A practical synthesis of long-chain iso-fatty acids (iso-C 12-C19) and related natural products

Article abstract of DOI:10.3762/bjoc.9.210

A gram-scale synthesis of terminally-branched iso-fatty acids (iso-C ₁₂-C₁₉) was developed commencing with methyl undec-10-enoate (methyl undecylenate) (for iso-C₁₂-C₁₄) or the C₁₅ and C₁₆ lactones pentadecanolide (for iso-C ₁₅-C₁₇) and hexadecanolide (for iso-C₁₈-C ₁₉). Central to the approaches outlined is the two-step construction of the terminal isopropyl group through addition of methylmagnesium bromide to the ester/lactones and selective reduction of the resulting tertiary alcohols. Thus, the C₁₂, C₁₇ and C₁₈ iso-fatty acids were obtained in three steps from commercially-available starting materials, and the remaining C₁₃-C₁₆ and C₁₉ iso-fatty acids were prepared by homologation or recursive dehomologations of these fatty acids or through intercepting appropriate intermediates. Highlighting the synthetic potential of the iso-fatty acids and various intermediates prepared herein, we describe the synthesis of the natural products (S)-2,15-dimethylpalmitic acid, (S)-2-hydroxy-15-methylpalmitic acid, and 2-oxo-14-methylpentadecane.

Full text of DOI:10.3762/bjoc.9.210

A practical synthesis of long-chain iso-fatty acids
(iso-C12–C19) and related natural products
Mark B. Richardson and Spencer J. Williams*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 1807–1812.
School of Chemistry, Bio21 Molecular Science and Biotechnology
Institute, University of Melbourne, Parkville, Victoria 3010, Australia
doi:10.3762/bjoc.9.210
Received: 11 June 2013
Email:
Accepted: 15 August 2013
Published: 04 September 2013
Spencer J. Williams* - sjwill@unimelb.edu.au
* Corresponding author
Associate Editor: H. Ritter
Keywords:
© 2013 Richardson and Williams; licensee Beilstein-Institut.
License and terms: see end of document.
chemoselective reduction; Evans’ auxiliary; Grignard addition;
homologation; ionic hydrogenation
Abstract
A gram-scale synthesis of terminally-branched iso-fatty acids (iso-C12–C19) was developed commencing with methyl undec-10-
enoate (methyl undecylenate) (for iso-C12–C14) or the C15 and C16 lactones pentadecanolide (for iso-C15–C17) and hexadecanolide
(for iso-C18–C19). Central to the approaches outlined is the two-step construction of the terminal isopropyl group through addition
of methylmagnesium bromide to the ester/lactones and selective reduction of the resulting tertiary alcohols. Thus, the C12, C17 and
C18 iso-fatty acids were obtained in three steps from commercially-available starting materials, and the remaining C13–C16 and C19
iso-fatty acids were prepared by homologation or recursive dehomologations of these fatty acids or through intercepting appro-
priate intermediates. Highlighting the synthetic potential of the iso-fatty acids and various intermediates prepared herein, we
describe the synthesis of the natural products (S)-2,15-dimethylpalmitic acid, (S)-2-hydroxy-15-methylpalmitic acid, and 2-oxo-14-
methylpentadecane.
Introduction
Long-chain iso-fatty acids occur in a broad range of organisms, septacidin [3], teicoplanins [4], tunicaminyluracil-based antibi-
and are especially abundant in bacteria where, through otics [5] (tunicamycins [6], corynetoxins [7], and strep-
incorporation into phospholipids, they influence membrane tovirudins [8]), the arylomycin glycopeptide antibiotics [9,10],
fluidity [1]. Emerging evidence has revealed unexpected roles maradolipids [11], plipastatin-type lipopeptides [12], Nod
for certain iso-fatty acids; for example iso-C15 and iso-C17 fatty factors [13], glycosylglycerides [14,15], phosphoglycolipids
acids have been shown to be essential in the development of [16], and various sphingolipids [17-19]. The terminal isopropyl
the model eukaryote Caenorhabditis elegans [2]. They are group of the iso-fatty acids arises from valine and leucine,
present as esters and amides in natural products including which through transamination and decarboxylation reactions
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yield isobutyryl-CoA and isovaleryl-CoA [20]. These starter available starting materials. To illustrate the opportunities that
units are elongated by fatty acid synthases to the final iso-fatty our approach provides, we demonstrate the elaboration of the
acids (even numbered for isobutyryl-CoA; odd-numbered for C17-iso-fatty acid 6 and an intermediate, 22, to several terminal-
isovaleryl-CoA) through extension with malonyl-CoA [21,22]. branched natural products that have not previously been synthe-
Long-chain iso-fatty acids are important analytical reference sized.
compounds owing to the presence of these materials in tobacco
[23], wool wax [24], butter fat [25], human sebaceous secre-
tions [26] (adult skin [27], meibum [28], cerumen [29], and
newborn vernix caseosa [30,31]), and a wide variety of microbi-
ological samples [1].
Previous syntheses of iso-fatty acids have typically utilized
extended, multi-step sequences. Two main approaches have
been used: (1) two-component cross-couplings that include
α-ketoester alkylation/decarboxylation [32,33], aldehyde–olefin
Figure 1: Structures of iso-fatty acids.
photoaddition [34], acetylide alkylation (sp3–sp) [35,36], Wittig
coupling [3,21,37-39], Kolbe electrosynthesis [35,40-42],
organocadmium (sp2–sp3) [43-46], organomagnesium (sp2–sp3) Results and Discussion
[47], or organocopper (sp3–sp3) [48,49] cross-couplings; or (2) Our approach to the iso-C12–C14 fatty acids 1–3 commenced
bidirectional extension of a central thiophene C4-fragment [50]. from methyl undec-10-enoate (methyl undecylenate) 9. Reac-
Two fundamentally different approaches worth special mention tion of 9 with methylmagnesium bromide afforded the tertiary
are the synthesis of the iso-C14 acid 3 by direct hydro-isopropy- alcohol 10 in 98% yield (Scheme 1). Selective reduction of the
lation of the terminal alkene of methyl undecylenate (available tertiary alcohol of 10 was achieved by ‘ionic hydrogenation’
as a pyrolysis product of ricinoleic acid) using isopropyl chloro- with triethylsilane and BF3·Et2O [53], affording 11. Oxidative
formate and ethyldichloroaluminium [51], and the synthesis of cleavage of 11 with KMnO4/Bu4NBr [54] afforded iso-C12 acid
the iso-C17 acid 6 from methyl ustilate (15,16-dihydroxypalmi- 1. Alternatively, anti-Markovnikov hydration of 11, using I2/
tate) [52]. Despite the interest in natural products containing NaBH4 then hydrogen peroxide [55], afforded the alcohol 12,
iso-fatty acids, these compounds are not readily acquired in and oxidation of 12 with KMnO4/Bu4NBr afforded iso-C13 acid
multigram quantities due to the complexity of the synthetic 2. Alternatively, alcohol 12 could be intercepted and converted
routes or limited availability of starting materials. To overcome to the mesylate 13 using MsCl/Et3N [56] and thence the nitrile
these problems, we report the scalable, gram-scale syntheses of 14 (KCN in DMSO/THF). Finally, hydrolysis of the nitrile 14
eight common iso-C12–C19 acids 1–8 (Figure 1), from readily with NaOH in H2O/EtOH afforded iso-C14 acid 3.
Scheme 1: Synthesis of iso-C12 1, iso-C13 2, and iso-C14 3 fatty acids from methyl undecylenate (9). Reagents and conditions: (a) MeMgBr, THF,
98%; (b) BF3·Et2O, Et3SiH, CH2Cl2, 99%; (c) KMnO4, Bu4NBr, AcOH, H2O, 88% for 1, 96% for 2; (d) i) I2, NaBH4, THF, ii) H2O2, 95%; (e) MsCl,
Et3N, CH2Cl2, 98%; (f) KCN, DMSO, THF, 72%; (g) aq NaOH, EtOH, 96%.
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Scheme 2: Synthesis of iso-C15 4, iso-C16 5, and iso-C17 6 fatty acids from pentadecanolide (15). Reagents and conditions: (a) MeMgBr, THF, 98%;
(b) BF3·Et2O, Et3SiH, CH2Cl2, 96%; (c) KMnO4, Bu4NBr, AcOH, H2O, 95% for 6, 93% for 5; 86% for 4; (d) NaH, CS2 then MeI; (e) reflux, 89% for 19
over 2 steps from 17, 98% for 22 over 2 steps from 20; (f) BH3·SMe2, THF, 78%.
The iso-C15–C17 fatty acids 4–6 were prepared from the readily MeI) [58] followed by Chugaev elimination afforded the
available C15 lactone pentadecanolide (exaltolide, 15) [57], a terminal alkene 19. Oxidative cleavage of 19 using KMnO4/
natural product that is produced industrially for use as a musk- Bu4NBr [54] afforded iso-C16 acid 5. Reduction of 5
odored perfumery fixative. Reaction of 15 with methylmagne- (BH3·Me2S) [59] afforded the alcohol 20 that when subjected to
sium bromide afforded the tertiary alcohol 16 in 98% yield the same transformations as before, via the xanthate ester 21,
(Scheme 2). Selective reduction of the tertiary alcohol of 16 delivered the terminal alkene 22. Finally, oxidative cleavage
was achieved using triethylsilane/BF3·Et2O [53], yielding 17. (KMnO4/Bu4NBr) [54] of 22 afforded iso-C15 acid 4.
Finally, oxidation of 17 with KMnO4/Bu4NBr [54] afforded the
iso-C17 acid 6. The iso-C15 acid 4 and iso-C16 acid 5 were The iso-C18 7 and iso-C19 8 fatty acids were synthesized
prepared by recursive dehomologation through intercepting the through similar approaches from the related C16 lactone hexade-
alcohol 17. Preparation of the xanthate ester 18 (NaH, CS2, then canolide 23 [60] (Scheme 3). Reaction of 23 with methylmag-
Scheme 3: Synthesis of iso-C18 7 and iso-C19 8 fatty acids from hexadecanolide 23. Reagents and conditions: (a) MeMgBr, THF, 97%; (b) BF3·Et2O,
Et3SiH, CH2Cl2, 95%; (c) KMnO4, Bu4NBr, AcOH, H2O, 82%; (d) MsCl, Et3N, 96%; (e) KCN, DMSO, THF, 75%; (f) aq NaOH, EtOH, 86%.
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nesium bromide afforded 24; triethylsilane/BF3·Et2O [53] although the absolute configuration has not been reported.
reduction gave 25; and KMnO4/Bu4NBr oxidation afforded iso- Diastereoselective hydroxylation [67] of the chelated Z-enolate
C18 acid 7 (Scheme 3). The iso-C19 acid 8 was readily prepared derived from 28 using the Davis oxaziridine [68] afforded the
by a three-step homologation through intercepting the alcohol 2-hydroxy compound 31 as a single diastereoisomer (as
25. Thus, mesylation of 25 (MsCl/Et3N [56]) afforded 26; determined by 1H NMR) in 71% yield. Esterification with
substitution (KCN/DMSO) afforded the nitrile 27; and hydro- MeOMgCl [69] (which has been shown not to cause epi-
lysis (NaOH in H2O/EtOH) of 27 afforded 8.
merization at the α-position [67]) and saponification [70]
afforded (S)-2-hydroxy-15-methylpalmitic acid (32). The
The above routes enable the acquisition of (multi)gram quan- ketone 33 was isolated from Xanthomonas campestris pv.
tities of the iso-C12–19 acids 1–8, and provide opportunities for vesicatoria 85-10 [71]. A direct one step synthesis of 33 was
their use as starting materials for the preparation of more com- achieved in 51% yield by Wacker oxidation using Pd/O2 [72] of
plex fatty acids. To illustrate their potential we undertook the the alkene 22, intercepted from the synthesis of the iso-C15
synthesis of several representative natural products (Scheme 4). acid 4.
2,15-Dimethylpalmitic acid has been isolated from a
microaerophilic subsurface microbial community [61], and is a Conclusion
component of human newborn vernix caseosa [31], although the We have accomplished a highly practical synthesis of the
absolute configuration of natural samples has not been deter- homologous iso-fatty acids 1–8. The iso-C12 1, iso-C17 6 and
mined. Conversion of iso-C17 acid 6 to the N-acyloxazolidi- iso-C18 7 acids were prepared from commercially-available
none 28 was achieved using pivalyl chloride/LiCl [62] and (S)- starting materials through three-step sequences and produced
4-benzyloxazolidinone. Diastereoselective methylation [63] of more than 1 g of each of the three iso-fatty acids in just 2 days
the chelated Z-enolate, derived from deprotonation of 28, using each. The remaining five fatty acids were each prepared on >1 g
NaHMDS, followed by addition of iodomethane, yielded 29 as scale by homologation or dehomologation reactions, or through
a single diastereoisomer (as determined by 1H NMR) in 80% the elaboration of intermediates in the synthesis of 1. Under-
yield. Cleavage of the chiral auxiliary using LiOH/H2O2 (which scoring the practicability of this approach, the iso-fatty acids or
occurs without racemization at the α-position) [64] afforded (S)- appropriate intermediates were used for the preparation of three
2,15-dimethylpalmitic acid (30) in 98% yield. 2-Hydroxy-15- natural products, enantiopure acids 30 and 32, and the ketone
methylpalmitic acid has been identified from a range of sources 33. The simplicity of our approach suggests that it will be of
[1] including the myxobacterium Stigmatella aurantiaca great utility in the preparation of iso-fatty acids for incorpor-
[21,65], and the oral bacterium Veillonella parvula [66], ation into more complex molecules.
Scheme 4: Synthesis of (A) 2-methyl- and 2-hydroxy-iso-fatty acids 30 and 32, and (B) the ketone 33. Reagents and conditions: (a) Et3N, PivCl, LiCl,
DMAP, (S)-4-benzyloxazolidinone, 71%; (b) NaHMDS, MeI, THF, 80%; (c) LiOH, H2O2, THF, H2O, 98%; (d) NaHMDS, Davis oxaziridine, THF, 71%;
(e) i) iPrMgCl, MeOH, 76%, ii) NaOH, MeOH, 83%; (f) O2, PdCl2, DMA, H2O, 51%.
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