Efficient synthesis of the C/D rings of atisine-type C20- diterpenoid alkaloids

Article abstract of DOI:10.1016/j.cclet.2012.11.004

A bicyclo[2.2.2]octane C/D ring system, with a lactonic ring at C-8 and C-9, of the atisine-type C₂₀-diterpenoid alkaloids, was successfully synthesized, using an oxidative dearomatization/intramolecular Diels-Alder reaction.

Full text of DOI:10.1016/j.cclet.2012.11.004

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 1378–1380
www.elsevier.com/locate/cclet
Efficient synthesis of the C/D rings of atisine-type
C20-diterpenoid alkaloids
*
De Lin Chen, Feng Peng Wang
Department of Chemistry of Medicinal Natural Products, West China College of Pharmacy, Sichuan University, Chengdu 610041, China
Received 20 September 2012
Available online 16 November 2012
Abstract
A bicyclo[2.2.2]octane C/D ring system, with a lactonic ring at C-8 and C-9, of the atisine-type C20-diterpenoid alkaloids, was
successfully synthesized, using an oxidative dearomatization/intramolecular Diels–Alder reaction.
# 2012 Feng Peng Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: C20-diterpenoid alkaloids; Oxidative dearomatization/intramolecular Diels–Alder reaction
C20-diterpenoid alkaloids, featuring polycyclic, highly bridged, and heavily substituted structures, constitute the
most structure types group of diterpeniod alkaloids [1]. The architectural features and the important pharmacological
activities of the C20-diterpenoid alkaloids pose an alluring target for synthetic chemists and medicinal chemists. The
atisine-type C20-diterpenoid alkaloids possess a relative simplicity and extensive structural data, made it an ideal first
synthesis target, such as atisine 1 (Fig. 1) was the first total synthesis of the diterpenoid alkaloids respectively by
Nagata group [2], Masamune group [3], Wiesner group [4], Fukumoto group [5]. Recently, we reported the total
synthesis of (Æ)-atisine and (Æ)-isoazitine [6], as well as constructing A/E ring systems [7], A/E/F ring systems [8] and
B/C/D ring systems of the C19-diterpenoid alkaloids [9].
So far, five different approaches to Pelletier’s synthetic intermediate (Fig. 1) for atisine has been reported [2–6], we
also expect to efficiently and convergently construct it. Our retrosynthetic analysis of the Pelletier’s synthetic
intermediate is shown in Scheme 1. The target compound 2 was proposed to proceed from the tetracyclic compound 3,
which constructed nitrogen heterocyclic ring through the double Mannich reaction [10]. The tetracyclic 3 could be
gained, using an intramolecular aldol condensation reaction of ketone 4 to form the key C9–C10 bond. The other key
C6–C7 bond was builded to afford the ketone 4 by a Witting reaction of phosphonium salt 5 with aldehyde 6. The
aldehyde 6 could be obtained through several steps from lactone 7. Here, we hope to report an efficient approach of
constructing the [2.2.2]octane C/D ring systems of atisine-type C20-diterpenoid alkaloids, using an oxidative
dearomatization/intramolecular Diels–Alder reaction, which was developed by Liao and co-workers in 2001 [11].
Lactone 7 was prepared as shown in Scheme 2. Reduction of commercially available aromatic aldehyde 8 with
NaBH4 under 0 8C provided benzyl alcohol 9, which reacted with acryloyl chloride to afford ester 10 in 70% yield over
* Corresponding author.
E-mail address: wfp@scu.edu.cn (F.P. Wang).
1001-8417/$ – see front matter # 2012 Feng Peng Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2012.11.004

D.L. Chen, F.P. Wang / Chinese Chemical Letters 23 (2012) 1378–1380
1379
O
1
3
2
OH
11
OH
9
10
Me
Me
15
16
5
4
OH
6
18
7
17
8
N
12
13
19
20
N
N
O
14
Me
Atisine (1)
Ac
Pelletier's key synthetic intermediate (2)
Fig. 1. Structures of atisine and Pelletier’s intermediate.
O
O
OMe
OMe
9
9
OMe
OMe
OH
O
10
10
Ac
NH
7
6
Me
2
OHC Me
MOMOH C Me
2
3
4
OTBS
OMe
OMe
OMe
OMe
PPh
3
O
AcO
6
Br
7
CHO
MOMOH C Me
2
O
5
6
7
Scheme 1. Retrosynthetic analysis for Pelletier’s intermediate.
OMe
MeO
O
MeO
MOMO
CHO
MOMO
CH2
MOMO
a
b
OH
O
9
8
10
OH
O
OMe
OMe
OMe
O
c
d
O
CH2
O
CH2
O
O
11
12
O
O
MeO
15
OMe
OMe
OMe
9
OMe
OMe
f
OMe
OMe
e
O
H
15
O
O
9
O
O
O
13
O
14
7
O
Scheme 2. Construction of C/D ring system of C20-diterpenoid alkaloids. Conditions and reagents: (a) NaBH4, MeOH, 0 8C, 90%; (b) acryloyl
chloride, Py, CH2Cl2, 0 8C, 78%; (c) CF3COOH, CH2Cl2, rt, 85%; (d) PIDA, MeOH, 0 8C, then xylene, reflux, 67%; (e) H2, Pd/C, MeOH, rt, 87%; (f)
Ph3PCH3Br, t-BuOK, Et2O, 40 8C, 53%.

1380
D.L. Chen, F.P. Wang / Chinese Chemical Letters 23 (2012) 1378–1380
2 steps. Using trifluoroacetic acid, was the MOM group removed to give phenol precursor 11 in 85% yield. With
phenol 11 in hand, we next explored the key oxidative dearomatization/intramolecular Diels–Alder reaction. Since a
significant amount of undesired intermolecular dimerization product was produced using the literature procedure [11],
we slightly modified the procedure as follows: the precursor 11 was oxidized with PhI(OAc)2 using methanol as
solvent in an ice-water bath, after 30 min switching the solvent from methanol to xylene, the resulting masked ortho-
quinone 12 was heated at 160 8C to provide the lactone 13 [12] in 67% yield. Compound 13 was assigned as the desired
endo product of the Diels–Alder reaction due to the critical correlation between H-9 and the methoxyl of C-15 in the
NOESY spectrum of the hydrogenation product 14 [13]. Finally, the lactone 7 [14] was yielded from ketone 14
through a Wittig methylenation.
In conclusion, an efficient construction of highly functionalized C/D rings of atisine-type C20-diterpenoid alkaloids
has been successfully accomplished within 6 steps from a know aromatic aldehyde 8, using oxidative dearomatization/
intramolecular Diels–Alder reaction, which demonstrate a convergent strategy to construct highly functionalized
bicyclo[2.2.2]octane systems by Liao and co-worker. Further elaboration into the A-, B-, E-rings base on the lactonic
ring of compound 7 are under investigated in our laboratory and will be published in due course.
Acknowledgment
We are grateful for the financial support provided by the National Science Foundation of China (No. 81273387).
References
[1] (a) F.P. Wang, Q.H. Chen, X.Y. Liu, Nat. Prod. Rep. 27 (2010) 529;
(b) F.P. Wang, X.T. Liang, in: G.A. Cordell (Ed.), The Alkaloids: Chemistry and Biology, vol. 59, Elsevier Science, New York, 2002, p. 1.
[2] (a) W. Nagata, T. Sugasawa, M. Narisada, et al. J. Am. Chem. Soc. 85 (1963) 2342;
(b) W. Nagata, T. Sugasawa, M. Narisada, et al. J. Am. Chem. Soc. 89 (1967) 1483.
[3] S. Masamune, J. Am. Chem. Soc. 86 (1964) 291.
[4] (a) R.W. Guthrie, Z. Valenta, K. Wiesner, Tetrahedron Lett. 7 (1966) 4645;
(b) R.W. Guthrie, W.A. Henry, et al. Collect Czech. Chem. Commun. 31 (1966) 602.
[5] (a) M. Ihara, M. Suzuki, K. Fukumoto, et al. J. Am. Chem. Soc. 110 (1988) 1963;
(b) M. Ihara, M. Suzuki, K. Fukumoto, et al. J. Am. Chem. Soc. 112 (1990) 1164.
[6] X.Y. Liu, H. Cheng, F.P. Wang, et al. Org. Biomol. Chem. 10 (2012) 1411.
[7] Z.K. Yang, Q.H. Chen, F.P. Wang, Tetrahedron 67 (2011) 4192.
[8] Z.G. Liu, L. Xu, F.P. Wang, et al. Tetrahedron 68 (2012) 159.
[9] H. Cheng, L. Xu, F.P. Wang, et al. Tetrahedron 68 (2012) 1171.
[10] F.F. Blicke, F.J. McCarty, J. Org. Chem. 24 (1959) 1069.
[11] (a) Y.K. Chen, R.K. Peddinti, C.C. Liao, Chem. Commun. 2001 (2001) 1340;
(b) H.Y. Shiao, H.P. Hsieh, C.C. Liao, Org. Lett. 10 (2008) 449.
[12] Spectra data of compound 13: IR (KBr, cmÀ1): 2978, 2911, 1776, 1738, 1466, 1053, 986; 1H NMR (400 MHz, CDCl3): d 1.78–1.84 (m, 1H),
2.22–2.29 (m, 1H), 3.16 (s, 3H), 3.20 (t, 1H, J = 2.8 Hz), 3.41 (dd, 1H, J = 6.8, 10.4 Hz), 3.50 (s, 3H), 4.50 (d, 1H, J = 8.8 Hz), 4.69 (d, 1H,
J = 8.4 Hz), 6.21 (d, 1H, J = 8.4 Hz), 6.53 (t, 1H, J = 7.6 Hz); 13C NMR (100 MHz, CDCl3): d 24.9 (CH2), 39.8 (CH), 47.8 (CH), 50.3 (CH3),
51.5 (CH3), 53.7 (C), 69.6 (CH2), 93.7 (C), 132.7(CH), 133.8 (CH), 175.0 (C), 201.7 (C); HR-ESIMS: m/z: C12H14O5 [M+Na]+ 261.0738
(calcd. 261.0739).
[13] Spectra data of compound 14: IR (KBr, cmÀ1): 2950, 2987, 1782, 1739, 1370, 1065, 976; 1H NMR (400 MHz, CDCl3): d 1.78–1.91 (m, 5H),
2.16 (dt, 1H, J = 2.0, 4.4 Hz), 2.42 (s, 1H), 3.14 (t, 1H, J = 10.0 Hz), 3.18 (s, 3H), 3.57 (s, 3H), 3.95 (d, 1H, J = 8.4 Hz,), 4.59 (d, 1H,
J = 8.8 Hz,); 13C NMR (100 MHz, CDCl3): d 21.2 (CH2), 21.9 (CH2), 22.8 (CH2), 39.5 (CH), 42.4 (CH), 48.1 (C), 51.0 (CH3), 51.9 (CH3), 70.2
(CH2), 96.9 (C), 175.8 (C), 208.0 (C); HR-ESIMS: m/z: C12H16O5 [M+Na]+ 263.0890 (calcd. 263.0895).
[14] Spectra data of compound 7: IR (KBr, cmÀ1): 2924, 1648, 1514, 1104; 1H NMR (400 MHz, CDCl3): d 1.61–1.72 (m, 4H), 1.83–1.95 (m, 2H),
2.47 (d, 1H, J = 2.8 Hz), 3.10–3.13 (m, 1H), 3.16 (s, 3H), 3.38 (s, 3H), 3.96 (d, 1H, J = 8.4 Hz), 4.60 (d, 1H, J = 8.4 Hz), 5.21 (s, 1H), 5.22 (s,
1H); 13C NMR (100 MHz, CDCl3): d 21.7 (CH2), 25.2(CH2), 26.4 (CH2), 38.2 (CH), 40.7 (CH), 47.9 (C), 50.0 (CH3), 50.6 (CH3), 71.2 (CH2),
+
100.0 (C), 113.3 (CH2), 148.9 (C), 177.4 (C); HR-ESIMS: m/z: C13H18O4 [M+Na] 261.1101 (calcd. 261.1103).