Methylated model studies

J. Rebek, Jr., (1987) first developed a new synthesis of Kemp s acid and then extensively explored its application in model studies. The synthesis involves the straightforward hydrogenation (A. Steitz, 1968), esterification and methylation of inexpensive 1,3,5-benzenetricar-boxylic acid (trimesic acid 30/100 g). The methylation of the trimethyl ester with dimethyl sulfate, mediated by lithium diisopropylamide (V. J. Shiner, 1981), produced mainly the desired aff-cis-1,3,5-trimethyl isomer, which was saponified to give Kemp s acid. 

In total synthesis, model studies are frequently performed on simpler systems prior to the final assault on the target molecule. In the synthesis of zaragozic acid A (1), 2-methyl-1,3-dithiane (92) was employed as a simple model for the more elaborate dithiane 67. Deprotonation of 92 with n-butyllithium under standard conditions47 and addition of the aldehyde provides a mixture of two diastereoisomers, 93 and 94 (Scheme 22), in approximately equal amounts. One of the diastereoisomers (93) lacks the TMS group,... 

As a reasonable biogenetie pathway for the enzymatic conversion of the polyunsaturated fatty acid 3 into the bicyclic peroxide 4, the free radical mechanism in Equation 3 was postulated 9). That such a free radical process is a viable mechanism has been indicated by model studies in which prostaglandin-like products were obtained from the autoxidation of methyl linolenate 10> and from the treatment of unsaturated lipid hydroperoxides with free radical initiators U). 

The synthesis of triquinane acids, initiated by the preparation of isocomenic acid [22], thus provided a general method for control of the stereochemistry of secondary methyl groups in these terpenes. The [4+1] annulation based on the dienes of type 23 then laid the groundwork for the first-generation design and a model study for the approach to retigeranic acid [23]. 

Molecular model studies confirm the possibility of the PPG chain to penetrate the [S-CD cavity, but not the a-CD cavity owing to the steric hindrance of the methyl groups on the chain. They also indicate that the [S-CD cavity can accommodate two propylene glycol units. 

Molecular modeling studies relative to both preinsertion intermediates and insertion states indicate that for all the metallocenes from 1 to 39 of Scheme 1.2 (independent of their structure and symmetry), when a substantial stereoselectivity is calculated for primary monomer insertion, this is mainly due to nonbonded energy interactions of the methyl group of the chirally coordinated monomer with the chirally oriented growing chain. 

Proton NMR studies of the methylated model porphyrin compound yielded the following results. When dissolved in the non polar... 

Mischnick, R, Kuhn, G. (1996). Model studies on methyl amyloses correlation between reaction conditions and primary structure. Carbohydr. Res., 290, 199-207. 

Several model studies on the separation of monosaccharides as their methyl glycosides have been published. These include three related papers, by Hough and coworkers,88,364,365 concerned with the determination of D-galactose, D-mannose, and L-fucose, and applied to membrane glycoproteins.366 Their method was also used in examining... 

Crystalline 2-methylimidazole exhibits different 13C (CPMAS) chemical shifts for C-4 and C-5 (125.0, 115.7 ppm). The average (120.3 ppm) is close to that reported for imidazole in deuterated DMSO (121.2 ppm). These results imply that solid state chemical shifts can be used instead of N-methyl models in tautomerism studies (87H(26)333). For imidazole the solid state l3C shifts are 137.6 (C-2), 129.3 (C-4), and 119.7 (C-5) (81JA6011). No proton exchange occurs in the solid, and the data support a structure resembling the crystal structure. Cooling imidazole solutions has not yet allowed the detection of individual tautomers, but by symmetry the compound exists in equal tautomeric forms, as does pyrazole (81CC1207). 

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