Cyclization for the Synthesis of

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Alternative Sm(II) Species-Mediated Cascade Coupling/Cyclization for the Synthesis of Oxobicyclo[3.1.0]hexane-1-ols Bingxin You,† Mengmeng Shen,† Guanqun Xie,†,‡ Hui Mao,† Xin Lv,† and Xiaoxia Wang*,†,‡ †

Department of Chemistry, College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, People’s Republic of China ‡ School of Environment and Civil Engineering, Dongguan University of Technology, Dongguan, 523808, People’s Republic of China S Supporting Information *

ABSTRACT: The allylSmBr/HMPA/MsOH system has been found to be an efficient reagent for the “ester-alkene” coupling/cyclization cascade of readily available α-allyloxy esters. Oxobicyclo[3.1.0]hexane-1-ols were thus prepared in good to excellent yields and diastereoselectivities. Investigation on the mechanism suggested the possible existence of a new Sm(II) species, namely, CH3SO3SmBr, which resulted from the reaction between allylSmBr and MsOH and may be the actual SET reagent. Scheme 1. AllylSmBr/Additives-Promoted “Ester-alkene” Coupling/Cyclization Reactions

D

ivalent samarium regents, especially samarium diiodide (SmI2) have been widely employed in C−C bondforming reactions important in the synthesis of natural products.1 The mechanism of these reactions continues to be an important area of study, given the value of transformations mediated by Sm(II)-based reagents.2 Among the reactions promoted by SmI2, the coupling of a ketone or an aldehyde with an alkene (the “carbonyl-alkene” coupling) has been welldocumented. In contrast, acid derivatives as the coupling partner with alkenes mediated by divalent samarium reagent are less known. Limited examples employed activated carboxylic acid derivatives, such as acid chlorides,3 and the acyl-type radical intermediate was involved. Unactivated esters and amides were inert to SmI2 alone, and their reduction by the reagent was reported to require the presence of either strong acid or base additive, or amine/H2O.4 The “ester-alkene” coupling mediated by SmI2 proved very challenging, since the first step is highly endergonic.5 In recent years, the Procter group6a−f and Szostak6g,h et al. have established that H2O or the combination of H2O with other co-additives could greatly enhance the reactivity of SmI2 and also achieve the intramolecular coupling of a lactone or an imide with unactivated CC bond (even an aromatic ring) to construct fused-ring or spirocyclic scaffolds. Coupling between unactivated esters and CC bond was alternatively achieved by allylSmBr in the presence of HMPA and other co-additives, such as HMPA/ H2O and HMPA/CuCl2·2H2O (Scheme 1, eqs 1 and 2).7 AllylSmBr has been used as a nucleophilic reagent8 and, only in recent years, the reagent’s inclination to revert to the more stable samarium(III) oxidation has drawn attention. The group of Zhang reported the dual role of allylSmBr in the preparation of 1,4-dienes and trienes starting from α-halo ketones and γhalo-α,β-unsaturated ketones.9a We unexpectedly found that allylSmBr could promote reductive debenzotriazoylation.9b Later on, the reactivity of the reagent to serve as the SET reagent was found to be tunable by additives, such as HMPA,10a © XXXX American Chemical Society

H2O,10b MeOH,10b and (EtO)2P(O)H.10c The combination of various additives may make allylSmBr a versatile SET reagent. To further explore the use of allylSmBr in the preparation of structurally diverse compounds, we envisioned that the readily available α-allyloxy esters 1 may be useful substrates for the preparation of oxobicyclo[3.1.0]hexane-1-ols 2, based on the allylSmBr/additive mediated “ester-alkene” coupling cascade (Scheme 1, eq 3). It is noteworthy that, although 1 seemed to be reasonable substrates for the preparation of 2 by Kulinkovich cyclopropanation strategy,11 the allylic ethers were unfortunately found to be inapplicable to such a tranformation,12 because of the efficient deprotection of allyl ethers into the corresponding alcohols under the Kulinkovich cyclopropanation conditions.13 Received: November 21, 2017

A

DOI: 10.1021/acs.orglett.7b03613 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Therefore, the successful intramolecular “ester-alkene” coupling in 1 would demonstrate the unique advantage of the allylSmBrbased reagent. As shown in Table 1, the reaction conditions were investigated using substrate 1a as the model substrate. Initially,

the employment of SmI2/HMPA/MsOH was not effective. When the reaction proceeded for 1 min, the starting materials remained almost intact and only a trace amount of the desired product could be detected. With the extension of time, the starting material was consumed gradually. However, only a very complicated mixture was obtained, which defied further analysis (Table 1, entry 17). Thus, the aged allylSmBr/HMPA/MsOH system was found to be a suitable reagent for the cascade cyclization/coupling reaction of 1a. With the optimized conditions in hand, the scope of the reaction was explored. The results are summarized in Scheme 2. A variety of methyl or ethyl esters 1, with R or R1 being aryl, allyl, or alkyl, all underwent the cascade ester-alkene radical cyclization, smoothly affording the desired oxobicyclo[3.1.0]hexane-1-ols 2 in moderate to excellent yields and diastereoselectivities. When the benzyl ester of mandelic acid 1a′ was used as the substrate, a complicated mixture was resulted. Reductive removal of the α-allyloxy did not occur to a significant extent, except for the tert-butyl ester 1a″, which afforded tert-butyl 2-phenylacetate in 50% yield. Slight debromination during the formation of 2g was detected, which was contaminated by 2a in 7% yield. It is noticeable that products 2q−2t were sensitive to common silica chromatography, and Et3N must be used to pretreat the silica column. As for the stereoselectivity, more than 80% selectivity was observed in most cases. To have deeper insight in the diastereoselectivity, the single crystal of 2j14 (Figure 1) was developed, and the X-ray diffraction analysis showed the hydroxyl and the naphthyl assumed a cis-configuration as the major diastereomer. The mechanism of the reaction was further investigated. At the beginning of the conditions optimization stage, MsOH was attempted as a proton source. However, the role of MsOH may be reconsidered, based on the observations that, when MsOH was added to the allylSmBr/HMPA reagent, an overnight stirring of the mixture was required to ensure better yields. It may imply MsOH was not simply a proton source and a different reducing species may be generated during the overnight “aging” process. AllylSmBr/HMPA might react with CH3SO3H gradually to generate a CH3SO3SmBr/HMPA complex,15 which could be indispensable for the coupling/ cyclization reaction here. The assumption may be supported by the control experiments. (1) 1a was subjected to allylSmBr/HMPA and allylSmBr/ MsOH reagent, respectively. In the former case, the diallylation product 3a was isolated as the major product, together with 2a and other unidentified byproducts, while no reaction could be observed between 1a and allylSmBr/MsOH (Table 1, entries 1 and 17; also see Scheme 3, eqs 1 and 2). The lack of nucleophilicity in the latter case may imply the exclusion of the nuleophilic allylSmBr species. (2) Instead of addition of HMPA first in the standard conditions, MsOH (2 equiv) was added prior to HMPA to the allylSmBr in THF. The characteristic purple color of allylSmBr faded immediately and a gray-blue suspension was obtained (it was proposed as CH3SO3SmBr). When HMPA was then added, the purple color regenerated immediately (the probable generation of CH3SO3SmBr/HMPA complex). (3) To the above recipe (namely, allylSmBr/MsOH/HMPA, where MsOH was added to the allylSmBr and the mixture was stirred 10 min before HMPA was introduced and the resulting mixture was stirred for 30 min more) was added 1a. In this

Table 1. Optimization of the Coupling/Cyclization Conditionsa

entry

additive (equiv)

temp (°C)

reaction time

yieldb (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

c H2O (1.6) CuCl2·2H2O (1.6) MeOH (1.6) t BuOH (1.6) CH3COOH (1.6) HCOOH (1.6) CF3COOH (1.6) TsOH·H2O (1.6) CF3SO3H (1.6) MsOH (1.6) MsOH (2.0) MsOH (1.0) MsOH (2.0) MsOH (2.0) MsOH (2.0)g MsOH (2.0)h

rt rt rt rt rt rt rt rt rt rt rt rt rt −20 −30 rt rt

overnight overnight overnight overnight overnight 10 min 10 min overnight 2h 10 min 1 min 1 min 20 min 20 min 20 min 1 min overnight

17d 40 8e 35 32 51 30 f 45 45 57 58 39 52 50 70 trace

a

Reaction conditions: a mixture of Sm powder (4.67 equiv), THF (15 mL), and allylBr (3.0 equiv) was stirred under N2 at rt for 1 h. Then, HMPA (12 equiv) was added via syringe (unless otherwise specified). After additional stirring for 10 min, CH3SO3H was added. Stirring was continued for another 10 min and 1a (1.0 mmol) in THF was added via syringe (unless otherwise specified). bIsolated yield. cNo other additive was used besides HMPA. dTogether with 30% of the diallylation products and other unidentified mixtures. eComplicated unidentified mixture. fNo reaction. gThe mixture of the allylSmBr/ HMPA/MsOH was stirred overnight before the introduction of 1a. h SmI2 was used instead of allylSmBr.

HMPA was attempted as the only additive and 17% yield of the desired product 2a was obtained (Table 1, entry 1). The addition of H2O as the co-additive afforded 40% yield (Table 1, entry 2). Using CuCl2·2H2O7b afforded an even poorer yield (Table 1, entry 3). However, the combination of MeOH or t BuOH as the proton source was less effective (Table 1, entries 4 and 5). Organic acids were also examined as co-additives. Satisfyingly, the use of CH3COOH afforded 2a in 51% yield (Table 1, entry 6). The use of HCOOH, CF3SO3H, or TsOH· H2O did not bring about any improvement (Table 1, entries 7, 9, and 10). CF3COOH seemed totally destructive, since the characteristic purple color of Sm(II)-HMPA complex disappeared upon its addition (Table 1, entry 8). MsOH afforded slightly better yield (57%) than CH3COOH (51%). Increasing the loading of the acid to 2 equiv led to a slightly higher yield of 2a, while decreasing the acid loading was unfavorable (Table 1, entries 12 and 13). Attempts to conduct the reaction at lower temperature did not afford better yields (Table 1, entries 14 and 15). However, when the mixture of allylSmBr/HMPA/ MsOH was stirred overnight before the introduction of 1a, a 70% yield of 2a was obtained (Table 1, entry 16). In contrast, B

DOI: 10.1021/acs.orglett.7b03613 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 2. Scope of the AllylSmBr/HMPA/MsOH Promoted Cascade Synthesis of Oxobicyclo[3.1.0]hexane-1-olsa,b

Figure 1. X-ray crystal structure of 2j.

Scheme 3. Control Experiments of the Reactivity of AllylSmBr/Additive(s)

Based on the above experiments, a possible mechanism was proposed in Scheme 4. AllylSmBr I reacted with HMPA and Scheme 4. Proposed Mechanism for the Formation of 2

a

Reaction conditions: a mixture of Sm powder (4.67 equiv), THF (15 mL) and allylBr (3.0 equiv) was stirred under N2 at rt for 1 h. Then HMPA (12 equiv) was added via syringe. After additional stirring for 10 min, MsOH (2.0 equiv, 129 μL) was added to the mixture and stirring at rt was continued overnight. Substrate 1a in 2 mL of THF was introduced and the reaction was completed immediately. bIsolated yield. cContaminated by 7% yield of 2a, which was generated by reductive removal of bromo. d2t tended to deteriorate quickly and the NMR of 2t was inevitably contaminated with minor impurities. Change of the deuterated solvent from CDCl3 to DMSO-d6 did not improve the spectra.

MsOH slowly to produce a CH3SO3SmBr/HMPA complex II.16 II may act as a more-powerful SET reagent than SmI2/ HMPA and was able to reduce the ester group to the ketyl III, which would take a more stable transition state and undergo the ketyl-alkene coupling to cyclize to IV. IV accepted a second electron from II and was transformed to the carbanion V. The subsequent intramolecular nucleophilic addition constructed the oxobicyclo[3.1.0]hexane-1-ol skeleton VI and finally afforded product 2 after workup. In summary, several acids could be used as the additive to allylSmBr to promote the reductive cyclization of α-allyloxy esters. The selective electron transfer to the unactivated ester carbonyl is impressive, and the oxobicyclo[3.1.0]hexane-1-ols

case, the desired 2a was produced in comparable yield (Scheme 3, eq 3) to that obtained from the optimized conditions (Table 1, entry 16). Besides, the generation of propene during the aging process has been detected by GC (comparison with authentic propene) and GC-MS (see the Supporting Information). C

DOI: 10.1021/acs.orglett.7b03613 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

4579. (b) Kolmar, S. S.; Mayer, J. M. J. Am. Chem. Soc. 2017, 139, 10687. (c) Huq, S. R.; Shi, S.; Diao, R.; Szostak, M. J. Org. Chem. 2017, 82, 6528−6540. (d) Chciuk, T. V.; Flowers, R. A., II. J. Am. Chem. Soc. 2015, 137, 11526. (3) Liu, Y. J.; Zhang, Y. M. Tetrahedron Lett. 2004, 45, 1295. (4) For selected examples, see: (a) Kamochi, Y.; Kudo, T. Chem. Lett. 1991, 20, 893. (b) Kamochi, Y.; Kudo, T. Bull. Chem. Soc. Jpn. 1992, 65, 3049. (c) Kamochi, Y.; Kudo, T. Chem. Lett. 1993, 22, 1495. (d) Szostak, M.; Spain, M.; Eberhart, A. J.; Procter, D. J. J. Am. Chem. Soc. 2014, 136, 2268. (e) Huq, S. R.; Shi, S.; Diao, R.; Szostak, M. J. Org. Chem. 2017, 82, 6528. (5) Mechanistic study of ester reductions: (a) Szostak, M.; Spain, M.; Eberhart, A. J.; Procter, D. J. J. Am. Chem. Soc. 2014, 136, 2268. (b) Szostak, M.; Spain, M.; Procter, D. J. Chem.Eur. J. 2014, 20, 4222. (c) Szostak, M.; Spain, M.; Procter, D. J. J. Am. Chem. Soc. 2014, 136, 8459. (d) Chciuk, T. V.; Anderson, W. R.; Flowers, R. A., II. J. Am. Chem. Soc. 2016, 138, 8738. (e) Kolmar, S. S.; Mayer, J. M. J. Am. Chem. Soc. 2017, 139, 10687. (6) For selected examples, see: (a) Huang, H.-M.; Procter, D. J. J. Am. Chem. Soc. 2017, 139, 1661. (b) Huang, H.-M.; Procter, D. J. J. Am. Chem. Soc. 2016, 138, 7770. (c) Szostak, M.; Sautier, B.; Spain, M.; Behlendorf, M.; Procter, D. J. Angew. Chem., Int. Ed. 2013, 52, 12559. (d) Parmar, D.; Matsubara, H.; Price, K.; Spain, M.; Procter, D. J. J. Am. Chem. Soc. 2012, 134, 12751. (e) Parmar, D.; Price, K.; Spain, M.; Matsubara, H.; Bradley, P. A.; Procter, D. J. J. Am. Chem. Soc. 2011, 133, 2418. (f) Parmar, D.; Duffy, L. A.; Sadasivam, D. V.; Matsubara, H.; Bradley, P. A.; Flowers, R. A., II; Procter, D. J. J. Am. Chem. Soc. 2009, 131, 15467. (g) Shi, S.; Szostak, M. Org. Lett. 2015, 17, 5144. (h) Shi, S.; Lalancette, R.; Szostak, R.; Szostak, M. Chem. Eur. J. 2016, 22, 11949. (7) (a) Tu, Y. W.; Zhou, L. J.; Yin, R. F.; Lv, X.; Flowers, R. A., II; Choquette, K. A.; Liu, H. L.; Niu, Q. S.; Wang, X. X. Chem. Commun. 2012, 48, 11026. (b) Shen, M. M.; Tu, Y. W.; Xie, G. Q.; Niu, Q. S.; Mao, H.; Xie, T. T.; Flowers, R. A., II; Lv, X.; Wang, X. X. J. Org. Chem. 2015, 80, 52. (8) For selected examples, see: (a) Li, Z.; Cao, X.; Lai, G.; Liu, J.; Ni, Y.; Wu, J.; Qiu, H. J. Organomet. Chem. 2006, 691, 4740. (b) Fan, X.; Zhang, Y. Tetrahedron Lett. 2002, 43, 5475. (c) Yu, M.; Zhang, Y. Gaodeng Xuexiao Huaxue Xuebao 2003, 24, 1618. (d) Gao, X.; Wang, X.; Cai, R.; Wei, J.; Wu, S. Huaxue Xuebao 1993, 51, 1139. (9) (a) Hu, Y.; Zhao, T.; Zhang, S. Chem.Eur. J. 2010, 16, 1697. (b) Zhong, Z.; Hong, R.; Wang, X. Tetrahedron Lett. 2010, 51, 6763. (10) (a) Yin, R.; Zhou, L.; Liu, H.; Mao, H.; Lü, X.; Wang, X. Chin. J. Chem. 2013, 31, 143. (b) Li, J. Y.; Niu, Q. S.; Li, S. C.; Sun, Y. H.; Zhou, Q.; Lv, X.; Wang, X. X. Tetrahedron Lett. 2017, 58, 1250. (c) Li, Y.; Hu, Y. Y.; Zhang, S. L. Chem. Commun. 2013, 49, 10635. (11) For general reviews, see: (a) Kulinkovich, O. G.; de Meijere, A. Chem. Rev. 2000, 100, 2789. (b) Sato, F.; Urabe, H.; Okamoto, S. Chem. Rev. 2000, 100, 2835. (12) Lecornué, F.; Ollivier, J. Org. Biomol. Chem. 2003, 1, 3600. (13) Lee, J.; Cha, J. K. Tetrahedron Lett. 1996, 37, 3663. (14) The crystal data of 2k have been deposited at Cambridge Crystallographic Data Center, U.K., and the CCDC reference number is 1582390. (15) The formula may not represent the correct structure of Sm(II) species. AllylSmBr and CH3SO3SmBr probably also coordinate with THF. (16) For selected relevant references on Sm(II)-HMPA, see: (a) Miller, R. S.; Sealy, J. M.; Shabangi, M.; Kuhlman, M. L.; Fuchs, J. R.; Flowers, R. A., II. J. Am. Chem. Soc. 2000, 122, 7718. (b) Knettle, B. W.; Flowers, R. A., II. Org. Lett. 2001, 3, 2321. (c) Prasad, E.; Flowers, R. A., II. J. Am. Chem. Soc. 2002, 124, 6895. (d) Sadasivam, D. V.; Antharjanam, P. K. S.; Prasad, E.; Flowers, R. A., II. J. Am. Chem. Soc. 2008, 130, 7228.

were prepared in moderate to good yields with good to excellent diastereoselectivities. Initial probe on the mechanism indicated the possible formation of a CH3SO3SmBr/HMPA complex, which may act as an important Sm(II) species for achieving such an “ester-alkene” coupling reaction. However, more investigations concerning the mechanism remain to be conducted. The applications of the allylSmBr-based divalent samarium reagent are still underway in our laboratory and will be reported in due course.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03613. Detailed experimental procedures; spectral data of new substrates and products (PDF) Accession Codes

CCDC 1582390 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiaoxia Wang: 0000-0001-5094-0269 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Natural Science Foundation of Zhejiang Province (Nos. LY14B020001 and LY16B020003), and National Natural Science Foundation of China (No. 21202152).

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REFERENCES

(1) For selected reviews on SmI2-mediated reactions, see: (a) Shi, S.; Szostak, M. Molecules 2017, 22, 2018. (b) Chciuk, T. V.; Flowers, R. A., II. The Role of Solvents and Additives in Reactions of Sm(II) Iodide and Related Reductants. In Science of Synthesis Knowledge Updates, Vol. 2; Marek, I., Ed.; Thieme: Stuttgart, Germany, 2016; 177 pp. (c) Just-Baringo, X.; Procter, D. J. Acc. Chem. Res. 2015, 48, 1263− 1275. (d) Choquette, K. A.; Flowers, R. A., II. Sm and Yb Reagents. In Comprehensive Organic Synthesis, 2nd Edition; Molander, G. A., Knochel, P., Eds.; Elsevier: Oxford, 2014; Vol. 1, 279 pp. (e) Szostak, M.; Fazakerley, N. J.; Parmar, D.; Procter, D. J. Chem. Rev. 2014, 114, 5959. (f) Szostak, M.; Spain, M.; Procter, D. J. Chem. Soc. Rev. 2013, 42, 9155. (g) Harb, H. Y.; Procter, D. J. Synlett 2012, 2012, 6. (h) Beemelmanns, C.; Reißig, H.-U. Chem. Soc. Rev. 2011, 40, 2199. (i) Procter, D. J.; Flowers, R. A., II; Skrydstrup, T. Organic Synthesis Using Samarium Diiodide: A Practical Guide; Royal Society of Chemistry: London, 2010. For selected recent examples concerning SmI2-mediated organic reactions, see: (j) Zhang, B.; Wang, X.; Cheng, C.; Sun, D.; Li, C. Angew. Chem., Int. Ed. 2017, 56, 7484. (k) Leng, L.; Zhou, X.; Liao, Q.; Wang, F.; Song, H.; Zhang, D.; Liu, X.-Y.; Qin, Y. Angew. Chem., Int. Ed. 2017, 56, 3703. (2) For selected mechanism studies, see: (a) Chciuk, T. V.; Anderson, W. R., Jr.; Flowers, R. A., II. Organometallics 2017, 36, D

DOI: 10.1021/acs.orglett.7b03613 Org. Lett. XXXX, XXX, XXX−XXX