Synthesis of Cyclopropanoids via Substrate-Based Cyclization Pathways

Article abstract of DOI:10.1021/acs.orglett.8b03537

A series of unexpected reactions triggered by the dimethyloxosulfonium methylide led to the discovery of unconventional approaches for the synthesis of cyclopropa-fused tetralones and indeno-spirocyclopropanes. These highly functionalized structures were further elaborated in one step to privileged scaffolds such as tetralones, indenones, and fluorenones. As a whole, the results presented herein establish new diversity-oriented folding pathways.

Full text of DOI:10.1021/acs.orglett.8b03537

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis of Cyclopropanoids via Substrate-Based Cyclization
Pathways
Uttam K. Mishra, Kaushalendra Patel, and S. S. V. Ramasastry*
Organic Synthesis and Catalysis Lab, Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER)
Mohali, Sector 81, Manauli PO, S. A. S. Nagar, Punjab 140306, India
S
* Supporting Information
ABSTRACT: A series of unexpected reactions triggered by
the dimethyloxosulfonium methylide led to the discovery of
unconventional approaches for the synthesis of cyclopropa-
fused tetralones and indeno-spirocyclopropanes. These highly
functionalized structures were further elaborated in one step
to privileged scaffolds such as tetralones, indenones, and
fluorenones. As a whole, the results presented herein establish
new diversity-oriented folding pathways.
Scheme 1. Some of the Common Synthetic Applications of
the Corey−Chaykovsky Reagent
mall molecules with broader scaffold diversity demon-
Sstrated their significance in several drug discovery
programs.1 A synthetic challenge therefore is to develop
contemporary and pertinent approaches to generate novel
molecular architectures which can cover large areas of chemical
space. Toward this end, the “folding strategy” in diversity-
oriented synthesis (DOS) is an efficient method to assemble
skeletally diverse compound libraries for chemical biology and
medicinal chemistry.2 According to Schreiber, the folding
strategy is the conversion of a collection of substrates with
preinstalled skeletal information into products having distinct
molecular frameworks by employing a common set of reaction
conditions.3
Among the privileged compound collections, cyclopropanes
undoubtedly receive serious consideration.4 The enhanced
metabolic stability and conformational rigidity of cyclo-
propanes over unsubstituted methylene units offer distinct
advantages to medicinal chemists. Further, cyclopropanes are
structural units of several bioactive natural products and
pharmaceutically important compounds including many
marketed drugs.5 In addition, due to their unique steric and
electronic properties, the strained three-membered carbocycles
can undergo a variety of ring transformations to generate new
molecular entities.6 These impressive features prompted
chemists to develop several inspirational methods for the
synthesis of cyclopropanes.7
In this context, it is worth mentioning about the versatile
methylene group-transfer agent, dimethyloxosulfonium meth-
ylide (DOSM) [famously known as the Corey−Chaykovsky
reagent], which can be conveniently prepared in situ from 1
(Scheme 1).8 By far, the most common application of this
reagent is toward the conversion of carbonyls, imines, and
electron-deficient olefins to epoxides, aziridines, and cyclo-
propanes, respectively. In addition to the traditional synthetic
transformations, a few unexpected reactions initiated by the
DOSM were also documented.9
Through the present work, we elucidate the development of
a new substrate-based diversity-oriented folding strategy10
leading to the synthesis of cyclopropa-fused tetralones (3) and
indeno-spirocyclopropanes (5 and 7),11 originating out of the
reaction between DOSM and the enone-tethered substrates 2,
4, and 6, respectively (Scheme 2). The cyclopropanoids were
subsequently elaborated in one step to privileged scaffolds such
as tetralones, indenones, and fluorenones. Remarkably, these
transformations take place under mild and straightforward
conditions and involve unusual rearrangements facilitated by
multiple proton transfers.
As part of our ongoing research programs in catalysis,12 it
necessitated us to access the cyclopropyl keto-aldehyde 8a, for
which we opted to perform cyclopropanation of the enone-
aldehyde 2a13 with 1 (Scheme 3a). However, an unexpected
product 3a was isolated in 94% yield, as a separable mixture of
diastereomers. The structure of the major diastereomer and
the relative stereochemistry were established from the X-ray
diffraction analysis of 3a (CCDC 1855673). Interestingly,
Received: November 5, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03537
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Unexpected Transformations Triggered by the
DOSM, and Elaboration of Cyclopropanoids: This Work
Scheme 4. Scope for Cyclopropa-Fused Tetralones and
a b c
,
,
Cyclopropa-Fused Dihydrodibenzothiophenones
Scheme 3. An Unexpected Reaction of DOSM with the
Enone-Aldehyde 2a
a
b
See the Supporting Information for details. Isolated yields.
c
1
Diastereomeric ratio (dr) is determined from the crude H NMR.
proton shift generates the enolate E, which, by displacing the
dimethylsulfoxonium group, delivers the product 5. In this
manner, a diverse set of indeno-spirocyclopropanes were
synthesized (5a−5j). The structures including the relative
stereochemistry of the major isomers were assigned based on
the X-ray diffraction analysis of 5i (CCDC 1855911). Some of
the salient features of this method are (i) both alkyl and aryl
ketones (R1) could be employed as substrates with only
marginal impact on the yield (5a−5d vs 5e−5j), (ii) enones
bearing alkyl, aryl, and heteroaryl groups at the β-position (R2)
fared equally well (5d vs 5b vs 5c), and (iii) even the presence
of strong-electron donating groups on the aromatic backbone
(5h and 5i), which could be undesired for nucleophile-
triggered reactions, displayed no noticeable impact; indeed, the
reactions were complete within 30 min.
Encouraged by the results obtained for enone-aldehydes (2)
and enone-ketones (4), the reaction of enone-eneone 6a under
the prototypical conditions was considered (Scheme 6). To
our surprise, the indeno-spirocyclopropane 7a was isolated in
90% yield.15 The structure and the relative stereochemistry of
the major isomer were assigned based on the X-ray structure
obtained for 7e (vide supra; CCDC 1855912). The formation
of 7a can be rationalized by considering a Michael/Michael
sequence on 6a, initiated by the DOSM.16 The so formed
zwitterion F then undergoes a 1,5-proton shift, and the
resultant enolate G facilitates the formation of 7a by
eliminating DMSO.
when 2a was treated with a stabilized ylide such as 9, the
cyclopropane 10a was realized (Scheme 3b).
Having realized the unprecedented nature of the formation
of 3a from 2a, the reaction parameters were optimized.14
Under the standardized conditions, the scope and generality of
the reaction were investigated, and the representative results
are summarized in Scheme 4. A variety of enone-aldehydes (2)
appended to the aromatic backbone generated the respective
cyclopropa-fused tetralones in good to excellent yields (3b−
3j). Both electron-withdrawing and -donating substituents
(including aliphatic groups) on the enone moiety as well as on
the aromatic backbone were well-tolerated. This method can
even be extended for the preparation of cyclopropa-fused
dihydrodibenzothiophenones (3k−3m). As of the mechanism
of the formation of 3, an initial aldol-type reaction of the ylide
(DOSM) with 2 generates the 1,4-zwitterionic species A. A
subsequent 1,3-proton shift followed by an intramolecular
Michael addition to the enone functionality provides the
enolate C, which enables the formation of 3 via the
nucleophilic displacement of the dimethylsulfoxonium group.
Interestingly, the reaction of enone-ketones 4 under the
optimized conditions generated indeno-spirocyclopropanes 5
in an unexpected manner (Scheme 5).15 The mechanism
leading to the formation of 5 involves a sequential
intermolecular Michael reaction of the ylide followed by an
intramolecular aldol reaction of the intermittent enolate,
thereby furnishing the 1,6-zwitterion D. An eventual 1,3-
Subsequently, few other enone−enones (6) were prepared
and the corresponding indeno-spirocyclopropanes (7b−7i)
B
DOI: 10.1021/acs.orglett.8b03537
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 5. Scope for Indeno Spirocyclopropanes from
Enone-Ketones
Scheme 6. Scope for Indeno Spirocyclopropanes from
a b c
a b c
, ,
, ,
Enone−Enones
a
b
See the Supporting Information for details. Isolated yields.
c
1
Diastereomeric ratio (dr) is determined from the crude H NMR.
and 2-spirocyclopropyl-cyclopenta[b]benzothiophene-1-ones
(7j) were obtained in excellent yields in less than 30 min
(Scheme 6). This method was realized to be less influenced by
the steric or electronic effects of the substituents and provided
products in consistently good yields. For example, (i) aryl or
alkyl groups at R1, (ii) aryl, heteroaryl, or alkyl groups at R2,
and (iii) electron-donating or -withdrawing groups on the
aromatic backbone were well-tolerated.
Having successfully established a short and efficient
synthesis of otherwise difficult-to-access cyclopropanoids
such as 3, 5, and 7,11 we turned our attention to further
demonstrate the generality and synthetic utility of the products
obtained herein by considering a few elaborations.
With the presence of a cyclopropyl ketone moiety in 3, ring
opening of the cyclopropane was expected under metal/liq.
NH3 conditions (Scheme 7a). It is well-established that the
cyclopropane bond that has maximum overlap with the π-
orbital system of the carbonyl group preferentially cleaves.17
Thus, when 3a was treated under Li/liq.NH3 conditions, the
trans-1,2-disubstituted tetralone 11a was isolated as hypothe-
sized, the structure of which was confirmed by the X-ray
diffraction analysis (CCDC 1855913). In a similar fashion, few
other analogues (11b−11e) were quickly assembled in high
yields.18
a
b
See the Supporting Information for details. Isolated yields.
c
1
Diastereomeric ratio (dr) is determined from the crude H NMR.
conditions, tetralones 12b−12e were also synthesized in good
yields (CCDC 1871641 for 12b). These results indicate that
the strategy could perhaps be applicable to other analogous
nucleophile-mediated ring openings for the synthesis of a
diverse range of tetralones.
Next, a synthetic elaboration of indeno-spirocyclopropanes 5
was considered (Scheme 8). It was hypothesized that the
bisbenzylic tert-alcohol could be activated under acidic
conditions and the cyclopropane moiety might anchimerically
assist the developing cationic center, thereby a skeletally
reorganized product could be observed. Accordingly, when 5a
was treated with a catalytic amount of PTSA at an elevated
temperature, the 2-styrylindenone 13a was realized. The
mechanistic rationale for the product formation is presented
in H. Under the optimized conditions, two more indenone
derivatives (13b and 13c) were prepared. This method thus
provides an efficient alternative to access 2-styryl-3-arylinde-
nones.20
It was also anticipated that the reaction of 3 in the presence
of an appropriate nucleophile under acidic conditions could
possibly undergo ring opening of cyclopropane (Scheme 7b).19
Indeed, we were delighted to find that the reaction of 3a in the
presence of a catalytic amount of PTSA in methanol generated
the trans-1,2-disubstituted tetralone 12a possessing three
contiguous stereogenic centers. Employing the same reaction
As part of our efforts to demonstrate the synthetic utility of
indeno-spirocyclopropanes 7 obtained herein, we made a
remarkable observation (Scheme 9). The reaction of 7a in the
presence of a catalytic amount of PTSA with azeotropic
removal of water furnished the 2,3-diarylfluorenone 14a in a
serendipitous manner. The structure was confirmed in analogy
with the single crystal X-ray diffraction analysis obtained for
C
DOI: 10.1021/acs.orglett.8b03537
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 7. Elaboration of Cyclopropa-Fused Tetralones 3 to
1,2-Disubstituted Tetralones 11
Scheme 9. An Unprecedented Conversion of Indeno-
a b
a b
,
,
Spirocyclopropanes 7 to Fluorenones 14
a
b
See the Supporting Information for the general procedure. Isolated
yields.
tetralones, indenones, and fluorenones incorporated with
unusual substitution patterns. The methods described herein
are operationally straightforward and mechanistically intriguing
and symbolize novel substrate-based diversity-oriented strat-
egies. We are in the process of applying these methods for the
synthesis of bioactive natural products. Efforts to extend the
concepts to new substrate classes are also in progress, and the
details will be communicated in due course.
a
b
See the Supporting Information for general procedures. Isolated
yields.
Scheme 8. Elaboration of Indeno-Spirocyclopropanes 5 to
2-Styrylindenones 13
a b
,
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03537.
Experimental procedures and spectral data for all new
compounds (1H NMR, 13C NMR) (PDF)
a
b
See the Supporting Information for the general procedure. Isolated
yields.
Accession Codes
CCDC 1855673, 1855911−1855913, 1855918, and 1871641
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.ca-
m.ac.uk/data_request/cif, or by emailing data_request@ccdc.
cam.ac.uk, or by contacting The Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
+44 1223 336033.
14b (CCDC 1855918). Having realized the significance of the
observation, a few more fluorenone analogues (14b−14f) were
synthesized. The occurrence of several bioactive natural
products and the relevance of fluorenones in materials
chemistry renders this an attractive strategy.12c,21
As indicated on the structure 7 (Scheme 9), the reaction is
believed to involve a sequential proton elimination, cyclo-
propane ring opening, and nucleophilic attack onto ketone to
generate the tetrahydrofluorenone I. The acid further
promotes water elimination to form the dihydrofluorenone J,
which upon oxidative aromatization produces 14.
In conclusion, we presented a series of unprecedented
diastereoselective transformations triggered by the DOSM for
the synthesis of complex and otherwise difficult-to-access
cyclopropanoids. Further, new one-step synthetic elaborations
were established to access privileged structures such as
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ramsastry@iisermohali.ac.in.
ORCID
S. S. V. Ramasastry: 0000-0001-5814-9092
Notes
The authors declare no competing financial interest.
D
DOI: 10.1021/acs.orglett.8b03537
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Chang, T.-C.; Liu, T.-C.; Hsieh, H.-P.; Liao, C.-C. Tetrahedron 2008,
64, 2586. (g) Miura, T.; Fujioka, S.; Iwasaki, H.; Ozeki, M.; Kojima,
N.; Yamashita, M. Tetrahedron Lett. 2014, 55, 1536. (h) Hajra, S.;
Roy, S.; Saleh, S. K. A. Org. Lett. 2018, 20, 4540.
(10) (a) Galloway, W. R. J. D.; Isidro-Llobet, A.; Spring, D. R. Nat.
Commun. 2010, 1, 80. (b) Kumar, K. In Concepts and Case Studies in
Chemical Biology; Waldmann, H., Janning, P., Eds.; Wiley-VCH:
Weinheim, 2014; p 391. (c) Garcia-Castro, M.; Kremer, L.;
Reinkemeier, C. D.; Unkelbach, C.; Strohmann, C.; Ziegler, S.;
Ostermann, C.; Kumar, K. Nat. Commun. 2015, 6, 6516. (d) Sawant,
ACKNOWLEDGMENTS
■
We thank IISER Mohali for funding and for the NMR, mass,
and departmental X-ray facilities. U.K.M. and K.P. thank IISER
Mohali for research fellowships.
REFERENCES
■
(1) (a) Tan, D. S. Nat. Chem. Biol. 2005, 1, 74. (b) Shaw, J. T. Nat.
Prod. Rep. 2009, 26, 11. (c) Schreiber, S. L. Proc. Natl. Acad. Sci. U. S.
A. 2011, 108, 6699. (d) O’Connor, C. J.; Beckmann, H. S. G.; Spring,
D. R. Chem. Soc. Rev. 2012, 41, 4444.
(2) (a) Burke, M. D.; Schreiber, S. L. Angew. Chem., Int. Ed. 2004,
43, 46. (b) Murrison, S.; Maurya, S. K.; Einzinger, C.; McKeever-
Abbas, B.; Warriner, S.; Nelson, A. Eur. J. Org. Chem. 2011, 2354.
̃
(c) Plesniak, M. P.; Garduno-Castro, M. H.; Lenz, P.; Just-Baringo,
X.; Procter, D. J. Nat. Commun. 2018, 9, 1.
(3) Oguri, H.; Schreiber, S. L. Org. Lett. 2005, 7, 47.
(4) (a) Watanabe, M.; Kobayashi, T.; Hirokawa, T.; Yoshida, A.; Ito,
Y.; Yamada, S.; Orimoto, N.; Yamasaki, Y.; Arisawa, M.; Shuto, S. Org.
Biomol. Chem. 2012, 10, 736. (b) Arisawa, M.; Sato, T.; Hoshiya, N.;
Al-Amin, M.; Kogami, Y.; Shuto, S. ACS Comb. Sci. 2014, 16, 215.
(c) Taylor, R. D.; MacCoss, M.; Lawson, A. D. G. J. Med. Chem. 2014,
57, 5845. (d) Talele, T. T. J. Med. Chem. 2016, 59, 8712.
̈
R. T.; Stevens, M. Y.; Skold, C.; Odell, L. R. Org. Lett. 2016, 18, 5392.
(11) The scaffolds 3, 5, and 7 are without precedence and can
present themselves as new pharmacophores.
(12) (a) Satpathi, B.; Ramasastry, S. S. V. Angew. Chem., Int. Ed.
2016, 55, 1777. (b) Satpathi, B.; Ramasastry, S. S. V. Synlett 2016, 27,
2178. (c) Satpathi, B.; Wagulde, S. V.; Ramasastry, S. S. V. Chem.
Commun. 2017, 53, 8042. (d) Singh, N. K.; Satpathi, B.; Balanarayan,
P.; Ramasastry, S. S. V. Org. Biomol. Chem. 2017, 15, 10212.
(e) Mondal, A.; Hazra, R.; Grover, J.; Raghu, M.; Ramasastry, S. S. V.
ACS Catal. 2018, 8, 2748.
(13) See the Supporting Information for the synthesis of 2a and
other analogues employed in this study.
(14) See the Supporting Information for details.
(15) For recent reviews on the synthesis of cyclopentanoids, see:
(a) Satpathi, B.; Mondal, A.; Ramasastry, S. S. V. Chem. - Asian J.
2018, 13, 1642. (b) Vivekanand, T.; Satpathi, B.; Bankar, S. K.;
Ramasastry, S. S. V. RSC Adv. 2018, 8, 18576.
(5) (a) Salaun, J. Top. Curr. Chem. 2000, 207, 1. (b) Faust, R. Angew.
̈
Chem., Int. Ed. 2001, 40, 2251. (c) Chen, D. Y.-K.; Pouwer, R. H.;
Richard, J.-A. Chem. Soc. Rev. 2012, 41, 4631. (d) Chawner, S. J.;
Cases-Thomas, M. J.; Bull, J. A. Eur. J. Org. Chem. 2017, 2017, 5015
and references cited therein .
(16) The preference for the DOSM to react at the R2-connected
enone over the R1-connected enone could be most likely due to steric
reasons.
(6) (a) Wong, H. N. C.; Hon, M.-Y.; Tse, C.-W.; Yip, Y.-C.; Tanko,
J.; Hudlicky, T. Chem. Rev. 1989, 89, 165. (b) Reissig, H.-U.; Zimmer,
R. Chem. Rev. 2003, 103, 1151. (c) Yu, M.; Pagenkopf, B. L.
Tetrahedron 2005, 61, 321. (d) Carson, C. A.; Kerr, M. A. Chem. Soc.
Rev. 2009, 38, 3051. (e) Tang, P.; Qin, Y. Synthesis 2012, 44, 2969.
(f) Talukdar, R.; Tiwari, D. P.; Saha, A.; Ghorai, M. K. Org. Lett.
2014, 16, 3954. (g) Schneider, T. F.; Kaschel, J.; Werz, D. B. Angew.
Chem., Int. Ed. 2014, 53, 5504. (h) Grover, H. K.; Emmett, M. R.;
Kerr, M. A. Org. Biomol. Chem. 2015, 13, 655. (i) Gharpure, S. J.;
Nanda, L. N. Tetrahedron Lett. 2017, 58, 711. (j) Verma, K.; Banerjee,
P. Adv. Synth. Catal. 2017, 359, 3848. (k) Meazza, M.; Guo, H.; Rios,
R. Org. Biomol. Chem. 2017, 15, 2479.
(7) (a) Donaldson, W. A. Tetrahedron 2001, 57, 8589. (b) Lebel, H.;
Marcoux, J.-F.; Molinaro, C.; Charette, A. B. Chem. Rev. 2003, 103,
977. (c) Papageorgiou, C. D.; Ley, S. V.; Gaunt, M. J. Angew. Chem.,
Int. Ed. 2003, 42, 828. (d) Kunz, R. K.; MacMillan, D. W. C. J. Am.
Chem. Soc. 2005, 127, 3240. (e) Reichelt, A.; Martin, S. F. Acc. Chem.
Res. 2006, 39, 433. (f) Xie, H.; Zu, L.; Li, H.; Wang, J.; Wang, W. J.
Am. Chem. Soc. 2007, 129, 10886. (g) Pellissier, H. Tetrahedron 2008,
64, 7041. (h) de Meijere, A.; Kozhushkov, S. I. Mendeleev Commun.
2010, 20, 301. (i) Kim, H. Y.; Walsh, P. J. Acc. Chem. Res. 2012, 45,
1533. (j) David, E.; Milanole, G.; Ivashkin, P.; Couve-Bonnaire, S.;
Jubault, P.; Pannecoucke, X. Chem. - Eur. J. 2012, 18, 14904.
(k) Thankachan, A. P.; Sindhu, K. S.; Krishnan, K. K.; Anilkumar, G.
Org. Biomol. Chem. 2015, 13, 8780. (l) Pons, A.; Poisson, T.;
Pannecoucke, X.; Charette, A. B.; Jubault, P. Synthesis 2016, 48, 4060.
(m) Ganesh, V.; Chandrasekaran, S. Synthesis 2016, 48, 4347.
(8) (a) Corey, E. J.; Chaykovsky, M. J. Am. Chem. Soc. 1962, 84, 867.
(b) Corey, E. J.; Chaykovsky, M. J. Am. Chem. Soc. 1965, 87, 1353.
(c) Gololobov, Y. G.; Nesmeyanov, A. N.; Lysenko, V. P.; Boldeskul,
I. E. Tetrahedron 1987, 43, 2609. (d) Li, A.-H.; Dai, L.-X.; Aggarwal,
V. K. Chem. Rev. 1997, 97, 2341. (e) Aggarwal, V. K.; Winn, C. L. Acc.
Chem. Res. 2004, 37, 611.
(17) (a) Norin, T. Acta Chem. Scand. 1963, 17, 738. (b) Dauben, W.
G.; Deviny, E. J. J. Org. Chem. 1966, 31, 3794. (c) Srikrishna, A.;
Ramachary, D. B. Tetrahedron Lett. 2002, 43, 2765. (d) Srikrishna, A.;
Dethe, D. H. Org. Lett. 2003, 5, 2295.
(18) (a) For trans-1,2-disubstituted tetralones and their synthetic
utility, see: Sun, F.-G.; Huang, X.-L.; Ye, S. J. Org. Chem. 2010, 75,
273. (b) For some natural products possessing this core, see: Zhou,
Y.-H.; Zhang, M.; Zhu, R.-X.; Zhang, J.-Z.; Xie, F.; Li, X.-B.; Chang,
W.-Q.; Wang, X.-N.; Zhao, Z.-T.; Lou, H.-X. J. Nat. Prod. 2016, 79,
2149.
(19) (a) Kalmode, H. P.; Handore, K. L.; Reddy, D. S. J. Org. Chem.
2017, 82, 7614. (b) Richmond, E.; Vukovic, V. D.; Moran, J. Org. Lett.
2018, 20, 574.
(20) For the synthesis of 2-styrylindenones and their biological
relevance, see: (a) Wang, S.; Zhu, Y.; Wang, Y.; Lu, P. Org. Lett. 2009,
11, 2615. (b) Li, B.-J.; Wang, H.-Y.; Zhu, Q.-L.; Shi, Z.-J. Angew.
Chem., Int. Ed. 2012, 51, 3948.
(21) For the synthesis and significance of 2,3-diarylfluorenones, see:
Akrawi, O. A.; Khan, A.; Hussain, M.; Mohammed, H. H.; Villinger,
A.; Langer, P. Tetrahedron Lett. 2013, 54, 3037.
(9) (a) Yurchenko, A. G.; Melnik, N. N. Tetrahedron Lett. 1989, 30,
3653. (b) Toda, J.; Seki, M.; Amano, K.; Oyama, T.; Sano, T.; Kiuchi,
F.; Tsuda, Y. Chem. Pharm. Bull. 1994, 42, 739. (c) Yamashita, M.;
Ohta, N.; Shimizu, T.; Matsumoto, K.; Matsuura, Y.; Kawasaki, I.;
Tanaka, T.; Maezaki, N.; Ohta, S. J. Org. Chem. 2003, 68, 1216.
(d) Bernard, A. M.; Frongia, A.; Piras, P. P.; Secci, F.; Spiga, M. Org.
Lett. 2005, 7, 4565. (e) Schomaker, J. M.; Bhattacharjee, S.; Yan, J.;
Borhan, B. J. Am. Chem. Soc. 2007, 129, 1996. (f) Chittimalla, S. K.;
E
DOI: 10.1021/acs.orglett.8b03537
Org. Lett. XXXX, XXX, XXX−XXX