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An Unexpected Ireland–Claisen Rearrangement Cascade During the Synthesis of the Tricyclic Core of Curcusone C

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An Unexpected Ireland–Claisen Rearrangement Cascade During the Synthesis of the Tricyclic Core of Curcusone C Subscriber access provided by Caltech Library Article An unexpected Ireland–Claisen rearrangement cascade during the synthesis of the tricyclic core of Curcusone C: Mechanistic elucidation by trial-and- error and automatic artificial force-induced reaction (AFIR) computations Chung Whan Lee, Buck L. H. Taylor, Galina P. Petrova, Ashay Patel, Keiji Morokuma, K. N. Houk, and Brian M. Stoltz J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. 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ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts. is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Page 1 of 12 Journal of the American Chemical Society 1 2 3 4 5 6 7 An unexpected Ireland–Claisen rearrangement cascade dur- 8 ing the synthesis of the tricyclic core of Curcusone C: Mech- 9 10 anistic elucidation by trial-and-error and automatic artificial 11 12 force-induced reaction (AFIR) computations 13 †║§ ‡ § ⊥ ‡# ⊥¶a 14 Chung Whan Lee, Buck L. H. Taylor, ∇ Galina P. Petrova, Ashay Patel, Keiji Morokuma, K. 15 N. Houk,‡* Brian M. Stoltz†* 16 † 17 Warren and Katharine Schlinger Laboratory for Chemistry and Chemical Engineering, Division of Chemistry and Chemical 18 Engineering, California Institute of Technology, Pasadena, California 91125, United States 19 ‡Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, United States 20 ⊥Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto 606-8103, Japan 21 22 a 23 Deceased November 27, 2017 24 KEYWORDS natural products, unexpected rearrangement, curcusone, total synthesis, divinylcyclopropane rearrangement 25 26 27 ABSTRACT: In the course of a total synthesis effort directed toward the natural product curcusone C, the Stoltz group discovered 28 an unexpected thermal rearrangement of a divinylcyclopropane to the product of a formal Cope/1,3-sigmatropic shift sequence. 29 Since the involvement of a thermally forbidden 1,3-shift seemed unlikely, theoretical studies involving two approaches, the “trial- 30 and-error” testing of various conceivable mechanisms (Houk group) and an “automatic” approach using the Maeda–Morokuma 31 AFIR method (Morokuma group) were applied to explore the mechanism. Eventually, both approaches converged on a cascade 32 mechanism shown to have some partial literature precedent: Cope rearrangement/1,5-sigmatropic silyl shift/Claisen rearrange- 33 ment/retro-Claisen rearrangement/1,5-sigmatropic silyl shift, comprising a quintet of five sequential thermally-allowed pericyclic 34 rearrangements. 35 36 37 1. Introduction likely to occur. Various mechanisms are then explored, guid- 38 ed by chemical intuition and prior results. Intermediates and 39 In the course of multistep total synthesis efforts directed to- ward a complex molecule, organic chemists sometimes seren- transition states are optimized to obtain energetics for each 40 dipitously discover unexpected structures that arise from un- proposed mechanism, and pathways with activation energies 41 known rearrangement cascades. It is often difficult to deter- too high to be possible are eliminated from consideration. We 42 mine the reaction mechanism. Besides the chemists’ intrinsic refer to this procedure as the “trial-and-error” approach. Gen- 43 curiosity about how the reaction occurs, it is important to in- erally, one mechanism consistent with experimental rates is 44 vestigate an unexpected reaction to understand its potential obtained in this way, but it is always possible that the actual 45 mechanism in order to expand the potential of the reaction. mechanism has not been discovered. 46 Computation has become an important tool to assist in mecha- Because of this lingering concern about overlooking the 1 47 nistic reasoning. As density functional theory (DFT) methods “global minimum mechanism,” or to avoid the use of chemical 48 have become more able to deal with the relatively large (for intuition, chemists have developed new algorithms to predict mechanisms.2,3 One of these, AFIR (artificial force-induced 49 theory) molecules studied by synthetic chemists, it has become an able partner with experiment in exploring mechanism. This reaction), devised by Maeda and Morokuma, involves apply- 50 paper describes a synthetic approach to curcusone C, inter- ing forces to stretch bonded atoms apart or force non-bonded 51 rupted by an unexpected skeletal rearrangemented. Because atoms together, and then a series of calculations to map out 2 52 the mechanism of this rearrangement was not clear, and the low energy mechanisms from reactant to product. This has 53 only likely possibility is forbidden by the Woodward– led to a family of such methods and reasonable success in 54 Hoffmann rules, computations were undertaken to uncover finding known mechanisms and sometimes discovering new 55 other plausible mechanisms. mechanisms. Other methods such as Martinez’s “nanoreactor” 56 The conventional approach to computational mechanism use high temperature molecular dynamics to determine reac- tion mechanisms, but this method has been applied only to 57 elucidation involves first selecting a method, nowadays one of 3 58 the DFT methods, that will give accurate (± 1 or 2 kcal/mol) small gas-phased processes. MNY methods, such as Chan- dler’s “throwing ropes over rough mountain passes – in the 59 energetics for the atoms involved and the types of mechanisms 60 ACS Paragon Plus Environment Journal of the American Chemical Society Page 2 of 12 dark,”4 or Zimmerman’s “Growing String Method”5 are more O O O H HH H 1 or less automatic ways of finding transition states for conver- 1 2 1 H H H R 10 8 10 8 sion of one molecule to another but require intensive calcula- 4 2 R1 2 4 3 tions to locate the lowest energy pathway. All of these meth- HO MeO ods are designed to find the best pathway across a potential O O O 4 Curcusone R1 R2 surface to convert reactants into products, but are less appro- Curcusone E (5) Curcusone F (6) 5 A (1) Me H 6 priate for multi-step mechanisms that often occur in organic B (2) H Me chemistry. C (3) Me OH 7 D (4) OH Me 8 In the course of synthetic studies by the Stoltz group toward O O O H 9 the natural product curcusone C, an unexpected reaction prod- H H H uct was encountered from a synthetic intermediate. Chal- H H H 10 OH R O O lenged to use computations to determine the mechanism of the OH 11 MeO MeO reaction, the Houk and Morokuma groups agreed to test the O O O 12 conventional “trial-and-error” and the automatic Maeda– Curcusone G (7) Curcusone H (8) R = β-Me : Curcusone I (9) R = α-Me : Curcusone J (10) 13 Morokuma AFIR methods. The calculations were performed originally reported structures 14 by the Houk and Morokuma groups, respectively. We wished 15 to see which method worked best and the relative time needed H 16 to determine an unknown mechanism for a real organic prob- H lem. In the event, Morokuma’s group using AFIR, and O 17 O Houk’s group, using the “trial-and-error” methods, converged 18 Spirocurcasone (11) 19 on the same mechanism for which experimental analogy was then found in the chemical literature. Both took about one 20 Figure 1. Curcusones A−J (1–10) and Spirocurcasone (11). year of part-time human effort and significant computer time. 21 This paper describes a general mechanism for the unusual Curcusones (1–10) possess novel tricyclic skeletons featur- 22 reaction discovered and provides an assessment of the apti- ing a 2,3,7,8-tetrahydroazulene-1,4-dione moiety with four 23 tudes of currently available methods to solve such problems. stereogenic carbon centers. Since the initial isolation in 1986, 24 2. Background a completed total synthesis has not been reported, although 25 Dai recently completed elegant syntheses of the proposed 26 Native to Central America, Jatropha curcas is a species of structures of curcusones I and J (9 and 10), but unfortunately flowering plant belonging to the Euphorbiaceae family that the data for the synthetic material did not match the isolation 27 can be found in many parts of the world. J. curcas has been 28 spectra, calling the original structural assignments into ques- used as a source of soap and lamp oil for hundreds of years, tion.7e Additionally, one methodological study for the con- 29 and recently J.
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