Dichromate-mediated, oxidation

A few years later, William H. Perkin,based on the amazing but erroneous idea, attempted a potassium dichromate-mediated oxidative dimerization of A-allyltoluidine (C10H13N) to produce quinine. Since A-allyltoluidine is structurally nothing like half a quinine molecule, this attempt was unsuccessful and futile. The entire idea was based on a newly reported empirical formula of quinine [2C10H13N -l- 30 = C20H24N2O2 + H2O] and although the arithmetic was suggestive, there was no chance that Perkin could have obtained quinine. 

The aldehyde function at C-85 in 25 is unmasked by oxidative hydrolysis of the thioacetal group (I2, NaHCOs) (98 % yield), and the resulting aldehyde 26 is coupled to Z-iodoolefin 10 by a NiCh/CrCH-mediated process to afford a ca. 3 2 mixture of diaste-reoisomeric allylic alcohols 27, epimeric at C-85 (90 % yield). The low stereoselectivity of this coupling reaction is, of course, inconsequential, since the next operation involves oxidation [pyridinium dichromate (PDC)] to the corresponding enone and. olefination with methylene triphenylphosphorane to furnish the desired diene system (70-75% overall yield from dithioacetal 9). Deprotection of the C-77 primary hydroxyl group by mild acid hydrolysis (PPTS, MeOH-ClHhCh), followed by Swem oxidation, then leads to the C77-C115 aldehyde 28 in excellent overall yield. 

In the case of potassium dichromate we did not observe modifications of UV absorption spectra (which means that no direct bond to DNA bases is formed), while the effect on thermal stability differed "in vivo" and "in vitro", in both cases suggesting a break down of the molecule, supported by viscosity variations, too. These data are in agreement with the mutagenic (14) and carcinogenic (15) action of Cr I compounds and strengthen the hypothesis that Cr effect on the cells is mediated by oxidizing molecules produced by its intracellular reduction to Cr (9). 

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