Reader Question: Steroidal Glycosides in Solanum Maternum Fruit

The tamarillo fruit is the fruit of a subtropical Solanum shrub

A reader contacted me via email recently posing a very intriguing question regarding the levels of steroidal glycoalkaloids present in a close relative of the tamarillo (tree tomato). I've reproduced an excerpt of his correspondence below and I ask any readers with practical experience in this area or with academic interest in characterizing the steroidal natural products in this wild fruit to get in touch with me (brian.heasley2009@gmail.com) so that I can relay the information to the relevant parties.

Hello Brian,

Thanks for your blog information in http://modernsteroid.blogspot.co.nz/2014/06/tomato-steroidal-glycoalkaloids.html

For context, I grow a wild South American Solanum species, Solanum maternum, which I doubt has ever been investigated for the glycoalkaloids present (it was discovered relatively recently).

It is the (extremely) close relative of the 'Tamarillo' ('tree tomato'), Solanum betaceum (synonym Cyphomandra betacea), a fruit in limited international commerce, and also grown here in New Zealand.

While in potatoes the 'normal' level of glycoalkaloids of 20-100 mg/kg is said to be 'not of concern' - albeit a 'safe' level has not been established - introducing genes from wild species of potato may inadvertently introduce undesirable higher levels of glycoalkaloids. In addition, new glycoalkaloids, 'untested' in human diet, may be introduced.

Solanum maternum has total resistance to the fungus disease powdery mildew; S. betaceum, the fruit of commerce, is susceptible. S. maternum is of interest for breeding for disease resistance - just as are wild potato species for the domesticate.

Fruit of S. maternum have a mix of acidity, unusual floral flavor notes - and a savage bitterness and throat-gripping 'acridity' that render it inedible. The Tamarillo has significant bitterness in the fruit wall, but not in the pulp cavity, and it is the pulp only that is eaten. Nevertheless, past consumer testing has shown some people report a 'catching' in the throat when they eat Tamarillo.

I haven't been able to find any internet information on the glycoalkaloid load in tamarillos, but assume, based on their historical wide acceptance, that it is relatively low. I assume that S. maternum has a 'high' glycoalkaloid load. There may be other chemicals present in the maternum fruit that adds to the general 'acridity' or 'heat'. But it has never been investigated...Checking the internet (I have no background in chemistry), I see there are some simple reagents which give a color precipitate in the presence of glycoalkaloids. An industrial chemist might make one or more for me, but these do not seem to be able to give a quantitative indication of levels, which is really what I am after. This is a faint hope - but I have to ask - do you know of any newly invented 'field kit' for a rough quantitative indication of glycoalkaloid levels? Alternatively, do you think there would be sufficient academic interest in characterizing the steroidal complexes in this wild fruit (ideally, from my point of view, in comparison with the domesticate, and also in comparison with fruit from a cross of the two species)?

Regiodivergent Outcomes in Metal-Catalyzed Aliphatic C-H Azidations of Estrone Derivatives

            New late-stage C-H functionalization reactions that are compatible with structurally complex and clinically useful natural products are of great interest to the pharmaceutical industry. This is due, in part, to the ability of nitrogen-containing functionality to modulate the binding affinity and physicochemical properties of drug candidate molecules. However, until recently, chemical methods to convert an alkyl (and, in particular, a tertiary) C-H bond into an alkyl carbon-nitrogen bond were lacking. This problem is further exacerbated by the fact that enzymes generally do not catalyze aminations of C-H bonds (for an exception to this rule, see this recent report). Earlier in 2015, two new synthetic methods for the metal-catalyzed azidation of aliphatic C-H bonds were received with great interest and enthusiasm by the chemical community. Interestingly, although the C-H azidation methodologies are mechanistically related, their application to differentially protected estrone derivatives produces regiodivergent outcomes (see Scheme above). In one case, the benzylic tertiary position (C9) reacts primarily and, in the other, the secondary C6 C-H position is functionalized (for details, see below).

            The method reported by the laboratory of John Groves at Princeton University is a manganese-catalyzed process that uses sodium azide as the azide source. Mechanistically, this chemistry is analogous to Groves’ previously disclosed manganese-catalyzed C-H fluorination wherein a fluoride ligand axial to manganese transfers fluorine to a carbon-centered radical derived from the hydrocarbon substrate. For azidation, the fluoride is simply replaced by an azide source (NaN₃). The substrate radical is then trapped by the in situ-generated Mn(IV)-azide complex to construct the carbon-nitrogen bond. Both manganese porphyrins as well as Mn salen-type Jacobsen catalysts are competent participants in the catalytic cycle and the novel azidation protocol can be run under air. Under the optimized reaction conditions, estrone acetate is converted predominantly to a C9-a-azide. A diazidation product wherein both benzylic positions (C6 and C9) are functionalized is also observed as a major side product.

            John Hartwig’s group has also developed a method for late-stage azidation of tertiary and benzylic C-H bonds using an iron catalyst and Zhdankin’s hypervalent azidoiodinane reagent. For a nice introductory overview of Hartwig’s technology, see this blog post. Azidation of TBS-protected estrone, under the iron-catalyzed conditions, furnishes the corresponding C6-a-azide in modest yield. The stereodivergent nature of the two related azide-forming processes is somewhat striking.

            On a somewhat unrelated note, the laboratory of Timothy Newhouse at Yale University has disclosed a new palladium-catalyzed methodology for a,b-dehydrogenation of esters and nitriles. The method nicely complements (and, in some cases, exceeds) earlier, more classical approaches to achieve a,b-unsaturation such as the Saegusa-Ito oxidation or the Sharpless selenoxide elimination (later extended by Grieco). The novel reaction from the Yale group is compatible with nitrile substrates derived from estrone (shown above), cholesterol and androstenedione-type steroids.