Some time ago my research group stumbled upon a family of macrocycles whose structure contained a somewhat uncommon stereogenic element, a plane of chirality. A plane of chirality arises when restricted rotation around a planar unit in a molecule leads to the formation of stereoisomers. The textbook example (seriously, look at Eliel) is E-cyclooctene. The chirality arises because of the twisting of the double bond and the asymmetry of the bridge that links its two sides. Individual enantiomers can be referred to either as P & M or pR & pS. The enantiomers of E-cyclooctene were resolved via transient formation of diastereomeric platinum complexes. In our system, we’ve explored in some detail how stereogenic centers at specific locations on the macrocycle backbone dictate which planar chirality it adopts as it forms in an RCM reaction. Kozlowski’s story turns that sequence upside down, or backwards. She leverages the chirality bestowed by a stereogenic axis to set two stereogenic centers on the seven-membered ring.
Dynamic Stereochemistry Transfer in a Transannular Aldol Reaction: Total Synthesis of Hypocrellin A
Erin M. O’Brien, Barbara J. Morgan, and Marisa C. Kozlowski*
Angew. Chem. Int. Ed. 2008, 47, 6877-6880
Title: I frequently start my graduate synthesis course with a standard introduction. I ask, “What are reasons for why humans do total synthesis?” It’s a roll call of sorts: • to provide material not available from its natural source; • to provide material for biological testing, including mechanism of action studies; • to prove the structure; or • to test an idea. This last one is interesting because it’s uncommon. But that’s what Kozlowski’s synthesis does. The questions to be dealt with are intertwined and revolve around stereochemistry and biosynthesis. The idea plays out over the course of the communication.
P1: Paragraph one contains pro forma items like the introduction of the title compound, a listing of biological activities, and observations on its structural details. Along with Hypocrellin A (1) (Scheme 1), the structures of other pyrelenequinones are shown. Kozlowski observes that three of the four compounds come from one producing organism (Shiraia bambusicola) and two come from another (Shiraia bambusae). An interesting coincidence is that for these two different organisms, each produces one complementary enantiomer of the same compound, where bambusicola produces Hypocrellin A (1) and bambusae makes Hypocrellin (4) (Mind the R/S and P/M notation.). And… And… The compound common to both organisms, Hypocrellin B (3), is racemic. The stereogenic centers have been removed and are replaced by a double bond and the stereogenic axis is of both configurations. She is using these observations to soften the reader up to the biosynthetic question she’ll be posing in a couple of paragraphs. It’s subtle and requires that you really focus in on the details in the structures.
The paragraph continues by listing the antiviral activity, immunotherapeutic properties, and “light induced activity against tumor cell lines,” of the perylenequinones. In the middle of those statements is an important clause that feels like it was just jammed in there. It’s a little awkward. She writes, “and elegant approaches to the calphostins have been reported.” As will become apparent, the calphostins are structurally similar to the perylenequinones and much of what was learned in pursuit of those targets will be leveraged by the team toward 1. Angewandte’s page limit may have precluded the consideration she was going to give to the calphostins here. Wrapping up the paragraph Kozlowski points out the seven membered ring, the quaternary stereogenic center, and the fact that, until this report, a synthesis of Hypocrelllin A was elusive. It ends by dangling the concept of dynamic stereochemistry transfer in front of the reader to keep the manuscript flowing.
P2: What “taut” and twisted story is told in paragraph two? It’s one that goes into greater detail about two specific structural features of the perylenequinones. The first is the tautomerization of the core polycyclic structure. Kozlowski calls Scheme 1 again and links the tautomerization of 1 and 1•taut (present at ~1:1) to other structural features such as the planarity of the core and intramolecular hydrogen bonding. It’s best to take a step back and really process that tautomerization. The overall result is easy to see – the quinol and quinolone rings within each of the napthyl units of the perylenequinone core essentially interconvert. That IS the tautomerization. It documents the delocalized nature of electrons in the ring. Explicitly charting that flow of electrons, however, requires a little effort. The second feature is the atropisomerism of the core; this means stereogenic axis talk! Calphostins come back into the story here too. While the helical configurations of calphostins are stable, those of the perylenequinones are not. The argument is that tying the pendant arms of the perylenequinone together enables it to wriggle more easily, or forces it to wriggle. At first that may seem counterintuitive because cyclization is usually equated with rigidification. Chris Farley’s, “fat guy in a little coat” scene comes to mind in an effort to come to terms with the idea. The analogy can’t be taken too far, though, because that might suggests bond breakage! At any rate, atropisomerism introduces yet another equilibrium, depicted in Scheme 2, between 1 and 1•atrop (Once again mind the P’s, M’s, R’s and S’s.). Thermodynamics places the equilibrium at 4:1, favoring the P-configured isomer, Hypocrellin A (1). There is more subtlety (and conservation of space, perhaps) at the end of this paragraph because Scheme 2 houses the atropisomer equilibrium as well as the upcoming retrosynthesis.
P3: If this paragraph had a title, it would be “The Question.” The question focuses on the origins of asymmetry in the molecule, which is what makes it chiral, and the sequence by which those stereogenic elements are installed in the biosynthesis. Kozlowski ponders whether the stereogenic “helical” axis or the centers are installed first. She envisions, “…a potentially biomimetic, dynamic stereochemistry transfer (DST) reaction,” as the driving force behind the design of her synthesis. As defined by her, a DST reaction takes one stereochemical element (center or axis) and leverages it for a diastereoselective reaction, but loses stereochemical integrity as a consequence (or subsequent to) the transformation. In a nutshell, “The Question,” in two parts, is: 1) Can the configuration of the helix in the perylenequinone drive the stereoselective formation of the centers distal to it via an aldol reaction; and 2) Is this transfer of asymmetry – axis to center – biomimetic? What better tool to answer the question than synthesis? The pursuit of an answer to The Question really dictates a certain pathway for the retrosynthetic analysis that follows.
P4: With an over-riding principle behind the synthesis in place, the issue of HOW to orchestrate it was now required. Paragraph four comes through with Kozlowski’s retrosynthetic analysis. We’re sent back to Scheme 2, now in its capacity of illustrating the retrosynthesis. The reaction to be implemented for the DST is an intramolecular aldol. This disconnection, that takes Hypocrellin A (1) to Intermediate 5 is shown with a clear indication that it is the DST and an aldol in the scheme. Note that the stereogenic axis of 5 is in the P configuration, like 1. With helical configuration set, the argument is that the aldol reaction will be selective for formation of the configurations of the centers as shown, which includes the stereogenic quaternary center. Compound 5 itself looks a bit like a calphostin, which makes sense; disconnection of the “2-propanoyl” pendants of 5 give di-iodo compound 6. If space had allowed, addition of 10 to the retrosynthetic scheme would have been instructive. It would have made the allylation/Wacker strategy readily apparent. Additional contemplation of 5 reinforces the biosynthesis argument because it is akin to the aforementioned calphostins and also a likely intermediate en route to the elsinochromes where a saturated six membered ring is fused to the perylenequinone core. The biosynthesis sub-theme keeps the storyline active within the communication.
P5: As intimated earlier, the extensive calphostin literature (including, logically, earlier contributions from the Kozlowski group) was to be utilized toward the synthesis of key intermediate 5 because of the structural similarities between the two. Attempts at developing diastereoselective oxidative aryl couplings, where the stereogenic centers on the pendants were supposed to drive the formation of atropisomers of a specific configuration were ineffective, even counterproductive. Count those failures as another argument in favor of the axis-to-center biosynthetic model. Kozlowski even adds more fuel to her argument by noting that enzymatic oxidative couplings of 2-napthols can be enantioselective. It’s here, essentially mid-paragraph, where a transition occurs. So far, the paper has really presented ideas and strategies. All that positioning set the foundation upon which the results, the synthesis, will be built. Enantionselective access to 5 was essentially the last piece of the retrosynthetic puzzle.
At this point an, “In the event…” phrase could be expected. Maybe it’s too cliché. Regardless, with the sentence, “In our approach, we employed an enantioslective oxidative coupling of 9…,” Kozlowski all but shouts to the reader, “Enough talk, let’s synthesis!” Getting to 9 requires doing some homework. Scheme 3 shows how compound 9 comes from 7 via 8, but the text makes no reference to the transformations at all. The reader is left to muddle through alone, so those who hadn’t already reached for the scrap paper to push electrons in the tautomerization may be reaching for it now. She then coolly picks out the perfect enantioselective oxidative coupling she needed – from her personal bag of tricks. If there is any self-aggrandizement, it is understated, with a citation and a “biaryl coupling with our copper diaza-cis-decalin catalyst (11) further highlights the utility of this method….” Reference 15b goes a long way in explaining the mechanism of the reaction and a rationalization fo the selectivity. An acetate ester to methyl ether switch followed by Suzuki coupling with allylB(pin) boronate delivers key intermediate 10. As is often the case, there is a nice symmetry between the end of the paragraph and the last compound of the scheme.
P6: “Our attention turned,” that’s another lovely phrase. The forward synthesis is now hitting its stride in paragraph six. The next stop is key intermediate 5 that will be used to interrogate The Question. Scheme 4 gets you from 10 to 16, which is the bis-ketal version of 5, so close. The marquee bond in the whole scheme is the one that closes the perylenequinone system. To begin, compound 10 from the previous paragraph is hydroxylated and then protected as the bis-benzyl ether smoothly giving 12. There are a few transformations throughout this synthesis whose reactivities/selectivities presumably rely on precedents from the calphostin literature. This hydroxylation is one of them. Wacker oxidation (pronounced “Vocker” if you’re German or you’re feeling confident) provides 13, and is followed by bis-ketal protection of the newly formed ketones. It’s noted that the ester units at C3 and C3’ were unreactive and required nucleophilic conditions for deprotection to 14; this cleavage is a minor bump in the road but one that was easily overcome. Bump number two now faces Kozlowski, and it is a little bigger. Those newly revealed acids have to be decarboxylated and standard conditions use temperatures that would allow the beloved perylenequinone enough energy to racemize. The solution: develop a new reaction; so a Pd-mediated carboxylation was consequently developed. This reaction could occurs a temperature low enough to keep the helical configuration intact. That meant 15 was in hand. After uneventful debenzylation of the ethers at C5 and C5’, more tweaking is required (bump number three) to make the oxidative cyclization to 16 efficient. The addition of base is telling when considering the napthol/quinol pKas. Critically, the twist (absolute configuration of the stereogenic axis) in 10 is maintained throughout the steps to 16, nearly setting the stage for The Question.
P7: A good story needs suspense and paragraph seven delivers. It’s like the indoor lacrosse ads that claim, “We’ll sell you the whole seat, but you’ll only need the edge!” So 16 is acid sensitive, having decomposed upon attempted ketal hydrolysis under standard acid conditions. To avoid this problem, Kozlowski resorts to another clever solution. The perylenequinone was reduced to the perylene bis-quinol 16b, (Scheme 5) and the ketals were then trans-ketalized using a mild Lewis acid in acetone; air oxidation of that species returns it to the appropriate oxidation state, perylenequinone 5. Now the reader is thinking, “Ok, here we go [with the DST]!” Instead, Kozlowski remarks that the “1,8-diketone aldol reaction that we have proposed to use had not been employed previously..” and leaves you hanging. Suspense.
P8: The manuscript takes a break from the excitement of the forward synthesis to reveal some additional details behind their confidence in the axis-to-center biosynthetic model based on molecular modeling studies. First, they found that the Z-enolate geometry would deliver the needed syn-aldol product via a closed transition state structure. Incidentally, the E-enolate gives rise to the other diastereomer, Shiraiachrome A (2). Second, energies of the same Z-enolate transition state structures showed that the correct diastereoface of the ketone electrophile of the aldol would also be in position based on the P configuration of the perylenequinone. Structures for the transition states are shown in Figure 1. The chemdraws at the bottom are slightly misleading because they aren’t exactly accurate representations of the structures that were in the calculation, and whose ball-and-stick model is right above them. It takes a little more work from the reader to make the jump here, but the message is clear nonetheless. The paragraph finishes with caveats about the need for a stable helical configuration over the reaction despite the fact that it atropisomerizes afterward. A search for the best Z-enolate selective base was on.
P9: And that base was found, “[a]fter considerable optimization.” One could wonder, “What does that mean in practical terms?” The phrase reads like a euphemism for the experimental equivalent of blood, sweat, and tears. Nonetheless, treatment of 5 with lithium disilazide base at low temp (Scheme 5) mediates enolate formation and ultimately the aldol cyclization. D.S.T. Selective deprotection of the C4 and C4’ methyl ethers (Thank you one more time calphostin.) follows the aldol and, “Voila!” Hypocrellin A (1). As may have been expected from the equilibria presented in the introductory paragpraphs, 1 is the major product, but of a mixture of products. Remember that Hypocrellin A naturally exists as a ~4:1 mixture of 1:1•atrop where the stereogenic axis makes them diastereomeric. Though there isn’t much explanation about why, the synthetic Hypocrellin A (1) sample is compared to a sample of 1 that was isolate from bambusicola. Rather, it is compared to the enantiomer, Hypocrellin (4) from bambusae. The spectra match and the CD in the supporting shows them to be enantiomers.
Also present in the mixture is Shiraiachrome A (2) and its atropdiastereomer, both of which probably arose from the E-enolate. The key ratio is for the yield of 1+1•atrop to 2+2•atrop, which is 10:1. This primarily reflects the selectivity of the aldol step. The details of Shiraiachrome A (2) are revealing. Remember 2 was the minor product in the aldol reaction, having arisen from the E-enolate attack on the “correct” face of the ketone. The helical configuration of the starting material, 5, was P. In the product structure the helical configuration switches to M to the tune of 10:1 (2:2•atrop).
P10: A routine summary paragraph wraps up this communication. Steps: 19; overall yield: 1.6%; average yield per step: 82%. Kozlowski also highlights the reaction development that went into this target including the napthol coupling, the Pd-mediated decarboxylation, and of course the DST aldol reaction. She aptly summarizes the key finding by writing, “In the aldol reaction, the two newly formed centrochiral stereocenters of the seven membered ring were dictated by the stable perylenequinone helical configuration and the enolate geometry.” For her this seals the biosynthetic model. From one perspective, the big twist was that the conversion of 5 to Shiraiachrome A (2) has even greater fidelity to DST as it was defined all the way back in paragraph three. This paper excels it posed The Question related to biosynthesis of the perylenequinones and how the target drove methods development, subtly presented. Kozlowski gives you total synthesis with a twist.
Note: A big thank you to Marisa Kozlowski for looking at the draft version of this post and for sharing the original figures and links to other coverage of this synthesis.