Progress toward the synthesis of a biomimetic membrane

Article abstract of DOI:10.1016/S0040-4039(00)00597-9

Ubiquinone has antioxidant properties and is important in the conversion of products from glycolysis and the citric acid cycle to ATP. We report the synthesis of the necessary components of a biological membrane mimic that can serve as a model system for elucidating the third step in the prokaryotic biosynthesis of ubiquinone. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00597-9

Tetrahedron Letters 41 (2000) 4247±4250
Progress toward the synthesis of a biomimetic membrane
Kyle W. Gano and David C. Myles*
Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095-1569, USA
Received 24 March 2000; revised 7 April 2000; accepted 9 April 2000
Abstract
Ubiquinone has antioxidant properties and is important in the conversion of products from glycolysis
and the citric acid cycle to ATP. We report the synthesis of the necessary components of a biological
membrane mimic that can serve as a model system for elucidating the third step in the prokaryotic bio-
synthesis of ubiquinone. # 2000 Elsevier Science Ltd. All rights reserved.
The electron-transport chain converts the products of glycolysis and the citric acid cycle into
energy in the form of ATP. A key component in this process is ubiquinone (coenzyme Q or Q).y
Existing as a nonpolar molecule that di€uses freely within the extensive mitochondrial frame-
work, ubiquinone is an essential lipid component of the mitochondrial electron-transport chain of
eukaryotes and the plasma membrane of many prokaryotes.1 Q is also present in other intra-
cellular membranes and human low-density lipoproteins.2 It has been shown that the reduced
form of Q (QH2), the hydroquinone, functions as an antioxidant by scavenging lipid peroxy
radicals, a class of oxidative products that represents a major cause of damage to cellular
membranes. Oxidative damage to mammalian cells has been linked to cancer and age-related
degenerative diseases.3 Despite the importance of Q/QH2, many aspects of Q biosynthesis are still
incomplete.
Our approach to investigate the biosynthesis of ubiquinone, speci®cally the step catalyzed by
an O-methyltransferase4 in the presence of S-adenosylmethionine, is to use self-assembled
monolayers (SAMs) to model the inner-mitochondrial membrane (Fig. 1). We report here the
synthesis of the key components of this biological membrane: an alkanethiol phosphonic acid,
which mimics the phosopholipid component,5 and an alkenethiol ubiquinone precursor tether.
The mixed monolayer assembly on gold provides an excellent sca€old to probe the prokaryotic
enzyme-binding site in the third step of ubiquinone biosynthesis.
* Corresponding author. Associate Director Organic and Medicinal Chemistry, Chiron Corporation, 5300 Chiron
Way, Mail Stop 4.5, Emeryville, CA 94608-2916, USA. E-mail: david_myles@cc.chiron.com
y
Abbreviations: Q, ubiquinone; QH2, ubiquinol.
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00597-9

4248
Figure 1. SAM of ubiquinone precursor
The synthesis of the alkanethiol phosphonic acid is initiated by the addition of p-toluenesulfonyl
chloride to the alcohol of 1 followed by displacement with sodium iodide (NaI) to furnish 10-
iododecene 2 (Scheme 1). Re¯uxing the iodo species in the presence of excess trimethyl phosphite,
a modi®ed Michaelis±Arbuzov reaction, gives the dimethyl phosphonate 3.6 The thiol linkage is
a€orded by hydrothioacetylation of alkene 3, thiolacetic acid and 2,20-azobisisobutyronitrile
(AIBN),7 to give 4.8 Methanolysis of the isolated thiolacetate followed by the facile dealkylation
of the resulting thiol dimethylphosphonate using bromotrimethylsilane yields 5, 10-thioldecane-
phosphonic acid, in 95% yield.9
Scheme 1. (a) TsCl, Et3N, DMAP, CH2Cl2, 96%; (b) NaI, acetone, 96%; (c) P(OMe)3 (10 equiv.), re¯ux, 95%; (d)
thiolacetic acid, AIBN, Tol, re¯ux, 87%; (e) acetyl chloride, MeOH, 0ꢀC, 85%; (f) TMS-Br; (g) H2O, 95% (two steps).
Synthesis of the ubiquinone analog tether begins with commercially available o-bromophenol 6
(Scheme 2). Using chloroform (CHCl3), then sodium hydroxide (NaOH), the o-aldehyde is
formed via the Reimer±Tiemann reaction.10 Dakin oxidation of this aldehyde with hydrogen
peroxide (H2O2) and NaOH a€ords 3-bromocatechol 7.11 Protection of the free hydroxyl groups
with acetic anhydride (Ac2O) yields the desired 1,2-diacetoxy-3-bromobenzene 8.12
Scheme 2. (a) CHCl3, NaOH, re¯ux; (b) H2O2, NaOH, 12% (two steps); (c) Ac2O, Et3N, DMAP, CH2Cl2, 85%

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The construction of the allylic stannane 13 also begins with commercially-available 9-decen-1-ol
1 (Scheme 3). Protection of this alcohol with triisopropylsilyl tri¯ate (TIPS-OTf) followed by
ozonolysis of the alkene gives 10. The aldehyde is homologated using triethyl phophonoacetate
to yield the allylic ester 11.13 Diisobutylaluminum hydride reduction (DIBAL-H) followed by
halogenation with N-chlorosuccinimide (NCS) and dimethyl sul®de (DMS) of the isolated alcohol
produces the allylic chloride 12.14 Other carbon-activating methods, such as Ph3P/Br2, Ph3P/
CBr4, and MsCl, were found to remove the -TIPS protecting group or, in the case of the latter,
serve in the promotion of an undesired SN20 reaction.15 The anion of tributyltin hydride is added
to 12 to a€ord the allylic stannane 13.13
Scheme 3. (a) TIPSOTf, Et3N, 0ꢀC!rt, CH2Cl2, 99%; (b) O3, CH2Cl2, ^78ꢀC, then nBu3P, 96%; (c) nBuLi, triethyl
phosphonoacetate, Et2O, 0ꢀC, 97%; (d) DIBAL-H, CH2Cl2, ^78ꢀC, 91%; (e) NCS, DMS, CH2Cl2, ^20ꢀC, 75%; (f)
LDA, Bu3SnH, THF, 0ꢀC, 80%
Arylbromide 8 and allylic stannane 13 are joined via a Stille coupling (Scheme 4).16 This reaction
a€ords the catechol derivative 14 after the selective deprotection of the triisopropylsilyl group
with tetrabutylammonium ¯uoride (TBAF). Upon activation of this alcohol with methanesulfonyl
chloride (MsCl), the protected thiol 15 is a€orded by the introduction of potassium thiolacetate.
The simultaneous deprotection of all acetyl groups by treatment with potassium carbonate
(K2CO3) in methanol17 yields the desired thiolated-alkenyl ubiquinone precursor 16.18
In summary, we have developed the components necessary to create a biological membrane
mimic. We are currently employing ubiquinone precursors of various lengths, synthesized by the
strategies described within, to probe the binding site of enzymes used in the biosynthesis of Q.
Scheme 4. (a) Pd(PPh3)4, HMPA, 65ꢀC, 78%; (b) TBAF, THF, 85%; (c) MsCl, TEA, CH2Cl2, 0ꢀC!rt; (d) potassium
thiolacetate, THF, 92% (two steps); (e) K2CO3, MeOH, 95%

4250
Acknowledgements
This work was supported by the National Science Foundation (NSF CHE-9501728), the Alfred
P. Sloan Foundation, and the Rohm and Haas Company. We wish to thank Dr. Jennifer A.
Shepherd for her advice and assistance in the synthesis of valuable intermediates.
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18. Satisfactory spectroscopic data was obtained for all intermediates. Acid 5: 1H NMR (400 MHz, CDCl3) ꢀ 1.46 (m,
18H), 2.52 (dt, 2H, J=7.4, 7.4 Hz); 13C NMR (100 MHz, CDCl3) ꢀ 24.7, 28.2, 28.4, 28.4, 28.7, 29.1, 29.4, 29.5,
32.8, 34.1; 31P NMR (160 MHz, MeOH, dicyclohexylammonium salt) ꢀ 24.3; HRMS m/z calcd for C10H23O3PS
1
(M+): 254.1106, found 254.1099. Thiol 16: H NMR (400 MHz, CDCl3) ꢀ 1.35 (m, 12H), 2.06 (q, 2H), 2.48 (dt,
2H, J=7.4, 7.4 Hz), 3.25 (d, 2H), 5.44 (br s, 2H), 5.61 (m, 1H), 5.76 (m, 1H), 6.72 (m, 3H); HRMS m/z calcd for
C17H26O2S (M+): 294.1637, found 294.1637.