Stereocontrolled synthesis of four isomeric linoleate triols of relevance to skin barrier formation and function

Article abstract of DOI:10.1016/j.tetlet.2018.11.033

Linoleate triol esters are intermediates along the pathway of formation of the mammalian skin permeability barrier. In connection with the study of their involvement in barrier formation we required access to isomerically pure and defined samples of four linoleate triol esters. A common synthetic strategy was developed starting from isomeric alkynols derived from D-tartaric acid and 2-deoxy-D-ribose.

Full text of DOI:10.1016/j.tetlet.2018.11.033

Accepted Manuscript
Stereocontrolled synthesis of four isomeric linoleate triols of relevance to skin
barrier formation and function
Robert W. Davis, Alexander Allweil, Jianhua Tian, Alan R. Brash, Gary A.
Sulikowski
PII:
S0040-4039(18)31361-3
https://doi.org/10.1016/j.tetlet.2018.11.033
TETL 50418
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
18 October 2018
7 November 2018
9 November 2018
Please cite this article as: Davis, R.W., Allweil, A., Tian, J., Brash, A.R., Sulikowski, G.A., Stereocontrolled
synthesis of four isomeric linoleate triols of relevance to skin barrier formation and function, Tetrahedron Letters
(2018), doi: https://doi.org/10.1016/j.tetlet.2018.11.033
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Tetrahedron Letters
Stereocontrolled synthesis of four isomeric linoleate triols of relevance to skin barrier
formation and function
Robert W. Davisa, Alexander Allweila, Jianhua Tiana, Alan R. Brashb and Gary A. Sulikowskia,b

aDepartment of Chemistry, Institute of Chemical Biology, Vanderbilt University, Nashville, TN 37235, U.S.A
bDepartment of Pharmacology, Institute of Chemical Biology, Vanderbilt University, Nashville, TN 37232, U.S.A
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Linoleate triol esters are intermediates along the pathway of formation of the mammalian skin
permeability barrier. In connection with the study of their involvement in barrier formation we
required access to isomerically pure and defined samples of four linoleate triol esters. A
common synthetic strategy was developed starting from isomeric alkynols derived from D-
tartaric acid and 2-deoxy-D-ribose.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Total Synthesis
Stereochemistry
Fatty acids
Synthesis strategy
———

Corresponding author.
E-mail address: kwangho.kim@vanderbilt.edu

2
Tetrahedron
The permeability barrier of the mammalian epidermis is
Figure 2. Synthetic strategy to access four linoleic acid triols starting isomer
alkynols (2).
critical to sustainability of life as it provides defense against
water loss and prevents the infiltration of harmful bacteria and
toxins.[1] Human and mouse genetic studies identify an
enzymatic pathway initiated by 12R-lipoxygenase (12R-LOX)
and followed by epidermal lipoxygenase-3 (eLOX3) as critical to
the formation of an intact epidermal barrier; genetic deficiencies
are neonatal lethal in mice and lead to congenital ichthyosis
(scaly skin) in afflicted human families.[2,3] 12R-LOX and
eLOX3 have been shown to oxidize the essential fatty acid
linoleate esterified in the skin-specific ceramide, Cer-EOS, and a
subsequent epoxide hydrolase step forms EOS-linoleate-triols
(Figure 1).[4,5,6] Overall the pathway presumably serves to
convert a hydrophobic fatty acid to hydrophilic triols leading to
membrane reorganization and further processing of the skin
barrier.[4]
Common among lipid oxidation products are 1,2-oxygenated
stereocenters, typically in the form of 1,2-diols with examples of
all
four
possible
O
I
O
4, PdCl2(PPh3)2
O
O
CuI, Et3N, 82%
OH
OH
4
5
CO2Me
(9R, 10R)-2
CO2Me
O
O
DMP, NaHCO3
CH2Cl2, 65%
H2, NiBr2
NaBH4, MeOH
> 95%
OH
Key steps of triol formation are summarized in Figure 1
starting from 12R-LOX mediated oxidation of linoleate ceramide
ester to the corresponding linoleate 9R-hydroperoxide.
Isomerization of the latter to an intermediate epoxy alcohol sets
the stage for epoxide hydrolysis leading to isomeric linoleate
triols. It is hypothesized that triol formation facilitates hydrolysis
of the ceramide ester resulting in release of the omega hydroxyl
group which then cross-link proteins by ester bond and ultimately
skin barrier formation.[4,5] In order to support mechanistic
studies of the epidermal LOX pathway we required defined
samples of the four isomeric triol carboxylic acids, the hydrolytic
products of the ceramide esters (Figure 1). Below we describe the
CO2Me
6
O
O
O
O
O
P(OMe)2
O
8, NaHMDS
CHO
THF, 0 °C
8
13
n-C5H11
82%
O
CO2Me
CO2Me
7
9
O
O
O
O
NaBH4-CeCl3
MeOH, 0 °C, 74%
HCl (aq)
Figure 1. 212R-LOX oxidation of linoleate ceramide to linoleate triol.
THF, 77-86%
C5H11
stereocontolled synthesis of these triols as their corresponding
methyl esters utilizing a common synthetic strategy from isomeric
starting materials. These specific epidermal triols are among the
sixteen regio- and stereo isomers produced through autoxidation
for which an LC-MS separation workflow was recently
reported.[7,8]
or
1:1 d.r.
HO
HO
C5H11
(S)-CBS, BH3•DMS
THF, 80%
CO2Me
CO2Me
(13R)-10
(13S)-10
9:1 d.r., 13R/13S
OH
OH
13
C5H11
13
C5H11
10
10
OH
OH
OH
OH
COOMe
COOMe
(10R, 13S)-11
(10R, 13R)-11
stereochemical perturbations. As part of our synthetic program to
provide investigators access to a broad spectrum of lipid
metabolites we have established synthetic routes to access all
four isomers of acetonide protected alkynols 2 (Figure 2).
Alkynols (9R, 10R)- and (9R, 10S)-2 are prepared starting from
D-tartaric acid and L-2-deoxyribose respectively (Scheme 1).
Alkynol (9R, 10R)-2 was prepared from D-tartaric acid in five
steps by way of acetonide
3 (Scheme 1).[9] Acetylide
displacement of the triflate derived from alcohol 3 followed a
slight modification of an earlier procedure reported by Mukai and
Hanaoka.[10] Alkynol (9R, 10S)-2 was prepared in 2 steps from
2-deoxy-L-ribose employing
a Colvin alkynylation of an
intermediate lactol as reported by Brimble.[11]
Scheme 1. Syntheses of akynols (9R, 10R)- and (9R, 10S)-2.
The synthesis of linoleic acid triols (9R, 10R, 13R)- and (9R,
10R, 13S)-1 started with a Sonagishira coupling of (9R, 10R)-2
and vinyl iodide 4[12] to afford enyne 5 in 82% yield. Saturation
of the enyne group of 5 under standard hydrogenation conditions
(e.g. H2, Pd/C, EtOAc) proved variable often leading to
incomplete reduction. In contrast, saturation of 5 by in situ
generated nickel(0) under an atmosphere of hydrogen

reproducibly delivered ester 6 in near quantitative yield. Dess-
Martin periodinane oxidation of 6 was followed by condensation
with the anion derived from ketophosphonate 8[13] to afford
enone 9. Luche reduction of 9 provided a 1:1 mixture of (13R)-
and (13S)-10, separated by flash chromatography. Alternatively,
reduction of 9 using (S)-CBS catalyst was demonstrated to afford
a 9:1 mixture of alcohols favoring (13R)-10. The configuration of
the alcohols (13R)- and (13S)-10 were assigned using the Mosher
ester analysis method, the major isomer produced from the (S)-
CBS reduction was in agreement with related enone
substrates.[14] Acetonide removal yielded the corresponding triol
esters (9R, 10R, 13R)- and (9R, 10R, 13S)-11. The individual
esters were saponified with aqueous potassium hydroxide in
methanol as needed for the analysis of epidermal lipid samples
the mammalian epidermal permeability barrier is formed. Studies
along this line will be reported in due course.
Acknowledgments
This work was supported by National Institute of General
Medical Sciences Grant GM-115722 (G.A.S.), National
Institute of Arthritis and Musculoskeletal and Skin Diseases
Grant AR-51968 (A.R.B.), and National Institutes of Health
Shared Resource Grant P30 CA068485.
References and notes
1. Madison, K. C. J. Invest. Dermatol. 2003, 121, 231-41.
2. Krieg, P.; Furstenberger, G. Biochim. Biophys. Acta 2014, 390-400.
3. Muñoz-Garcia, A.; Thomas, C. P.; Keeney, D. S.; Zheng, Y.; Brash, A. R.
Biochim. Biophys. Acta 2014, 401-408.
Scheme 2. Synthesis of triol esters (10R, 13R)- and (10R, 13S)-11.
Synthesis of the remaining pair of linoleic acid triols were
prepared from alkynol (9R, 10S)-2 using the same series of
reactions as employed for the (9R, 10R, 13R)- and (9R, 10R,
13S)-11 pair of esters (Scheme 3). Once again cross-coupling of
(9R, 10S)-2 and vinyl iodide 4 was followed by hydrogenation
over nickel(0) to afford alcohol 12. The latter was subjected to
the earlier described oxidation-olefination sequence to give enone
13. Luche reduction again provided a separable mixture of allylic
alcohols (13R)- and (13S)-14, assignment of C13 alcohol
stereochemistry was again based upon Mosher ester analysis.
Finally, acetonide hydrolysis (13R)- and (13S)-14 afforded
methyl esters (10S, 13R)- and (10S, 13S)-11, respectively.
4. Zheng, Y.; Yin, H.; Boeglin, W. E.; Elias, P. M.; Crumrine, D.; Beier, D.
R.; Brash, A. R. J. Biol. Chem. 2011, 286, 24046-24056.
5. Chiba, T.; Thomas, C. P.; Boeglin, W. E.; O'Donnell, V. B.; Brash, A. R.
J. Biol. Chem. 2016, 291, 14540-14554.
6. Yamanishi, H.; Boeglin, W. E.; Morisseau, C.; Davies, R. W.; Sulikowski,
G. A.; Hammock, B. D.; Brash, A. R. J. Lipid Res. 2018, 59, 684-695.
7. Fuchs, D.; Hamberg, M.; Sköld, C. M.; Wheelock, A. M.; Wheelock, C. E.
J. Lipid Res. 2018, 59, 2025-2033.
8. A racemic synthesis of linoleic acid triols has been reported, see: Tadario,
K.; Hirukawa, T.; Yano, M. Bull. Chem. Soc Jpn. 1994, 67, 839-842.
9. Iida, H.; Yamazaki, N.; Kibayashi, C. J. Org. Chem. 1987, 52, 3337-3342.
10. Mukai, C.; Kim, J. S.; Uchiyama, M.; Sakamoto, S.; Hanaoka, M. J. Chem.
Soc. Perk. Trans 1 1998, 2903-2915.
11. Yuen, T. Y.; Brimble, M. A. Org. Lett. 2012, 14, 5154-5157.
12. Reddy, C. R.; Suman, D.; Rao, N. N., Helv. Chim. Acta 2015, 98, 967-972.
13. Hayashi, K.; Tanimoto, H.; Zhang, H.; Morimoto, T.; Nishiyama, Y.;
Kakiuchi, K., Org. Lett. 2012, 14, 5728-5731.
14. Pandya, B. A.; Snapper, M. L., J. Org. Chem. 2008, 73, 3754-3758.
Supplementary Material
Supplementary data associated with this article can be found
in the online version, at do:XXXXX.
Scheme 3. Synthesis of triol esters (10S, 13R)- and (10S, 13S)-11.
In summary, we have developed stereocontrolled routes to
four linoleic ester triols. The synthetic products and intermediates
will find utility in studies aimed at defining the process by which

4
Tetrahedron
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Stereocontrolled synthesis of four Isomeric linoleate triols of relevance to
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Robert W. Davis, Alexander Allweil, Jianhua Tian, Alan R. Brash and Gary A. Sulikowski

Highlights
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Lipids
Chiral pool
Mosher Esters
Stereoselective