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Synthesis of the Southern Tripeptide (C1–N12) of Sanglifehrins Using Asymmetric Organocatalysis

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 α-amination was used in the synthesis of the m-hydroxyphenylalanine part, whereas α-hydrazination was used for the piperazic ester part. GRAPHICAL ABSTRACT


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 actinomycete strains based on their affinity of binding to cyclophilin A, a cytosolic protein binding to cyclosporin. These macrolides are found to have 20-fold greater affinity to cyclophilin-A over the marketed drug cyclosporin A.
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 Phe 60 , Met 61 , Phe 113 , and Leu 122 of cyclophilin A.
The profound biological activities attributed to these macrolides [4] along with unprecedented type of structure having macrocyclic lactone core with two uncommon amino acids, (S)-3-carboxy-piperazine and (S)-m-tyrosine, naturally attracted interest from the synthetic organic chemistry community. To date, two total syntheses [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 C 1 -N 12 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 organocatalytic a-amination and a-hydrazination respectively (Scheme 1).
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 protected 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 NaBH 4 reduction furnished (S)-hydrazino alcohol 12. The optical purity of 12 was determined as its benzoate ester 12a, which was synthesized 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.
EXPERIMENTAL column chromatography were distilled prior to use.

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, 1 H and 13 C NMR, HPLC, and HRMS reports of all the compounds for this article can be accessed on the publisher's website.