A cascade synthesis of: S -allyl benzoylcarbamothioates via Mumm-type rearrangement

Article abstract of DOI:10.1039/c8ob02293c

A catalyst and solvent free synthesis of S-allyl benzoylcarbamothioates has been achieved from the in situ generated benzoylcarbonimidothioates obtained by reacting MBH alcohols with aroyl isothiocyanates. An intramolecular thia-Michael addition of the in

Full text of DOI:10.1039/c8ob02293c

Organic &
Biomolecular Chemistry
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A cascade synthesis of S-allyl
benzoylcarbamothioates via Mumm-type
rearrangement†
Cite this: Org. Biomol. Chem., 2018,
16, 7787
Received 17th September 2018,
Accepted 8th October 2018
Anjali Dahiya,
Wajid Ali,
Tipu Alam
and Bhisma K. Patel
*
DOI: 10.1039/c8ob02293c
rsc.li/obc
A catalyst and solvent free synthesis of S-allyl benzoylcarba-
The synthetic utility and tendency of the Morita Baylis
mothioates has been achieved from the in situ generated benzoyl- Hillman (MBH) adducts to afford multifunctional products
carbonimidothioates obtained by reacting MBH alcohols with aroyl have become increasingly evident in past decades. As under-
isothiocyanates. An intramolecular thia-Michael addition of the stood by the numerous protocols that highlight its ability to
in situ generated adduct triggers a Mumm-type rearrangement undergo an array of chemical transformations for the construc-
leading to a stereoselective synthesis of highly functionalised tion of a diverse molecular targets.4 Taking cues from the reac-
S-allyl benzoylcarbamothioates.
tivity of aroyl isothiocyanate,3 we articulated that this could be
the precursor to afford S-allyl benzoylcarbamothioate upon
reaction with MBH alcohol. Thus, a reaction was carried out
between methyl 2-(hydroxy(phenyl)methyl)acrylate (1) and
benzoyl isothiocyanate (a) in toluene (2 mL) at room tempera-
ture. A new allyl thioether (1a) containing two important moi-
eties viz. imide and 2-methylthioacrylate, was isolated in 56%
yield possibly via a Mumm type of rearrangement.
Among the sulphur based compounds allylic thioethers
exhibits a broad range of biological activities. Despite the
remarkable progress made to date, construction of allylic
thioethers via metal-free C–S bond forming reactions is still
lacking. Several strategies, including transition metal-catalysed
substitution reaction, thiocarbonylation, insertion reaction,
and transition metal-catalysed or organocatalysed Michael
additions, have been well recognised over the years to con-
struct C–S bonds.5 Thus, the development of efficient and sus-
The progress of cascade synthetic methods, where structural
divergence is selectively congregated at the molecular level
from multifunctional small molecules in one pot, has been
established as one of the key epitomes of modern organic
chemistry. Toward this endeavor, cascade processes have
found wide applications because they successfully implement
the goal and principle of sustainability.1 In many cascade
approaches, aroyl isothiocyanates are important synthons and
have found extensive use in the construction of biologically
active acyclic and cyclic frameworks.2a,b Aroyl isothiocyanate, a
hetero-cumulene, contains an acyl group and a thiocyanate
group. Due to the presence of four reactive sites viz. the
nucleophilic S and N-atoms, the electrophilic carbonyl and
thiocarbonyl groups they serve either as an electrophile or an
ambedient nucleophile. Especially, the carbonyl group in aroyl
isothiocyanates divulges unique structural features and shows
differential reactivity under various reaction conditions.2c In
our previous report, we have established the efficacy of aroyl
isothiocyanates as
a
nucleophile with α-bromoketones.3a
While, both as an electrophile and a nucleophile during the
ring opening of oxiranes3b and aziridines.3c In the former a
concomitant transfer of a thiocyanate (as nucleophile) and an
aroyl moiety (as electrophile) occurs (Scheme 1a) while in the
latter a domino ring opening cyclisation of aziridine takes
place (Scheme 1b).3
Department of Chemistry, Indian Institute of Technology Guwahati, 781 039 Assam,
India. E-mail: patel@iitg.ernet.in; Fax: +91-3612690762
†Electronic supplementary information (ESI) available: 1H and 13C NMR spectra.
CCDC 1844331. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c8ob02293c
Scheme 1 Differential reactivity of aroyl isothiocyanate.
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Table 1 Optimisation of reaction conditionsa,b
tainable methods for the construction of C–S bonds is deemed
worthy of investigation.
Imides are significant structural motifs found in various
natural products and pharmaceutical agents.6a,b As shown in
Fig. S1 (see the ESI†), selected examples include diacetazotol
(Dermagan),6c tetraacetylethylenediamine (TAED)6d and the
anxiolytic drug aniracetam (Ampamet).6c Such an imide motif
also appears in certain natural products such as fumaramid-
mycin,6e,f coniothyriomycin6g and unnatural compound
SB-253514.6h They also appear as precursors in a variety of
reactions, such as condensation, alkylation, acylation and
cycloaddition.7 The classical methodology for the synthesis of
imides relies on the condensation of amides with carboxylic
acids and its derivatives. N-Acylation of amides using alde-
hydes as the acyl source is one of the important synthetic tools
in the construction of imides. There are limited precedents
for the synthesis of imides based on metal-catalysed cross
coupling strategies such as (i) Rh(II)-catalysed sulfamidation
of aldehydes;8a–c (ii) Fe/Cu catalysed coupling of aldehydes
and thioesters with carboxamides;8d–g (iii) oxidation of
N-alkylbenzamides;8h–j and (iv) ceric ammonium nitrate (CAN)-
promoted oxidation of 4,5-diphenyloxazoles.8k Though the
Entry
Solvent
Temp. (°C)
Time
Yieldb (%)
1
2
3
4
5
6
7
8
Toluene
1,4-Dioxane
DCE
DCM
Chlorobenzene
DMF
DMSO
CH3CN
H2O
—
—
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
50
60
80
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
9 h
56
35
59
43
60
39
12
9
64
70
83
96
91
9
10
11
12
13
—
—
5 h
5 h
a Reaction conditions: (1) (0.2 mmol), (a) (0.2 mmol), solvent (2 mL),
under air. b Isolated yield.
above-mentioned protocols describe elegant methods for the time giving 83% and 96% of the product in 9 h and 5 h,
construction of imide motifs but the limitation is, it is appli- respectively (Table 1, entries 11 and 12). However, no further
cable for the derivatisation of sulfonamides or carboxamides improvement in the yield or reduction in reaction time was
only.9 Despite several procedures available for their synthesis, observed when the reaction was performed at 80 °C (Table 1,
the use of designer substrates, poor yields, cumbersome multi- entry 13).
stepped processes and inadequate product diversity are some
Encouraged by this catalyst and solvent free synthesis,
of the limitations. Thus, the development of innovative various aroyl isothiocyanates (a–k) were reacted with MBH
methods for the synthesis of imides from readily available sub- alcohol (1) to enhance the scope and generality of this
strates under a mild condition is highly desirable.
rearrangement reaction (Scheme 2). As can be seen from
In the pursuit to accomplish an appropriate reaction con- Scheme 2, differently functionalised aroyl isothiocyanates
dition for this unparalleled transformation, other reaction bearing electron neutral –H (a), electron-donating (b–d) as well
parameters such as solvents and temperature were screened as electron-withdrawing groups reacted smoothly with (1)
using methyl 2-(hydroxy(phenyl)methyl)acrylate (1) and affording their desired products (1a–1k) in good to excellent
benzoyl isothiocyanate (a) as the reacting partners. Initially, yields. Benzoyl isothiocyanate (a) when reacted with (1) gave
different nonpolar solvents such as 1,4-dioxane, DCE, DCM the product (1a) in 96% yield. The structure of the product
and chlorobenzene were tested. When the reaction was carried (1a) has been unambiguously established by single crystal
out in 1,4-dioxane instead of toluene (Table 1, entry 2) the X-ray crystallography (Fig. 1).
yield dropped to 35%, whereas the use of 1,2-dichloroethane
Aroyl isothiocynates bearing electron-donating (EDG) sub-
(Table 1, entry 3) gave a comparable yield (59%). When the stituents viz. p-Me (b), p-Et (c), and p-OMe (d) when reacted
reaction was performed in DCM and chlorobenzene the with (1) provided their corresponding products (1b, 92%), (1c,
product was isolated in 43% and 60% yields, respectively 90%) and (1d, 81%) in good yields. This protocol was also
(Table 1, entries 4 and 5). Further, the use of polar solvents equally successful for aroyl isothiocyanates bearing moderately
such as DMF (39%), DMSO (12%) and acetonitrile (9%) [p-F (e)] and strongly [p-CF3 (f), and p-NO2 (g)] electron-with-
(Table 1, entries 6, 7 and 8) were found to be inferior to that of drawing groups (EWG) giving their respective products (1e,
chlorobenzene (Table 1, entry 5). This reaction in water gave a 95%), (1f, 98%), (1g, 91%) in excellent yields (Scheme 2).
64% yield of the product (1a) (Table 1, entry 9) which however, 2-Napthoyl isothiocyanate (h) reacted effectively with (1) giving
was associated with number of other side products. In pre- its product (1h, 93%) in good yield. Heteroaromatic isothio-
vious reports, maximum yield of products was obtained under cyanates such as thiophenoyl (i) also reacted competently with
a solvent free condition using aroyl isothiocyanates.3 Thus, a (1) affording its S-allyl benzoylcarbamothioate (1i) in 78%
reaction under a neat condition was attempted at room temp- yield. In addition, aliphatic isothiocyanates such as cinnamoyl
erature. To our delight, an improved yield (70%) of the product ( j) and cyclohexoyl (k) furnished their rearranged products (1j,
(1a) was obtained (Table 1, entry 10). Interestingly, performing 74%) and (1k, 89%) in slightly lesser yields.
a neat reaction at elevated temperatures of 50 °C and 60 °C,
The substrate scope and generality of this protocol was
not only improved the yield but also shortened the reaction further investigated by treating other Morita–Baylis–Hillman
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Scheme 2 Substrate scope for the synthesis of S-allyl benzoyl-
carbamothioates. Reaction condition: (1) (0.5 mmol), (a–k) (0.5 mmol),
under air at 60° C for 5–6 h. Isolated yield.
Scheme 3 Scope of synthesis of S-allyl benzoylcarbamothioate with
different MBH alcohols. Reaction conditions: (2–16) (0.50 mmol), a–f
(0.50 mmol), under air at 60 °C for 5–6 h. Isolated pure product.
Fig. 1 ORTEP view of (1a) with 50% thermal ellipsoid probability.
(MBH) alcohols (2–16) with various aroyl isothiocyanates (a–f) ring R1 of MBH alcohol is substituted with strongly electron-
and the results are summarised in Scheme 3. The MBH withdrawing groups such as p-CF3 (5) and p-NO2 (6) their
alcohol (2) reacted with a variety of aroyl isothiocyanates (a–f) respective products (5a, 93%) and (6a, 88%) were obtained in
bearing electron-neutral (a), electron donating [p-Me (b), good yields. A napthyl substituted MBH alcohol (7), when
p-OMe (c), p-Et (d)] and electron withdrawing [p-F (e), p-CF3 reacted with benzoyl isothiocyanate (a) afforded the product
(f)] groups affording their anticipated S-allyl benzoylcarba- (7a) in 95% yield. Aliphatic MBH alcohol such as methyl
mothioates (2a, 84%), (2b, 79%), (2c, 80%), (2d, 71%), (2e, 2-(cyclohexyl(hydroxy)methyl)acrylate (8) produced its rearranged
91%), (2f, 90%) in good to excellent yields (Scheme 3). A product (8a) but in a relatively lower yield (73%). Moderate
p-OMe substituted MBH alcohol (3) upon reaction with yields of products (9a, 81%) and (10a, 83%) were obtained
benzoyl isothiocyanate (a) gave the desired product (3a) in when furan (9) and thiophene (10) containing MBH alcohols
77% yield. However, p-F (4) substituted MBH alcohol when were reacted with benzoyl isothiocyanate (a) under the opti-
reacted with different aroyl isothiocyanates (a–f) afforded their mised reaction condition.
rearranged products (4a, 95%), (4b, 93%), (4d, 86%), (4e, 97%)
and (4f, 96%) in excellent yields (Scheme 3). When the phenyl as methyl ester (–CO2Me), other esters such as –CO2Et (11) and
After successfully demonstrating the present strategy for R2
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i
–CO2 Bu (12) were also screened and their corresponding pro-
ducts (11a, 94%), (12a, 89%) were obtained in good yields
(Scheme 3). However, when R2 as an ester was replaced by a
–CN group (13) and reacted with aroyl isothiocyanates (a) and
(b) their rearranged products (13a, 77%) and (13b, 71%) were
obtained in moderate yields. The feasibility of the present
strategy was further surveyed by replacing R2 with –SO2Me (14)
and –COMe (15) and reacted with different aroyl isothio-
cyanates. To our delight their corresponding products (14a, 69%)
and [(15a, 78%), (15b, 74%) and (15e, 81%)] were obtained in
good to moderate yields. When cyclohex-2-en-1-one (16)
derived MBH alcohol was reacted with 4-methylbenzoyl iso-
thiocyanate (b) afforded its anticipated product (16b) in 55%
yield. The reaction was not productive at all when isatin
derived MBH alcohols (17 and 18) were reacted with benzoyl
isothiocyanate (a) (Scheme 3). This might be due to the steric
hindrance of MBH alcohols that prevents nucleophilic
addition at the sp2 carbon of the –NvCvS bond. Unlike aroyl
isothiocyanates, both starting materials remain unconsumed
even after 24 h. The configuration of the double bond of the
S-allyl benzoylcarbamothioate was confirmed by 1D NOE
experiment. It was appealing to note that the Z-isomer is
obtained exclusively (see the ESI†). To demonstrate the scal-
ability of the present methodology a reaction was carried out
with methyl 2-(hydroxy(phenyl)methyl)acrylate (1) (5 mmol,
963 mg) and benzoyl isothiocynate (a) (5 mmol, 815 mg) under
the standard optimised reaction condition giving 87% yield of
the product (1a).
Further, systematic investigations were carried out to depict
a plausible mechanism for this transformation. A reaction was
carried out with a preformed 18O-labeled MBH alcohol10 and
benzoyl isothiocyanate (a) under the optimised condition. A
18O-labeled S-allyl benzoylcarbamothioate (1a″) was obtained
as confirmed by HRMS analysis (see the ESI†). Further,
13CNMR analysis of 1a″ shows two signals (δ 169.622 and
169.658 ppm) due to both labeled and unlabeled carbonyl
groups of carbamothioate. This observation suggests that the
carbonyl oxygen is originating from the –OH of MBH alcohol
(Scheme 4).
Scheme 5 Plausible reaction mechanism.
Scheme 6 Synthetic transformations of 1a.
vals rules out the possible formation of any cyclic intermediate
(B) (see ESI†). The other possibility is that the intermediate (A)
undergoes a thia-Michael addition with concurrent Mumm-
type of rearrangement (Scheme 5, path b). The Mumm
rearrangement is a significant part of the Ugi reaction that
involves the 1,3-acyl migration of an acyl imidate to an
imide.11 The above labeling experiment supports the occur-
rence of the Mumm-type rearrangement (Scheme 4).10
To demonstrate the applicability of our protocol, the S-allyl
benzoylcarbamothioate (1a) was subjected to a few useful
organic transformations (Scheme 6). An acid hydrolysis of (1a)
was carried out which was furnished (1ab) by cleavage of the
imide bond. When (1a) was treated with NaBH4, selective clea-
vage of the C–S bond takes place giving (1ac) in moderate
yield. Other transformations are still under process.
In summary, we have developed an elegant approach for
the synthesis of S-allyl benzoylcarbamothioate via a Mumm-
type rearrangement. This methodology allows the useful syn-
thesis of many valuable S-allyl benzoylcarbamothioate under
mild condition and avoids the use of costly and harmful
materials or cumbersome multi-stepped processes. The 18O
labeling experiments support the proposed mechanistic
pathway. In this protocol, C–O and C–S bonds are assembled
simultaneously and have the merits of wide range of substrate
scope, neat condition and simple purification.
There are two possible paths that can account for this
migration. The first possible route is the generation of a ben-
zoylcarbonimidothioate intermediate (A) after nucleophilic
addition of MBH alcohol (1) with benzoyl isothiocyanate (a)
which undergo thia-Michael addition to form a cyclic inter-
mediate 1,3-oxathiane (B). The intermediate (B) opens up to
give an S-allyl benzoylcarbamothioate (1a) (Scheme 5, path a).
However, a careful examination of 1H NMR of the reaction
mixture obtained by reacting (1) and (a) at different time inter-
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
B. K. P acknowledges support of this research by the
Department of Science and Technology (DST/SERB) (EMR/
2016/007042), DST-FIST and MHRD: 5-5/2014-TS-VII. CIF, IIT
Guwahati, for instrumental facility.
Scheme 4 18O labeling experiment.
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Y. Miyasaka, K. Yokose, H. Shirai, K. Takano, P. Quitt and
P. J. Lanz, Antibiot., 1975, 28, 648; (g) K. Krohn, C. Franke,
P. G. Jones, H.-J. Aust, D. Draeger and B. Schulz,
Liebigs Ann. Chem., 1992, 789; (h) I. L. Pinto, H. F. Boyd
and D. M. B. Hickey, Bioorg. Med. Chem. Lett., 2000, 10,
2015.
7 (a) I. T. Raheem, S. N. Goodman and E. N. Jacobsen, J. Am.
Chem. Soc., 2004, 126, 706; (b) C. D. Vanderwal and
E. N. Jacobsen, J. Am. Chem. Soc., 2004, 126, 14724;
(c) M. Gandelman and E. N. Jacobsen, Angew. Chem., Int.
Ed., 2005, 44, 2393; (d) E. P. Balskus and E. N. Jacobsen,
J. Am. Chem. Soc., 2006, 128, 6810; (e) S. Thompson,
S. A. McMahon, J. H. Naismith and D. O’Hagen, Bioorg.
Chem., 2016, 64, 37; (f) C. Feng, D. Feng and T.-P. Loh,
Chem. Commun., 2015, 51, 342; (g) A. C. Wright,
C. K. Haley, G. Lapointe and B. M. Stoltz, Org. Lett., 2016,
18, 2793.
8 (a) J. Chan, K. d. Baucom and J. A. Murry, J. Am. Chem.
Soc., 2007, 129, 14106; (b) L. Wang, H. Fu, Y. Jiang and
Y. Zhao, Chem. – Eur. J., 2008, 14, 10722; (c) J. Wang, C. Liu,
J. Yuan and A. Lei, Chem. Commun., 2014, 50, 4736;
(d) F. Wang, H. Liu, H. Fu, Y. Jiang and Y. F. Zhao, Adv.
Synth. Catal., 2009, 351, 246; (e) H. Yu and Y. Zhang,
Eur. J. Org. Chem., 2015, 1824; (f) L. Wang, H. Fu, Y. Jiang
and Y. Zhao, Chem. – Eur. J., 2008, 14, 10722; (g) H. Yu,
Y. Chen and Y. Zhang, Chin. J. Chem., 2015, 33, 531;
(h) K. C. Nicolaou and C. J. N. Mathison, Angew. Chem., Int.
Ed., 2005, 44, 5992; (i) L. Xu, S. Zhang and M. L. Trudell,
Chem. Commun., 2004, 1668 and references therein;
( j) D. A. Evans, P. Nagorny and R. Xu, Org. Lett., 2006, 8,
5669; (k) Y. Zhang, L. Pan, Y. Zou, X. Xu and Q. Liu, Chem.
Commun., 2014, 50, 14334; (l) X. Yan, K. Fang, H. Liua and
C. Xi, Chem. Commun., 2013, 49, 10650.
9 (a) H. Li, K. Dong, H. Neumann and M. Beller, Angew.
Chem., Int. Ed., 2015, 54, 10239; (b) Y. Lv, Y. Li, T. Xiong,
Y. Lu, Q. Liu and Q. Zhang, Chem. Commun., 2014, 50,
2367; (c) Y. Li, F. Zhu, Z. Wang, J. Rabeah, A. Brückner and
X.-F. Wu, ChemCatChem, 2017, 9, 915; (d) A. O. Gálvez,
C. P. Schaack, H. Noda and J. W. Bode, J. Am. Chem. Soc.,
2017, 139, 1826.
Notes and references
1 (a) A. Padwa, Chem. Soc. Rev., 2009, 38, 3072;
(b) R. Ardkhean, D. F. J. Caputo, S. M. Morrow, H. Shi,
Y. Xiong and E. A. Anderson, Chem. Soc. Rev., 2016, 45,
1557; (c) S. Blouin, G. Blond, M. Donnard, M. Gulea and
J. Suffert, Synthesis, 2017, 49, 1767; (d) H. Pellissier, Chem.
Rev., 2013, 113, 442; (e) M. J. Climent, A. Corma and
S. Iborra, Chem. Rev., 2011, 111, 1072.
2 (a) A. K. Mukerjee and R. Ashare, Chem. Rev., 1991, 91, 1;
(b) T. Castanheiro, J. Suffert, M. Donnard and M. Gulea,
Chem. Soc. Rev., 2016, 45, 494; (c) V. Ficeri, P. Kutschy,
M. Dzurilla and J. Imrich, Collect. Czech. Chem. Commun.,
1994, 59, 2650; (d) K. G. Bedane and G. S. Singh,
ARKIVOC, 2015, 5, 206 and references cited therein;
(e) R. K. Saunthwal, M. Patel, R. K. Tiwari, K. Parang
and A. K. Verma, Green Chem., 2015, 17, 1434;
(f) R. K. Saunthwal, M. Patel, S. Kumar and A. K. Verma,
Tetrahedron Lett., 2015, 56, 677; (g) R. K. Saunthwal,
M. Patel, A. K. Danodia and A. K. Verma, Org. Biomol.
Chem., 2015, 13, 1521.
3 (a) C. C. Palsuledesai, S. Murru, S. K. Sahoo and B. K. Patel,
Org. Lett., 2009, 11, 3382; (b) A. Modi, W. Ali and
B. K. Patel, Org. Lett., 2017, 19, 432; (c) A. Dahiya, W. Ali
and B. K. Patel, ACS Sustainable Chem. Eng., 2018, 6, 4272.
4 (a) D. Basavaiah, A. J. Rao and T. Satyanarayana, Chem.
Rev., 2003, 103, 811; (b) D. Basavaiah, B. S. Reddy and
S. S. Badsara, Chem. Rev., 2010, 110, 5447; (c) W. Yang,
D. Tan, R. Lee, L. Li, Y. Pan, K.-W. Huang, C.-H. Tan and
Z. Jiang, Chem. – Asian J., 2012, 7, 771; (d) L. D. S. Yadav,
R. Patel and V. P. Srivastava, Tetrahedron Lett., 2009, 50,
1335; (e) M. Ciclosi, C. Fava, R. Galeazzi, M. Orenaa and
J. Sepulveda-Arquesb, Tetrahedron Lett., 2002, 43, 2199;
(f) S. Bhowmik, A. Mishra and S. Batra, RSC Adv., 2011, 1,
1237; (g) J. Qi, J. Zheng and S. Cui, Org. Lett., 2018, 20,
1355; (h) Y. Wei and M. Shi, Chem. Rev., 2013, 113, 6659.
5 (a) P. Zhu, X. He, X. Chen, Y. You, Y. Yuan and Z. Weng,
Tetrahedron, 2014, 70, 672; (b) P. Singh, R. Bai,
R. Choudhary, M. C. Sharma and S. S. Badsara, RSC Adv.,
2017, 7, 30594; (c) L. Zhang, J. Zhu, J. Ma, L. Wu and
W.-H. Zhang, Org. Lett., 2017, 19, 6308; (d) Q.-L. Xu, 10 (a) B. Zhu, L. Yan, Y. Pan, R. Lee, H. Liu, Z. Han,
W.-B. Liu, L.-X. Dai and S.-L. You, J. Org. Chem., 2010, 75,
4615; (e) P. S. Reddy, M. A. Reddy, B. Sreedhar and
M. V. B. Rao, Synth. Commun., 2010, 40, 2075.
K.-W. Huang, C.-H. Tan and Z. Jiang, J. Org. Chem., 2011,
76, 6894.
11 (a) V. Mercalli, A. Nyadanu, M. Cordier, G. C. Tron,
L. Grimaud and L. E. Kaim, Chem. Commun., 2017, 53,
2118; (b) J. Chen, Y. Shao, L. Ma, M. Ma and X. Wan, Org.
Biomol. Chem., 2016, 14, 10723; (c) H.-W. Liang, Z. Yang,
K. Jiang, Y. Ye and Y. Wei, Angew. Chem., Int. Ed., 2018, 57,
5720; (d) A. L. Chandgude and A. Dömling, Org. Lett., 2017,
19, 1228; (e) H. Harayama, T. Nagahama, T. Kozera,
M. Kimura, K. Fugami, S. Tanaka and Y. Tamaru, Bull.
Chem. Soc. Jpn., 1997, 70, 445; (f) M. Sakamoto,
M. Yoshiaki, M. Takahashi, T. Fujitaa and S. Watanabe,
J. Chem. Soc., Perkin Trans. 1, 1995, 373.
6 (a) Y. Takeuchi, T. Shiragami, K. Kimura, E. Suzuki and
N. Shibata, Org. Lett., 1999, 1, 1571; (b) S. M. Capitosti,
T. P. Hansen and M. L. Brown, Org. Lett., 2003, 5, 2865;
(c) The Merck Index, version 12:3, Merck&Co Inc.,
Whitehouse Station, NJ, USA, 1999; (d) E.-S. Kim,
K. Y. Park, J.-M. Heo, B. J. Kim, K. D. Ahn and J.-G. Lee,
Ind. Eng. Chem. Res., 2010, 49, 11250; (e) H. B. Maruyama,
Y. Suhara, J. Suzuki-Watanabe, Y. Maeshima, N. Shimizu,
M. Ogura-Hamada, H. Fujimoto and K. J. Takano, Antibiot.,
1975, 28, 636; (f) Y. Suhara, H. B. Maruyama, Y. Kotoh,
This journal is © The Royal Society of Chemistry 2018
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