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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Am Chem Soc. Author manuscript; available in PMC 2010 September 23.
Published in final edited form as:
PMCID: PMC2774260
NIHMSID: NIHMS142516

Enantioselective Rhodium-Catalyzed [4+2+2] Cycloaddition of Dienyl Isocyanates for the Synthesis of Bicyclic Azocine Rings

Transition metal-catalyzed cycloadditions have proven among the most attractive methods to construct medium-sized ring systems.1 Although [4+4],2 [6+2],3 [5+2+1],4 and [4+2+2]5 cycloadditions have been elegantly demonstrated to assemble various eight-membered carbocycles, formation of eight-membered nitrogen-containing rings (azocines) has not been explored. In addition, there are no reported examples of successful enantioselective cycloadditions to construct eight-membered rings.6 We have recently demonstrated that Rh(I) catalysts are capable of effecting enantioselective [2+2+2] cycloadditions with the use of alkenyl heterocumulenes.7 Herein we describe a highly asymmetric rhodium-catalyzed [4+2+2] cycloaddition of terminal alkynes and dienyl isocyanates to afford bicyclo[6.3.0] azocine derivatives (eq 1).

equation image
(1)
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Bicyclo[6.3.0] azocine ring systems are unique architectures found in several biologically active compounds. Wang and coworkers have recently designed a potent XIAP antagonist, a small molecule consisting of the bicyclic azocine as the basic template.8 A number of manzamine alkaloids such as nakadomarin A and manzamine A, which exhibit potent antimalarial and antituberculosis activity, are equipped with such ring systems.9 Previous approaches to bicyclo[6.3.0] heterocycles have been stepwise including a ring-closing metathesis to afford the eight-membered ring.10

Our initial efforts to effect the [4+2+2] cycloaddition focused on 1-octyne 1a and the dienyl isocyanate 2 as a mixture of E/Z isomers (Table 1, entry 1). Treatment of the substrates with [Rh(C2H4)2Cl]2 modified with phosphoramidite L1 furnishes both the [4+2+2] cycloadduct 3a and the [2+2+2] cycloadduct 4a in 40% yield as an inseparable 4:1 mixture.11 Further investigation led to the isomerically pure diene (E)-2 as the optimal substrate to provide 3a selectively (entry 2).12 Despite a significant amount of unreacted isocyanate 2, the desired bicyclic azocine 3a is obtained with an exceptional enantioselectivity (99% ee). Replacing the pyrrolidinyl group on the phosphoramidite ligand with either the piperidine (L2) or azepine (L3) dramatically increases reactivity toward azocine ring formation while maintaining the high level of enantioselectivity (entries 3 – 4).13

Table 1
Development of the Rh-catalyzed [4+2+2] Cycloaddition
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With optimal conditions in hand, a variety of substituted bicyclic azocines can be synthesized in good yields and superb enantioselectivities (Chart 1). Alkyl alkynes bearing a chloride, a methyl ester, or an unprotected terminal alkyne (1b1d) all participate smoothly to provide the corresponding cycloadducts (3b3d). Alkynes possessing functionalities such as silyl ether, phthalimide, phenyl, and Boc-protected indole at the propargylic positions (1e1h) are well tolerated to furnish the [4+2+2] cycloadducts (3e – 3h) in good yields and identical enantioselectivities.14

Chart 1
Enantioselective Synthesis of [6.3.0] Bicyclic Azocines

Cycloaddition of isocyanates with substitution at the diene portion is also feasible. For example, when 2-methyl dienyl isocyanate 5 is reacted under the standard conditions, [4+2+2] cycloadditions with various alkynes all proceed uneventfully (6a, 6e, 6j).15 Reactions with aryl alkynes, however, proceed only in moderate yield. With 1-bromo-4-ethynylbenzene (1i), cycloadduct 3i can only be obtained in 35% isolated yield with the same high enantioselectivity.

Several aspects of these findings suggest that there may be a mechanistic divergence from our previously developed reaction. Prime among these is the invariant enantioselectivity with regard to alkyne structure as well as the failure to observe any vinylogous amide adducts in this chemistry. In order to gain insight into the reaction mechanism, we conducted a competition experiment between dienyl isocyanates 2 and 5. If oxidative cycloaddition occurs between the alkyne and isocyanate first (path a in Scheme 1), the ratio of products 3 and 6 should be 1:1.7h In the event, 3 is formed with 2:1 selectivity over 6.16 We suggest that this is most consistent with initial oxidative cyclization between the diene and isocyanate following path b to form V. Coordination and insertion of alkyne then provides the [4+2+2] adduct. With more reactive nucleophilic alkynes, path a becomes competitive forming rhodacycle II. Diene coordination and insertion is slow, presumably for steric reasons, allowing competitive alkyne insertion to form pyridone. The diene found in Z-2 is a poor ligand for Rh and thus prefers path a, leading to increased amounts of both 4 and pyridone.17

Scheme 1
Proposed Mechanism
equation image
(2)

The Rh-catalyzed cycloaddition protocol allows access to synthetically useful bicyclic azocines. Dihydroxylation affords diol 7 in 72% yield for the major diastereomer (7:1 dr, eq 2). Alternately, an α,β-unsaturated aldehyde functionality can be readily unmasked in two simple steps from 3e, eq 3.

equation image
(3)

In conclusion, we have developed the first enantioselective rhodium-catalyzed [4+2+2] cycloaddition of terminal alkynes and dienyl isocyanates. The process provides access to highly functionalized bicyclo[6.3.0] azocine ring systems with exceptional enantioselectivities. Further studies on the full scope of this new process are in progress.

Acknowledgments

We thank NIGMS (GM080442) for support. We gratefully acknowledge Johnson Matthey for a loan of Rh salts and BioTools for VCD experiments.

Footnotes

Supporting Information Available: Experimental procedures, characterization, 1H and 13C NMR spectra are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

Supplementary Material

1_si_001

References

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2. For selected references, see: Wender PA, Ihle NC. J Am Chem Soc. 1986;108:4678.(b) Wender PA, Tebbe MJ. Synthesis. 1991:1089. (c) Wender PA, Nuss JM, Smith DB, Suárez-Sobrino A, Vågberg J, Decosta D, Bordner J. J Org Chem. 1997;62:4908.
3. Wender PA, Correa AG, Sato Y, Sun R. J Am Chem Soc. 2000;122:7815.
4. Wender PA, Gamber GG, Hubbard RD, Zhang L. J Am Chem Soc. 2002;124:2876. [PubMed]
5. (a) Evans PA, Robinson JE, Baum EW, Fazal AN. J Am Chem Soc. 2002;124:8782. [PubMed] (b) Gilbertson SR, DeBoef B. J Am Chem Soc. 2002;124:8784. [PubMed] (c) Varela JA, Castedo L, Saá C. Org Lett. 2003;5:2841. [PubMed] (d) Evans PA, Baum EW. J Am Chem Soc. 2004;126:11150. [PubMed] (e) Evans PA, Baum EW, Fazal AN, Pink M. Chem Commun. 2005:63. [PubMed] (f) Lee SI, Park SY, Chung YK. Adv Synth Catal. 2006;348:2531. (g) Murakami M, Ashida S, Matsuda T. J Am Chem Soc. 2006;128:2166. [PubMed] (h) Wender PA, Christy JP. J Am Chem Soc. 2006;128:5354. [PubMed] (i) DeBoef B, Counts WR, Gilbertson SR. J Org Chem. 2007;72:799. [PubMed] (j) Hilt G, Janikowski J. Angew Chem Int Ed Engl. 2008;47:5243. [PubMed]
6. In their full paper, Gilbertson and coworkers reported a single example of 41% ee as the highest selectivity observed. See: ref 5h.
7. (a) Yu RT, Rovis T. J Am Chem Soc. 2006;128:2782. [PubMed] (b) Yu RT, Rovis T. J Am Chem Soc. 2006;128:12370. [PubMed] (c) Yu RT, Rovis T. J Am Chem Soc. 2008;130:3262. [PubMed] (d) Lee EE, Rovis T. Org Lett. 2008;10:1231. [PubMed] (e) Yu RT, Lee EE, Malik G, Rovis T. Angew Chem Int Ed. 2009;48:2379. [PMC free article] [PubMed] (f) Oberg KM, Lee EE. Tetrahedron. 2009;65:5056. [PMC free article] [PubMed] (g) Friedman RK, Rovis T. J Am Chem Soc. 2009;131:10775. [PubMed] (h) Dalton DM, Oberg KM, Yu RT, Lee EE, Perreault S, Oinen ME, Pease ML, Malik G, Rovis T. Submitted. [PMC free article] [PubMed]
8. Sun H, Nikolovska-Coleska Z, Lu J, Meagher JL, Yang C-Y, Qiu S, Tomita Y, Ueda Y, Jiang S, Krajewski K, Roller PP, Stuckey JA, Wang S. J Am Chem Soc. 2007;129:15279. [PMC free article] [PubMed]
9. For their representative total syntheses, see: Winkler JD, Axten JM. J Am Chem Soc. 1998;120:6425. [PMC free article] [PubMed](b) Humphrey JM, Liao Y, Ali A, Rein T, Wong Y-L, Chen H-J, Courtney AK, Martin SF. J Am Chem Soc. 2002;124:8584. [PubMed] (c) Nagata T, Nakagawa M, Nishida A. J Am Chem Soc. 2003;125:7484. [PubMed] (d) Young IS, Kerr MA. J Am Chem Soc. 2007;129:1465. [PubMed]
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11. This reaction also forms ~4% pyridone. Conducting this reaction at 0.06 M in 2 leads to 25% combined yield of 3 and 4 in a 3:1 ratio along with ~10% pyridone.
12. Further studies on [2+2+2] cycloadditions with various 1,2-disubstituted alkenyl isocyanates are ongoing.
13. We observe symmetrical ureas derived from the isocyanate as the only significant byproduct. No regioisomers have been observed.
14. Larger scale reactions may be conducted with lower catalyst loading and slightly higher concentration; with 3 mol % [Rh(C2H4) 2Cl]2 and 6 mol% L3 at 0.073M using 1.5 mmol of 2, 3e is formed in 68% yield and 99% ee.
15. Substitution at the terminus of the diene leads to only [2+2+2] adduct under these conditions (E,E-octa-4,6-dienyl isocyanate and 1a afford 4a’ in 46% yield, 46% ee). Bicyclo [6.4.0] systems are not accessible under these conditions.
16. At higher catalyst loading (25 mol% [Rh(C2H4)2Cl]2), 3 and 6 are formed quantitatively in a 4:1 ratio.
17. At 0.02 M, no pyridone is observed with E-2. At 0.1 M, we see <5% pyridone (75% yield of 3a). Also see entry 1, Table 1 and footnote 11.