Stereoselective cathodic synthesis of 8-substituted (1R,3R,4S)-menthylamines

Article abstract of DOI:10.3762/bjoc.11.34

The electrochemical generation of menthylamines from the corresponding menthone oximes equipped with an additional substituent in position 8 is described. Due to 1,3-diaxial interactions a pronounced diastereoselectivity for the menthylamines is found.

Full text of DOI:10.3762/bjoc.11.34

Stereoselective cathodic synthesis of 8-substituted
(1R,3R,4S)-menthylamines
Carolin Edinger, Jörn Kulisch and Siegfried R. Waldvogel*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 294–301.
Institut für Organische Chemie, Johannes Gutenberg-Universität
Mainz, Duesbergweg 10-14, 55128 Mainz, Germany
doi:10.3762/bjoc.11.34
Received: 28 December 2014
Accepted: 11 February 2015
Published: 27 February 2015
Email:
Siegfried R. Waldvogel* - waldvogel@uni-mainz.de
* Corresponding author
This article is part of the Thematic Series "Electrosynthesis".
Associate Editor: J. A. Murphy
Keywords:
cathodic reduction; chiral amines; electrosynthesis; menthylamines;
oximes
© 2015 Edinger et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The electrochemical generation of menthylamines from the corresponding menthone oximes equipped with an additional
substituent in position 8 is described. Due to 1,3-diaxial interactions a pronounced diastereoselectivity for the menthylamines is
found.
Introduction
Optically active amines serve as powerful and versatile tools in pinene [17,18] or camphor [19] are used as precursors for chiral
organic synthesis. Among numerous applications they are β-amino alcohols. As precursors for α-chiral primary amines,
applied as chiral ligands [1], as catalysts for various asym- fenchone [20] and camphor [21,22] are typically employed.
metric transformations [2], and as building blocks for alkaloid Furthermore, optically pure dehydroabietylamine is readily
and pharmaceutical drug synthesis [3,4]. The increasing number available and applicable without further modification [23-25].
of applications leads to a growing interest in the stereoselective
preparation of such amines. Throughout the last decades, Among the terpenoid derived amines, menthylamine and its
several strategies [3] such as stereospecific amination via C–H 8-substituted derivatives 1 represent a particularly interesting
insertion [5-8] or asymmetric olefin hydroamination [9-13] candidate. Due to the strong steric influence in the vicinity of
have been investigated in order to obtain access to optically the amino functionality, high selectivity can be expected for
pure amines. However, the major fraction of starting materials asymmetric transformations [26,27], since appropriate moieties
for the synthesis of these compounds is still provided by the as substituents R create a molecular U-turn (see Scheme 1).
chiral pool. Usually, optically active alcohols or amino acids
serve as starting material for such amine syntheses [14]. Natu- However, while optically pure menthol derivatives play a
rally occurring terpenes such as carene [15], limonene [16], significant role in organic synthesis [28], menthylamines have
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Beilstein J. Org. Chem. 2015, 11, 294–301.
In the past, we elaborated straightforward and efficient
approaches to optically pure (−)-menthylamine (3, see
Scheme 2, pathways II, III, IV).
One protocol involves conversion of the inexpensive technical
intermediate racemic neomenthol to menthylamine in a three-
step sequence, followed by enantiomeric resolution employing
tartaric acid (see Scheme 2, pathway IV) [43]. By the amount
of water in the crystallisation mixture the precipitation of
the desired diastereomeric salt can be chosen [44]. Furthermore,
we developed a Bouveault–Blanc-type protocol where
Scheme 1: Structural features of 8-substituted menthylamines 1.
only been used in a few situations, which is attributable to their (−)-menthone oxime is converted to 3 in attractive diastereose-
rather poor availability in optically pure manner. Among the lectivity (see Scheme 2, pathway II) [45]. However, the neces-
applications reported so far, are the uses as building blocks in sity for excess amounts of sodium metal constitutes a major
supramolecular receptors [29-34] or the synthesis of high- drawback. Consequently, we also investigated on electrochem-
performance stationary phases for liquid chromatography ical alternatives for the reduction of (−)-menthone oxime. In
[35-38]. (−)-Menthone (2) can be converted to menthylamine this context, we found that it can be efficiently converted to the
by different methods: A general way to convert naturally corresponding diastereomeric amines with an excess of 3a in a
occurring terpenoids is the reductive amination under divided cell under galvanostatic conditions employing an Hg
Leuckart–Wallach conditions (see Scheme 2, pathway I) [39]. pool cathode (see Scheme 2, pathway III) [26]. Here, we report
This method was applied to convert 2 to N-alkyl substituted a new synthetic route to optically pure 8-substituted menthyl-
menthylamines [40]. However, a significant disadvantage of amines 1 starting from commercially available (+)-pulegone (4,
this method is the lack of stereocontrol and partial inversion of Scheme 3). An important feature of this sequence is the initial
the configuration at position 4 resulting in a complex diastereo- introduction of a sterically demanding moiety R in position 8
meric mixture. Alternatively, reduction of menthone oxime can via cuprate addition. After conversion to menthone oximes 7
be achieved, either employing Bouveault–Blanc conditions within the second step, R is supposed to enhance the stereose-
[41], or via hydrogenolysis at a transition metal catalyst [42]. lectivity of the following electrochemical reduction process
Both approaches lead to the desired product as a diastereomeric (Scheme 3, step 3). Concomitantly, the resulting amines 1 are
mixture.
expected to have improved properties as catalysts/auxiliaries for
Scheme 2: Synthetic strategies to menthylamines.
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Beilstein J. Org. Chem. 2015, 11, 294–301.
Scheme 3: Stereoselective synthesis of 8-substituted (1R,3R,4S)-menthylamines.
asymmetric transformations due to the increased sterical from aryl bromide 5, yielding the corresponding menthones
demand in the vicinity of the amine group.
6a–c in good yields. In all cases, simple distillation is sufficient
to isolate 6 in high purity.
Results and Discussion
Exploratory work
Subsequent treatment of epimeric mixtures 6a–c with
Our previous studies revealed that menthone oxime can be elec- NH2OH∙HCl and NaOH provides the corresponding (1R,4S)-
trochemically reduced on either mercury pool or lead cathode menthone oximes in good yields (68–85%). For both, cuprate
(see Scheme 4) [26]. Under optimized reaction conditions, the addition and oxime formation, conversions of the compounds
use of a mercury pool cathode renders compound 3 in 86% exhibiting the diphenyl moiety in position 8 (6c and 7c,
yield with a diastereomeric ratio (dr) 3a:3b of 2.4. In contrast, R2 = Ph) require prolonged reaction times and render slightly
using a lead cathode under optimized conditions leads to a lower yields.
reversed diastereomeric ratio of 0.6 in 99% yield. Notably,
Electrochemical reduction of 8-substituted
methyltriethylammonium methylsulfate (MTES) was used as
additive in the latter case in order to suppress lead corrosion and menthone oximes
to improve the current efficiency [26,46,47]. These promising The electrolyses were carried out in a divided cell using a
results prompted us to study the stereoselectivity of the reduc- Nafion® sheet as separator and platinum as anodic material.
tion in the presence of a sterically demanding group in position Since previous studies revealed that the reduction of the non-
8 of the substrate molecule.
substituted menthone oxime was most effective on mercury or
lead as cathodic material [26], we were prompted to study the
influence of such cathodes onto yield and stereoselectivity in
Synthesis of the menthone oxime substrates
In order to obtain the desired 8-substituted (1R,4S)-menthone the conversion of 7a (see Scheme 6).
oxime substrates 7 we started from commercially available (+)-
pulegone (4) in technical grade (92%) (see Scheme 5). First, the The results clearly indicate that lead represents the superior
desired aryl moiety was installed by cuprate addition generated cathode for this application. Compared to mercury, the use of a
Scheme 4: Influence of the cathode system onto the stereoselectivity of the reduction of (1R,4S)-menthone oxime.
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Beilstein J. Org. Chem. 2015, 11, 294–301.
Scheme 5: Preparation of 8-substituted (1R)-menthones 6 and the corresponding oximes 7.
Scheme 6: Influence of cathode material on the preparation of (1R,3R,4S)-menthylamine 8a.
lead cathode renders significantly higher yields (93% vs 19%).
Table 1: Effect of temperature onto yield and stereoselectivity of the
electrochemical reduction of 7a on a lead cathode.
Furthermore, an improved dr of 8:1 in favor of the desired
isomer 8a is obtained (compare dr = 6:1 obtained at Hg). Both
results clearly demonstrate the positive effect of the substituent
in position 8 onto the diastereoselectivity of the reduction
(compare Scheme 4, dr = 2.4). Since lead was distinctly the
prime cathode material it was used throughout all further
investigations. To determine the influence by the temperature
onto the selectivity, the reaction was carried out at 20, 40, and
60 °C (see Table 1). Methyltriethylammonium methylsulfate
(MTES) serves as additive in order to suppress electrode corro-
sion due to PbSO4 formation [26,46,47].
Entrya
T [°C]
yieldb [%] c.e.c [%]
drd [8a:8b]
1
2
3
60
40
20
33
85
93
13
34
38
6.2
5.4
5.2
aCatholyte: 0.5% MTES and 2% H2SO4 in MeOH, current density:
12.5 mA cm–2, passed charge: 10 F mol–1, anode: platinum; byield
determined by GC (internal standard); cc.e. = current efficiency; dratio
determined by NMR spectroscopy.
ionic coating on the electrode surface and concomitant inhibi-
With increasing temperature higher diastereoselectivity can be tion of cathodic corrosion and evolution of molecular hydrogen
observed, but the product yield strongly decreases. At 60 °C, [46,47].
hydrolysis of the oxime becomes the predominant reaction,
yielding large amounts of menthone 6a and the desired product The chosen additives differ in size and number of ammonium
8 in only 33% yield (Table 1, entry 1). Moreover, the influence groups. The use of additive 11 leads to the highest product yield
of alkylammonium salt additives on the reaction was studied (Table 2, entry 3) but with a dr of 4.7 the lowest stereoselec-
(see Table 2). Such additives are known to have a positive tivity. However, optimal compromise is given by MTES (addi-
effect on reductions on lead cathodes due to the formation of an tive 10) as additive with a yield of 93% and a diastereomeric
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Beilstein J. Org. Chem. 2015, 11, 294–301.
Table 2: Influence of alkylammonium salts on the electrochemical preparation of 8a.
Entrya
1
additive
yieldb [%]
83
c.e.c [%]
drd [8a:8b]
33
37
7.5
9
2
3
93
95
6.2
4.7
10
38
11
aCatholyte: 0.5% additive and 2% H2SO4 in MeOH, current density: 12.5 mA cm−2, passed charge: 10 F mol−1, anode: platinum; byield determined by
GC (internal standard); cc.e. = current efficiency; dratio determined by NMR spectroscopy.
ratio of 6.2 (Table 2, entry 2). The compact cation 9 renders the of 6:1. However, the yield can be significantly improved using
best result with regard to diastereomeric ratio, but with inferior other alkylammonium additives, such as tetramethylammonium
yield of 83%.
salt 9, BQAOH (11) and cyclic alkylammonium salt 13 (see
Table 3, entries 1, 3 and 4), whereas 10 and 13 render im-
proved yields along with lower diastereoselectivity, the use of
additive 11 provides the highest yield with best stereoselec-
tivity (see Table 3, entry 3). Again, the positive effect of the
Next, the elaborated conditions were applied to oxime 7b
(Table 3, entry 2). The corresponding menthylamines 12a and
12b were obtained in only 42% yield in a diastereomeric ratio
Table 3: Electrochemical synthesis of 12a using different additives.
Entrya
additive
yieldb [%]
c.e.c [%]
drd [12a:12b]
1
2
3
9
63
42
78
25
17
31
6.8
5.6
8.6
10
11
4
73
29
4.4
13
aAnode: platinum; byield determined by GC (internal standard); cc.e. = current efficiency; dratio determined by NMR spectroscopy.
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Beilstein J. Org. Chem. 2015, 11, 294–301.
substituent in position 8 onto the diastereoselectivity of the
reduction is clearly demonstrated (compare Scheme 4).
Since the achieved results with additive 11 are very promising
(78%, dr 8.6) we were prompted to perform further studies on
the effect of the concentration of 11 contained in the electrolyte.
The results reveal that a concentration of 1 wt % renders
optimum values for product yield, current efficiency and dia-
stereomeric ratio (see Table 4).
Scheme 7: Protection of the oxime functionality in 7c due to the steri-
cally demanding diphenyl moiety in 8-position.
Diastereomerically pure 8-substituted
menthylamines
To obtain diastereomerically and analytically pure 8-substi-
tuted (1R,3R,4S)-menthylamines, each diastereomeric mixture
can be transformed to the corresponding hydrochlorides by
passing HCl through a solution of 8 or 12 in diethyl ether
followed by a selective crystallization of diastereomers 8c and
12c from 1,4-dioxane or CH2Cl2/heptane, respectively (see
Scheme 8). 8c and 12c can thus be obtained in 73% and 88%
yield.
Table 4: Influence of the concentration of additive 11 on the electrore-
duction of 7b.
Entrya c (additive 11)
yieldb
[%]
c.e.c
[%]
drd
[12a:12b]
[%]
1
2
3
0.5
1
78
89
85
13
36
37
8.6
8.9
7.8
4
aCatholyte: 2% H2SO4 in MeOH, current density: 12.5 mA cm−2,
passed charge: 10 F mol−1, anode: platinum; byield determined by GC
(internal standard); cc.e. = current efficiency; dratio determined by
NMR spectroscopy.
Conclusion
We presented an efficient strategy to synthesize (1R,3R,4S)-
menthylamines with aryl substituents in position 8. Starting
from commercially available (+)-pulegone, the desired aryl
Increasing the additive amount to 4 wt % does not have any moiety can be installed in position 8 by cuprate addition. Subse-
further benefit onto the oxime conversion, while simultane- quent treatment with NH3OH∙HCl at alkaline conditions
ously the stereoselectivity decreases slightly (Table 4, entry 3). provides the enantiomerically pure (1R,4S)-menthone oximes in
Employing oxime 7c demonstrates the limitations of our good yields. In the final step of the reaction sequence, the
methodology. After passing 10 F under the optimized reaction oxime is electrochemically reduced. We found that the use of a
conditions using additive 10, non-converted oxime 7c was fully lead cathode in combination with alkylammonium additives
recovered. A possible explanation is a strong decrease of the render the desired 8-substituted (1R,3R,4S)-menthylamines in
electron transfer rate due to steric shielding of the oxime func- high yields and good diastereoselectivity. Compared to non-
tionality (see Scheme 7). Also the low solubility of the sub- substituted (1R,3R,4S)-menthylamine, the diastereoselectivity is
strate is obstructive.
significantly improved, owing to the increased steric demand of
Scheme 8: Separation of the diastereomeric 8-substituted menthylamines by crystallization of their hydrochlorides.
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