical transmitter. The cyclohexane machinery can also mimic an allosteric effect by transmitting a conformational change (signal) from one complexing center (e.g.…”

Cyclohexane-Based Conformationally Controlled Crowns and Podands

“…Although the major part of the macrocycle beyond the bridge fragment XCCY is not characterized by the conformational parameters of the cyclohexane moiety, this approach allows obtaining important information ( Table 1) As could be expected, the conformer B with equatorial orientation of C-O bonds is predominant for both transcyclohexano-15-crown-5 (3) [9] and trans-cyclohexano-18- [20,21] a) Integration in the low-temperature 13 C NMR; b) Measurement of the conformationally averaged couplings (signal widths) in the room-temperature 1 H NMR; c) Integration in the low-temperature 1 H NMR; d) K eq = 0.1 [22]; e) K eq < 0.02 [22]; f) K eq = 0.08 [25]; g) K eq = 0.05 [25].…”

“…This conformational shift was described as a 'contraction effect' of the macrocycle [4][5][6][7]9,11,[13][14][15][16][19][20][21]. Its magnitude was measured as a deviation of the free energy difference for the A B equilibrium (∆G B-A ; see Table 1 ) from the analogous parameter for t r a n s -1 , 2dimethoxycyclohexane, and was estimated as 2.7 kJ/mol for 3 [9] and 3.7 kJ/mol for 4 [4].…”

“…Conformational control via introduction of various substituent(s) into a trans-fused six-membered cycle was proposed by us as a new principle for modification of the complexing ability of (cyclohexano)crown compounds and non-macrocyclic ionophores (podands) [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Similar idea was suggested for cyclohexane-based podands by Raban et al [22][23][24][25].…”