Towards an enantiospecific total synthesis of garsubellin a and related phloroglucin natural products: The α-pinene approach

Article abstract of DOI:10.1016/j.tetlet.2003.12.023

The first enantiospecific approach to garsubellin A and related phloroglucin natural product nemorosone, of contemporary interest from (-)-α-pinene, has been delineated. Through a series of stereospecific operations, the requisite stereochemistry of the prenyl groups has been secured. Kende cyclization has been employed as the key step to construct the functionalized bicyclo[3.3.1]nonane core.

Full text of DOI:10.1016/j.tetlet.2003.12.023

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 1113–1116
Towards an enantiospecific total synthesis of garsubellin A
and related phloroglucin natural products: the a-pinene approach
Goverdhan Mehta* and Mrinal K. Bera
Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, India
Received 27 October 2003; revised 28 November 2003; accepted 5 December 2003
Abstract—The first enantiospecific approach to garsubellin A and related phloroglucin natural product nemorosone, of contem-
porary interest from ())-a-pinene, has been delineated. Through a series of stereospecific operations, the requisite stereochemistry of
the prenyl groups has been secured. Kende cyclization has been employed as the key step to construct the functionalized bicy-
clo[3.3.1]nonane core.
Ó 2003 Elsevier Ltd. All rights reserved.
In 1997, Fukuyama et al. reported the isolation of a
novel polyprenylated phloroglucin natural product
garsubellin A 1 from the wood of Garcinia subelliptica
(Guttiferae).1 These authors also reported that 1
enhanced in vitro choline acetyltransferase (ChAT)
activity in P10 rat septal neuron cultures by 154% at
10 lM concentration.1 This was a very significant
observation as many neurodegenerative disorders of
intense contemporary concern like AlzheimerÕs disease
have been attributed to deficiencies in the levels of
neurotransmitter acetylcholine (ACh).2 Consequently,
inducers of the enzyme (ChAT), which is involved in the
biosynthesis of ACh, have potential in developing
therapies for AlzheimerÕs disease. Structurally, garsu-
bellin A 1 (Fig. 1) belongs to a small but growing family
of phloroglucins, characterized by the presence of a
highly oxygenated and densely functionalized bicy-
clo[3.3.1]- nonane-1,3,5-trione core embellished with one
or more hydrophobic prenyl groups. Other prominent
members of this structural class are hyperforin3 from
Hypericum perforatum and nemorosone 34 from the
floral resins of several Clusia species and among them,
the former is widely recognized as the beneficial ingre-
dient of St. JohnÕs wort. On the other hand, nemorosone
3 has been shown to exhibit a promising activity profile
against epitheloid carcinoma (HeLa), epidermoid car-
cinoma (Hep-2), prostate cancer (PC-3) and CNS cancer
(U251).4c
O
O
OH
OH
O
O
O
O
Ph
2 Hyperforin
3 Nemorosone
OH
O
5
It is therefore hardly surprising that both on account of
structural complexity and biological potential, these
polyprenylated pholoroglucins have received consider-
able attention from synthetic organic chemists in the
past few years.5 However, to date no total synthesis of
any member of this natural product family has been
achieved though many interesting and novel strategies
have been described towards garsubellin A 1 and related
compounds.5a–d We report here the first enantiospecific
approach to garsubellin A 1 and nemorosone 3 from the
O
2
4
8
7
O
O
Figure 1. Structure of garsubellin A 1.
* Corresponding author. Tel.: +91-80-293-2850; fax: +91-80-360-0936;
e-mail: gm@orgchem.iisc.ernet.in
0040-4039/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.12.023

1114
G. Mehta, M. K. Bera / Tetrahedron Letters 45 (2004) 1113–1116
O
O
a
EtO
O
a
HO
HO
10
4
5
6
7
COOR
b
12 R = Et
b
13 R = H
c
c
HO
HO
O
I
8
O
d
e
d
O
O
O
O
O
HO
16
15
O
RO
14
e
Scheme 2. Reagents and conditions: (a) CH3C(OEt)3, CH3CH2-
COOH, 180 °C, 24 h, 75%; (b) 1 N KOH, MeOH–H2O, 60 °C, 6 h,
80%; (c) I2, KI, NaHCO3, THF–H2O, 0 °C, 12 h, 85%; (d) nBu3SnH,
AIBN, C6H6, 80 °C, 1 h, 95%; (e) DIBAL-H,THF, )78 °C, 1 h, 78%.
9
10 R = H
f
11 R = PNB
Scheme 1. Reagents and conditions: (a) OsO4, NMMO, (CH3)2CO–
H2O–tBuOH (5:5:2) rt, 3 days; 70%; (b) Ph3PþCH(CH3)2Brꢀ, KOtBu,
THF, 0 °C, 1 h, 65%; (c) NaIO4, THF–H2O (1:1), 0 °C, 1 h, 88%; (d)
1 N KOH, MeOH, 0 °C, 1 h, 60%; (e) NaBH4, CeCl3, MeOH, 0 °C,
10 min, 82%; (f) p-NO2C6H4COCl, pyridine, DMAP, DCM, rt, 80%.
a
b
16
HO
O
readily and abundantly available monoterpene chiron
())-a-pinene 4 that has culminated in the generation of
the bicyclo[3.3.1]nonane core and the stereoselective
installation of the key prenyl subunits. To the best of
our knowledge, the absolute configuration of garsubellin
A 1 and related phloroglucins has not been determined.
Thus, the choice of a-pinene, available in both enan-
tiomeric forms, as the starting material for synthesis is
particularly appropriate.
17
18
c
d
O
TMSO
o-
α- attack
21
20
19
())-a-Pinene 4 was converted into (+)-campholenic
aldehyde 5 through epoxidation and Lewis acid medi-
ated fragmentation as described in the literature,6 and
this aldehyde served as the starting point of our syn-
thesis. Catalytic OsO4-mediated dihydroxylation of 5
furnished 6 as a single diastereomer (Scheme 1). Wittig
olefination on 6 led to 77 and installed the key C(8)
prenyl side arm (see numbering in 1). The cis-diol moiety
in 7 was cleaved with periodate and the resulting 1,5-
dicarbonyl compound 8 on base mediated intramolec-
ular aldol cyclization furnished the cyclohexenone 9.
Luche reduction8 of 9 was stereoselective and exclusively
delivered the b-hydroxy compound 10 with hydride
addition from the a-face opposite to the bulky prenyl
side-chain.7 The stereochemical assignment in 10 was
secured through a single crystal X-ray structure deter-
mination9 of the p-nitrobenzoate ester 11 prepared from
10 (Scheme 1).
e
O
O
22
Scheme 3. Reagents and conditions: (a) Ph3PCH(CH3)þ2 Brꢀ, KOtBu,
THF, 0 °C, 1 h, 65%; (b) PCC, DCM, 0 °C, 1 h, 98% (c) NaH, allyl
bromide, THF, 60 °C, 4 h, 70%; (d) LDA, TMSCl, THF, )78 °C, 1 h;
(e) Pd(OAc)2, CH3CN–DCM, rt, 12 h, 30% from 20 after two steps.
15 provided the lactol 16 with a masked aldehyde
functionality required for the generation of the C(4)
prenyl moiety (Scheme 2). Wittig isopropenylation of
the bicyclic lactol proceeded as planned to yield 17 and
installed the second prenyl unit (Scheme 3). PCC oxi-
dation of 17 gave 18 and set the stage for the intro-
duction of the allyl side chain required for the
generation of the bridged bicyclo[3.3.1]nonane frame-
work. Allylation of 18, employing NaH as the base was
stereoselective and furnished a single diastereomer 20.7
The enolate 19 derived from 18 encounters considerable
Allylic alcohol 10 was subjected to a stereospecific
orthoester Claisen rearrangement (Johnson modifica-
tion)10 to install the second prenyl unit and it smoothly
delivered 12 (Scheme 2).7 Further hydrolysis of ester 12
to the acid 13 and iodolactonization led to 14. Reductive
deiodination of 14 with TBTH was straightforward and
furnished the bicyclic lactone 15.7 Dibal-H reduction of

G. Mehta, M. K. Bera / Tetrahedron Letters 45 (2004) 1113–1116
1115
6. (a) Chapuis, C. Helv. Chim. Acta 1992, 75, 1527; (b)
Mehta, G.; Nandakumar, J. Tetrahedron Lett. 2001, 42,
7667.
steric hindrance on the top face due to the gem-dimethyl
substitution and a-face attack to give 20 is clearly
favored (Scheme 3).
7. All new compounds were fully characterized on the basis
of IR, 1H and 13C NMR and HRMS data. Selected
spectral data for key compounds: 10: IR (cmꢀ1): 3391; 1H
NMR (300 MHz, CDCl3): d 5.72–5.68 (m, 1H), 5.50 (d,
J ¼ 9:9 Hz, 1H), 5.12–5.06 (m, 1H), 3.87 (br s, 1H), 2.25–
2.05 (m, 2H), 1.84–1.67 (m, 2H), 1.69 (s, 3H), 1.61 (s, 3H),
1.43–1.37 (m, 1H), 1.06 (s, 3H), 0.76 (s, 3H); 13C NMR
(75 MHz, CDCl3): d 132.6, 130.3, 128.8, 123.8, 76.5, 43.1,
In our synthetic strategy, Kende cyclization11 had been
identified as the pivotal step to construct the bicy-
clo[3.3.1]nonane framework. Consequently, cyclohexa-
none 20 was transformed to the TMS enol ether 21 then
Pd(OAc)2 mediated cyclization was gratifyingly suc-
cessful to furnish 22 in modest yield (Scheme 3).7 The
structure of 22 was in full conformity with its spectral
characteristics and its bicyclic skeleton and prenyl and
gem-dimethyl substitution pattern correspond to that
present in garsubellin A 1 and nemorosone 3. Further
efforts are ongoing to adapt this sequence to build the
functionalization pattern of the natural products on to
the bicyclo[3.3.1]nonane core.
37.7, 29.3, 28.1, 26.2, 25.1, 18.2, 13.5. (+)-12: IR (cmꢀ1):
25
D
1738; ½aꢁ +13.9° (c 1.01, CHCl3); 1H NMR (300 MHz,
CDCl3): d 5.41–5.33 (m, 2H), 5.11 (br s, 1H), 4.14 (q,
J ¼ 6:9 Hz, 2H), 2.55–2.49 (m, 1H), 2.24–2.22 (m, 2H),
2.14–2.05 (m, 1H), 1.70 (s, 3H), 1.59 (s, 3H), 1.36–1.19 (m,
1H), 1.25 (t, J ¼ 7:2 Hz, 3H), 1.01 (s, 3H), 0.97–0.85 (m,
2H) 0.82 (s, 3H); 13C NMR (75 MHz, CDCl3): d 172.7,
139.8, 132.2, 127.1, 124.0, 60.2, 44.6, 41.1, 34.8, 34.0, 30.8,
28.7, 28.5, 25.8, 22.8, 17.8, 14.2; HRMS (ES) m=z calcd for
C17H28NaO2 [M+Na]þ: 287.1987 found: 287.2016; (+)-15:
In short, employing ())-a-pinene as the chiron, we have
achieved the first enantiospecific construction of the
bicyclic core present in garsubellin A and nemorosone 3
with appropriate positioning of the C(4) and C(8) prenyl
chains. Kende cyclization has been employed as the key
step for the generation of the functionalized bicy-
clo[3.3.1]nonane core.
25
D
IR (cmꢀ1): 1773; ½aꢁ +42.7° (c 0.82, CHCl3); 1H NMR
(300 MHz, CDCl3): d 5.08–5.03 (m, 1H), 4.50 (br s, 1H),
2.65 (dd, J ¼ 16:5, 6.6 Hz, 1H), 2.35–2.01 (m, 4H), 1.72–
1.61 (m, 2H), 1.7 (s, 3H), 1.58 (s, 3H), 1.46–1.4 (m, 1H),
1.13–0.91 (m, 2H), 0.96 (s, 3H), 0.87(s, 3H); 13C NMR
(75 MHz, CDCl3): d 177.4, 132.7, 123.5, 80.0, 44.8, 41.9,
38.2, 35.6, 31.9, 30.7, 28.7, 28.1, 25.8, 20.8, 17.8; HRMS
(ES) m=z calcd for C15H24NaO2 [M+Na]þ: 259.1674
25
found: 259.1689; (+)-17: IR (cmꢀ1): 3488; ½aꢁ +32.2° (c
D
1.18, CHCl3); 1H NMR (300 MHz, CDCl3): d 5.14–5.12
(m, 2H), 3.89 (s, 1H), 2.14–1.96 (m, 3H), 1.70 (br s, 6H),
1.67–1.66 (m, 2H), 1.63 (s, 3H), 1.59 (s, 3H), 1.42–1.34 (m,
4H), 1.20–1.07 (m, 1H), 0.97 (s, 3H), 0.91 (s, 3H); 13C
NMR (75 MHz, CDCl3): d 132.4, 131.7, 124.4, 122.8, 69.7,
48.0, 47.7, 42.5, 33.0, 31.2, 28.8, 28.3, 25.8 (2C), 22.4, 17.8
Acknowledgements
We would like to thank the Chemical Biology Unit of
JNCASR for the support of this research. One of us
(M.K.B.) thanks the UGC for the award of a research
fellowship.
(2C); HRMS calcd (ES) m=z for C18H32NaO [M+Na]þ:
25
D
287.2351, found: 287.2339; (+)-18: IR (cmꢀ1): 1713; ½aꢁ
33.9° (c 0.62, CHCl3); 1H NMR (300 MHz, CDCl3): d
5.15–5.08 (m, 2H), 2.40–2.19 (m, 4H), 2.08–2.03 (m, 2H),
1.95–1.85 (m, 1H), 1.72 (s, 3H), 1.68 (s, 3H), 1.68–1.59 (m,
2H), 1.58 (s, 6H), 1.15–1.11 (m, 1H), 1.06 (s, 3H), 0.75 (s,
3H); 13C NMR (75 MHz, CDCl3): d 212.4, 132.7, 132.5,
123.5, 122.1, 56.6, 50.4, 46.7, 39.5, 34.9, 29.8 (2C), 28.2,
27.5, 25.8, 25.7, 19.9, 17.8; HRMS (ES) m=z calcd for
References and notes
C18H30NaO: 285.2194 [M+Na]þ, found: 285.2194. (+)-20:
25
D
1. Fukuyama, Y.; Kuwayama, A.; Minami, H. Chem.
Pharm. Bull. 1997, 45, 947. For related garsubellins B–E,
see: Fukuyama, Y.; Minami, H.; Kuwayama, A. Phyto-
chemistry 1998, 49, 853.
IR (cmꢀ1): 1706; ½aꢁ +56.9° (c 1.16, CHCl3); 1H NMR
(300 MHz, CDCl3): d 5.65–5.51 (m, 1H), 5.12–4.98 (m,
4H), 2.46–2.38 (m, 2H), 2.22–2.06 (m, 4H), 2.00–1.96 (m,
1H), 1.76–1.64 (m, 2H), 1.72 (s, 3H), 1.70 (s, 3H), 1.60 (s,
3H), 1.59 (s, 3H), 1.41–1.25 (m, 2H), 1.04 (s, 3H), 0.74 (s,
3H); 13C NMR (75 MHz, CDCl3): d 215.0, 134.3, 133.5,
132.8, 123.9, 120.4, 118.3, 54.3, 52.6, 42.7, 41.2, 39.0, 37.8,
33.3, 30.1, 28.7, 26.4, 26.2, 20.5, 18.4, 18.2; HRMS calcd
2. Hefti, F. J. Neurobiol. 1994, 25, 1418.
3. Gurevich, A. I.; Dobrynin, V. N.; Kolosov, M. N.;
Poprako, S. A.; Ryabova, I.; Chernov, B. K.; Debrentz-
eva, N. A.; Aizeman, B. E.; Garagulya, A. D. Antibiotiki
(Moscow) 1971, 16, 510.
4. (a) de Oliveira, C. M. A.; Porto, A. L. M.; Bittrich, V.;
Marsaioli, A. J. Phytochemistry 1999, 50, 1073; (b) Cuesta-
Rubio, O.; Velez-Castro, H.; Frontana-Uribe, B. A.;
Cardenas, J. Phytochemistry 2001, 57, 279; (c) Cuesta-
Rubio, O.; Frontana-Uribe, B. A.; Ramirez-Apan, T.;
Cardenas, J. Z. Naturforsch 2002, 57c, 372.
5. (a) Nicolaou, K. C.; Pfefferkorn, J. A.; Kim, S.; Wei, H. X.
J. Am. Chem. Soc. 1999, 121, 4724; (b) Usuda, H.; Kanai,
M.; Shibasaki, M. Org. Lett. 2002, 4, 859; (c) Usuda, H.;
Kanai, M.; Shibasaki, M. Tetrahedron Lett. 2002, 43,
3621; (d) Spessard, S. J.; Stoltz, B. M. Org. Lett. 2002, 4,
1943; (e) Young, D. G. J.; Zeng, D. J. Org. Chem. 2002,
67, 3134; (f) Kraus, G. A.; Nguyen, T. H.; Jeon, I.
Tetrahedron Lett. 2003, 44, 659.
for C21H34NaO: 325.2507 [M+Na]þ, found: 325.2509. 22:
25
D
IR (cmꢀ1): 1718; ½aꢁ )22.2° (c 0.72, CHCl3); 1H NMR
(300 MHz, CDCl3): 5.83 (dt, J ¼ 9:3, 3.3 Hz, 1H), 5.64–
5.57 (m, 1H), 5.18–5.08 (m, 2H), 2.41–2.39 (m, 3H), 2.23–
2.06 (m, 4H), 1.92–1.87 (m, 1H), 1.84–1.78 (m, 1H), 1.71
(s, 6H), 1.57 (s, 6H), 0.98 (s, 3H), 0.9–0.78 (m, 1H), 0.8 (s,
3H); 13C NMR (75 MHz, CDCl3): d 216.4, 133.7, 132.3,
129.9, 126.3, 123.7, 119.9, 60.1, 49.2, 43.5, 42.9, 42.4, 38.9,
35.1, 28.0, 26.1, 26.0, 25.8, 20.8, 17.9, 17.8; HRMS (ES)
m=z calcd for C21H32NaO: 323.2351 [M+Na]þ, found:
323.2354.
8. Gemal, A. L.; Luche, J. L. J. Am. Chem. Soc. 1981, 103,
5454.
9. Crystal data: X-ray data were collected at 293 K on a
SMART CCD–BRUKER diffractometer with graphite

1116
G. Mehta, M. K. Bera / Tetrahedron Letters 45 (2004) 1113–1116
ꢀ
monochromated MoKa radiation (k ¼ 0:7103 A). Struc-
ture was solved by direct methods (SIR92). Refinement
was by full-matrix least-squares procedures on F2 using
SHELXL-97. Compound 11: C20H25NO4 MW ¼ 343,
Crystal system: triclinic, space group: P-1, cell parameters:
ꢀ
ꢀ
ꢀ
a ¼ 6:169 (4) A, b ¼ 11:196 (6) A, c ¼ 14:154 (8) A,
a ¼ 89:277 (9)°, b ¼ 82:668 (10)°, c ¼ 77:000 (9)°,
V ¼ 944:57 A , Z ¼ 2, Dc ¼ 1:207 g cmꢀ3, F ð000Þ ¼367:9,
3
ꢀ
l ¼ 0:08 mmꢀ1. Total number of l.s. parameters ¼ 326,
R1 ¼ 0.0500 for 2742 Fo > 4sig(Fo) and 0.0625 for all
3457 data. GOF ¼ 1.042, Restrained GOF ¼ 1.042 for
all data. An ORTEP drawing of compound 11 with 50%
ellipsoidal probability has been shown below.
Crystallographic data is deposited with the Cambridge
Crystallographic Data Centre, CCDC 220642.
10. Johnson, W. S.; Werthemann, L.; Bartlett, W. R.; Brock-
som, T. J.; Faulkner, D. J.; Petersen, M. R. J. Am. Chem.
Soc. 1970, 92, 741.
11. Kende, A. S.; Roth, B.; Sanfilippo, P. J. J. Am. Chem. Soc.
1982, 104, 1784.