Flattened chair

The tetraoxahydrospirophosphorane (57) has been isolated in 66% yield from the reaction of (55) with triethylammonium perfluor-opinacolate (56). Hexafluoroacetone inserts into the P-H bond of (57) to form (58) which may also be obtained from (59) as shown1 1. The 1H and 19F n.m.r. spectra of the phosphoranes reveal rapid pseudorotational processes and a time-averaged conformation of a flattened chair for the six-membered rings. 

The structure of the anion [Be3(OH)3(malonate)3]3 (95) is illustrated in Fig. 18. The Be3(OH)3 ring has a flattened chair conformation with no crystallographic symmetry. Nevertheless, the arrangement of oxygen atoms around each beryllium is near tetrahedral. 

For an ideal chair with = 60°, R = 2 (Table VI). Deviations from R = 2 are taken as indications of departure from ideal chair conformation. For R > 2.5, an appreciably puckered-chair conformation is indicated and for R < 1.8, a flattened-chair.70,71... 

V,/V-Dinitrosopiperazine (409) exists in CH2C12 solution at 40°C as a mixture of the syn and anti forms in which the anti form predominates. The R value (1.5) of the anti form indicates its adoption of a boat form or a very flattened chair.333... 

When the starting material is not a chair but a flattened chair, the first priority is to make a proper chair for the product. Strangely this means axial attack by nucleophiles on such electrophiles as... 

The tubulin-bound conformation of DDM in solution has been determined from tr-NOE data [112], Sample conditions were similar to those used previously to determine the bioactive conformation of EpoA. Distance restraints were obtained from a series of tr-NOE spectra recorded at several mixing times and were used in the structure calculation based on the complete relaxation matrix methodology [37], The NMR-derived bioactive conformation is quite similar to the crystal structure except for the conformation of the 8 lactone ring, that is close to a flattened chair in solution but a twisted boat in the crystal (Fig. 18). 

The restricted rotation about the exocyclic N-R bond in N-nitroso and N-formyl-cA-2,6-diphenylpiperidines was studied (cf. Scheme 38 and Table XIII) (91JOC4833, 99IJC(B)325). Flattened boat conformers with the phenyl substituents in quasi-<2x/<2/ positions (100a,b) (99IJC(B)325) and flattened chair conformers with axial phenyl substituents (101 a,b) (95JOC7411) were observed as preferred conformations. The energy minima... 

To get at the explanation we need to look at the conformation of the enamine intermediate. At this point we shall generalize a bit more and write a structure that represents any enol derivative where X may be OH, CT, OSiMe3) NR2> and so on. The conformation has a double bond in the ring, and is a partially flattened chair, as described in Chapter 18. 

This is also axial addition to form a chair directly (rather than a twist boat) with the nucleophile approaching from the bottom. We must draw the ring as a flattened chair. 

L-shape of the steroid skeleton. The geometry actually found is a flattened chair for ring C and a nearly planar skeleton ... 

Of the factors concluded from calculations to be important in determining the magnitudes of barriers to inversion of six-membered rings, only that of 1,2 interactions is clearly illustrated by appropriately chosen experimental values. Several compounds with flattened chair conformations due to widely different causes invariably seem to have low barriers to inversion. Several other effects relevant in certain situations can be demonstrated from results for suitable sets of compounds. If a prediction of a barrier be sought, it is my opinion that a better value will be obtained by examining barriers experimentally determined in as similar group of compounds as is available, than by consideration of calculations. 

The skeletal geometry of cyclohexene and 1,2-epoxy cyclohexane systems corresponds to a partially flattened chair or half-chair (8). The alternative "half-boat conformation has... 

The lactone ring in 17j8-hydroxy-17o -methyl-2-oxa-5a-androstan-3-one (1) has a flattened chair-like conformation, with a torsion angle of 11—12° about the 2,3-bond. Clearly this structure represents an energetic compromise between the normal preferences for a chair ring and a coplanar ester group, and may have some... 

A series of 1,2,6-triphenylphosphorinan-4-ones has been prepared and an analysis of the h-, C-, and P-nmr spectral data of all these compounds indicates that a flattened chair is the major conformation in each case (J.B. Rampel et at, J. org. Chem., 1981, 46, 1156). Similar analysis has been carried out on a series of substituted phosphorinanones and all the compounds appeared to be chair forms in solution as indicated by the chemical shifts and coupling constants (idem, ibid., p.1166). 

The joint effects of both a 4,6-O-benzylidene and a 1,2-O-benzyl-idene ring were examined by Coxon.96 As the coupling constants J2,3 and J3A of the dibenzylidene derivatives are, in all cases, significantly larger than those of the 1,2-O-alkylidene- and -arylidene-a-D-glucopy-ranoses, there is evidently less conformational distortion of the diacetal than of the monoacetal. The pyranoid part was assigned a flattened-chair form, and the 1,3-dioxane ring, a chair form. 

A flattened CC conformation has been assigned for the 3,7-diazabicyclo[3.3.1]nonan-9-ones 156-159 in (CD3)2SO and in CDC13 on the basis of H-and 13C-NMR, IR, and Raman spectral data (111,114). H-and 13C-NMR studies of compounds 156 and 160-162 in CDC13 solution have confirmed for 156 and shown for 160-162 a flattened chair-chair conformation with the N-substituents in the equatorial position. Additionally, an increase in distortion of the A-alkylpiperidine ring was observed as the size of the JV-substituent was increased (115). 

An interesting result was obtained for compound 255 bearing an endo-1 hydroxy group. In the solid state, it adopts an envelope-flattened chair conformation (from X-ray data) (194). 

The bis-(3,5-dimethylpyrazolyl)borane dimer, [(CsN2H7)2BH]2 contains a ring of the type (49), in a flattened-chair conformation (dihedral angle of 12.7 between the NBN plane and the central plane).Steric effects of the bulky terminal pyrazolyl units force this structure to be adopted, by contrast to the boat conformation of pyrazolyldideuteriogallane. 

Substituents in 2,4-position generaly prefer equatorial orientations in a chair confonnation of the six-membered ring. Bis-axial orientation of these substituents would lead to strong repulsion, but in some examples an axial-equatorial confonnation is observed, as will be disscussed below. The X-ray analysis of the 2,4-diaryl-3,7-dimethyl-l,5-dimethoxycarbonylbispidin-9-one (3) reveals a flattened chair-chair conformation with the aryl substituents, as expected, in the equatorial position. From comparison of the IR-, H-, and NMR spectra with those of the analoguous p-methoxyphenyl- and p-chlorophenyl-substituted derivatives, it follows that these all adopt a double-chair conformation (56). The same behavior was also found for the bispidones 14 and 35 (70) 46-48 and 104 (see Chart 16) (160). The PM3 calculations for various bispidones with aromatic substituents have shown that the cc confonnation is slightly more stable than the cb form, and this is much more stable than the bb conformation in all cases (161). 

The structure of the 2,4,4-tris(trifluoromethyl)-4//-l,3,5-oxadiazine formed by condensation of 0,0-diethyl-l-(2-cyanoguanidino)-l-methylpropyl phosphonate with 2-(acylimino)hexafluoro-propanone has been assigned unequivocally on the basis of an x-ray crystal structure of the trifluoroacetyl derivative (27) <88DOK(302)107>. A crystal structure of the related diazaphospholidene derivative (28) reveals that the 1,3,5-oxadiazine ring adopts a markedly flattened chair conformation with O, and C4 at 3.8° and —4.5°, respectively, out of the plane formed by C2—N3—N5—Ce... 

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