Synthesis of the southern tripeptide (C1-N12) of sanglifehrins using asymmetric organocatalysis

Article abstract of DOI:10.1080/00397911.2014.947653

The tripeptide southern region of the novel cyclophilin binding natural product macrolides, namely sanglifehrins, is synthesized involving asymmetric organocatalysis as chirality-inducing step. List's asymmetric α-amination was used in the synthesis of th

Full text of DOI:10.1080/00397911.2014.947653

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Synthesis of the Southern Tripeptide
(C1–N12) of Sanglifehrins Using
Asymmetric Organocatalysis
Laghuvarapu Radhika a & Srivari Chandrasekhar a
a Division of Natural Products Chemistry , CSIR Indian Institute of
Chemical Technology , Hyderabad , India
Accepted author version posted online: 05 Sep 2014.Published
online: 15 Oct 2014.
To cite this article: Laghuvarapu Radhika & Srivari Chandrasekhar (2014) Synthesis of the Southern
Tripeptide (C1–N12) of Sanglifehrins Using Asymmetric Organocatalysis, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry, 44:24, 3602-3609,
DOI: 10.1080/00397911.2014.947653
To link to this article: http://dx.doi.org/10.1080/00397911.2014.947653
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Synthetic Communications1, 44: 3602–3609, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2014.947653
SYNTHESIS OF THE SOUTHERN TRIPEPTIDE (C1–N12)
OF SANGLIFEHRINS USING ASYMMETRIC
ORGANOCATALYSIS
Laghuvarapu Radhika and Srivari Chandrasekhar
Division of Natural Products Chemistry, CSIR Indian Institute of Chemical
Technology, Hyderabad, India
GRAPHICAL ABSTRACT
Abstract The tripeptide southern region of the novel cyclophilin binding natural product
macrolides, namely sanglifehrins, is synthesized involving asymmetric organocatalysis as
chirality-inducing step. List’s asymmetric a-amination was used in the synthesis of the
m-hydroxyphenylalanine part, whereas a-hydrazination was used for the piperazic ester
part.
Keywords a-Amination; a-hydrazination; asymmetric organocatalysis; peptide coupling;
tripeptide
INTRODUCTION
The immunosuppressive activity of cyclosporin A,[1a] FK 506, and
rapamycin[1b] is attributed to immunophilin modulation, wherein the cyclosporin
operates through cyclophilin binding.[1] Thus an exhaustive screening was initiated
to identify novel natural products having cyclophilin binding properties. In this
process, a novel class of macrolides has been isolated by Sanglier et al.[2] from acti-
nomycete strains based on their affinity of binding to cyclophilin A, a cytosolic pro-
tein binding to cyclosporin. These macrolides are found to have 20-fold greater
affinity to cyclophilin-A over the marketed drug cyclosporin A.
Received June 12, 2014.
Address correspondence to Srivari Chandrasekhar, Division of Natural Products Chemistry, CSIR
Indian Institute of Chemical Technology, Hyderabad 500007, India. E-mail: srivaric@iict.res.in
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/lsyc.
3602

SYNTHESIS OF SOUTHERN TRIPEPTIDE OF SANGLIFEHRINS
3603
Figure 1. Structures of sanglifehrins A, B, C, and D.
Further, Kallen et al.[3] have provided mechanistic insights of this higher
binding affinity through cocrystal studies and also observed that the piperazic acid
moiety present in sanglifehrins is deeply buried in the hydrophobic cavity formed
by the amino acids Phe60, Met61, Phe113, and Leu122 of cyclophilin A.
The profound biological activities attributed to these macrolides[4] along with
unprecedented type of structure having macrocyclic lactone core with two uncom-
mon amino acids, (S)-3-carboxy-piperazine and (S)-m-tyrosine, naturally attracted
interest from the synthetic organic chemistry community. To date, two total synth-
eses[5] and several partial syntheses[6] of sanglifehrin A 1a (Fig. 1) are in print. The
piperazic acids are common constituents of monamycins[7] whereas m-tyrosine has
a profound role in probing the pathways in the central nervous system (CNS).[8]
Our group has a long-term objective of evaluating the synthesis of natural products
incorporating uncommon amino acids[9] and studying their biological activity.
Towards this goal, we identified the southern tripeptide core of sanglifehrin A as a
powerful privileged fragment for evaluating as a key synthon toward SAR studies.
Herein, the synthesis of the C1-N12 fragment 2 of sanglifehrins is presented using
asymmetric organocatalysis, wherein chirality is introduced via (R)-proline.
RESULTS AND DISCUSSION
The classical retrosynthetic analysis of tripeptide 2 produced two synthetic
fragments 3 and 5, which in turn could be obtained from 6 and 7 through organo-
catalytic a-amination and a-hydrazination respectively (Scheme 1).
The synthesis of (S)-m-benzyloxy tyrosine methyl ester (3) is described in
Scheme 2. The 3-(3-(benzyloxy)phenyl)propanal (6) was prepared following litera-
ture procedure from 3-hydroxybenzaldehyde.[10] The aldehyde 6 underwent a very

3604
L. RADHIKA AND S. CHANDRASEKHAR
Scheme 1. Retrosynthetic analysis of tripeptide 2.
Scheme 2. Synthesis of fragment 3.

SYNTHESIS OF SOUTHERN TRIPEPTIDE OF SANGLIFEHRINS
3605
highly enantioselective organocatalytic a-amination[11] under List’s conditions[11a] in
the presence of (R)-proline (10 mol%) and di-tert-butyl azodicarboxylate to furnish
hydrazino aldehyde 8, which was reduced to 8a using NaBH4 for characterization.
The ee of the product 8a was determined to be 93%.[12] The hydrazino aldehyde 8
was oxidized to carboxylic acid, which was esterified immediately as methyl ester
9. The catalytic hydrogenation of 9 furnished phenol 10 in 86% yield. The vicinal
Boc groups on hydrazine 10 were knocked down with trifluoroacetic acid (TFA),
which followed Raney–Ni hydrogenation to generate the amino ester, which was
protected as a Boc amide 11.[13] All these three transformations were carried out
without engaging any purification steps with an overall yield of 67%. Rebenzylation
of phenolic group in 11 was achieved in quantitative yields to furnish benzyl pro-
tected meta-tyrosine methyl ester 3, for which the spectral data was matched with
that reported.[14] A direct conversion of 9 to 11 was attempted, albeit in poor yields
in the presence of Raney-Ni (Scheme 2).
(3S)-Methyl piperazate 15 was synthesized using organocatalytic
a-hydrazination from 5-bromopentanal 7, following the procedure reported by
Hamada et al.,[15] except that (R)-proline (10 mol%) and di(Boc)-hydrazine were used
to synthesize (S)-enantiomer. Thus, 7 on treatment with di(Boc)-hydrazine and
(R)-proline[16] followed by NaBH4 reduction furnished (S)-hydrazino alcohol 12.
The optical purity of 12 was determined as its benzoate ester 12a, which was synthe-
sized by treating 12 with BzCl and pyridine in 97% yield. The ee of the product 12a
was found to be 81%.[17] Exposure of 12 to TBSOTf and 2,6-lutidine followed by
treatment with NaH provided piperazine derivative 13 in 95% yield for two steps.
The silyl group in 13 was cleaved with TBAF to furnish alcohol 14 (96% yield),
Scheme 3. Synthesis of (3S)-methyl piperazate 15.

3606
L. RADHIKA AND S. CHANDRASEKHAR
Scheme 4. Synthesis of southern tripeptide 2.
which was oxidized to known methyl ester 5 in two steps (87% overall yield) and
whose identity was fully confirmed.[6n] Exposure of 5 to TFA in CH2Cl2 furnished
di-TFA salt of piperazic ester 15 (Scheme 3).
After establishing the synthetic procedures for the synthesis of uncommon
amino acid derivatives 3 and 15, compound 3 was exposed to TFA in CH2Cl2, which
was further coupled with N-Boc (S)-valine 4 using HOBt (1-hydroxybenzotriazole)
and EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide) to furnish the required
dipeptide 16 in 89% yield with excellent diastereoselectivity. The major diastereomer
of the dipeptide was separated through column chromatography and subjected to
hydrolysis with LiOH to furnish dipeptide acid 17 in 87% yield. Further, the dipep-
tide acid 17 was coupled selectively at N(1) of piperazic ester[5a,5b] 15 using HATU
(N,N,N0,N0-tetramethyl-O-7-azabenzo-triazol-1-yluroniumhexafluorophosphate) and
DIPEA to afford the fully and differentially protected desired tripeptide 2 (76% yield)
in 4:1 diastereomeric ratio,[18] which was separated by silica-gel column chromato-
graphy. Spectral data are in agreement with the reported data[6g](Scheme 4).
EXPERIMENTAL
1H and 13C NMR spectra were recorded in CDCl3=dimethylsulfoxide (DMSO)
with 300-, 400-, 500-, or 700-MHz spectrometers at ambient temperature. Chemical
shifts were measured in parts per million (ppm) and coupling constants in hertz (Hz);
shifts were measured relative to the signals for residual CHCl3 (d ¼ 7.26 ppm), CDCl3
(d ¼ 77.0 ppm), DMSO (d ¼ 2.50 ppm), and DMSO (d ¼ 39.43 ppm). Optical rota-
tions were measured with an Anton-Paar MCP 200 digital polarimeter by using a
1-mL cell and a 1-dm path length. FT-IR spectra were recorded as KBr discs or neat.
High-resolution mass spectra (HRMS) were recorded with an Agilent Technologies

SYNTHESIS OF SOUTHERN TRIPEPTIDE OF SANGLIFEHRINS
3607
6510 Q-TOF spectrometer. Technical-grade ethyl acetate and hexanes used for
column chromatography were distilled prior to use.
Synthesis and Characterization Data of Southern Tripeptide 2[6g]
HATU (48 mg, 0.127 mmol) and DIPEA (0.06 mL, 0.37 mmol) were added to a
solution of compound 17 (0.050 g, 0.01 mmol) in dry DMF (3 mL) at room tempera-
ture and stirred for 10 min. To this reaction mixture, compound 15 (16 mg,
0.01 mmol) in dry DMF (2 mL) was added and stirred for 2 h. The reaction mixture
was diluted with ethyl acetate (20 mL), washed with brine (10 mL), and dried over
sodium sulfate. The combined organic layer was concentrated under vacuo and pur-
ified through silica-gel column chromatography (100–200 mesh) to furnish required
20
diastereomer 2 as a liquid (48 mg, 76%). ½aꢀD ¼ ꢁ22.4 (c ¼ 3.6, MeOH). IR (KBr):
nꢂmax ¼ 3347, 2950, 2923, 1699, 1440, 1219, 1115, 772 cmꢁ1
.
1H NMR (DMSO,
500 MHz): d 7.66 (d, J ¼ 11.2 Hz, 1H), 7.46–7.31 (m, 5H), 7.16 (m, 1H), 7.04–6.73
(m, 3H), 6.58 (d, J ¼ 9.4 Hz, 1H), 5.55–5.41 (m, 1H), 5.07–4.98 (m, 2H), 3.78 (s,
3H), 3.76–3.72 (m, 1H), 3.69–3.56 (m, 1H), 3.15 (br, 1H), 2.99–2.93 (dd, J ¼ 13.2,
3.5 Hz, 1H), 2.73–2.65 (dd, J ¼ 8.3, 12.1 Hz, 1H), 2.47–2.41 (m, 1H), 1.91–1.83 (m,
1H), 1.80–1.72 (m, 1H), 1.70–1.47 (m, 3H), 1.35 (s, 9H), 0.76–0.72 (m, 6H) ppm.
13C NMR (DMSO, 125 MHz): d 171.4, 171.3, 170.5, 158.1, 155.2, 139.2, 137.0,
128.9, 128.3, 127.7, 127.5, 121.7, 115.7, 112.4, 78.0, 68.9, 59.9, 52.0, 51.6, 49.5, 40.4,
38.2, 30.3, 28.0, 22.0, 19.1, 19.0, 18.0 ppm. HRMS (ESI): calcd. for C32H45N4O7
[MþH]þ 597.3282; found 597.3281.
CONCLUSION
In conclusion, organocatalytic asymmetric a-amination and hydrazination
have been utilized to build the privileged tripeptide fragment of sanglifehrins.
FUNDING
L. R. thanks the University Grants Commission (UGC), New Delhi, for a
research fellowship. This work was supported by the Department of Science and
Technology, New Delhi (SR=S1=OC-65=2009).
SUPPLEMENTAL MATERIAL
Full experimental details, 1H and 13C NMR, HPLC, and HRMS reports of all
the compounds for this article can be accessed on the publisher’s website.
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L. RADHIKA AND S. CHANDRASEKHAR
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3609
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7.467 min, major tR 8.291 min.
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150 ꢃ 4.6, 5U; mobile phase: 60% ACN in H2O (0.1% FA); detection: 210 nm; flow rate:
1.0 mL=min; major isomer tR 9.23 min, minor isomer tR 9.71 min.