Conformity principles

The replacement of carbon by other elements produces changes in several structural parameters and consequently affects the conformational characteristics of the molecule. In this section, we will first describe some stereochemical features of heterocyclic analogs of cycloalkanes. For the purpose of elaborating conformational principles, the discussion will focus on six-membered rings, so that the properties may be considered in the context of a ring system possessing a limited number of low-energy conformations. 

The equilibria of the diasteromers of 2-Alk-2,4,6-tri-Me-l,3-dioxanes and 2-Ph-2,4,6-tri-Me-l,3-dioxanes were studied by equilibration (in ether at 25°C (78JA2202) (Scheme 12) the more bulky 2-Alk substituent goes into the equatorial position in line with well-known conformational principles [78ACSA(B)769 95H2233]. The configuration and conformation of a multitude of 5-alkyl-, 5,5-dialkyE, 2,2,-di-Me-5-alkyl-, and 2,2,4,5-tetra-Me-5-alkyl-l,3-dioxanes were studied by H NMR spectroscopy (75T489) the... 

Also for 1,2,4-trioxane, from MM3 calculations, a structure close to a chair with the protons and substituents in axial and equatorial positions, respectively, was suggested [92JCS(CC)1689]. The substituted derivatives 63 (Scheme 25) have substituents R [Me, iPr, CH2HgBr, CH(HgBr)Me] in an equatorial position (all in agreement with standard conformational principles), and only in 64-66 were axial methyl substituents reported, based on NOE measurements and 7c-h coupling constants [92JCS(CC)1689]. 

This can be achieved only in favorable cases, e.g., by differences in stcric hindrance at both reactive sites in compound 1 or by annulating the epoxide to a cyclohexane chair with locked conformation (principle of diaxial attack4). 

H-NMR studies on 2,6-dimethylmorpholine314 and N-substituted 2,6-dimethylmorpholines315 are consistent with the single diequatorial conformation 391 for the cis isomers and with the equilibrium 392 393 for the trans isomers. The 13C-NMR spectrum of the monomethyl-, 2,3-, 2,5-, 2-6-, 3-5-dimethyl-, 2,3,4-, 2,3,6-trimethyl-, and 2,3,5,6-tetramethylmorpholines are largely consonant with expectations based on conformational principles derived from cyclohexane conformational analysis.316 The all-cis 2,3,5,6-isomer, which did not prove amenable to analysis by H-NMR spectro-... 

In most reactions of tram-decalins, the conformational principles of simple six-inembered rings can be used, but you may expect lighter control from the greater rigidity. If you wish to design a molecule where you are quite certain of the conformation, a frarrs-decalin is a better bet than even a t-butyl cyclohexane as frans-decalins cannot flip. 

Before discussing the main features of the spectra in terms of stereochemical concepts, let us review some conformational principles in oligosaccharide chemistry, more specifically in disaccharides. It will then be possible to relate the salient points of the spectra directly to these conformational principles. 

Figure 12 shows how glycerol can bind to a (1 - 4)-/S-linked disaccharide via the ring oxygen (0-5 ), thus preventing the involvement of mechanism B it also depicts the impossibility for glycerol to bind to the a counterpart. These statements would also hold true for any values of [Pg.386]

The conformational principles developed earlier appear to control septanose conformations. In both solution and in crystals, TC conformations are preferred that have hydroxymethyl, alkoxy groups, and hydroxyl groups in axial or isoclinal positions [307,317,318,319,320,321,322,323, 324]. The favored TC conformations have oxepane conformations of type TCb or TCc. The equilibria between conformers are often closely balanced as indicated by the significant change in conformers populated when 0-5 of methyl /3-D- /ycero-D-guloseptanoside was methylated [319]. 

Cyclic acetals of ketoses are important and useful compounds. Among other applications, they can be used as intermediates in the synthesis of numerous, useful sugar derivatives and as substrates for studies of conformational principles in fused-ring, heterocyclic A personal contribution by the author. 

The conformational principles which apply to rings other than six-membered ones are rather more sketchy because only six-membered rings adopt well-defined chair (or, for cyclohexenes, half-chair) conformations. But we can still give you some general guidelines and principles, and illustrate them with some important examples. We will look in detail at four- and five-membered rings. 

So far we have been stayed in a quiet harbour named equilibrium armed with old reliable tools of classical thermodynamics. Everything was steady so far. The conformity principle was unshaken, that is, the newly derived equatimis comprised the former well-known relations as specific cases. Hereafter, we are going outwards this peaceful place to non-equilibrium ocean, where these old tools will be of less and less use. 

Simple hydrocarbons represent rather well-behaved extensions of the conformational principles illustrated previously in the analysis of rotational equilibria in ethane and n-butane. The staggered conformations correspond to potential energy minima, the eclipsed conformations to potential energy maxima. Of the staggered conformations, anti forms are more stable than gauche. The magnitudes of the barriers to rotation of many small organic molecules have been measured. Some representative examples are listed in Table 3.3. The experimental techniques used to study rotational isomerism include microwave spectroscopy, electron diffraction, ultrasonic absorption, and infrared spectroscopy. ... 

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