二環性β-プロリンオリゴマーによる剛直なトランスアミドヘリックスの創製

論 文 の 内 容 の 要 旨

論 文 題 目

Generation of Robust trans-Amide Helix by Using

Bicyclic β-Proline Oligomers

(二 環 性 β −プ ロ リ ン オ リ ゴ マ ー に よ る 剛 直 な ト ラ ン ス ア ミ ド ヘ リ ッ ク ス の 創 製 )

Introduction

It has been shown that non-natural β-amino acids can form regular structures such as helices. Development of robust helical structures that are relatively independent of environmental changes and interaction with short β-peptides is a challenging goal in the area of peptide engineering. To this aim, it is necessary to control amide cis-trans equilibrium as well as main chain conformation, because unbiased rotamers are present in the case of tertiary amides. It was reported that β-amino acid of bicyclic 7-azabicyclo[2.2.1]heptane derivatives bearing with a C4-bridgehead methoxymethyl group sides with the cis-amide completely(Figure 1(a))1_{. Thus I expected that}

introduction of a substituent at the C1-bridgehead position can bias the equilibrium to the trans-amide conformation. In this work I show substitution at the bridgehead position C1 close to the carboxylic functionality can indeed shift the amide equilibrium to trans amide (Figure 1(b)). Thus a method for synthesis of β-proline mimics, bridgehead-substituted 7-azabicyclo[2.2.1]heptane-endo-carboxylic acid and the homooligomers ((R)-2, 3, 4, 6, 8, (S)-2h and (S)-3h) was established and their structures were investigated (Figure 1(c)).

Figure 1. (a) cis-Amide homooligomers. (b) Controlling amide cis-trans equilibrium by steric hindrance. (c) Target trans-amide homooligomers.

N O R N R cis trans O O O ∗ ∗ N MeO O n n=2, 3, 4, 6, 8 CH2OCH3 Boc ∗ ∗ N MeO O n Boc CH₂OCH₃ (R)-2: n=2 (R)-3: n=3 (R)-4: n=4 (R)-6: n=6 (R)-8: n=8 (a) _{(b) (c)} (R)-1: n=1 1 2 4 1 3 4 2 1 1

Results and Discussion

1. Synthesis of monomers and homooligomers

A synthetic route of β-amino acid unit ((R)-1) is established for the first time (Scheme 1):

  Scheme 1. Synthesis of homooligomers with bridgehead-substituted bicyclic β-amino acid.

the two diastereoisomers of α-methoxy-α-(trifluoromethyl)phenylacetyl (MTPA) ester (1R,2R,4S,2’R)-3 and (1S,2S,4R,2’R)-32 _{were separated by recrystallization from n-octane to afford two optically pure}

diastereoisomers. The stereochemical structure of (1S,2S,4R,2’R)-3 in the solid state was confirmed by X-ray crystallographic analysis. Diol 4 was obtained from ester 3 by reduction, and the primary alcohol was selectively protected with a TBDMS group. Oxidation of this secondary alcohol 5 gave ketone 6, followed by Wittig reaction to give exo-olefin 7. The silyl group was deprotected with Amberlyst®_{-15, followed by methyl}

etherification to give 9. Hydroboration-oxidation reaction of exo-methylene 9 was performed with a combination of BH3·THF and 2,3-dimethyl-2-butene to afford alcohol 10. The bulky 2,3-

dimethylbutan-2-ylborane (thexyl borane) improved the facial selectivity, affording an endo/exo ratio of 93:7. The resulting mixture of alcohols 10 was subjected to sequential oxidation reactions; Swern oxidation, and following Pinnick oxidation gave the corresponding aldehyde 11 first and the carboxylic acid 12, which was transformed to methyl ester by the action of (trimethylsilyl)diazomethane (TMSCHN2). At this stage, the endo/exo isomers of the

methyl ester were separated by column chromatography on silica gel to give (R)-1.

MeOOC NHBz + OMe TMSO Hydroquinone 120 oC, 24 h OTMS COOMe ref.2 Boc N MeOOC O O F3C OMe Boc N COOMe O O CF3 1 (±)!2 _{(1R,2R,4S,2'R)-3 (1S,2S,4R,2'R)-3} 1 4 2' 8 steps oil solid MeO OMe BzHN + Boc N O O COOMe MeO CF3 NaBH4, CaCl2 THF/EtOH Boc N OH CH2OH TBDMSCl, Et3N DMAP, DMF r.t.,19 h, 90% Boc N OH CH2OTBDMS (COCl)2, DMSO Et3N, CH2Cl2 -78 °_{C, 3 h, 94%} Boc N CH2OTBDMS O t-BuOK, MePh3PBr Et2O r.t., 3 h, 96% Boc N CH2OTBDMS Amberlyst® 15 MeOH r.t., 2.5 h, 95% Boc N CH2OH NaH, MeI THF r.t., 12 h, 92% Boc N CH2OCH3 BH3 ·THF 2N NaOH, 30% H2O2 r.t., 1 h, 93% (COCl)2, DMSO Et3N, CH2Cl2 -78 °_{C, 3 h, 88%} Boc N CH2OCH3 OHC NaClO2 NaH2PO4 ·2H2O t-BuOH r.t., 2 h, 100% Boc N CH2OCH3 HOOC toluene/MeOH r.t., 15 min, 80% ∗ ∗ Boc N CH2OCH3 MeO _O + Boc N CH2OCH3 MeOOC (1R, 2R,4S, 2'R)-3 (1S, 2R, 4S)-4 (1S, 2R, 4S)-5 (1S, 4S)-6 (1S, 4S)-7 (1S, 4S)-8 (1S, 4S)-9 11 12 _{(R)-1 (1S, 2S, 4S)-13} r.t., 48 h, 89% 2,3-dimethyl-2-butene THF, 0 °_{C, 3.5 h} 1) 2) 1) 2) separation by column chromatography endo/exo= 93:7 endo/exo= 93:7 TMSCHN2 ∗ ∗ Boc N MeO O CH2OMe TFA, CH2Cl2 r.t. , 20 min endo-12, HATU DIPEA, CH₂Cl₂ 0 °_{C- r.t.} 12 h, 85% (2 steps) (R)-1 ∗ ∗ N MeO O Boc 2 CH2OMe ∗ ∗ N MeO O Boc n CH2OMe (R)-2 1) 2) Boc N CH2OCH3 HOH2C 10 endo/exo=93:7 (R)-3: n=3 (R)-4: n=4 (R)-6: n=6 (R)-8: n=8

With the building block in hand, sterically congested homooligomers, dimer (R)-2, trimer (R)-3, tetramer (R)-4, hexamer (R)-6 and octamer (R)-8, were successfully synthesized by means of a solution-phase coupling procedure using O-(7-azabenzotriazol-1-yl)-N,N,N’,N’-tetramethyluronium hexafluorophosphate (HATU) and N,N-diisopropylethylamine (DIPEA). All oligomers were obtained in moderate to good yields (52-85%).

2. Structural Analysis of Homooligomers

With the homooligomers in hand, structural studies of the homooligomers in solution and solid states were performed. I investigated the solution conformation of the oligomers by 2D-NMR in various kinds of protic and aprotic solvents. The observed NOE signals in all of the solvents are consistent with the postulate that the oligomers take trans amide conformation (Figure 2(a)), while the cis conformer is practically not detected. Single crystal structure analyses of the optical pure (R)-2 and p-BrBz-(R)-3-OH were successfully conducted. Both the (R)-2 and p-BrBz-(R)-3-OH take trans amide conformation, which is consistent with the 2D-NMR results. And main chain conformation is well regulated. The energy-minimized structure of the octamer (R)-8 was obtained by Monte Carlo conformation search followed by DFT geometry optimization. The energy-minimized structure bears all-trans amide and takes a left-handed helical structure, which coincided well with the crystal structure of p-BrBz-(R)-3-OH. In addition to the NMR spectra, circular dichroism (CD) spectra also highlighted a similar consistent folding property of these series of compounds (Figure 2(b)). The overall shapes of the signals and intensities per residue were similar throughout the range of oligomers.

3. Effect of Hydrogen Bonding on Oligomer Structures

Conclusion

I established a synthetic route to bridgehead-substituted 7-azabicyclo[2.2.1]heptane ester as a building block for oligomer formation.3 _{Sterically congested homooligomers were also obtained in good yields by a solution}

coupling procedure, using HATU as a coupling reagent. I found that the homooligomers take robust helical structure with all-trans amide linkages. Crystal structural analysis of the dimers and the trimer showed that the conformation of each monomeric unit is highly preorganized, indicating that the oligomers take consistent helical structure from the dimer to the octamer. I believe these oligomers will be useful as scaffolds for functional helical molecules with a range of potential biochemical applications, e.g., as modulators of protein-protein interactions and reliable spacers with well-defined lengths, upon installing various functional groups at the bridgehead positions.

-35000! -30000! -25000! -20000! -15000! -10000! -5000! 0! 5000! 10000! 200! 220! 240! 260! 280! 300! [θ ] (d e g ·c m 2·dmol -1·residue -1)! wavelength(nm)! (R)-2! (R)-3! (R)-4! (R)-6! (R)-8! (a) (b)

Figure 2. (a) Diagnostic inter-residue NOE of (R)-3. (b) CD spectra of (R)-2, (R)-3, (R)-4, (R)- 6, and (R)-8 at 100 µM in methanol. N   N Boc O O N MeO N MeOOC β1 β2 α'2 β3 α'3 OMe OMe α'1 NOE NOE

Reference: