The First Total Synthesis of (±)-Ingenol
Jeffrey D. Winkler,* Meagan B. Rouse, Michael F. Greaney,Sean J. Harrison, and Yoon T. Jeon
J. Am. Chem. Soc. 2002, 124, 9726-9728 DOI:10.1021/ja026600a
Note: Many, many thanks go to Jeff Winkler for looking over the post and also sharing an original copy of the manuscript.
Reading a research article is an active process. It’s completely different from leisure reading like a novel or magazine. It’s more like a special – extreme – case of textbook reading. The information has to be unpacked, worked-over and constantly grappled with. During the first meeting of my Organic Synthesis class this year, my students and I worked through the Winkler synthesis of Ingenol. The objectives of the activity were three-fold: 1 – I wanted to compare the strategy used by Winkler to that the recent Baran synthesis. (Which was itself spurred by Carmen Drahl’s reporting on Baran’s synthesis.) 2 – I wanted to analyze how the authors used text and figures to relay the information about natural product synthesis. This helps me develop dos and don’ts for my own writing and a strategy I wanted to share with my students. 3 – I wanted to illustrate how much work an “active” reading of a manuscript should actually be. Below is a play-by-play of the manuscript. It analyzes how Winkler et al. communicated their results in the manuscript and how the reader should interact with the text.
Paragraph 1: Straightforward opening for a total synthesis paper. The authors mention the textbook reasons of “biological activity” and “structural complexity” that inspired their efforts. The inside-outside, C8/C10 trans intrabridgehead stereochemistry linking the BC rings is the most noteworthy feature. Structure 16 is also identified as an important intermediate in this paragraph. It is likely singled out because its preparation completes the carbon skeleton of Ingenol 1, a milestone in the synthesis.
Paragraph 2: This paragraph details the retrosynthetic strategy of the authors and is accompanied by Scheme 1. “Dioxenone photoaddition-fragmentation product 2” is the first intermediate offered in the retrosynthesis. The authors don’t elaborate on what is required to convert 2 to 1, so it is up to the reader to work it out. Inspection of the structures shows that, regardless of order, that: • the A ring of must be methylated at C2 and the C1-C2 alkene must be formed; • both C4 and C5 must be oxygenated; • the C6-C7 alkene must be formed; and • the ester attached to C6 must be reduced down to the corresponding alcohol. Additionally, the authors’ description of 2 leaves a little bit to be desired. It describes how the intermediate was prepared, but little else. To understand, the active reader has to look forward to Scheme 2 to see the intermediates 13/14 to see the photoaddition product and also back at the earlier work from the laboratory where the tandem photoaddition-fragmentation strategy was developed. Once the reader is familiar with the two-step sequence, intermediate 3 becomes a logical precursor for 2. The dioxenone unit in 3 can be simplified to the keto unit in 4 via a retro-carboxylation and enolization process. Reductive enolate formation and alkylation of 5 would lead to 4. The origins of enone 5 are not completely explained in the main text, but reference 6 states, “We thank Professor Phil Eaton (Univeristy of Chicago) for providing an unpublished procedure for the conversion 6 to 5.” Compound 6 is the deoxygenated version of 5; it lacks Ingenol’s C3 hydroxy group. The note in reference 6 points the reader in the right direction, but it still requires work to understand where 6 itself came from. In this case, reference 4 leads the way. Tetrahedron 1968, 24, 553 (ref 4c) shows that 6 can be synthesized via elimination and acylation of lactone X or its corresponding hydroxy acid using polyphosphoric acid. The precursors were probably prepared from beta-keto ester Y and methyl/ethyl methacrylate based on the J. Org. Chem. 1999, 64, 3770 reference (4a).
Paragraph 3: The forward synthesis of intermediate 16 from 5/6 depicted in Scheme 2 begins here. The paragraph is dedicated to one reaction in the synthesis: the reductive alkylation of 6 using methyl crotonate as the electrophile. In the reaction, three contiguous stereocenters are set including a quaternary center right in the middle. Two relative stereochemical relationships arise from the three centers. The authors deal with each separately. The cis relationship between the bridgehead carbons in 7/8 is because the beta face of the enolate is less sterically hindered than the alpha face. This argument should probably conjure an image of a bis-envelope conformation of a cis-[3.3.0]bicyclooctane. The C10-C11 stereochemistry requires transition state structures to rationalize the diastereoselectivity. Figure 1 shows transition states, labeled A and B, and they are “called” in the paragraph; that is, the writing makes specific reference to the items in the figure by name or letter. This connection between text and figures is common and effective technique to facilitate reader understanding. Figure 1 uses arrows to draw attention to the interaction that disfavors TS B; it’s the C4 methine and the alpha hydrogen of crotonate (both TS have a gauche interaction between the electrophile and the enolate). This paragraph is a good example of using a figure to rationalize a key principle that supplements the reaction sequence presented in a scheme. A structure for the product of alkylation discussed in paragraph 3 is not actually shown in Scheme 2. Instead, the product of a two-step sequence, alkylation and silyl enol ether formation, is shown. When 6 was used as the starting material of the sequence, 7 was the product. The sequence yielded a mixture of diastereomers in a 14:1 a:b (C11 methyl) ratio.
Paragraph 4: If the same two-step sequence was conducted on the more highly functionalized enone 5, an unacceptably low yield and diastereoselectivity was obtained. The authors consequently elected to continue forward from compound 7. The paragraph continues on to describe the reactions that converted 7 first into ketone 9 and ultimately to dioxenone. Conversion of 9 to 10 included carboxylation of an in situ generated lithium enolate with Mander’s reagent. This reagent gave the beta keto (methyl) ester as the product. It is converted to the PMB ester in the proceeding step before dioxenone formation without explanation. We reasoned that the transesterification was necessary (Why else would they add a step?) and that it implicated the ester oxygen as a nucleophile in the dioxenone formation. In the case of a PMB ester, the resulting carbocation would be relatively stable especially when compared to a methyl carbocation. The paragraph ends with the formation of compound 10. This makes good sense because they have taken the starting material forward to an intermediate that secures the functionality (the dioxenone) of one half of the photocycloaddition reaction.
Paragraph 5: The manuscript switches focus to the other end of the molecule that will participate in the upcoming photocycloaddition. They install a hydroxyl group onto C14 of 10 by an allylic oxidation to give compound 11. One unanswered question is why that couldn’t have been done earlier, perhaps before the sequence that formed dioxenone 10. The allylic oxidation is necessary to introduce functionality into this segment of the molecule that can be parlayed toward the cyclopropane unit of Ingenol. Photocycloaddition of 10 was low yielding so they refunctionalized to the corresponding allylic chloride. The chloride was considerably more efficient in the cycloaddition, providing 14 in 60% yield. Compound 14 is the first of the two showcase reactions that will ultimately deliver the carbon skeleton of Ingenol. The enthusiasm of the authors is apparent by the exclamation point used to finish the sentence, “…we were delighted to find that photocycloaddition of the derived allylic chloride 12 proceeded in 60% yield to give the desired photoadduct 14, accompanied by the C13 chloro-isomer (5:2 ratio)!” They are pleased with the reaction. In reference 8 they also promise to explain the origins of the C13 chloro isomer, which here goes without further comment.
Compound 14 is amongst the “perilous heights of an advanced intermediate” as has been stated about so many compounds in natural product synthesis. There is an abundance of functionality waiting to be unleashed en route to key intermediate 16. The cyclobutane, just formed in the cycloaddition, will be unraveled in the formation of the challenging inside-outside bicycle ring junction between rings B and C. The carboxylate will eventually become the C6 hydroxymethyl group. The C14 chloride will enable the cyclopropanation via the intermediacy of the alkene. Based on the group’s previous work and the enthusiasm about the success of the cycloaddition, they are confident that the fragmentation would be successful. They give the fragmentation a clause and then continue tidying up the structural elements until they arrive at 15. In the footnotes there is the comment on the C6 stereochemistry – remember that that gives an indication of the electron motion of the fragmentation.
Stylistically, it may have been nicer to highlight the two-step sequence by making them the sole subjects of one paragraph. Perhaps paragraph 4 could have simply ended with the synthesis of the chloride, 12. I would have relegated the cycloaddtion of 11 to the notes and then let paragraph 5 shine in the brilliance of the photocycloaddition-fragmentation sequence.
Paragraph 6: The key steps necessary for the synthesis of the carbon skeleton of Ingenol are now behind them. A dihalocyclopropanation of alkene in 15 is followed by bis-alkylation with methyl cuprate to give the landmark intermediate 16. This paragraph wraps up the presentation of Scheme 2. To get to this point in the synthesis, there have been 18 linear steps.
Comparing structure 2 and 16 shows that they are very similar. The main differences are the oxygenation at C3 and the redox state of the group attached to the C6 carbon. It could even seem that the rest of the synthesis is endgame.
Paragraph 7: The payment for using 6 in place of 5 has come due. In paragraph 7, the authors quickly move through a series of transformations that convert 16 to 22. The key player that enables these transformations is the hydroxymethyl group attached to C6. Oxidation of that group to the corresponding aldehyde allows sequential eliminations that create the diene in 22. The authors report flatly in this paragraph that the seven steps reported are “to introduce the A ring functionality present in ingenol”. They don’t put emphasis on it, but it’s logical to think that they’d have preferred to carry an oxygenated C3 up to this point and done only one or two steps to be in a much better position than they presently are. So it goes.
Paragraph 8: The alkenes have been introduced into 22 so they can be oxygenated. It must be a “controlled burn”, however. They first reduce the aldehyde attached at C6 and then do a dihydroxylation on the C5-C6 alkene. The regioselectively is ascribed to greater steric accessibility of this alkene relative to the C3-C4 alkene. Similarly, the beta selectivity is also due to the steric accessibility of reagents from the beta face according to the authors. Building a model is an obvious way to get a sense of the 3D shape of the molecule as well as the associated selectivities of reactions associated with intermediates along the way. Unfortunately it’s not always put into practice. It definitely counts as “active” reading and will help understanding.
Paragraph 9: This paragraph finishes up the B ring synthesis. Dihydroxylation of the C3-C4 alkene and protection of the secondary hydroxyl as the benzoate give 28 and set up the linkage of the two tertiary alcohols into cyclic sulfate, 29. Elimination of the cyclic sulfate to provide the C6-C7 alkene was based on previous work in a closely related system. The remainder is some typical plug-and-chug to get to structure 31. As the authors write at the end of the paragraph, “it remained only to introduce the requisite A ring functionality to complete the synthesis of Ingenol.”
Paragraph 10: Structurally the paragraphs in the manuscript are a little clunky. They seem to tease the subject of the proceeding paragraph at the end of a given paragraph and then jump straight into the details at the start of that next paragraph. I don’t know if it’s accidental or by design. The authors walk the reader through C4-C5 diol protection, oxidation, Pd(0) beta-keto ester formation, alkylation, decarboxylation/oxidation to give enone 35. Whew. Steppy? They follow with Luche reduction of the enone and deprotections to ultimately deliver the title compound, Ingenol 1. They remind the reader that this was a racemic synthesis by reporting that the material prepared by them was identical to an authentic sample except for optical rotation.
Paragraph 11: The wrap-up. The synthesis was 43 steps from 6 with an average yield of 80%. That’s an 0.0068% overall yield using their numbers. It would be nice to have the authors report that number themselves. They complete the paragraph with the highlights of the synthesis. The best of these highlights is the penultimate sentence, “The establishment of the C8/C10 trans intrabridgehead stereochemistry serves as a testament to the utility of the intramolecular dioxenone photoaddition-fragmentation approach to the synthesis of structurally and stereochemically complex natural products.” Agreed. This paper showcases that tandem approach to the carbon skeleton AND continues to complete the total synthesis of Ingenol 1.