The variation of the relative abundance of [MH]' and M" in the methane chemical ionization mass spectra of six polymethylbenzenes [C,H,-m(CH3),; n = 1 4 as a function of methane pressure is reported. The relevance of these results to the suggestion that M" arises from [MH]' by H loss in violation of the even-electron rule is discussed. An analysis of the energetics of three possible routes for M" formation ( H loss from (MH]', charge transfer ionization of M accompanied by dissociation of [CH,]' to CH, and H , and residual electron ionization of M) is presented. The importance of considering residual electron ionization in chemical ionization spectra is emphasized.
We describe an efficient, six -stage synthesis of 2 -methyl -4-deoxyisopicropodophyI lotoxin, which proceeds via a Diels-Alder cycloaddition between 2-methylmaleic anhydride and the orthoquinodimethane generated by treatment of 5-[ 1 -acetoxy-1 -(3',4',5'-trimethoxyphenyl)methyl] -6-trimethylsilylmethyl-l,3-benzodioxole with silica gel at 60 "C. The podophyllin was converted into a number of 4'-ester derivatives. This procedure was also used to prepare a diazapodophyllin via the use of 4-phenyl-1,2.4-triazoline-3,5-dione as dienophile.
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