Selective 0-methylation

The yield of the more active RRR-a-tocopherol can be improved by selective methylation of the other tocopherol isomers or by hydrogenation of a-tocotrienol (25,26). Methylation can be accompHshed by several processes, such as simultaneous halo alkylation and reduction with an aldehyde and a hydrogen haUde in the presence of staimous chloride (27), amino alkylation with ammonia or amines and an aldehyde such as paraformaldehyde followed by catalytic reduction (28), or via formylation with formaldehyde followed by catalytic reduction (29). 

As seen in Figure 1, the organo sulfur compounds are methylated at the boiling point (90°C) of dimethyl carbonate, whereas methylation (or alkylation with other alkyl groups) of other functional groups requites higher temperatures. This has resulted in the selective methylation of sulfhydryl groups of compounds that contain other substituents that can be alkylated. The other substituents can then be alkylated at elevated temperatures (63). 

Chromone, 2,3-dichloro-7-methoxy-synthesis, 3, 825 Chromone, dihydromass spectra, 2, 23 Chromone, 3,5-dihydroxy-2-methyl-selective methylation, 3, 716 Chromone, 5,7-dihydroxy-2-methyl-prenylation, 3, 716 Chromone, 3,7-dimethoxy-2-methyl-reactions... 

Oxidation of /U-cresol afforded a triphenol 22 which is approximately half the molecule. The central hydroxyl of the triphenol could be selectively methylated and then the compound was ort/ro-brominated and bridged using 1,3-dibromopropane to give 23. Metallation with butyllithium followed by iron catalyzed coupling afforded the macrocycle as indicated. 

Selective methylation of catechol catalyst development and characterisation... 

An alternative approach (Matveev et al., 1995), which avoids the selectivity problem mentioned above, is shown in Fig. 2.19. In this route, 1-naphthol is selectively methylated at the 2-position using methanol over a solid catalyst in the vapour phase. The product undergoes selective oxidation to menadione with O2 and a heteropolyanion catalyst. 

Af-methylation of the 3-amino-substituted THD 66 has been reported <2005JOC6230, 2005JMC2266>. Due to the calculated p-ATa s of the two nitrogens, selective methylation of the nitrogen attached to the THD nucleus is only possible for the synthesis of compound 214 via intermediate 213 in a two-step protocol (Scheme 13). 

M. Farnik, M. Weimann, C. Steinbach, U. Buck, N. Borho, T. B. Adler, and M. A. Suhm, Size selected methyl lactate clusters Fragmentation and spectroscopic fingerprints of chiral recognition. Phys. Chem. Chem. Phys. 8, 1148 1158 (2006). 

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