Permethylation

NR qq g relative stability of iminoboranes and amino iminoboranes shows strong dependence on the stetic bulk offered by R and R7 This is illustrated by the stabiUty of at 0°C whereas the permethyl analogue, (CH3) S[(CH3), decomposes above —110°C (72). The... 

MeSSiMe3 or PhSSiMe3, Znl2, Bu4N I. In this case the 6-O-methyl ether was cleaved selectively from permethylated glucose. 

Figure 10.2 MDGC-MS differentiation between the enantiomers of theaspiranes in an aglycone fraction from puiple passion fruit DB5 pre-column (25 m X 0.25 mm i.d., 0.25 p.m film thickness canier gas He, 0.66 ml/min oven temperature, 60-300 °C at 10 °C/min with a final hold of 25 min) permethylated /3-cyclodextrin column (25 m X 0.25 mm i.d., 0.25 p.m film thickness canier gas He, 1.96 ml/min 80 °C isothermal for 20 min and then programmed to 220 °C at 2 °C/min). Reprinted from Journal of High Resolution Chromatography, 16, G. Full et al., MDGC- MS a powerful tool for enantioselective flavor analysis , pp. 642-644, 1993, with permission from Wiley-VCH.

Similarly, when both the Cp and arene ligands are permethylated, the reaction of 02 with the Fe1 complex leads to C-H activation of the more acidic benzyl bond [57]. When no benzylic hydrogen is present, superoxide reacts as a nucleophile and adds onto the benzene ligand of the FeCp(arene)+ cation to give a peroxocyclohexadienyl radical which couples with a Fe Cp(arene) radical. A symmetrical bridging peroxo complex [(Fe"Cp)2(r 5-C6H60)2] is obtained. The C-H activation reactions of the 19e Fe1 radicals BH can be summarized as follows... 

Tschiersky and Baltes96 prepared di-D-fructose dianhydrides by heating fructose. Mass spectra of the permethyl ethers were obtained. Although the dianhydrides were of unknown structure, examination of the spectra indicate that all are difuranose dianhydrides (mlz 101 as the base peak, and low intensities of the ions mlz 88 and 277) and at least one is reminiscent of a 2,3-linked difuranose structure (relatively intense mlz 363 [M — 45]+).95-98... 

Following the first of these two strategies, the whole series of permethylated [nlpericyclines 3-6 (n=5-8) has been synthesized starting from a single building block - dimethylpropargyl alcohol 10 - which is commercially available and can easily be prepared from acetone and acetylene (Scheme 2) [4,6]. 

This reaction sequence was also found to be convenient and versatile for preparing the potential synthetic precursors to permethylated 86 and spiro-cyclopropanated exploded [n]rotanes (Scheme 26) [48]. 

Scheme 26. Acyclic oligoynes for the preparation of permethylated and spirocyclopropanated expanded [n]rotanes...

The fact that only ethylene and tetramethylethylene are evolved from exp-[8]rotane 168 and permethyl-exp-[6]rotane 173 upon thermal decomposition leads to the conclusion that the spirocyclopropane moieties in these expanded [n]rotanes fragment only externally and leave carbene moieties behind. Indeed, the MALDI-TOF mass spectra of several exp-[ ]rotanes show fragment ions with M minus 28. Thus, if this fragmentation in an exp-[n]rotane were to continue n times, a cyclic C carbon cluster would be left over. So far, however, a fragment ion with m/z = 480 corresponding to 182 has not been recorded in the mass spectrum of exp-[8]rotane 168 and it remains to be seen whether a Cgo cluster 183 will be detected in the mass spectrum of exp-[12]rotane 171 (Scheme 35). 

Three types of carbohydrate sample have been reported to give data above mass 4000, namely, permethylated polysaccharides, permethylated glycosphingolipids, and naturally acylated forms of a mycobacterial O-methyl-D-glucose polysaccharide. All are hydrophobic, and desorption is probably facilitated by their inability to form strongly hydrogen-bonded aggregates, either with themselves or with the matrix. 

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