Cis relationship

The methyl groups are described as cis because both are up relative to the hydrogen present at each carbon If both methyl groups were down they would still be cis to each other Notice that ring flipping does not alter the cis relationship between the methyl groups Nor does it alter their up versus down quality substituents that are up m one conformation remain up m the ring flipped form... 

The relative configurations of vicinal protons follow from the characteristic values of their coupling constants. Thus 16.1 Hz confirms the trans relationship of the protons on C-8 and C-9, 10.8 Hz confirms the cis relationship of the protons on C-6 and C-1. The 2.0 Hz coupling is common to the oxirane protons at = 3.00 and i.27 this value fixes the trans relationship of the protons at C-4 and C-5 following a comparison with the corresponding coupling in the methyloxirane (2.6 Hz). The anti relationship of the protons A-H and h-H can be recognised from their 8.7 Hz coup-... 

The relative configuration is derived from the NOE difference spectra. Significant NOEs are detected owing to cis relationships within the neighbourhood of non-geminal protons ... 

If mutarotation were not a factor, the threo pair would give the isomer having the phenyl group and nitrogen atom in a cis relationship and the erythro pair the isomer with the phenyl and carbonyl groups cis. Since it is doubtful, at least in some cases, that steric integrity is maintained in acetic anhydride prior to elimination, the subsequent... 

A Assume that you have a variety of cvclohexanes substituted in the positions indicated. Identify the substituents as either axial or equatorial. For example, a 1,2-cis relationship means that one substituent must be axial and one equatorial, whereas a 1,2-trans relationship means that both substituents are axial or both are equatorial. 

An important stage in the synthesis has been reached. It was anticipated that cleavage of the trimethylsilyl enol ether in 18 using the procedure of Binkley and Heathcock18 would regiospecifically furnish the thermodynamic (more substituted) cyclopentanone enolate, a nucleophilic species that could then be alkylated with iodo-diyne 17. To secure what is to become the trans CD ring junction of the steroid nucleus, the diastereoisomer in which the vinyl and methyl substituents have a cis relationship must be formed. In the... 

In contrast, the fluoride ion induced cyclization proceeds via an anticlinal approach which minimizes steric congestion between the allylsilane and the substituent in the 3-position of the cyclohexenone50. Thus, the product having a cis relationship between the vinyl group and R2 at the ring junction is formed preferentially 48 " 51. The controlling element is the substituent R2. In its absence (R2 — H) the diastereoselectivity disappears (see Table 1). 

Thus the trans relationship between the hydroxymethyl group and the C-l hydroxy group in a-D-glucopyranose, and the cis relationship between the methyl group and the C-l hydroxy group in P-L-fucopyranose, are clearly shown. Note that representation of ketoses may require a different modification of the Fischer projection, as shown in the fructofuranose example above. Here C-2 is rotated about the bond with C-3 to accommodate the long bond to C-2 from the oxygen at C-5. 

Deoxy-L-galactose (L-fucose) is common, and has only been found as the a- or )3-pyranoside. The rare D-fucose has, however, been found both as a-pyranoside, in the LPS frorn Pseudomonas cepacia serotypes B and E, and as a-furanoside, in the cell-wall antigen from Eubacterium saburreum L 452 and the O-antigens from different strains of Psuedomonas syrin-gae The a-furanoside, as in 3, has a cis relationship between the aglycon and OH-2. The corresponding P form has not yet been found. 6-Deoxy-o-and -L-talose are components of the extracellular polysaccharides from some strains of Butyrivibrio fibrisolvens and of the LPS from some strains of E. coli respectively. 

This procedure illustrates a fundamentally new method for constructing substituted tetrahydrofurans.5-10 This practical method assembles the tetrahydrofuran ring from allylic diol and carbonyl components and in the process forms three ring bonds C(2)-C(3), C(4)-C(5) and 0-C(5). Both aldehydes (eq 1) and ketones (illustrated in the present procedure) can be employed as the carbonyl component. Although it is often convenient to isolate the acetal intermediate, conversion to the 3-acyltstrahydrofuran can also be accomplished in many cases by the direct reaction of the diol and carbonyl components.8 High ds stereoselectivity (at least 20 1) is observed in the preparation of tetrahydrofurans that contain single side chains at carbons 2 and 5 (eq 1). The kinetically controlled product also has the cis relationship of these side chains and the 3-acyl substituent. 

The 10,11-epoxide of chlorovulone I (107) was also obtained in low yield (0.05%) from the hexane extract of C. viridis [124], Its structure was assembled from spectroscopic data which showed a high degree of similarity to that obtained for chlorovulone I (100) except for UV and 1H NMR features due to the Cl0,11-olefin. Confirmation of structure came from synthesis of 107 by epoxidation of chlorovulone I. Epoxy-chlorovulone I (107) was found to possess the same 12R hydroxyl stereochemistry. The cis relationship of the epoxide and hydroxyl group was indicated by an intramolecular hydrogen bond as revealed by characteristic IR absorptions at high dilution. This 10,11-epoxide derivative... 

Although styrene is not a 1,3-diene, the cathodic reduction of a solution containing styrene and an ester with magnesium electrode interestingly affords a single stereoisomer of 2-phenylcyclopropanol derivative in which the phenyl and the alkyl (R2) groups are stereoselectively located in a cis relationship on the cyclopropane ring (equation 25). 

A difference in conformation of molecules in the ervafolene and ervafolidine series emerged from X-ray analysis. In ervafolene (246), where C-3 is linked to N1, a cis relationship between N4 lone pair and the bond between C-14 and C-17 was observed. Also the lone pair of N4 was cis to the ethyl chain at C-20. In 3-epiervafolidine and (19 R)-19 -hydroxyervafolidine, the molecule adopted an identical conformation with the lone pair of N4 and N4, trans with respect to the C-14—C-17 bond and the ethyl chain, respectively. The absolute configurations depicted in formulas 242-249 were deduced from relative configurations and based on the assumption that the ibogan half of the molecule was derived from 20-epipandoline (165) whose absolute configuration is known. 

Complete O-deprotection of the hexofuran-3-yl isothiocyanates led to three types of compounds, depending on the stereochemical relationship of the isothiocyanato group and the reactive OH. Deprotection of d-alio and d-galacto derivatives is a good illustration of the influence of a cz s-relationship between those two groups on the geometry of the molecule. The 2,3-cis relationship in d-alio series induced the formation of a 2,3-OZT imposing a... 

Rothwell and colleagues352 studied the titanium mediated [2 + 2 + 2] cycloaddition of alkenes with monoynes and diynes. Among the reactions studied, the reaction between styrene (29) and diyne 609 in the presence of titanium catalyst 610 proved cleanest (equation 175). The reaction yielded 614 via a [2 + 2 + 2] cycloaddition followed by a titanium mediated suprafacial [1,5] H-shift involving 611-613. The cis relationship between the trimethylsilyl group and the phenyl group indicated that the initially formed titananorbornene 611 had an endo stereochemistry. 

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