Enantiospecific total synthesis of (?)-crotanecine

Article abstract of DOI:10.1016/j.tetlet.2017.08.056

Crotanecine is the necine base component of a number of pyrrolizidine alkaloids. This necine subunit is an amino triol bearing a primary allylic alcohol characterized by an all-cis relationship of its stereocenters. The synthesis of crotanecine has been accomplished using simple yet versatile ring construction approach using ribose as chiral starting point.

Full text of DOI:10.1016/j.tetlet.2017.08.056

Accepted Manuscript
Enantiospecific total synthesis of (-)-Crotanecine
Yugandhar Kothapalli, Ravi Kumar Puthukanoori, Mahesh Ranga, Srinivasa
Rao Alapati
PII:
S0040-4039(17)31063-8
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.08.056
TETL 49246
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
25 June 2017
21 August 2017
22 August 2017
Please cite this article as: Kothapalli, Y., Puthukanoori, R.K., Ranga, M., Alapati, S.R., Enantiospecific total
synthesis of (-)-Crotanecine, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.2017.08.056
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Graphical Abstract
Enantiospecific total synthesis of (-)-Crotanecine
Leave this area blank for abstract info.
a,b
Yugandhar Kothapalli , Ravi Kumar Puthukanoori* , Mahesh Ranga , Srinivasa Rao Alapati .
a
a
a
a
Medicinal chemistry laboratory, GVK Biosciences Pvt. Ltd., Plot No 28, IDA Nacharam, Hyderabad,
5
00076,Telangana, India.
b
Department of Chemistry, Jawaharlal Nehru Technological University, Hyderabad, 500085, Telangana, India.

Enantiospecific total synthesis of (-)-Crotanecine
a,b
Yugandhar Kothapalli , Ravi Kumar Puthukanoori , Mahesh Ranga , Srinivasa Rao
a*
a
a
Alapati .
a
Medicinal chemistry laboratory, GVK Biosciences Pvt. Ltd., Plot No 28, IDA Nacharam, Hyderabad, 500076,
Telangana, India
b
Department of Chemistry, Jawaharlal Nehru Technological University, Hyderabad, 500085, Telangana, India.
Abstract: Crotanecine is the necine base component of a number of pyrrolizidine alkaloids. This necine subunit is an
amino triol bearing a primary allylic alcohol characterized by an all-cis relationship of its stereocenters. The synthesis
of crotanecine has been accomplished using simple yet versatile ring construction approach using ribose as chiral
starting point.
8
OH
OH
OH
OH
HO
HO
1
HO
HO
H
N
H
H
N
H
N
7a
7
HO
6
N
2
5
3
Retronecine (2)
Turnefocidine (3)
Platynecine (4)
(
+)-Crotanecine (1)
H
H
HO Me
HO Me
O
Me
O
OH
OH
OH
Me
HO
HO
H
N
H
N
Me
H
Me
H
O
O
O
O
HO
AcO
N
N
Rosmarinecine (5)
Heliotridine (6)
Anacrotine (7)
Acetylanacrotine (8)
OH
Me
Me
Me
H
HO Me
HO Me
HO Me
HO
H
H
H
Me
O
O
O
O
O
Me
H
Me
H
Me
H
Me
H
HO
AcO
HO
HO
O
O
O
O
O
O
O
N
N
N
N
madurencine (9)
acetylmadurencine (10) acetyl-cis-madurencine (11)
Crotaflorine (12)
Figure-1: Structures of pyrrolizidine alkaloids and few of its derivatives
Introduction and Background:
+)-Crotanecine 1 is found conjugated to a variety of necic acids in many pyrrolizidine alkaloids found in plants of the
Crotalaria species. Historically pyrrolizidine alkaloids referred to as necine bases, share common
azabicyclo[3.3.0]octane core but differ based on the oxygenation pattern and whether or not saturated. Anacrotine 7
(
a
1
isolated from the seeds of C. anagyroids obtained from Sri Lanka and from C. incana shrub grown in South Africa
,2
3
afforded senecic acid and a new amino triol named crotanecine. Upon re-examination of leaves and twigs gathered
Key words: Anacrotine, crotanecine, Baylis-Hillman Reaction, Ring closing metathesis, Total synthesis.
*
Corresponding author. Tel.: +91 9866240780. e-mail: pravik1003@gmail.com

from C. agatiflora grown in Australia six new alkaloids were isolated, all containing (+)-crotanecine as the base
4
Figure 1). Crotanecine bears a double bond at the C(6)-C(7) position; therefore, all the alkaloids (7-12) that contain
(
crotanecine should be hepatotoxic, the most common biological effect of pyrrolizidine alkaloids.
As part of our research objectives directed at development of a general synthetic methodology for
5
functionalised indolizidine and pyrrolizidine alkaloids , we have become interested in developing approach for
6a-c
enantioselective synthesis of Crotanecine. There have been three previous synthesis of crotanecine 1 along with one
6
formal synthesis . As outlined in Scheme 1 retrosynthetic analysis indicated that intermediate 13 can serve as good
d
7
chiral synthon as it can further manipulated using aldehyde functional group to extend the side chain to build upon.
We have contemplated that Baylis-Hillman as a stereochemical determining reaction could serve the purpose to
introduce the desired side chain to give 14. Intermediate 15 can be derived from 14 via converting alcohol to amine
followed by building the five membered ring using ring closing metathesis. After obtaining 15, enforced cyclisation
would produce crotanecine carbon framework.
Scheme 1.Retrosynthetic plan for the Crotanecine
Based on retrosynthetic analysis, L-ribose was recognised as suitable chiral starting pool; however it was
found to be very expensive. Hence to help our exploration efforts we have chosen the cheaper and readily available
alternative chiral pool D-ribose as starting material, which eventually would lead to optical antipode of natural
8
,9
Crotanecine 1. The synthesis was started with preparation of desired aldehyde 13 from known synthetic route .
However, for the practical purpose we have made several modifications in the experimental condition in order to
make this route more viable and scalable. The synthesis of key intermediate 14 was started from the commercially
1
available inexpensive D-ribose. The treatment of D-ribose with catalytic amount of concentrated sulfuric acid and
0
acetone resulted in acetonide 16 in quantitative yields (Scheme 2). Sequential reduction of 16 with NaBH followed
4
8
,9
by oxidative cleavage of the diol with NaIO
4
provided hemiacetal 17. The acetonide group was deprotected by using
6
0% aqueous acetic acid to yield D-erythrose 18. As the synthesis of diethyldithioacetate could not be conveniently
scaled up due to its water solubility, we have prepared the more lipophilic dibenzyldithioacetal. Acid catalysed
dithioacetalisation of the hemiacetal 18 with benzyl mercaptane, followed by chromatography on a short silica gel
column gave 19. The primary hydroxyl group in 19 was then selectively protected as silyl ether using TBDMSCl
providing 20, which on isopropylidenation using 2,2-dimethoxypropane, camphor sulfonic acid afforded 21 in good

yield. Dethioacetalisation of 21 in aqueous acetonitrile in the presence of mercury (II) chloride, gave crude aldehyde
3. It was purified over short silica plug to obtain pure aldehyde 13 in 55% yield.
1
2 4 4
Scheme 2: Reagents and conditions:(a) Acetone, conc. H SO , 2h, 87%; (b) i) NaBH , MeOH, 0°C to rt, 1.5h; ii)
NaIO , t-BuOH, H O, rt, 16h, 71%; (c) 60% aq. AcOH, H O, 90°C,6h; (d) Con.HCl, BnSH, -60°C to rt, 3h, 52%; (e)
4
2
2
TBDMSCl, imidazole, DMF, RT, 3h, 58%; (f) 2,2-DMP, CSA, acetone, rt, 5h, 77%; (g) HgCl
RT, 2h, 55%.
2 3 2
, CaCO , MeCN, H O,
1
Accordingly, the Baylis-Hillman reaction of 13 with ethyl acrylate in the presence of DABCO gave 14
1
(
Scheme 3) as a mixture of two diastereomers which were acetylated to obtain inseparable diasteromeric mixture (5:1)
of acetate 22. Hence this mixture was preserved as such and considered for next reactions until a practical separation
is achievable. Treatment of 22 with 4-nitrobenzesulfonamide and DABCO underwent successive S 2'-S 2'
N
N
displacement reaction stereo specifically to give the sulfonamide 23 whose treatment with allybromide yielded
inseparable diastereomeric mixture of diene 24 in 96% yield. Reduction of ester group in a mixture of 24 with
diisobutylaluminium hydride at -78°C gave the primary alcohol 25 in 54% yield which was fully characterised as its
acetate 26.
O
O
O
O
OEt
OEt
OEt
OH
OEt
TBSO
O
a
c
TBSO
NH
S
O
TBSO
N
TBSO
TBSO
OAc
d
b
O
O
O
O
S
13
O
O
O
O
O
O
O
O
O
1
4
22
2
3
24
(5:1)
NO2
NO2
HO
O
AcO
AcO
O
AcO
O
TBSO
N
TBSO
N
TBSO
H
N
O
N
O
S
O
f
S
O
g
O
+
TBSO
H
e
S
O
S
O
O
O
O
O
O
O
2
6
15a
2
5
15
NO2
NO2
major
NO2
minor
NO2
Scheme 3. Reagents and conditions:(a) Ethyl acrylate, DABCO, DMSO, rt, 64%; (b) Ac
to rt, 4h, 92%; (c) 4-nitrobenzenesulfonamide, DABCO, DCM, 0°C to RT, 4h, 67%; (d) allyl bromide, K
°C tort, 4h, 96%; (e) DIBAL-H, DCM, -78°C, 15 min, 54%; (f) Ac O, NEt , DMAP, DCM, 0°C to rt, 4h, 80%; (g)
Grubb’s-II (5 mol%), toluene, 65°C,2h.
2
3
O, NEt , DMAP,DCM, 0°C
2
CO , DMF,
3
0
2
3

1
Initially, the monoacetate 26 was treated with catalytic amount of Grubbs first generation ruthenium
2
complex, but no progress of the reaction was observed even under refluxing toluene. On the other hand treatment of
°
monoacetate 26 with more reactive second generation Grubb’s catalyst (5 mol%) in toluene at 65 C for 2h resulted
cyclic allylic acetate 15 and 15a as a well separable mixture of diastereomers. The separation of these diastereomers
were achieved by normal silica chromatography and major isomer was considered as required diastereomer 15 and
taken forward for proceeding steps.
Scheme 4.Reagents and conditions:(a) TBAF in THF, -40°C to -10°C, THF, 1h, 80%; (b) p-TsCl, NEt
DCM, 0°C to rt, 4h, 78%; (c) 3N aqHCl, THF, 70°C,5h; (d) Ac O, NEt , DMAP, DCM, 0°C to rt, 4h, 75%; (e)
Cs CO , thiophenol, MeCN, rt, 4h, 55%; (f) NaOMe, MeOH, THF (1:3), rt, 4h, 64%.
Removal of silyl protecting group in 15 with 1M tetra-n-butylammonium fluoride solution in THF, followed
3
, DMAP,
2
3
2
3
by tosylation of the resulting primary alcohol in 27 furnished the tosylate 28 over two steps. For the removal of nosyl
group in 28, reaction was attempted using thiophenol and cesium carbonate in acetonitrile, which did not provide any
cyclisation product, possibly due to ring strain imposed by the acetonide group. Hence, acetonide group in 28 was
1
3
removed by acid catalysed hydrolysis using 3N aq. HCl in THF to obtain 29. It was then protected as acetate using
acetic anhydride, triethylamine and DMAP at 0 °C at room temperature to furnish triacetate 30. (Scheme 4)
Removal of nosyl group was achieved by treatment of 30 with thiophenol, cesium carbonate in acetonitrile,
which resulted in concomitant base mediated cyclisation to afford bicyclic product 31 in 55% yield. The bicyclic
triacetate 31 was fully characterized to confirm the structure. The configuration of new stereogenic centre at C(7a) in
3
1 was established on the basis of extensive nOe experiments (see the supporting information) and the results are
displayed in (Fig. 2). Thus definite nOe interactions between C(1)H and C(2)H indicated these hydrogens are in same
plane as expected based on original chiral pool stereochemistry. Further extension of nOe studies revealed strong nOe
interactions between C(7a)H and C(1)H which confirmed S configuration at C7a centre. These studies have confirmed
the all-cis relationship between C2, C1 and C7a as needed for crotanecine framework. Finally deprotection of cyclic
triacetate 31 was carried out using catalytic amount of sodium methoxide in methanol/THF (1:3) at room temperature
over 4 hours furnished an 64% yield of crotanecine 1, in optical antipode form. The purification of crotanecine posed

several difficulties and it was observed that side products generated in the solvolysis promoted the decomposition of
crotanecine, thereby complicating the purification. For analytical purpose flash chromatography was carried out over
silica gel followed by recrystallization from ethanol/ether furnished an analytical pure sample of crotanecine (-)-1, an
2
5
6c
21
-65.9 (c 0.5, EtOH), mp 195-205 °C; Reported [α] D : +38.7 (c 0.52, EtOH),
optical antipode of natural (+)-1.{[α
]
D
1
mp 192-195). Based on H NMR analytical data, the changes in chemical shift values for C(1)H, C(2)H and C(8)H are
within the expected ranges. Contrary to the expectations, protons on C(3), C(5) and C(7a) have displayed the most
1
13
dramatic chemical shift variances in H NMR and minor variances in C NMR spectra and it was understood that
6c,e
14
these proton chemical shifts are very sensitive to traces of moisture and method of purification .
Figure 2. Observed nOe interactions in compounds 31 and Crotanecine (-)-1
To further confirm the carbon framework and relative stereochemistry of (-)-1, additional nOe studies (see
the supporting information) have been considered. HMBC studies on C(8)H presented connectivity to C(8), C(6) and
C(7a). In other hand C(6)H pointed strong connectivity to C(7), C(7a), C(5), C(8). The nOe studies have revealed all-
cis relationship between C(7a)H, C(1)H and C(2)H indicating S,R,S configuration respectively. Thus 2D NMR
experiments provided unambiguous structural confirmation of carbon framework and stereochemistry of synthetic
crotanecine.
In summary, a short and enantiospecific synthesis of Crotanecine (-)-1 has been achieved by deploying
Baylis-Hillman reaction and ring closing metathesis as key steps. Further synthetic applications of the strategy
delineated above to the indoziline and pyrrolizidine alkaloids will be reported in due course.
Acknowledgement:
Authors are grateful to the management GVK Biosciences Private Ltd., for the financial support and
encouragement. We thank Dr. Sudhir Kumar Singh for his invaluable support and motivation.
Supplementary Material:
1 13
Analytical data is provided for all key intermediates along with H, C, NOE, HMBC and HSQC spectra for
compound 31 and crotanecine 1.

References:
1
2
.
.
Atal, C. K.; Kapur, K. K.; Culvenor, C. C. J.; Smith, L. W. Tetrahedron Lett. 1966, 537.
(a) Culvenor, C. C. J.; Smith, L. W.; Willing, R. I. Chem. Commun. 1970, 65. Anacrotine has in addition been
isolated from C. laburnifolia: (b) Sawhney, R. S.; Girotra, R. N.; Atal, C. K. Culvenor, C. C. J.; Smith, L. W.
Indian J. Chem. 1967, 5, 655. (c) Crout, D. H. G. J. Chem. Soc., perkin Trans. 11972, 1602.
This report contains the only reported physical data of analytically pure crotanecine: Mattocks, A. R. J.
Chem.Soc. C 1968, 235.
3
.
4
5
6
.
.
.
Culvenor, C. C. J.; Smith, L. W. Anal. Quim. 1972, 68, 883.
Jeremy Robertson and Kiri Stevens, Nat. Prod. Rep.,2014, 31, 1721-1728.
(a) Yadav, V. K.; Rueger, H.; Benn, M. Heterocycles,1984,22, 2735; (b) Bennett, R. B. III: Cha, J. K.
Tetrahedron Lett.1990,31, 5437; (c) Scott E. Denmark and Atli Thorarensen, J. Am. Chem. Soc.1997,119, 125-
137; (d) Buchanan, J. G.; Jigajinni, V.B.; Singh, G.; Wightman, R. H. J. Chem. Soc., Perkin Trans.1,1987, 2377;
(
e) Roder, E.; Liu, K.; Bourauel, T. Fresenius’ Anal. Chem. 1992, 342, 719.
7
8
9
.
.
.
Li, Yuan et al, Tetrahedron, 2004, 60, 6523-6531.
Thomas Storz, Bruno Bernet and AnreaVasella.; Helvetica chemical acta, 1999, 82, 2380.
Kin-ichi Tadano, Hiroo Maeda, Masahide Hoshino, YouichiIimura, Tetsuo Suami; J. Org. Chem.1987, 52, 1946-
1956.
1
1
1
1
1
0. Nagarapu, L.; Shuklachary, K.; Rajashekar, B.; Tetrahedron, 2012, 68, 5829-5832.
1. Palakodety Radhakrishna, Raghu Ram, K.; Tetrahedron letters 2012, 53, 744-747.
2. VenkataRamana D,; and Yashwant D. Venkar.; Eur.J. Org. Chem.2007, 5583-5589.
3. Yoshiyasu Ichikawa, Takashi Ito, Minoru Isobe.;Chem. Eur. J.2005, 11, 1949-1957.
4. For the sample obtained after silica chromatography, H-C7a signal shifted down field by 0.40 ppm with
1
comparison to crude sample H NMR value. When this purified sample is stirred with saturated methanolic
ammonia for about 3-4h, for the resulting material the same signal is again shifted up field by about 0.20 ppm and
similarly H-C3 & H-C5 have shown minor changes. To substantiate further; the pure sample was further treated
1
with dry HCl in dioxane to form its HCl salt. The H NMR spectrum of the HCl salt clearly indicated major
change in the values of the CH and CH attached to nitrogen (peaks at δ ppm 2.80, 3.11, 4.38 were shifted to δ
2
ppm 3.95, 4.30 and 5.20 respectively, see the supporting information). We are believing these chemical shift
variances are due to several factors such as presence of traces of salts or hydration or protonation on ring nitrogen
2
which might be causing the shift variances for CH and CH groups attached to nitrogen.
1
5. Spectral data for selected compounds:

1
Compound 31: H NMR (DMSO-d
6
, 400 MHz): δ = 5.78 (s, 1H, H-C6), 5.10 (d, J = 4.4 Hz, 1H, H-C2), 5.03 (d,
J = 2.8 Hz, 1H, H-C1), 4.51 (s, 2H, H-C8), 4.25 (br s, 1H, H-C7a), 3.75 (br d, J = 15.2 Hz, 1H, H-C5), 3.29-3.31
(
m, 1H, H-C5), 3.04 (d, J = 12.0 Hz, 1H, H-C3), 2.84 (dd, J = 12.0, 4.4 Hz, 1H, H-C3), 1.99 (s, 3H), 1.96 (s, 3H),
1
3
1
6
.94 (s, 3H). C NMR (DMSO-d , 100 MHz): δ = 170.0, 169.5, 169.1, 132.7 (C-7), 128.0 (C-6), 78.5 (C-2), 74.3
(
C-1), 73.4 (C-7a), 62.4 (C-5), 60.3 (C-8), 58.0 (C-3), 20.6.
1
Crotanecine-(-)-1: White solid, mp 195-205 °C; H NMR (CD
3
OD, 500 MHz): δ = 5.66 (s, 1H, H-C6), 4.38 (br
s, 1H, H-C7a), 4.18-4.21 (m, 3H, H-C2 & 8), 4.03 (dd, J = 4.0, 1.0 Hz, 1H, H-C1), 3.81-3.85 (dt, J = 15.5, 2.0
Hz, 1H, H-C5), 3.31-3.36 (m, 1H, H-C5), 3.11 (d, J = 11.0 Hz, 1H, H-C3), 2.80-2.83 (dd, J = 11.0, 4.0 Hz,1H, H-
1
3
C3). C NMR (CD
3
OD, 125 MHz): δ = 139.8 (C-7), 125.6 (C-6), 79.7 (C-2), 77.0 (C-7a), 76.6 (C-1), 63.5 (C-5),
61.9 (C-4), 59.8 (C-8).

Highlights
1.
Abundantly available D-Ribose is used as chiral starting material.
2
.
A Baylis-Hillman reaction has been used to generate new stereo-genic centre.
Ring closing metathesis was deployed for construction of pyrrolidine ring.
Base mediated method was used to remove nosyl group with concomitant cyclisation.
3.
4.