Oxygen conformers

A study195 based on the NMR lanthanide-induced shifts (LIS) for a series of cis- and trans-3-substituted, and 3,3-disubstituted thietane oxides concluded that all cis-substituted oxides (5c R = CH3, t-Bu and aryl) exist exclusively in the diequatorial conformation. The trans-3-substituted isomers (185) prefer the equatorial oxygen conformation (R = CH3,86% t-Bu, 65-75% aryl, 75%), which means an axial preference for the substituents (e.g. 185d), at least when they are bound to a shift reagent (equation 75). 

The previously discussed conformational study of 3-substituted thietane oxides using lanthanide shift reagents185 corroborates the conclusions derived from other NMR studies and suggests that all rrans-3-substituted thietane oxides prefer an equatorial oxygen conformation when the thietane oxides are bound to shift reagents. 

The dipole moments of the two conformers 146 and 147 of 1-rm-butyl-piperidine-4-spiro-4 (l, 3 -dioxolan) were calculated by vector addition of the moments of 1-fert-butylpiperidine and 4,4-dimethyl-1,3-dioxolan, making reasonable assumptions regarding geometries. This gave 1.15 and 1.93 D as the moments of the two conformers. The observed moment of 1.47 D gave AG° of 0.37 kcal mol-1 in favor of the axial oxygen conformer 146 in cyclohexane solution. AG%2- of 0.26 kcal mol-1 was estimated from low-temperature H-NMR studies.121 The differences between the piperidine and... 

The increased axial preference relative to that observed in thiane-1-oxide, but comparable to that observed in the oxathiane analogue (see Section 3.3.3), could support the influence of an electrostatic attraction between the sulfinyl oxygen atom and the syn-diaxial hydrogen atoms (Scheme 3.48). Interestingly, trans-1,4-dithiane-1,4-dioxide (37) possesses a diaxial oxygen conformation in the solid state [80] and appears to exist preferentially in the same conformation in solution [81] (Scheme 3.49). 

Schumacker PT, Chandel NS, Agusti AG. Oxygen conformance of cellular respiration in hepatocytes. Am J Physiol 1993 265 L395-L402... 

The large sulfur atom is a preferred reaction site in synthetic intermediates to introduce chirality into a carbon compound. Thermal equilibrations of chiral sulfoxides are slow, and parbanions with lithium or sodium as counterions on a chiral carbon atom adjacent to a sulfoxide group maintain their chirality. The benzylic proton of chiral sulfoxides is removed stereoselectively by strong bases. The largest groups prefer the anti conformation, e.g. phenyl and oxygen in the first example, phenyl and rert-butyl in the second. Deprotonation occurs at the methylene group on the least hindered site adjacent to the unshared electron pair of the sulfur atom (R.R. Fraser, 1972 F. Montanari, 1975). 

Typical carbon-oxygen bond distances m ethers are similar to those of alcohols (—142 pm) and are shorter than carbon-carbon bond distances m alkanes (—153 pm) An ether oxygen affects the conformation of a molecule m much the same way that a CH2 unit does The most stable conformation of diethyl ether is the all staggered anti conformation Tetrahydropyran is most stable m the chair conformation—a fact that has an important bearing on the structures of many carbohydrates... 

The most stable conformation of acetone has one of the hydrogens of each methyl group eclipsed with the carbonyl oxygen Construct a model of this conformation... 

Films or membranes of silkworm silk have been produced by air-drying aqueous solutions prepared from the concentrated salts, followed by dialysis (11,28). The films, which are water soluble, generally contain silk in the silk I conformation with a significant content of random coil. Many different treatments have been used to modify these films to decrease their water solubiUty by converting silk I to silk II in a process found usehil for enzyme entrapment (28). Silk membranes have also been cast from fibroin solutions and characterized for permeation properties. Oxygen and water vapor transmission rates were dependent on the exposure conditions to methanol to faciUtate the conversion to silk II (29). Thin monolayer films have been formed from solubilized silkworm silk using Langmuir techniques to faciUtate stmctural characterization of the protein (30). ResolubiLized silkworm cocoon silk has been spun into fibers (31), as have recombinant silkworm silks (32). 

The configuration at the chiral centers C-4a, C-5a, and C-12a determine the conformation of the molecule. In order to retain optimum in vitro and in vivo activity, these centers must retain the natural configuration. The hydrophobic part of the molecule from C-5 to C-9 is open to modification ia many ways without losing antibacterial activity. However, modification at C-9 may be critical because steric iateractions or hydrogen bonding with the oxygen atom at C-10 may be detrimental to the activity. 

The traditional definition of a barrier polymer requited an oxygen permeabihty less than 2 nmol /(m-s-GPa) (originally, less than (1 comil)/(100 in. datm)) at room temperature. This definition was based pardy on function and partiy on conforming to the old commercial unit of permeabihty. The old commercial unit of permeabihty was created so that the oxygen permeabihty of Saran Wrap brand plastic film, a trademark of The Dow Chemical Company, would have a numerical value of 1. 

Reactivity. Hemoglobin can exist ia either of two stmctural coaformatioas, corresponding to the oxy (R, relaxed) or deoxy (T, tense) states. The key differences between these two stmctures are that the constrained T state has a much lower oxygen affinity than the R state and the T state has a lower tendency to dissociate into subunits that can be filtered in the kidneys. Therefore, stabilization of the T conformation would be expected to solve both the oxygen affinity and renal excretion problems. 

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