Diversity-oriented approach to spirooxindoles: Application of a green reagent 'rongalite'

Article abstract of DOI:10.1016/j.tetlet.2015.04.095

Abstract A range of functionalized spirooxindole derivatives have been assembled via the Diels-Alder (DA) reaction. Here, rongalite has been used to generate the key sultine building blocks which are useful latent diene equivalents in the DA chemistry. The di-bromo intermediates used here are produced by reacting the 1,2,4,5-tetrakis(bromomethyl)benzene with protected oxindole derivatives under operationally simple reaction conditions. In our study, we avoided the isolation of various intermediates, and thus reduced the cost and efforts related to the overall process.

Full text of DOI:10.1016/j.tetlet.2015.04.095

Accepted Manuscript
Diversity-oriented approach to spirooxindoles: application of a green reagent
“
rongalite”
Sambasivarao Kotha, Rashid Ali
PII:
S0040-4039(15)00742-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.04.095
TETL 46235
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
24 February 2015
21 April 2015
23 April 2015
Please cite this article as: Kotha, S., Ali, R., Diversity-oriented approach to spirooxindoles: application of a green
reagent “rongalite”, Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.04.095
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Diversity-oriented approach to spirooxindoles:
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Application of a green reagent “rongalite”
Sambasivarao Kotha,* Rashid Ali

1
Tetrahedron Letters
journal homepage: www.els evier.com
Diversity-oriented approach to spirooxindoles: Application of a green reagent
rongalite”
Sambasivarao Kotha,* Rashid Ali
“
Department of Chemistry, Indian Institute of Technology-Bombay, Powai, Mumbai-400076 India, Phone: +91-22-2576 7160, Fax: +91(22)-2572 7152; E-mail:
srk@chem.iitb.ac.in
ARTICLE INFO
ABSTRACT
Article history:
Received
A range of functionalized spirooxindole derivatives have been assembled via the Diels–Alder
(
DA) reaction. Here, rongalite has been used to generate the key sultine building blocks which
Received in revised form
Accepted
are useful latent diene equivalents in the DA chemistry. The di-bromo intermediates used here
are produced by reacting the 1,2,4,5-tetrakis(bromomethyl)benzene with protected oxindole
derivatives under operationally simple reaction conditions. In our study, we avoided the
isolation of various intermediates, and thus reduced the cost and efforts related to the overall
process.
Available online
Keywords:
1
,2,4,5-Tetrakis(bromomethyl)benzene
Diels−Alder chemistry
Rongalite
Spirooxindole
Sultine derivatives
2009 Elsevier Ltd. All rights reserved.
Introduction
One of the major goals of the organic as well as medicinal
chemists is to synthesize simple molecules that are useful as
therapeutic agents. Spirooxindole unit has been found to be core
structural element in several natural and non-natural products
1
Figure 1). Recently, a great deal of attention has been paid for
(
the synthesis of spirooxindoles due to their structural relevance
2
with bioactive molecules. Among the “drug-like” small
Figure 1: Structures of biologically active compounds containing
spirooxindole unit
molecules, functionalized spirooxindole synthesis is rapidly
Sulfones are considered to be useful synthons for the creation of
3
growing. Construction of spirooxindole framework is
a
C‒C bonds via cationic, anionic, and radical intermediates in
8
preparative organic chemistry. Moreover, fused 3-sulfolenes are
challenging task because of the creation of a quaternary center
which itself is considered a difficult task in synthetic organic
latent source of conjugated dienes, and consequently they are
useful partners in DA reaction for the construction of diverse and
4
chemistry. Therefore, it is desirable to design simple strategies
5
involving “one-pot” synthesis or without isolating the
9
intricate targets containing six-membered rings. Interestingly,
intermediates during the overall process. Ultimately, such
approaches would result in reducing the cost and also the waste
associated with the synthesis of these bioactive molecules.
neighbouring methylene moiety present in the sulfone can be
alkylated with various electrophiles due the electron withdrawing
nature of the sulfone group. This unique reactivity combined with
the ease of desulfonylation has been exploited for assembling
6
In spite of several reports for the synthesis of spirooxindole
derivatives, new and general approaches with minimum number
of steps from simple starting materials are highly desirable. Here,
we report a new and useful strategy to deliver diverse
1
0
various interesting targets. In addition, α-halogenated sulfones
1
1
are valuable precursors for the Ramberg‒Bäcklund reaction to
generate carbon-carbon double bonds. Although, several methods
are reported for the synthesis of sulfone derivatives, most of them
7
spirooxindole derivatives by using DA reaction as a key step.

2
Tetrahedron
12
involve multi-step processes. Due to this reason, simple and
gave the corresponding cycloadducts. The DA adduct with
efficient methods to assemble sulfones are in great demand.
DMAD was aromatized with MnO in refluxing toluene to
2
Therefore, we are interested to synthesize the sulfone derivatives via
deliver the dehydrogenated product 16c in 20% overall yield
involving four steps (Table 1 and Entry 3). To further expand the
utility of this strategy, we selectively protected the oxindole 12
1
3
a green reagent such as rongalite. Rongalite is commonly used as a
bleaching agent in printing and dyeing industry and it is also very
useful reagent in the synthetic organic chemistry.
2 3 2
with Boc-group by using K CO /(Boc) O in MeCN to afford the
expected product 13b. Later, the Boc-protected oxindole
derivative 13b was reacted with the tetra-bromo compound 11 by
using K CO as a base in MeCN to deliver the di-bromo
Results and Discussion
As a part of major program directed towards the development of
14,15
2
3
new methodologies to spirocyclic frameworks,
we identified
intermediate 14b (Scheme 1). Next, the crude product 14b was
treated with rongalite to furnish the sultine derivative 15b, which
on treatment with tetracycnoethylene gave the desired DA adduct
functionalized spirooxindole derivatives as our target. The
documented difficulties in their synthesis prompted us to develop
an operationally simple method from commercially available
starting materials. In this context, we started our journey with the
preparation of 1,2,4,5-tetrakis(bromomethyl)benzene 11 by using
1
6d in good yield (Table 1 and Entry 4). Surprisingly, the DA
adducts with N-phenylmaleimide and DMAD gave the un-
protected DA products due to the de-protection of the Boc-group
under thermal reaction conditions (Table 1, Entry 5 and 6). In
addition, the scope of this methodology has been expanded by
starting with the propargyl protected oxindole derivative 13c and
applying the same sequence of reactions to deliver the
propargylated spirooxindole derivative 16g in good overall yields
1
6
a known procedure. Having the tetra-bromo intermediate 11 in
hand, our next task was to selectively protect the NH-group of
oxindole 12. To this end, N-methyl derivative was prepared from
the oxindole 12 by using K
generate the desired compound 13a, which on treatment with
tetra-bromide 11 in the presence of K CO at room temperature
rt) gave the di-bromo building block 14a (Scheme 1). Later, the
2 3 2 4
CO /Me SO in refluxing toluene to
2
3
(
Table 1 and Entry 7).
(
Table 1: Structures of dienophiles and the DA products assembled in our study
crude product 14a was directly transformed into the sultine
derivative via rongalite. The sultine derivative 15a was then
subjected to the DA reaction with tetracyanoethylene to furnish
the required cycloadduct 16a in 33% overall yield (three steps)
Entry
Dienophiles
Products
Yields (%)a
NC
NC
NC
CN
CN
CN
CN
33
1
N
N
CN
O
O
1
6a
(
Table 1 and Entry 1). The structure of the compound 16a was
O
O
N
1 13
confirmed by H, C NMR spectral data and further supported
N
Ph
26
2
3
Ph
by the HRMS data.
O
O
1
6b
CO2Me
CO2Me
CO2Me
CO2Me
20
N
N
O
O
1
6c
CN
NC
NC
CN
CN
CN
4
5
6
27
29
Boc
HN
CN
Scheme 1. General scheme for the synthesis spirooxindole
derivatives; Reagents and conditions: (a) N-Bromosuccinimide,
CN
1
6d
O
O
CH
toluene, 2 h, 76% (compound 13a); ii) K
hydrogen sulphate (TBAHS), (Boc) O, MeCN, 12 h, 50%
compound 13b); iii) K CO , TBAHS, propargyl bromide, MeCN,
5 h, 42% (compound 13c); (c) K CO , TBAHS, MeCN, 15-24, not
isolated; (d) rongalite, tetrabutylammonium bromide (TBAB), DMF,
h, not isolated; (e) dienophiles, toluene, reflux, 10-24 h, 20-33%
2
Cl
2
, reflux, 500 W lamp, 24 h, 47%; (b) i) NaH, Me
2 4
SO ,
N
Ph
N
2
CO , tetrabutylammonium
3
O
O
O
1
6e
2
CO2Me
CO2Me
(
2
3
CO2Me
CO2Me
HN
25
20
1
2
3
O
16f
NC
6
CN
CN
NC
NC
CN
CN
overall yields (three steps).
N
7
CN
O
Similarly, the DA reaction with different dienophiles such as N-
phenylmaleimide and dimethyl acetylenedicarboxylate (DMAD)
16g
aYields refers to the isolated yields after column chromatography

3
Later, the sultine intermediates have also been rearranged to sulfone
derivatives under toluene reflux conditions (Scheme 2 and Figure 2).
Conclusions
Here, we have established a simple strategy to assemble various
functionalized spirooxindole derivatives under operationally
simple reaction conditions. Moreover, the intermediates were not
isolated during the synthetic sequence. In our study, the sultine
derivatives have been prepared by using rongalite, and
additionally we have demonstrated that they are useful latent
diene equivalents in the DA sequence. We have also shown that
synthesis of compound 16a involving tetra-bromo compound 11
is more effective than that of a [2+2+2] co-trimerization route.
The methodology developed here may be useful for generating
diverse and intricate spiro-targets of biological interest.
Scheme 2. Synthesis of sulfones containing spirooxindole unit
Reagents and conditions: (a) toluene, reflux, 10-16 h, 30-41%
overall yields (three steps).
Acknowledgments
We are grateful to DST-New Delhi for the financial support. R.
A. thanks University Grants Commission, New Delhi for the
award of research fellowship. S. K. thanks the Department of
Science and Technology for the award of a J. C. Bose fellowship.
We also thank to IRCC (IITB) for financial support.
Figure 2. List of sulfone derivative assembled in our strategy
The efficiency of this methodology has been compared by
synthesizing the compound 16a following an alternative route
and isolating the key intermediates during the sequence. To this
end, we began with the N-methylated compound 13a by
converting it into the di-propargylated compound 18 (70%) with
NaH/propargyl bromide in THF (Scheme 3). Later, the di-
propargylated building block 18 was subjected to a [2+2+2] co-
trimerization with the aid of Wilkinson’s catalyst in dry EtOH in
the presence of a catalytic amount of titanium isopropoxide to
Supplementary data
Supplementary data (synthetic procedures, characterization
13
1
data and the H, C NMR spectra for all the new compounds)
can be found at http://dx.doi.org/.............
References and notes
yield the diol intermediate 20, which was treated with PBr
CH Cl to afford the di-bromo building block 14a in 48% yield
two steps). Later, the di-bromo compound 14a was converted to
3
in
1
.
(a) Sebahar, P. R.; Williams, R. M. J. Am. Chem. Soc. 2000, 122,
666; (b) Collins, M. A.; Hudak, V.; Bender, R.; Fensome, A.;
2
2
5
(
Zhang, P.; Miller, L.; Winneker, R. C.; Zhang, Z.; Zhu, Y.; Cohen,
the sultine derivative 15a (76%) by using rongalite in DMF,
which was then reacted with tetracyanoethylene to deliver the
DA adduct 16a in 72% yield (Scheme 3).
J.; Unwalla, R. J.; Wrobel, J. Bioorg. Med. Chem. Lett. 2004, 14,
2
185; (c) Fensome, A.; Bender, R.; Cohen, J.; Collins, M. A.;
Mackner, V. A.; Miller, L. L.; Ullrich, J. W.; Winneker, R.;
Wrobel, J.; Zhang, P.; Zhang, Z.; Zhu, Y. Bioorg. Med. Chem.
Lett. 2002, 12, 3487; (d) Sheikh, Z.; Steel, R.; Tasker, A. S.;
Johnson, A. P. J. Chem. Soc., Chem. Commun. 1994, 763; (e)
Bian, Z.; Marvin, C. C.; Pettersson, M.; Martin, S. F. J. Am.
Chem. Soc. 2014, 136, 14184.
Scheme 3. An alternative route for the synthesis of compound 16a
Reagents and conditions: (a) NaH, propargyl bromide, THF, 2 h,
2
.
(a) Han, C.; Zhang, T.; Zhang, A.; Wang, D.; Shi, W.; Tao, J.
Synthesis 2014, 1389; (b) Matsumaru, T.; McCallum, M. E.;
Enquist Jr., J. A.; Smith, G. M.; Kong, K.; Wood, J. L.
Tetrahedron 2014, 70, 4089; (c) Santos, M. M. M. Tetrahedron
i
0%; (b) Wilkinson’s catalyst, Ti(O Pr)
7
4
, EtOH, reflux, 18 h (c)
3 2 2
PBr , CH Cl , rt, 15 h, 48% (two step); (d) rongalite, TBAB, DMF, 0
o
C-rt, 6 h, 76%; (e) tetracyanoethylene, toluene, reflux, 12 h, 72%.
By comparison of the two strategies for the formation of
compound 16a, it is clear that the earlier strategy involving tetra-
bromide 11 is far superior than that of the later route because of
the involvement of inexpensive chemicals as well as lesser
number of steps. In addition, the overall yield is also better in the
previous case (33%) as compared to the later route (24%).
2
014, 70, 9735; (d) Zhao, Y.; Liu, L.; Sun, W.; Lu, J.; McEachern,
D.; Li, X.; Yu, S.; Bernard, D.; Ochsenbein, P.; Ferey, V.; Carry,
J.-C.; Deschamps, J. R.; Sun, D.; Wang, S. J. Am. Chem. Soc.
2
013, 135, 7223.
3. For reviews on spirooxindole synthesis, see: (a) Honga, L.; Wang,
R. Adv. Synth. Catal. 2013, 355, 1023; (b) Dalpozzo, R.; Bartolib,
G.; Bencivenni, G. Chem. Soc. Rev. 2012, 41, 7247; (c) Singh, G.
S.; Desta, Z. Y. Chem. Rev. 2012, 112, 6104; (d) Voituriez, A.;
Marinetti, A.; Gicquel, M. Synlett 2015, 142; (e) Chen, J.-R.; Yu,

4
Tetrahedron
X.-Y.; Xiao, W.-J. Synthesis 2014, DOI: 10.1055/s-0034-
Chang, M.-Y.; Chen, Y.-C.; Chan, C.-K. Synlett 2014, 1739; (e)
Pandya, V. G.; Mhask, S. B. Org. Lett. 2014, 16, 3836; (f) Feng,
X.-W.; Wang, J.; Zhang, J.; Yang, J.; Wang, Na.; Yu, X.-Q. Org.
Lett. 2010, 12, 4408.
1
378944; (f) Klein, J. E. M. N.; Taylor, R. J. K. Eur. J. Org.
Chem. 2011, 6821.
4
5
.
.
Hudlicky, T.; Reed, J. W. The Way of Synthesis; Wiley-VCH:
Weinheim. 2007, p. 98.
13. (a) Kotha, S.; Khedkhar, P. Chem. Rev. 2012, 112, 1650; (b)
Kotha, S.; Krishna, N. G. Curr. Sci. 2011, 101, 923; (c) Kotha, S.;
Chavan, A. S. J. Org. Chem. 2010, 75, 4319; (d) Kotha, S.;
Khedkhar, P. J. Org. Chem. 2009, 74, 5667; (e) Kotha, S.;
Kashinath, D.; Khedkhar, P. Synthesis 2007, 3357.
(a) Saifuddin, M.; Agarwal, P. K.; Kundu, B. J. Org. Chem. 2011,
7
6, 10122; (b) Samala, S.; Mandadapu, A. K. Saifuddin, M.;
Kundu, B. J. Org. Chem. 2013, 78, 6769; (c) Srinivas, S.; Kundu,
B. Eur. Org. Chem. 2013, 3797; (d) Rajesh, R.; Periyasami, G.;
Raghunathan, R. Tetrahedron Lett. 2010, 51, 1896.
14. For our group contribution to spirocycles see (a) Kotha, S.;
Mandal, K. Tetrahedron Lett. 2004, 45, 1391; (b) Kotha, S.; Deb,
A. C.; Chattopadhyay, S. Lett. Org. Chem. 2006, 3, 128; (c)
Kotha, S.; Dipak, M. K.; Mobin, S. M. Tetrahedron 2011, 67,
4616; (d) Kotha, S.; Manivannan, E.; Ganesh, T.;
Sreenivasachary, N.; Deb, A. C. Synlett 1999, 1618; (e) Kotha, S.;
Deb, A. C. Indian J. Chem. 2008, 47B, 1120; (f) Kotha, S.;
Manivannan, E. Arkivoc 2003, (iii), 67; (g) Kotha, S.; Deb, A. C.;
Kumar, R. V. Bioorg. Med. Chem. 2005, 15, 1039; (h) Kotha, S.;
Mandal, K.; Tiwari, A.; Mobin, S. M. Chem. Eur. J. 2006, 12,
8024; (i) Kotha, S.; Manivannan, E. J. Chem. Soc. Perkin Trans.
1, 2001, 2543; (j) Kotha, S.; Deb, A. C.; Lahiri, K.; Manivannan,
E. Synthesis 2009, 165; (k) Kotha, S.; Deb, A. C.; Lahiri, K.;
Manivannan, E. Synthesis 2009, 165; (l) Kotha, S.; Ali, R.;
Chinnam, A. K. Tetrahedron Lett. 2014, 55, 4492; (m) Kotha, S.;
Ali, R. Heterocycles 2014, 88, 789; (n) Kotha, S.; Ali, R.; Tiwari,
A. Synlett 2013, 1921; (o) Kotha, S.; Ali R.; Tiwari, A. Synthesis
2014, 2471; (p) Kotha, S.; Ali, R.; Srinivas, V.; Krishna, N. G.
Tetrahedron 2015, 71, 129.
6
.
(a) Ramachary, D. B.; Venkaiah, C.; Madhavachary, R. Org. Lett.
2
013, 15, 3042; (b) Hilton, S. T.; Ho, T. C. T.; Pljevaljcic, G.;
Jones, K. Org. Lett. 2000, 2, 2639; (c) Kang, S. R.; Lee, Y. R.
Synthesis 2013, 2593; (d) Han, C.; Meng, W.; Liu, H.; Liu, Y.;
Tao, J. Tetrahedron 2014, 70, 8768; (e) Tan, B.; Hernandez-
Torres, G.; Barbas, C. F., III. J. Am. Chem. Soc. 2011, 133, 12354;
(
f) Zhu, S.-L.; Ji, S.-J.; Zhang, Y. Tetrahedron 2007, 63, 9365; (g)
Badillo, J. J.; Ribeiro, C. J. A.; Olmstead, M. M.; Franz, A. K.
Org. Lett. 2014, 16, 6270; (h) Sun, J.; Gong, H.; Yan, C.-G.
Tetrahedron 2013, 69, 10235; (i) Li, J.; Wang, J.; Xu, Z.; Zhu, S.
ACS Comb. Sci. 2014, 16, 506; (j) Liang, Y.-R.; Chen, X.-Y.; Wu,
Q.; Lin, X.-F. Tetrahedron 2015, 71, 616.
7
.
(a) Lee, H.-Y.; Kim, H. Y.; Tae, H.; Kim, B. G.; Lee, J. Org. Lett.
2
003, 5, 3439; (b) Bentz, D.; Laschat, S. Synthesis 2000, 1766; (c)
Zeng, Y.; Reddy, D. S. Org. Lett. 2004, 6, 4993; (d) Kotha, S.;
Ganesh, T.; Ghosh, A. Bioorg. Med. Chem. Lett. 2000, 10, 1755;
(
e) Kotha, S.; Brahmachary, E.; Sreenivasachary, N. Tetrahedron
Lett. 1998, 39, 4095.
8
.
(a) Chinchilla, R.; Nájera, C. Recent Res. Devel. Org. Chem. 1997,
15. Some selected papers on spirocycles (a) Han, B.; Huang, W.; Ren,
W.; He, G.; Wang, J.-H.; Peng, C. Adv. Synth. Catal. 2015, 357,
561; (b) Pradhan, R.; Patra, M.; Behera, A. K.; Mishra, B. K.;
Behera, R. K. Tetrahedron 2006, 62, 77; (c) Kita, Y.; Higuchi, K.;
Yoshida, Y.; Lio, K.; Kitagaki, S.; Shuji, A.; Fujioka, H. Angew.
Chem. Int. Ed. Engl. 1999, 38, 683; (d) Kuck, D. Chem. Ber.
1994, 127, 409; (e) Thommen, M.; Keese, R. Synlett 1997, 231; (f)
Keese, R. Chem. Rev. 2006, 106, 4787; (g) Zhao, Y.; Liu, L.; Sun,
W.; Lu, J.; McEachern, D.; Li, X.; Yu, S.; Bernard, D.;
Ochsenbein, P.; Ferey, V.; Carry, J-C.; Deschamps, J. R.; Sun, D.;
Wang, S. J. Am. Chem. Soc. 2013, 135, 7223; (h) Sannigrahi, M.
Tetrahedron 1999, 55, 9007; (i) Sawada, T.; Nakada, M. Org.
Lett. 2013, 15, 1004; (j) Liang, J.-J.; Pan, J.-Y.; Xu, D. C.; Xie, J.
W. Tetrahedron Lett. 2014, 55, 6335; (k) Rios, R. Chem. Soc. Rev.
2012, 41, 1060; (l) Dyakonov, V. A.; Trapeznikova, O. A.; de
Meijere, A.; Dzhemilev, U. M. Chem. Rev. 2014, 114, 5775.
16. Kawai H.; Umehara, T.; Fujiwara, K.; Tsuji, T.; Suzuki, T. Angew.
Chem. Int. Ed. Engl. 2006, 45, 4281.
1
, 437; (b) Nájera, C.; Sansano, J. M. Recent Res. Devel. Org.
Chem. 1998, 2, 637; (c) Chinchilla, R.; Nájera, C. Chem. Rev.
2
000, 100, 1891; (d) Prilezhaeva, E. N. Russ. Chem. Rev. 2001,
0, 897; (e) Back, T. G. Tetrahedron 2001, 57, 5263; (f) Buynak,
7
J. D.; Vogeti, L.; Chen, H. Org. Lett. 2001, 3, 2953; (g) Back, T.
G.; Clary, K. N.; Gao, D. Chem. Rev. 2010, 110, 4498.
9
.
(a) Carruthers, W. Tetrahedron Organic Chemistry Series, Vol 8,
Cycloaddition Reactions in Organic Synthesis; Baldwin, J. E.;
Magnus, P. D.; Eds; Pergamon: Oxford, U. K. 1990; (b) Paquette,
L. A. Comprehensive Organic Synthesis; Pergmon: Oxford, Vol.
5
, 1991; (c) Crew, A. P. A.; Storr, R. C.; Yelland, Y. Tetrahedron
Lett. 1990, 31, 1491; (d) Sandulache, A. Silva, A. M. S. Cavaleiro,
J. A. S. Tetrahedron 2002, 58, 105.
1
0. (a) Nájera, C.; Yus, M. Tetrahedron 1999, 55, 10547; (b) Wang,
G.; Mahesh, U.; Chen, G. Y. J.; Yao, S. Q. Org. Lett. 2005, 5, 737;
(
c) Brant, M. G.; Wulff, J. E. Org. Lett. 2012, 14, 5876; (d)
Buynak, J. D.; Vogeti, L.; Chen, H. Org. Lett. 2001, 3, 2953.
1. (a) Paquette, L. A. Org. React. 1977, 25, 1; (b) Zhang, H.; Zhang,
L-X.; Yang, F.; Liu, F-L. J. Mol. Struc–Theochem. 2010, 950, 41;
1
1
(
c) Cooke Jr., M. P. J. Org. Chem. 1981, 46, 1747.
2. (a) Shank, K. The Chemistry of Sulphones and Sulphoxides; Patai,
S.; Rappoport, Z.; Stirling, C. Eds; John Wiley: New York, 1988,
pp 165-231; (b) Colquhoun, H. M.; Zhu, Z.; Williams, D. J. Org.
Lett. 2001, 3, 4031; (c) Yao, Q. Org. Lett. 2002, 4, 427; (d)