Acyclic systems

By means of 250 MHz H n.m.r. spectra, the solution conformations of stereoisomers of peracetylated aldohexose dimethyl acetals and diethyl acetals have been determined. With the exceptions of the T -galacto- and D-man/io-isomers, which were extended planar, all isomers were in the sickle conformation. The results were interpreted on the basis of avoidance of parallel 1,3-substituent interactions. 

Fourier Transform n.m.r. at 90 MHz has been used to probe the conformations o per-acetylated L-arabinononitrile, D-mannononitrile, D-glucononitrile, D-galactononitrile, 6-deoxy-L-galactononitrile, 2-deoxy-D-amhino-hexononitrile, and 2-deoxy-D-/yxo-hexononitrile. Zig-zag conformations were adopted for all except D-glucononitrile, which exists in the sickle form. Conformational studies on the four 1-amino-l-deoxy-D-pentitols and their hydrochlorides by 250 MHz H n.m.r. in D2O have shown that the D-arabino- and the D-lyxo-isomers adopt the extended zig-zag conformation, whereas the D-xylo- and D-nho-isomers, which have unfavourable 1,3-interactions, adopt the non-planar sickle conformation the results are similar to those obtained by J f-ray crystallo- 

Samek and M. Budesinsky, Collect. Czech. Chem. Commun., 1979, 44, 558. 

The difluorophosphoranes (18),17 (19),18 and (20)18 have been obtained using xenon difluoride as shown. The most stable conformations of the trifluorophosphoranes (21)19 and (22)20 have apical fluorines pseudorotation was slow on the n.m.r. time-scale below — 60 °C (21) and — 40 °C (22). The intermolecular exchanges of fluorines 

The trifluoromethylphosphoranes (CF3)2PMe3 and (CF3)3PMe2 have been obtained as stable, unreactive, white solids from the reactions of tetramethyl-lead with the corresponding chlorophosphoranes.23 Pseudorotation of (CF3)3PMe2 is slow on the n.m.r. time-scale at 100 °C, indicating a very considerable difference in apicophilicity between methyl and trifluoromethyl groups. Among other acyclic phosphoranes prepared are (23),24 (24),25 and (25).26 

PhP(OPh)4 Ki = (3 5)xl0-10 and Kz x. 10 3 mol l-1. In general, the more oxygens attached to phosphorus the less the dissociation of the phenoxyphosphorane. The rate constants for the dissociation of a number of the phenoxyphosphoranes (26) were obtained from a study of thevariable-temperature n.m.r. of equimolar mixtures of the phosphoranes and the related phosphonium triflates RwP(0Ph)4-nCF3S03-.28 Together with the equilibrium constants for these dissociations, these led to the conclusion that the reactions of the cations Rw (OPh)4- with phenoxide ion in acetonitrile proceed with the speed of collision. 

The penta-alkyl derivatives of Group V elements have been reviewed. The synthesis of pentaethoxyphosphorane from triethyl phosphite and ethyl benzenesulphenate has been extended to the phosphoranes (RO)sP, where R = Me, PrS PhCHg, MegCCHg, cyclo-CgHg, or cyclo-CgHn.i  

Pseudorotation of the tetra-2,6-dimethylphenoxyphosphorane (25) is slow on the n.m.r. time-scale below — 38 °C, presumably because of steric crowding in the square-pyramidal intermediate. The methyl protons of (25) are rapidly exchanged for deuterium in CDClg, probably via the ylide (26). 

Aminotetrafluorophosphorane has been prepared by amination of the corresponding chloro-compound in the vapour state. Rotation round the PN bond is slow on the n.m.r. timescale at 30 °C and analysis of the and F n.m.r. spectra of the isomer shows that in the ground state the hydrogens and apical fluorines are coplanar, as in (2), with strong intramolecular hydrogen-bonding. 

Full details have appeared of the preparation of aryloxyfluoro-phosphoranes (3) according to the general equation 

The methoxyphosphorane (5 R = Me) is in rapid equilibrium with the ylide and methanol in non-polar solvents at room temperature, but with the phenoxyphosphorane (5 R = Ph) this equilibration is slow on the n.m.r. timescale under the same conditions. Methylmethoxytriphenyl-phosphorane is covalent in the crystalline state, but its solutions are tinged with the yellow of the ylide. 

Schmidbauer, K.-H. Mitschke, and J. Weidlein, Angew. Chem. Internat. Edit., 1972, 

The equilibrium between phosphonium methoxide and methoxyphos-phorane has been observed in some cases by n.m.r. Thus the chemical shift of a solution of the salt (9) in methanol changes from + 14.5 to + 91.7 p.p.m. as the methoxide ion content is increased to 3 molar 

A comparative study of MM2 and MM3 force fields for all 16 diastereomeric peracetylated 1-deoxy-l-nitroheptitols and peracetylated 2-deoxyaldooctoses has 

The differentiation of threo- and erythro-isomers of 1-substituted glycerol obtained by thermodynamically controlled isopropylidenation has been achieved by observation of values for the acetal methyl groups in H n.m.r. spectra for terminal isopropylidene is 0.05 p.p.m., since only one methyl group is shielded by the chairs, whereas for the at-threo isopropylidene group aS 4. 0.05 The values of aS were obtained for twenty-eight alditol isopropylidene derivatives. Observed coupling constants for protons in tetritol and 

Kuroda, and K. Osaki, Bull. Chem. Soc. Japan, 1977, 50, 3079. 

Accurate n.m.r. parameters for four acetylated thiazolidines [e.g. (165)] derived from L-arabinose and L-cysteine have been obtained by computer-assisted analysis of the spectra. The polyacetoxyalkyl side-chains of (165 R = Me, R = Ac) and its (iS)-diastereoisomer adopt a planar zig-zag conformation, which tends to become somewhat distorted in certain derivatives [e.g. 3-acetyl-(J )-(L-fl A flZ mo-2,3,4-triacetoxy-1 -hydroxybutyI)thiazolidine-4(R)- 

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The grid. search technique can easily be applied to acyclic systems. Ring sy.slems can be treated as pseudo-acyclic by cutting one ring bond. The major drawback of this technique is that, because of the restricted number of degrees of freedom in... 

Careful conformational analysis of acyclic systems is needed. Hornoallylie Systems... 

When the aldehyde group is directly attached to a carbon atom of a ring system, the suffix -carbaldehyde is added to the name of the ring system, e.g., 2-naphthalenecarbaldehyde. When the aldehyde group is separated from the ring by a chain of carbon atoms, the compound is named (1) as a derivative of the acyclic system or (2) by conjunctive nomenclature, for example, (1) (2-naphthyl)propionaldehyde or (2) 2-naphthalenepropionaldehyde. 

Nucleophilic attack on ring atoms of large heterocycles is largely confined to saturated systems, saturated parts of partially unsaturated systems, and to carbonyl functions and the like. These reactions are not fundamentally different from those of corresponding acyclic systems, except for transannular reactions. 

The anomeric effect is also present in acyclic systems and stabilizes conformations that allow antiperiplanar (ap) alignment of the C—X bond with a lone-pair orbital of the heteroatom. Anomeric effects are prominent in determining the conformation of acetals and a-alkoxyamines, as well as a-haloethers. MO calculations (4-3IG) have found 4kcal/mol as the difference between the two conformations shown below for methoxy-methyl chloride. ... 

Although there is usually a preference for anti elimination in acyclic systems, syn elimination is competitive in some cases. In acyclic systems, the extent of anti versus syn elimination can be determined by use of stereospecifically deuterated substrates or by use of diastereomeric reactants which will give different products by syn and anti elimination. The latter approach showed that elimination from 3-phenyl-2-butyl tosylate is a stereospecific anti process. ... 

It has generally been assumed that phosphorous oxychloride-pyridine dehydrations, the elimination of sulfonates, and other base catalyzed eliminations (see below) proceed by an E2 mechanism (see e.g. ref. 214, 215, 216). Concerted base catalyzed eliminations in acyclic systems follow the Saytzelf orientation rule i.e., proceed toward the most substituted carbon), as do eliminations (see ref 214). However, the best geometrical arrangement of the four centers involved in 2 eliminations is anti-coplanar and in the cyclohexane system only the tran -diaxial situation provides this. 

Another way of removing the six translational and rotational degrees of freedom is to use a set of internal coordinates. For a simple acyclic system these may be chosen as 3N — I distances, 3N — 2 angles and 3N -3 torsional angles, as illustrated in the construction of Z-matrices in Appendix E. In internal coordinates the six translational and rotational modes are automatically removed (since only 3N — 6 coordinates are defined), and the NR step can be formed straightforwardly. For cyclic systems a choice of 3A — 6 internal variables which span the whole optimization space may be somewhat more problematic, especially if symmetry is present. 

For acyclic systems, the anti diastereoselectivity of the (i )-enolates is lower than the syn diastereoselectivity of comparable (Z)-enolates. For example, carboxylic acid esters, which form predominantly ( )-enolates, react with aldehydes with high anti selectivity only in those cases where bulky aromatic substituents are in the alcoholic part of the ester22 25. 

Direct alkylation of allylic alcohols via the (allyloxy)phosphonium ion intermediate normally proceeds with anti-y selectivity for the Cyclic system, and sy/i-y selectivity for the acyclic system (see Table l)35 36. 

Localized (n-type) Carbanions 1.5.2.1.1.1. Stereogenic Centers at the y-Position (Acyclic Systems)... 

The intramolecular Michael addition of acyclic systems is often hampered by competing reactions, i.e., aldol condensations. With the proper choice of Michael donor and acceptor, the intramolecular addition provides a route to tram-substituted cyclopentanones, and cyclopentane and cyclohexane derivatives. Representative examples are the cyclizations of /3-oxo ester substituted enones and a,/J-unsaturated esters. 

In all of the above, the activating, directive and stabilizing destabilizing effects are similar in principle to those in the acyclic systems. However, the magnitude of these effects per se, or in conjunction with other characteristics of the systems in point, are considerably different and, consequently, the ultimate chemical results may be different. 

In principle, the properties and chemical behavior of cyclic sulfoxides and sulfones having a ring size of seven and up are expected to be quite similar to those of the analogous acyclic systems. 

In 1982 the present author discovered cyclic orbital interactions in acyclic conjugation, and showed that the orbital phase continuity controls acyclic systems as well as the cyclic systems [23]. The orbital phase theory has thus far expanded and is still expanding the scope of its applications. Among some typical examples are included relative stabilities of cross vs linear polyenes and conjugated diradicals in the singlet and triplet states, spin preference of diradicals, regioselectivities, conformational stabilities, acute coordination angle in metal complexes, and so on. 

Phosphorylation of enolate ions by dialkyl or diaryl phosphorochlori-dates gives exclusive O-phosphorylation and it appears that the product geometry in acyclic systems is determined by the polarity of the solvent and... 

Several trends have emerged in the extensive carbon-13 NMR spectroscopy data that have been accumulated for sulfones and sulfoxides. Based on many studies of cyclic systems—particularly five- and six-membered ring sulfur compounds—these trends were shown to generally apply equally to both the cyclic and acyclic systems . Thus (a) oxidation of a sulfide to a sulfone results in a 20-25 ppm downfield chemical shift for sp -hybridized a-carbon atoms and 4-9 ppm upfield shift for / -carbons , and (b) there is very little difference between the chemical shifts of a-carbon atoms of sulfones and sulfoxides despite the difference in the inductive effects of these two functional groups . A difference is observed, however, in the H chemical shift of related cyclic sulfoxides and sulfones . 

Oxygen-17 NMR spectroscopy has an immense potential for structural analysis of cyclic sulfoxides and sulfones as well as for providing insight into the nature of bonding within these two functional groups . Indeed, in addition to data concerning the NMR chemical shifts for several cyclic sulfoxides and sulfones, NMR chemical shift differences between several diastereotopic sulfonyl oxygens in both cyclic and acyclic systems have been reported . 

In acyclic systems, the enolate conformation comes into play. p,(3-Disubstituted enolates prefer a conformation with the hydrogen eclipsed with the enolate double bond. In unfunctionalized enolates, alkylation usually takes place anti to the larger substituent, but with very modest stereoselectivity. 

In acyclic systems, the stereochemistry of alkylation depends on steric factors. Stereoselectivity is low for small substituents.71... 

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