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Biomimetic Kinetic Resolution: Highly Enantio- and Diastereoselective Transfer Hydrogenation of Aglain Ketones To Access Flavagline Natural Products

We have previously reported asymmetric syntheses and absolute configuration assignments of the aglains (+)-ponapensin and (+)-elliptifoline and proposed a biosynthetic kinetic resolution process to produce enantiomeric rocaglamides and aglains. Herein, we report a biomimetic approach for the synthesis of enantiomerically enriched aglains and rocaglamides via kinetic resolution of a bridged ketone utilizing enantioselective transfer hydrogenation. The methodology has been employed to synthesize and confirm the absolute stereochemistries of the pyrimidone rocaglamides (+)-aglaiastatin and (−)-aglaroxin C. Additionally, the enantiomers and racemate of each metabolite were assayed for inhibition of the heat-shock response, cytotoxicity, and translation inhibition.

In previous studies, we proposed 11a that ketoreductase or reductoisomerase enzymes 12 could differentiate between the enantiomers of ketone 5a, selecting for either direct reduction or rearrangement followed by reduction as part of a parallel kinetic resolution 13 process ( Figure 2). In the case of kinetic resolution of aglain ketone 5a, both enantiomers may be converted to bioactive natural products which led us to investigate the application of a reductive, chemical kinetic resolution approach to enantiopure ketone 5a.
Noyori and co-workers have previously used chiral ruthenium complexes in the kinetic resolution of racemic secondary alcohols. 14 Metz and co-workers 15 recently demonstrated the utility of this approach for the reductive, kinetic resolution of racemic flavanone substrates. Herein, we report the kinetic resolution of aglains such as 5a via reduction of a chiral, racemic bridged ketone and its application toward the total syntheses of the pyrimidones (+)-aglaiastatin (2) and (−)-aglaroxin C (3).

■ RESULTS AND DISCUSSION
Biomimetic Kinetic Resolution. Our initial investigations utilized conditions recently reported by Metz and co-workers 15 for asymmetric transfer hydrogenation of racemic flavanones via Rh (III) catalysis employing hydrate 5b 11a which to our delight afforded (−)-6 in 95% ee after 40% conversion (Table 1, entry 1). The absolute stereochemistry was determined through comparison to (+)-6, previously assigned through X-ray crystal structure analysis and synthesis of (−)-methyl rocaglate from a common intermediate. 11a Even after a more than 5-fold increase in reaction time (entry 2), we could not push the reaction to 50% conversion. After further optimization, CH 2 Cl 2 was found to be the solvent of choice, providing optimal conversion (entry 5). Low conversions observed (entries 1, 2, and 4) were likely due to the insolubility of hydrate 5b in both ethyl acetate and THF. In situ dehydration of 5b with 4 Å molecular sieves (entry 3) led to no reaction, suggesting that the molecular sieves were interfering with the process. Furthermore, subjection of ketone 5a to the reaction conditions did not produce significantly different results from use of hydrate 5b. Other metal hydride and ligand systems were tested (entries 6−8), each affording lower enantioselectivity or in one case no reactivity (entry 8).
What was perhaps more intriguing than the excellent enantioselectivity was the complete diastereoselectivity observed. Based on our previous reductions of ketone 5a and hydrate 5b, 11a we hypothesized that non-coordinating reducing agents would preferably attack the in situ generated ketone away from the phloroglucinol ring. Since the rhodium catalyst system is sterically encumbered, we believe that other modes of stabilization enhance attack from the phloroglucinol face. In the aforementioned example of kinetic resolution of flavanones through catalytic asymmetric transfer hydrogenation, Metz and co-workers 15 proposed an assembly for kinetic resolution of flavanones featuring stabilizing CH−π interactions 16 between the methyl groups on the Cp* ligand and the aromatic substituent of the ketone. Figure 3 depicts models generated using Spartan '10 17 for both favored and disfavored enantiomers of aglain ketone substrates. In the case of the favored substrate/catalyst assembly ( Figure 3A) we propose that, in addition to the stabilizing CH−π interaction, a hydrogen bond between the tertiary alcohol of substrate (−)-5a and the sulfonyl oxygen of the catalyst helps to direct the chiral reducing agent. In the case of the disfavored substrate/catalyst ( Figure 3B), this hydrogen bond interaction is not evident which allows (−)-5a to be selectively reduced through assembly A. This is in contrast to the assembly proposed by Metz and co-workers in which a destabilizing steric interaction led to selective reduction of one enantiomer. 15 To evaluate the scope of the kinetic resolution on different substrates, we synthesized a variety of aglain ketone analogs and subjected them to the optimized conditions (Table 2). Our aim was to manipulate the electronics of the aromatic system and   observe its effect on the kinetic resolution. Desmethoxy substrate 7 produced both a lower conversion and decreased enantioselectivity (entry 2). When the aryl ring was not directly adjacent to the reactive ketone, the reaction did not occur as demonstrated by attempted reduction of β-keto ester 8 (Table  2, entry 3). Results using the 4′-Br-substituted derivative 9 18 are identical to that of the trimethoxy system suggesting that the substitution of the aryl group coplanar to the ketone has little effect. Difluorinated substrate 10 19 led to an erosion of ee more so than the desmethoxy system. This is likely due to the less electron rich π system being unable to form a strong CH−π bond with the Cp* ligand 20 thus slowing conversion. To evaluate the ee of the starting materials, the reisolated material was converted to methyl rocaglate analogs 14 and 15.
Scheme 1. Synthesis of (+)-Aglaiastatin (2) and (−)-Aglaroxin C (3) from Hydrate (+)-5b favoring (±)-2 when heated in the presence of ammonium acetate (60°C, 12 h). Since the formation of aglaiastatin (2) and epi-aglaiastatin 18 is irreversible, the diastereoselectivity of these products may be controlled through the Curtin− Hammett principle 24 wherein cyclization occurs faster when the N-acyliminium reaction center is above the convex face of the rocaglate scaffold. Figure 5 shows ground state conformers of N-acyliminium intermediates leading to aglaiastatin (2) and epi-aglaiastatin 18 calculated in the gas phase using the B3LYP/ 6-31G* level of theory. 17 These N-acyliminiums feature acetate counterions that are hydrogen-bonded to both the hydroxyl 25 and enamine on the convex face of the rocaglate scaffold. Intermediate 20a (leading to aglaiastatin) is 2.0 kcal/mol lower in energy than its corresponding rotamer 20b. This energetic difference appears to be due to steric repulsion between the 3‴methylene of the pyrrolidine and the acetate counterion, and the N-acyliminium reaction center residing within the concave face of the cyclopenta[b]benzofuran scaffold. Through invoking the Hammond postulate, these intermediates provide a rationale for the observed 10:1 diastereoselectivity using acetate as counterion. When rotamers 20a and 20b were subjected to a similar computation with formate as the counterion, the corresponding formate structures were found to have an energy difference of 1.0 kcal/mol, 19 presumably because of the smaller counterion size. Furthermore, acetate is more basic than formate for deprotonation of the enamine as the nitrogen attacks the N-acyliminium. This explains the greater production of aglaiastatin (2) vs oxazinone 19 when acetate is the counterion as compared with formate (cf. Scheme 2). We also considered that the N-acyliminium cyclization may also occur through 6π-electrocyclization ( Figure 6) in which formation of aglaiastatin (2) is torquoselective through the aforementioned steric effects. 26 However, this mechanism would require Scheme 2. Experiments To Probe the Mechanism of N-Acyliminium Cyclizations a a Conditions: (a) ammonium acetate (10 equiv), THF, 60°C, 2 h; (b) formic acid (1 equiv), ammonium formate (15 equiv), THF, rt, 6 d (69% from (±)-5b; no conversion from (±)-2; (c) ammonium formate (10 equiv), THF, 60°C, 12 h (58% from (±)-5b; no conversion from (±)-2); (d) ammonium acetate (10 equiv), THF, 60°C, 12 h.   tautomerization and flattening of the six participating atoms in order to achieve the orbital alignment necessary for electrocyclization.

■ CONCLUSION
In summary, we have accomplished a highly enantio-and diastereoselective kinetic resolution of aglain ketones using Rh(III)-mediated transfer hydrogenation. This process allows rapid access to bioactive rocaglate natural products in a biomimetic fashion. From the substrate scope, it is also evident that this process can be used to access a variety of enantiopure aglain and rocaglate analogues. We further demonstrated the utility of the methodology by enantioselective syntheses of (+)-aglaiastatin (2) and (−)-aglaroxin C (3). Finally, we tested both enantiomers of aglaiastatin (2) and aglaroxin C (3) for several of the most prominent bioactivities reported for this compound class and found that the natural enantiomers were most potent. Moreover, we observed a concordance in potency for all derivatives across the three bioassays, suggesting there may be a single, conserved molecular target responsible for their activity. Further work toward the synthesis and biological evaluation of aglaiastatin and rocaglate analogs is currently in progress and will be reported in due course.

Notes
The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS
We thank the National Institutes of Health (R01 GM073855 and R01 CA175744) for research support, Dr. Jeffrey Bacon (Boston University) for X-ray crystal structure analyses, and Dr. Norman Lee (Boston University) for high-resolution mass spectrometry data. NMR (CHE-0619339) and MS (CHE-0443618) facilities at Boston University are supported by the NSF. Research at the Center for Chemical Methodology and Library Development at Boston University (CMLD-BU) was supported by NIH grant GM-067041. We also thank Professor John Snyder (Boston University), Prof. Amir Hoveyda (Boston College), Mr. Tian Qin (Boston University), Ms. Sarah Skraba-Joiner (University of New Hampshire), and Mr. Wenyu Wang (Boston University) for extremely helpful and stimulating discussions.