Cis relationship between

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... 

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. 

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. 

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. 

Benzodiazepines [95, 96] and triethylamine in CH2CI2 treated with acetoxyace-tyl chloride or phthalimidoacetyl chloride at 0°C afforded exclusively the p-lactam-fused 1,4-benzodiazepines [97]. In all the cases studied, the reaction provided the final tricyclic systems in very good yields. A high level of diastereoselectivity was achieved and the final products were isolated as single diastereomers, with a cis relationship between the aryl group from the benzodiazepine and the substituent of the ketene. 

C(20), C(14) configuration. Since velbanamine, like quebrachamine, adopts the 16/ -substitution type of conformation shown in [445], the conformational identity of [448] and [449] also shows the cis relationship between 14-H and 16-H, permitting assignment of stereochemistry to pandoline. (268) Other 13C NMR data on pandoline derivatives are available. (269)... 

Three other new 7-hydroxylated aporphines have also been isolated from the tuberous roots of S. venosa they are all identified to be the /V-oxides of identical configuration, namely, (-)-sukhodianine /3-N-oxide (46), (-)-ushinsunine /3-,/V-oxide (47), and (-)-stephadiolamine f3-N-oxide (48). The cis relationship between H-6a and H-7 is indicated from the NMR spectrum, and a partial NMR NOE study clarified the configuration of the N-oxide function (30). 

It is interesting to note that the occurrence of C-7 oxygenated aporphine alkaloids with the C-6a R configuration is limited to the families Annonaceae, Lauraceae, Magnoliaceae, and Menispermaceae. Aporphine alkaloids oxygenated at both C-4 and C-7 have been found in the Annonaceae, but (-)-stephadiolamine /3-A-oxide (48) is the first known alkaloid hydroxyl-ated at both C-4 and C-7 and having a cis relationship between H-6a and H-7. (-)-O-Acetylsukhodianine is the first known example of a naturally occurring 7-acetoxylated aporphine (30). 

Disubstituted /3-sultams can be prepared by treatment with lithium diisopropylamide (LDA) followed by reaction with an alkyl iodide. Alkylation of 3,4-diphenyl /3-sultams proceeds stereoselectively to give products with a cis-relationship between the two aryl groups as the major products (Equation 8) <1998T8941>. 

The unequivocal cis relationship between the diaryl groups of the 3,4-dihydro-2,4-diaryl-2//-l-benzothiopyran 51 was determined from a crystal study. The assignment could not be made from the analysis of the vicinal coupling constants from the H NMR spectra of the cis and trans isomers because of their similarity (Equation 4) <2003T3621>. 

In the ISOC process alkenic silyl nitronates (649) are the reactive intermediates which undergo cycloaddition. This species is generated by reacting the nitroalkene (647) (Scheme 86) or nitroalkyne (651) (Scheme 87) with triethylamine and trimethylchlorosilane in benzene at room temperature for 48 h to give the fused isoxazole systems (648) or (652). A cis relationship between the less bulky C-7 and the hydrogen at C-3a in the products (648) is preferred. The yields of the reactions average... 

Two minor processes sometimes operate competitively with that illustrated in the scheme. One of these involves 1,4-addition of the second vinyl anion to give a reactive intermediate that differs structurally from 1, but is capable of setting into motion a closely related sequence of chemical events leading to an isomeric diquinane.4 This is the route followed to produce the minor product characterized here. The other option consists of cis-t, 2-addition, an event that is followed by a dianionic oxy-Cope rearrangement via a boat-like transition state.4 When sufficient substitution is present to allow the installation of multiple stereogenic centers, the adoption of this pathway is easily distinguished from the electrocyclic alternative since a cis relationship between relevant substituents is in place, instead of the trans arrangement required by the electrocyclization cascade. 

The initial adduct 9 undergoes a series of sigmatropic rearrangements under the reaction conditions to give tetra- and dihydrothiophene 1-oxides 11 and 1289. The cis relationship between the sulfinyl oxygen and the adjacent methyl group in 11 and 12 is a consequence of the stereoelectron-ic requirements of the addition of the intermediate sulfenic acid 10 to the C-C double bond. 

An alternative sequence involves the hydroboration of an alkene (Scheme 2.10c). This reaction has the overall effect of producing the anti-Markownikoff hydration of the double bond. A further characteristic of this pathway is that there is a cis relationship between the added hydrogen derived from the borane and the hydroxyl group. 

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