Conformational analysis envelope

The isoxazolidine ring exists primarily as an envelope (77AHQ2l)207) and the nitrogen lone pair can occupy an axial or equatorial position. Photoelectronic spectroscopy is a useful tool to determine conformational analysis of molecules possessing vicinal electron lone-pairs. Rademiacher and Frickmann (78TL841) studied isoxazolidine and 2-methyl- and 2-t-butyl-isoxazolidine and found mixtures of equatorial and axial (e/a) compounds. The ratios of H, Me and Bu in the efa position were 1 3, 4 1 and 10 1, respectively. 

Conformational analysis of 1,3-thiazolidines and other thiazole derivatives has been carried out by NMR methods (see Section 3.06.3.6.1). Thus, the analysis of the coupling constants of the H NMR spectra <74JA1465> permitted to postulate envelopes with either C-4 or C-5 as the flap atom and with the 2-substituent anti to the flap as the predominant conformations (Equation (2)). 

Fig. 9.12. Scattergram of the scores on the first two factors from a principal component analysis of the torsion angles of 181 furanose rings. The two factors include essentially 100% of the variance of the ring torsion angles. The numbers show the position of some selected ring conformations. 1 envelope (e) with mirror plane through C(2 ) endo 2 e C(3 ) endo 3 e C(3 ) exo 4 e C(2 ) exo 5 twist with Ci axis through C(T) 6 e 0(T) endo...

The lactarane sesquiterpenes 10.2, 10.10, 10.13, containing the 2(9),7(8)-cycloheptadiene ring were submitted to conformational analysis by molecular mechanics and HNMR spectroscopy (9). It was observed that the conformational mobility of each compound is practically restricted to the interconversion of the envelope forms of the cyclopentene ring by contrast, essentially only a single conformation of the seven membered ring, with the 3-methyl group pseudo-equatorially oriented, is populated. This is due to the planarity of either the diene in 10.13 or the diene-carbonyl double bonds in 10.2, 10.10. 

Aspects of cycloalkane conformational analysis. A. The two puckered conformations of cyclobutane. Note how the substituents (X and Y) exchange positions when puckered forms interconvert. B. Left The envelope form of cyclopentane—the conformer contains a mirror plane in the plane of the page. Right The half-chair form of cyclopentane, looking down the two-fold rotation axis. 

Dioxolane also pseudorotates essentially freely in the vapor phase. 2,2 -Bi-l,3-dioxolane (128) has been shown by X-ray crystallography to have a conformation midway between the half-chair and envelope forms. The related compound 2-oxo-l 3-dioxolane (129) shows a half-chair conformation. This result is confirmed by microwave spectroscopy and by NMR data. Analysis of the AA BB NMR spectra of the ring hydrogen atoms in some 1,3-dioxolane lerivatives is in agreement with a puckered ring. Some 2-alkoxy-l,3-dioxolanes (130) display anti and gauche forms about the exocyclic C(2)—O bond. 

Figure 4 The energy landscape of the pnon protein (Pi P) (residues 124-226) in vacuum, obtained by principal coordinate analysis followed by the minimal energy envelope procedure. Two large basins are seen. One basin is associated with the native Pi P conformation the other is associated with partially unfolded conformations.

The most elaborate use of MM calculations in the LIS analysis was described by DeTar and Luthra (298). Their approach was based on the traditional relative shift method, wherein lanthanide parameters are adjusted to give optimum agreement with the observed relative LIS values. Based on their previous analysis of proline conformations (228), they determined that A-acetylproline methyl ester (71) exists in CDQ3 as a 60 40 mixture of half-chair and envelope conformations by simultaneously adjusting the substrate geometries and the conformer mole fractions, in addition to the lanthanide parameters (298). 

Deficiencies in intensities, which occur in x-ray powder dififiaction as well as in single crystal electron diffiaction, may cause problems even in early stages of ab initio structure analysis. Nevertheless, examples for successful use of the tangent formula or Sayre equation for structure determination from ED data have been worked out [14]. Other direct methods, like maximum entropy can provide us with an envelope of the molecules in the cell, which delivers an idea of its orientation [15]. An alternative approach to ab initio structure determination is the calculation of the gas phase conformation of an initial model for subsequent refinement by energy minimization [16]. 

A number of l,3-dioxan-2-ylium ions have been studied with respect to the preferred conformation. From a detailed H and C NMR study (790MR616) and the X-ray crystal structural analysis (94CJC2084), it was concluded that the 1,3-dioxan-2-ylium ions adopt the envelope conformation with the C —O —C —O—moiety in one plane (due to mesomerism) and C —C —forming the flap (Scheme 16). Ring interconversion 19a 19b and a 1 1 conformational equilibrium was assumed. 

Tetrafluoro-1,3,2-dithiazoldine (7) has an envelope conformation in the gas phase (electron diffraction study) and in the crystal (x-ray structural analysis) <93JPC9625). The S atom is located in the flap in the first case S and N play the part alternatively in the second case, both conformational varieties comprising the unit cell while being interconnected by N H—N bonds. A quantum chemical calculation was made to rationalize these structural features (Section 4.12.2). 

The FOZ 310, derived from the corresponding hydrocarbon bearing a C=C link, is unusually stable (m.p. 159-161 °C) and resistant to reduction by NaBHt. Its strucmre was elucidated by XRD analysis, showing the same O envelope conformation of 288,... 

The ozonide of 2,3-diphenyl-2-bomene can be prepared by photosensitized addition of oxygen to the corresponding epoxide (313), as shown in equation 103. The structure of product 314 was determined by single-crystal XRD crystallographic analysis. It should be pointed out that both 302 and 314 have Oe envelope conformation for the 1,2,4-trioxolane... 

The X-ray crystal structure of fZ)-4-[(5)-2,2-dimethyl-l,3-dioxolan -ylmethyl-ene]-2-phenyl-5(4//)-oxazolone has been determined. " The analysis shows an almost planar disposition for the entire molecule with the exception of the dioxolane ring that adopts an envelope conformation. As such, the dioxolane ring is mainly situated on the si,si diastereotopic face of the olefinic bond, a situation that accounts for the observed diastereoselectivity in Diels-Alder reactions. 

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