Facile synthesis of deuterium-labelled geranylgeraniols

Article abstract of DOI:10.1246/bcsj.20140384

Facile and stereoselective syntheses of three different kinds of deuterium-labelled geranylgeraniol analogs have been achieved. LiAlD₄ is used as the deuterium source to ensure high deuterium incorporation. [8,8-d₂]- and [9,9-d₂]- geranylgeraniols have been prepared for the first time. [10-d]- geranylgeraniol was efficiently prepared with a high degree of deuterium incorporation.

Full text of DOI:10.1246/bcsj.20140384

Short Articles
labelled geranylgeraniol (GGOH) analogs in which a proton of
GGOH (2) is replaced with a D have become valuable probes
Facile Synthesis of Deuterium-
Labelled Geranylgeraniols
1
7­25
to trace the cation migration with hydride shifts.
In
addition, the D-labelled analogs have been supplied as useful
analytical standards for mass spectrometry analysis of the
2
6
metabolic pathways.
Yusuke Totsuka, Shota Ueda,1
1
The D-labelling of acyclic terpenes, such as GGOH (2)
Tomohisa Kuzuyama, and Tetsuro Shinada*1
2
20,21,24
and farnesol are divided into three categories: Type 1,
1
7­19,24,25
22­24
Type 2,
theses of D-labelled GGOHs have been performed by using
-oxo-[4,4,4-d ]-butanoate (3), 3-oxo-[2,2-d ]-butanoate (4),
and Type 3
(Figure 1). Previously, syn-
1Graduate School of Science, Osaka City University,
3
3
2
3
-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585
2Biotechnology Research Center, The University of Tokyo,
-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657
LiAlD , or AlD as the D source. Despite these previous con-
4
3
tributions, improved methods with high degrees of D-incorpo-
ration, E-stereoselectivity, and practical convenience are still
required. In this paper, we report efficient and robust syntheses
1
E-mail: shinada@sci.osaka-cu.ac.jp
Received: December 11, 2014; Accepted: January 7, 2015;
Web Released: January 16, 2015
of three different types of D-labelled GGOH analogs, [8,8-d ]-
2
GGOH (5), [10-d]-GGOH (6), and [9,9-d ]-GGOH (7), from
2
readily available starting materials. The use of LiAlD as the D
4
source ensured efficient D-incorporation into the target analogs
5
­7.
The synthesis of [8,8-d ]-5 commenced with reduction of
Facile and stereoselective syntheses of three different
kinds of deuterium-labelled geranylgeraniol analogs have
been achieved. LiAlD4 is used as the deuterium source to
ensure high deuterium incorporation. [8,8-d2]- and [9,9-d2]-
geranylgeraniols have been prepared for the first time. [10-d]-
geranylgeraniol was efficiently prepared with a high degree
of deuterium incorporation.
2
27
compound 8 using LiAlD (Scheme 2). A tert-butyldiphenyl-
4
silyl (TBDPS) group which could be removed with tetrabutyl-
ammonium fluoride (TBAF) was chosen as a protecting group
of 8 in consideration of the lability of the isoprene unit under
acidic conditions. Ester 8 was reduced at ¹40 to ¹20 °C to give
9
in 92% yield. When the reaction was performed at elevated
temperatures, the undesired 1,4-reduction product was ob-
served as a minor and inseparable by-product. Resultant allyl
alcohol 9 was transformed to bromide 10 using the Appel
reaction. The bromide 10 was reacted with allylsulfone 11
in the presence of t-BuOK to give coupling product 12. The
TBDPS group was then removed with TBAF to give 13 in 67%
Naturally occurring cyclic diterpenes are a structurally
diverse class of terpenoids derived from the ubiquitous
precursor geranylgeranyl diphosphate (GGDP, 1) and have
received significant attention due to their potential utilities in
medicinal and agricultural research areas.1 Cyclic diterpenes
are biosynthesized by diterpene cyclases (Scheme 1). The bio-
synthesis consists of multisteps: an initial activation of GGDP
by diterpene cyclases to form highly reactive carbocation a,
sequential cyclization, cation migrations involving hydride
shifts and alkyl group rearrangements, and termination reaction
by deprotonation or nucleophilic addition. These reactions are
precisely controlled at the reactive site of diterpene cyclases to
provide a single cyclic diterpene product. Further functional-
ization of the product by enzymes, such as oxidase, provides
highly oxidized cyclic diterpene derivatives.
2
8
,2
1
OH
4
2
geranylgeraniol (GGOH) (2)
Type 1: [4n+4] Type 2: [4n+2] Type 3: [4n+1]
n = 0: C4
n = 1: C8
n = 2: C12
n = 0: C2
n = 1: C6
n = 2: C10
n = 3: C14
n = 0: C1
n = 1: C5
n = 2: C9
n = 3: C13
O
O
O
O
LiAlD4/AlD3
D3C
OR
H3C
OR
D
4
D
3
Biosynthetic reaction mechanisms of cyclic diterpenes have
attracted considerable attention in recent years.1
­16
Many
efforts have been devoted to structural and mechanistic studies
8
Type 1
of diterpene cyclases.1
­16
OH
OH
In these studies, deuterium (D)-
D
D
[
8,8-d ]-geranylgeraniol (5)
diterpene
cyclase
2
OPP
10
D
Type 2
Type 3
O
O
P
a
geranylgeranyl
diphosphate (GGDP) (1)
PP = P O
OH
OH OH
[10-d ]-geranylgeraniol (6)
cationic cyclization
hydride shift
skeletal rearrangement
D D
oxidation
enzyme(s)
highly oxidized
cyclic diterpene
derivatives
OH
9
cyclic
diterpenes
[9,9-d2]-geranylgeraniol (7)
deprotonation
nucleophilic addition
Figure 1. Classification of D-labelling patterns and struc-
tures of synthetic targets 5­7.
Scheme 1. Biosynthetic pathway of cyclic diterpenes.
Bull. Chem. Soc. Jpn. 2015, 88, 575–577 | doi:10.1246/bcsj.20140384
© 2015 The Chemical Society of Japan | 575

a
a
X
OTBDPS
b
OHC
OTBDPS
2
HO C
EtO2C
OTBDPS
9
2%
D D
1
4
15
8
9
1
: X = OH
0: X = Br
b
c
SO2Ph
HO
ODC
X
1
1
SO2Ph
2 steps
5
4%
D
D
17
75%
16
c
OR
D
D
d
e
EtO2C
1
1
2: R = TBDPS
3: R = H
d
72%
8
7%
D
D
3 steps, 67%
18
1
2
9: X = OH
0: X = Br
f
e
8%
SO2Ph
OH
SO Ph
2
g
1
2
5
D
D
5
OR
D
Scheme 2. Synthesis of Type 1 analog 5. Reaction condi-
tions: a) LiAlD4 (2.0 equiv), Et2O, ¹40 to ¹20 °C; b) CBr4
2
2: R = TBDPS
h
23: R = H
3 steps, 45%
(
1.3 equiv), PPh3 (1.3 equiv), CH2Cl2, rt; c) 11 (1.2 equiv),
i
t-BuOK (4.0 equiv), THF, ¹20 °C; d) TBAF (4.0 equiv),
THF, rt; e) LiBHEt3 (5.0 equiv), [Pd(dppp)Cl2] (5 mol %),
THF, 0 °C.
OH
80%
D
6
Scheme 3. Synthesis of Type 2 analog 6. Reaction condi-
yield over three steps. The phenylsulfonyl group was then
tions: a) NaClO2 (9.0 equiv), NaH PO4 (7.0 equiv), 2-
2
removed using LiBHEt3 in the presence of [Pd(dppp)Cl ]
(
yield.
methyl-2-butene (24 equiv), t-BuOH/H O, rt; b) LiAlD
4
2
2
dppp: 1,3-bis(diphenylphosphino)propane)29 to give 5 in 58%
(2.0 equiv), Et2O, 0 °C; c) PCC (2.0 equiv), CH2Cl2, rt;
d) ethyl 2-(triphenylphosphoranylidene) propionate (1.5
equiv), CH2Cl2, rt; e) LiAlH4 (2.0 equiv), Et2O, ¹40 to
In accordance with previous studies to prepare Type 2
1
9,24
¹20 °C; f) CBr4 (1.3 equiv), PPh3 (1.3 equiv), CH2Cl2,
analogs,
we initially attempted the synthesis of 6 using 4
rt; g) 21 (1.2 equiv), t-BuOK (4.0 equiv), THF, ¹20 °C;
h) TBAF (4.0 equiv), THF, rt; i) LiBHEt3 (5.0 equiv),
to D-label at position C10. However, in our experiments, 4 was
found to be labile and allowed the rapid deuterium­proton
exchange at the methylene. To establish the more robust D-
incorporation method, we turned our attention to a synthetic
[Pd(dppp)Cl2] (5 mol %), THF, 0 °C.
LiAlD4
D D
2
7,30
route using LiAlD4 (Scheme 3). Known aldehyde 14
was
CO2Me
OH
D-incorporation
oxidized to carboxylic acid 15, which was then reduced with
b
c
LiAlD to give 16 in 75% over two steps. Alcohol 16 was
4
O
C5-elongation:
oxidized to aldehyde 17 using PCC. The Wittig reaction using
a stable ylide (ethyl 2-(triphenylphosphoranylidene) propio-
nate) secured the E-stereoselective formation of 18 with high
yield. Reduction of 18 at low temperature gave 19 in 72%
yield. Alcohol 19 was then transformed into 23 in three steps:
CO2Me
CO2Me
(i) bromination
(ii) C–Cbond formation
with d
d
(iii) phosphorylation
(
iv) coupling reaction
with Me CuLi
without purification of e
O
OEt
2
P
CH3
O
OEt
i) Appel type bromination,28 (ii) coupling with sulfone 21, and
iii) removal of the TBDPS group with TBAF. Desulfonylation
D
D
D D
(
(
CO2Me
CO2Me
f (E:Z = ~95:5)
e (E:Z = ~5:95)
unstable to isolate
of 23 using LiBHEt /[Pd(dppp)Cl ] afforded 6 in 80% yield.
3
2
difficult to separate the Z-isomer
from mixuture by chromatograpy
Previous syntheses of Type 3 analogs have been performed
by the reduction of α,β-unsaturated ester b with LiAlD to give
4
Scheme 4. Previous synthesis of Type 3 analog via
phosphonate.
c, followed by C5-chain elongation of c using as an isoprene
equivalent d (Scheme 4).2
0­22,31
To remedy the practical draw-
backs in regard to the instability of vinyl phosphate e and
preparation of all E-isomers as a single product, we applied an
alternative approach via a vinyl tosylate32 to the stereoselective
synthesis of Type 3 analog 7 (Scheme 5).
¹20 °C provided D-labelled alcohol 27 in high yield. Allyl
alcohol 28 was then subjected to a series of reactions: (i)
bromination using PBr , (ii) C­C bond formation with the
3
A dienolate of methyl acetoacetate was reacted with bromide
4 to give 25 in 84% yield (Scheme 5). Ketoester 25 was
dianion enolate of methyl acetoacetate, (iii) conversion to Z-
tosylate, (iv) Negishi cross-coupling, and (v) reduction of the
resultant α,β-unsaturated ester at low temperature to give 29.
The same reaction sequence was repeated to give 7 in good
overall yield.
2
transformed to Z-tosylate 26 under Tanabe’s mild conditions
in a highly stereoselective manner (95:5).3
3,34
Z-Tosylate 26
was found to be more stable than vinyl phosphonate e and
the undesired E-vinyl tosylate enabled removal by silica gel
column chromatography. The Negishi coupling35 was per-
In summary, we have developed practical synthetic routes to
1
access different types of D-labelled GGOHs 5­7. H NMR and
1
3
formed using commercially available Me Zn in the presence of
C NMR data of 5­7 revealed a high D-incorporation ratio
2
a Pd catalyst at ambient temperature to give 27 as a single
(Supporting Information). These methods could be applied to
the synthesis of various D-labelled GGOH analogs. Attempts
stereoisomer. Careful reduction of 27 with LiAlD at ¹40 to
4
576 | Bull. Chem. Soc. Jpn. 2015, 88, 575–577 | doi:10.1246/bcsj.20140384
© 2015 The Chemical Society of Japan

a
O
b
OTs
9
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CO2Me
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25
26
1
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e, a, b, c, f
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1.5 equiv), LiCl (5.0 equiv), Et3N (1.5 equiv), NMI (1.5
equiv), CH2Cl2, rt; c) Me2Zn (2.0 equiv), [Pd(PPh3)2Cl2]
3 mol %), THF, rt; d) LiAlD4 (1.5 equiv), Et2O, ¹40 to
20 °C; e) PBr3 (0.5 equiv), Et2O, ¹15 to 0 °C; (f) LiAlH4
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Bull. Chem. Soc. Jpn. 2015, 88, 575–577 | doi:10.1246/bcsj.20140384
© 2015 The Chemical Society of Japan | 577