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Benzofuranoids ustusoranes A (1), B (2), and E (3) were isolated from Aspergillus ustus 094102. If these compounds were separated by column chromatography on silica gel, what would be the elution order? (top — comes off the column first, bottom — comes off last).

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As I get it, column chromatography is like TLC where in a silica gel the nonpolar compound comes out first.

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    $\begingroup$ The comparison with TLC is valid, and your judging of polarity, based on the interaction of the $\ce{OH}$ groups with the silica sounds good. So, yes, I would come to the same conclusion. If you would perform the separation in the lab, you would nevertheless keep all three fractions and determine the structures by independent methods (NMR). $\endgroup$ – Klaus-Dieter Warzecha Mar 3 '17 at 9:11
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Your conclusion is absolutely correct. Column chromatography can be thought of as three-dimensional version of TLC (and vice-versa). So the most polar compound which interacts with silica gel most elutes slowest and the least polar compound leaves the column first.

I also support your suggested order as an initial starting point. However, As Klaus mentioned in a comment, unless one has performed the same separation multiple times one should never rely on paper-chemical polarity analysis alone and always confirm structures independently — typically that means ‘measure an NMR of all fractions’.

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  • $\begingroup$ Why three-dimensional? $\endgroup$ – orthocresol♦ Mar 3 '17 at 22:48
  • $\begingroup$ @orthocresol Diffusion in a column can go $x$ and $y$, elution is $-z$. In a TLC, elution is also $z$, but diffusion can only happen in $x$ direction. $\endgroup$ – Jan Mar 3 '17 at 23:27
  • $\begingroup$ I always thought 2D TLC referred to the thing where you develop along $z$ direction, then rotate it by $90^\circ$, stick it in a different solvent system, and develop along $x$ direction. $\endgroup$ – orthocresol♦ Mar 3 '17 at 23:33
  • $\begingroup$ @orthocresol Yes, that’s the method of two-dimensional TLC evolution. But even in standard, single evolution TLCs you get a certain diffusion perpendicular to the elution direction and spot broadening. $\endgroup$ – Jan Mar 3 '17 at 23:37
  • $\begingroup$ Ah, okay; I wasn't meaning to nitpick, I was just curious as to what you meant. $\endgroup$ – orthocresol♦ Mar 3 '17 at 23:39

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