Stereoselective preparation of 3-amino-2-fluoro carboxylic acid derivatives, and their incorporation in tetrahydropyrimidin-4(1H)-ones, and in open-chain and cyclic β-peptides

Article abstract of DOI:10.1002/hlca.201100340

The preparation of (2S,3S)- and (2R,3S)-2-fluoro and of (3S)-2,2-difluoro-3-amino carboxylic acid derivatives, 1-3, from alanine, valine, leucine, threonine, and β³h-alanine (Schemes 1 and 2, Table) is described. The stereochemical course of (diethylamino)sulfur trifluoride (DAST) reactions with N,N-dibenzyl-2-amino-3-hydroxy and 3-amino-2-hydroxy carboxylic acid esters is discussed (Fig. 1). The fluoro-β-amino acid residues have been incorporated into pyrimidinones (11-13; Fig. 2) and into cyclic β-tri- and β-tetrapeptides 17-19 and 21-23 (Scheme 3) with rigid skeletons, so that reliable structural data (bond lengths, bond angles, and Karplus parameters) can be obtained. β-Hexapeptides Boc[(2S)-β³hXaa(αF)]₆OBn and Boc[β³hXaa(α,αF₂)]₆-OBn, 24-26, with the side chains of Ala, Val, and Leu, have been synthesized (Scheme 4), and their CD spectra (Fig. 3) are discussed. Most compounds and many intermediates are fully characterized by IR- and ¹H-, ¹³C- and ¹⁹F-NMR spectroscopy, by MS spectrometry, and by elemental analyses, [α]_D and melting-point values. Copyright

Full text of DOI:10.1002/hlca.201100340

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Helvetica Chimica Acta – Vol. 94 (2011)
Stereoselective Preparation of 3-Amino-2-fluoro Carboxylic Acid
Derivatives, and Their Incorporation in Tetrahydropyrimidin-4(1H)-ones, and
in Open-Chain and Cyclic b-Peptides
by Tomohiro Yoshinari1), FranÅois Gessier2), Christian Noti3), Albert K. Beck, and Dieter Seebach*
Laboratorium fꢀr Organische Chemie, Departement fꢀr Chemie und Angewandte Biowissenschaften,
ETH-Zꢀrich, Hçnggerberg HCI, Wolfgang-Pauli-Strasse 10, CH-8093 Zꢀrich
(phone: þ 41-44-632-2990; fax: þ 41-44-632-1144, e-mail: seebach@org.chem.ethz.ch)
The preparation of (2S,3S)- and (2R,3S)-2-fluoro and of (3S)-2,2-difluoro-3-amino carboxylic acid
derivatives, 1 – 3, from alanine, valine, leucine, threonine, and b3h-alanine (Schemes 1 and 2, Table) is
described. The stereochemical course of (diethylamino)sulfur trifluoride (DAST) reactions with N,N-
dibenzyl-2-amino-3-hydroxy and 3-amino-2-hydroxy carboxylic acid esters is discussed (Fig. 1). The
fluoro-b-amino acid residues have been incorporated into pyrimidinones (11 – 13; Fig. 2) and into cyclic
b-tri- and b-tetrapeptides 17 – 19 and 21 – 23 (Scheme 3) with rigid skeletons, so that reliable structural
data (bond lengths, bond angles, and Karplus parameters) can be obtained. b-Hexapeptides Boc[(2S)-
b3hXaa(aF)]6OBn and Boc[b3hXaa(a,aF2)]6-OBn, 24 – 26, with the side chains of Ala, Val, and Leu,
have been synthesized (Scheme 4), and their CD spectra (Fig. 3) are discussed. Most compounds and
many intermediates are fully characterized by IR- and 1H-, 13C- and 19F-NMR spectroscopy, by MS
spectrometry, and by elemental analyses, [a]D and melting-point values.
1. Introduction. – The effect of backbone substitution A by F-atom(s) (or other
heteroatoms) of a peptide consisting of proteinogenic a-amino acids cannot be studied
due to hydrolytic instability4). In constrast, F-substituted b- and g-amino acid
derivatives B [1][3 – 6] and C [7] are stable. For incorporation of 3-amino-2-fluoro and
3-amino-2,2-difluoro acid moieties into b-peptides D-F and D-F2, or for syntheses of b-
peptides E-F and E-F2 bearing F-atom in each residue, we needed an access to
configurationally pure building blocks of type 1, 2, and 3 with N-Boc protection for
solution synthesis [4] and N-Fmoc protection for solid-phase peptide synthesis [5].
One aim of the present paper is to describe the preparation of such a-fluoro-b-
amino acid derivatives in full detail5), and to discuss some mechanistic aspects. A
1
)
Postdoctoral Research Fellow at ETH Zꢀrich (2010/2011), financed by the Japan Society for the
Promotion of Science for Young Scientists (JSPS No. 200909778) and by the Swiss National Science
Foundation (SNF-Project No. 200020-126693).
Postdoctoral Research Fellow at ETH Zꢀrich (2001 – 2003), financed by ETH Zꢀrich, Swiss
National Science Foundation (SNF-Project No. 2000-058831) and Novartis Pharma AG, Basel.
Part of the Master Thesis (Diplomarbeit) of C. N., ETH Zꢀrich, 2002/2003.
See the discussion in [1]. For a g,g-difluoro-d-amino acid, see [2].
Only the preparation of the alanine-derived compounds 1 – 3, R1 ¼ Me, has been described in full
detail [4]. For the NMR-structural analyses of four peptides with a central fluoro- or difluoro-
substituted building block, see: [1][5][6]. Polyfluorinated peptides of type E-F and E-F2 are
mentioned in a preliminary communication [3], and in the hitherto unpublished master thesis of C.
Noti (see Footnote 3). For determination of proteolytic stabilities of F-substituted b-peptides, see
[4].
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3
4
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ꢁ 2011 Verlag Helvetica Chimica Acta AG, Zꢀrich

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second purpose is to present cyclic and macrocyclic F-substituted b-amino acid
derivatives F-F and F-F2, which fulfill the following stringent requirements: i) they
should be crystalline (for X-ray analysis) and ii) they are likely to have the same
conformation in solution (for NMR analysis) and in the solid state; this would provide
experimental data (Karplus parameters) for the interpretation of NMR spectra and for
comparison with structures calculated by ab initio or molecular-dynamics methods6).
6
)
It turned out that both the NMR analysis and the MD calculation of F-substituted b-peptides gave
confusing results, due to lack of reliable parameters [5][6][8].

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2. Preparation of the F-Substituted b-Amino Acid Derivatives 1 – 3. – We have
chosen three stereospecific methods for the introduction of F-substituents, all starting
from natural a-amino acids alanine, valine, leucine, and threonine7). The F-atoms were
introduced nucleophilically with (diethylamino)sulfur trifluoride (DAST; ꢂFꢀꢃ) [9] and
electrophilically with (PhSO2)2NF (ꢂFþꢃ) [9b][10].
The ꢂwork-horseꢃ route (Scheme 1) started from the aldehydes 4a – 4c [11] obtained
by N,N-dibenzylation and CO2H reduction of the corresponding amino acids. Reetzꢃs
diastereoselective non-chelation-controlled (BF3) and chelation-controlled (TiCl4)
cyanohydrin reaction with Me3SiCN (! 5a – 5c) [12] was followed by the Pinner
reaction with aqueous workup to give the 3-amino-2-hydroxy carboxyclic acid esters
6a – 6c, treatment of which with DAST led to mixtures of the constitutional isomers 7
and iso-7 with higher regioselectivities in the l-series of compounds (i.e., 6 ! 7) than in
the u-series (i.e., epi-6 ! epi-7; see mechanistic discussion in Sect. 3). For the
preparation of the geminal difluoro-b-amino acid esters, a ꢂcheaperꢃ non-diastereose-
lective version of the cyanohydrin reaction was used [13] (Scheme 1; lower part). Swern
oxidation of the mixture of diastereoisomers 6/epi-6 to the a-keto esters and in situ
treatment with DAST provided the difluoro esters 8 in enantiomerically pure form8).
For preparation of the butanoate derivative epi-7a, there is a shorter route
(Scheme 2, a) [15]: the N,N-dibenzylthreonine benzyl ester (9) undergoes a fluorinating
rearrangement when treated with DAST to give epi-7aBn of (2R,3S)-configuration as
the major product. As with the other mixtures of isomers 7/iso-7 (Scheme 1, upper part)
chromatographic separation from the ꢂdirectꢃ substitution product iso-epi-7aBn and
isolation of the pure a-fluoro-b-amino acid ester epi-7aBn was straightforward.
Finally, the methyl Boc- and Cbz-(S,S)-3-amino-2-fluorobutanoates, 1abb and 1acb,
are accessible by direct fluorination [16] of the corresponding doubly lithiated [17]
b3hAla derivatives with (PhSO2)2NF (Scheme 2,b).
Having established the preparation of the 3-amino-N,N-dibenzyl-2-fluoro- and -2,2-
difluoro-alkanoates 7 and 8, only simple functional-group manipulations were needed
to arrive at the desired building blocks 1 – 3 for peptide synthesis. The reagents and
solvents used are collected in the Table, together with the starting materials and
products of the various conversions. The absolute configurations of compounds 1 – 3
follow from the use of (S)- or l-a-amino acid starting materials, the relative
configurations are deduced from an X-ray crystal structure (bottom part of the Table),
from NMR data and by analogy9).
7
)
Since these amino acids are all available in both enantiomeric forms, the enantiomers of the
compounds reported herein will be accessible by exactly the same methodology.
Purification of the keto esters by chromatography turned out to lead to partial racemization. The in
situ procedure led to the N-Boc-amino-difluoro carboxylic acids 3aba, 3bba, and 3cba with er>
97:3, > 99:1, and > 98:2, respectively, as derived from HPLC on chiral column material of
precursor esters. For previous preparations of a,a-difluoro-b-amino acid derivatives, see [14a,b]. For
a comprehensive review on the methods of synthesis of geminal difluoro methylene compounds, see
[14c].
The NMR-structural analyses [1][5][6] of b-hepta- and b-tridecapeptides of type D-F and D-F2 with
central F- or F2-b-amino acid residues 1, 2, and 3 (R1 ¼ Me, R2, Y¼ (bhXaa)n) provide further
support for the correctness of the relative and absolute configurational assignment of these building
blocks.
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Table. Reagents Used for the Functional-Group Manipulations to Convert the Primary Products 7 and 8
of Fluorination to the Starting Materials 1 – 3. Interconversions of compounds of type 1 and 2 are also
included in this Table. For the specification with the letters a, b, c, d, see the Formulae in the Introduction.
The X-ray data for compounds 1aba, 1abb, and 3cbb have been deposited with the Cambridge
Crystallographic Data Centre.
Starting material Reaction conditions Product Starting material Reaction conditions Product
7a
7b
7c
epi-7a
epi-7aBn
epi-7aBn
epi-7b
epi-7c
8a
1, 2, 3
1, 2, 3
1, 2, 3
1, 2, 3
2, 3
1aba
1bba
1cba
2aba
2aba
2aca
2bba
2cba
3aba
3aca
3bbb
3cbb
1abb
1acb
1cba
2cba
3cba
1aca
2aca
3aca
1cbc
2cbc
3cbc
3bbb
3cbb
1
1
5
5
1aba
1aca
1cbc
2cbc
3cbc
1aad
2aad
3aad
1cac
2cac
3cac
3bba
3cba
5
2, 4
6, 2
6, 2
6, 2
7
7
7
1, 2, 3
1, 2, 3
1, 2, 3
1, 2, 4
2, 3
8a
8b
8c
2, 3
1
1
Before describing the use of the fluorinated b-amino acids for tetrahydropyrimi-
dinone and peptide synthesis, some comments about the mechanistic course of the
DAST reaction are appropriate.
3. Stereo- and Regiochemical Course of the DAST Reactions 6 ! 7. – As we can see
from Schemes 1 and 2,a, the desired b-amino-a-fluoro-acid esters 7 are formed with

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Scheme 2. Alternative Routes to a-Fluoro-b-amino Acid Derivatives. a) Treatment of an N,N-
dibenzylthreonine ester with DAST, and b) direct electrophilic fluorination of an N-carbamoyl-b-amino
acid ester through a doubly lithiated species.
retention of configuration10), when we start from the b-amino-a-hydroxy-acid esters 6,
and with inversion on both stereogenic centers, when we start from the a-amino-b-
hydroxy ester 9. The isomeric a-amino-b-fluoro carboxylic acid esters iso-7 and iso-epi-7
are formed with inversion on both centers and with retention on the F-substituted C-atom,
respectively. This, at first sight, somewhat confusing stereochemical outcome is caused by
the intermediacy of N,N-dibenzylaziridinium ions [15a,b], as outlined in Fig. 1,b–f. Both,
the ring closure to, and the ring opening of the three-membered ring occur with an SN2-
type inversion of configuration, and the regiochemical course is determined by the relative
rates of CꢀN bond cleavage next to the ester group and next to the substituent R on the
three-membered ring. As can be seen from the table in Fig. 1, the carbonyl-assisted
opening of the aziridinium ring is preferred in the trans-substituted series, while there is a
delicate dependence on the type of substituent in the cis-series of aziridinium
intermediates. Concomitantly, the yields of the desired b-amino-a-fluoro-acid derivatives
of (2R,3S)-configuration (epi-7) are lower than those of the (S,S)-isomers 7.
4. Tetrahydropyrimidin-4(1H)ones and Cyclo-b-peptides Containing the F-Substi-
tuded Amino Acid Residues of 1a, 2a, and 3a. – As mentioned in the Introduction, there
is lack of structural and NMR data for F-substituted carbonyl compounds. To correlate
X-ray-crystal solid-state structures with NMR-solution structures, the compounds should
have rigid skeletons. Thus, we turned to previous work on non-fluorinated b-amino acid
derivatives, which had been shown to have a high degree of crystallinity, and which had
structures that are likely not to be subject to dynamic conformational equilibrations on
10
)
Note that the CIP convention may not be useful for assigning retention or inversion of
configuration because priority orders may change. A useful test is to assign whether the new
substituent is located in the same or in the opposite half-space of the stereogenic center as
compared to the replaced substituent:

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Fig. 1. Steric and regiochemical course of the DAST reactions with dibenzylamino hydroxy esters 6 and 9.
a) Reaction with DAST converts the OH group to a leaving group, which is not replaced
intermolecularly by Fꢀ but intramolecularly by Bn2N. b) The resulting aziridinium ion can react with
Fꢀ in the a-carbonyl (a) or in the b-carbonyl position (b). c) In the trans-substituted aziridinium ring, an
SN2-assisting conformation of the ester group is possible (high regioselectivity of a attack). d) In the cis-
substituted aziridinium ring, there are three substituents on the same face of the three-membered ring,
which causes steric hinderence of the SN2-assisting ester-group conformation, allowing for competing
ring-opening (b) next to the R group (poor regioselectivity (epi-7/iso-epi-7)). e) and f) trans- and cis-
aziridinium-ion intermediates, relative rates of ring opening, and yields of purified products 7 and epi-7.
The high selectivity of formation of epi-7b through as cis-aziridinium ion might be due to a kind of
i
neopentyl effect: a Me group of the Pr-substituent blocks the SN2 trajectory of the attacking Fꢀ
nucleophile, thus favoring the ring opening next to the ester group.
the NMR time-scale. These were hexahydropyrimidin-4-ones G [18], and cyclo-b-tri-
and -tetrapeptides H [19] and I, respectively [20]11) (Fig. 2).
Tetrahydropyrimidin-4(1H)-ones 11 – 13. Thus, we treated the amino acid amides
1aad, 2aad, and 3aad with pivalaldehyde in refluxing CH2Cl2 (with azeotropic removal
11
)
These cyclo-b-peptides assemble to stacks in the solid state. A cyclo-b-tripeptide has recently been
used to construct a mimic of CD40L with a KD value of 2.4 nM [21].

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Fig. 2. Cyclic derivatives G, H, and I of b-amino acids and F-substituted tetrahydro-pyrimidinones 11 – 13.
The numbers in the formulae of 11 – 13 are bond distances in ꢄ, determined by X-ray crystal-structure
analysis13). For 12, two sets of bond lengths are given, since there are two different conformations in the
crystal unit cell of this compound. Presentation J indicates the nN ! s*(CꢀN) interaction facilitating ring
opening to the imino amides 14 and 15.
of H2O), which led to the tetrahydropyrimidin-4(1H)-ones 11, 12, and 13, respectively
(Fig. 2). The yields of purified, crystalline products were moderate or low (52, 43, and
16%, resp.), which was, at least partially, due to their lability: when dissolved in a protic
solvent such as MeOH, they underwent ring opening to yield imino amides 14 and 15
(NMR-tube experiment). Especially the F2-substituted compounds of type 13 are
prone to undergo this isomerization. The cis- and trans-isomers 13a and 13b co-
crystallized in a 1:1 ratio and could not be separated. The crystals of 12 contain two
conformers in the unit cell. Benzyloxycarbonylation of the tetrahydropyrimidin-4(1H)-

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ones 11 – 13 gave more stable12) derivatives Cbz-11, Cbz-12, and Cbz-13, respectively.
X-Ray crystal structures13) of compounds 11 – 13 exhibit pronounced pyramidalization
of the amino N(5) (D between 0.30 and 0.44 ꢄ), such that the virtual lone-pair lobe is
antiperiplanar to the C(6)ꢀN(1) bond, with concomitant bond-length shortening and
lengthening. In the Cbz derivatives, the N(5)-atom is, of course, sp2-hybridized, and the
bond lengths (in Cbz-12) are more or less normal14). The instability of tetrahydropyr-
imidin-4(1H)-ones is clearly caused by a stereoelectronic effect (nN ! s*(CꢀN)), as
indicated by the presentation J in Fig. 2.
Cyclo-b-tripeptides 17 – 19. The synthesis was accomplished according to traditional
protocols (Scheme 3). Coupling of the N-Boc-fluoro-amino acids with the dipeptide
ester H-bGly-bGly-OMe using HBTU/NMM15) furnished the N-Boc-b-tripeptides
16a – 16c in high yields. There are numerous approaches for the cyclization of peptides,
many with miserable yields. The route via pentafluorophenyl esters, as developed by U.
Schmidt et al. [24], was the method of choice, as it had been successfully employed for
b-peptides before [19b,d]: transesterification of 16a – 16c to the active esters, Boc-
deprotection, and slow addition (via syringe pump) of solutions of the resulting TFA
salts of the tripeptide pentafluorophenyl esters to a dilute (3.3 mm) solution of Hꢀnig
base (DIPEA in MeCN, 708) gave the cyclo-b-tripeptides 17 – 19 in good overall yields
(Scheme 3, left). Like the non-fluorinated analogs [19b,c], the cyclization products are
insoluble in essentially all common solvents and precipitated from the hot, dilute
reaction mixture, to be isolated by simple filtration. NMR Spectra were recorded in a
mixture of CDCl3 and TFA.
Cyclo-b-tetrapeptides 21 – 23. In an attempt to prepare cyclo-b-dipeptides consisting
of one F-substituted residue and one b-Gly moiety, we prepared the N-Boc-protected
peptide methyl esters 20a – 20c. By the same procedure as for the tripeptides, the
dipeptide pentafluoro-phenyl esters were prepared, and their solutions were added to
the Hꢀnig-base solution (MeCN, 708). To our surprise16), the cyclo-b-tetrapeptides
12
)
)
)
The F2-substituted Cbz-13 was still quite unstable and could be isolated in only 6% yield as an oil
(see Exper. Part). We had noticed the instability of such hydro-pyrimidinones with non-F-
substituted derivatives before [18f].
The crystal structures of the heterocycles 11 – 13 of type G and of the cyclo-b-tri- and -tetrapeptides
H and I will be published separately, together with detailed NMR analyses and ab initio calculations
[22].
The dihydrobenzopyrimidin-1(1H)-ones of type i are also somewhat unstable. In crystal structures,
their N(1)-atom is, however, only slightly or not at all pyramidalized (p-conjugation is stronger than
n ! s* interaction) [23].
13
14
15
16
)
)
For abbreviations not specified in the Schemes, Table, and Figures, see introduction of the Exper.
Part.
A ꢂreverseꢃ case was reported by Vilarrasa and co-workers, who tried to prepare a cyclo-b-
tetrapeptide under the same conditions and were disappointed to obtain the eight-membered ring
of a cyclo-b-dipeptide [20b].

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21 – 23 were isolated in acceptable yields (30 – 40%; Scheme 3, right). Again, these
cyclic peptides have high melting points and poor solubilities in common organic
solvents, like the non-fluorinated counterparts [19d][20c], and as with those, we were
not able to prepare suitable single crystals for conventional X-ray analyses.
There are ongoing attempts to determine the structures of some of the F-substituted
cyclo-b-peptides 17 – 19 and 21 – 23 by powder-diffraction techniques13), a method that
had been successful with the simple (b3hAla)4 derivatives [20c].
5. Synthesis and Spectroscopic Characterization of b-Hexapeptides 24 – 26 with F-
Substitution in Each Residue. – From the NMR analyses of b-peptides containing a
single, central fluoro- or difluoro-b-amino acid residue 1, 2, or 3 [1][5][6], we would
expect that a b-hexapeptide 24b, built of (S,S)-2-fluoro-bhVal, -bhAla, and -bhLeu
residues does not fold to an (M)-314-helix [25]. The isomeric hexapeptide 25b with the
corresponding (2R,3S)-fluoro-amino acid building blocks, on the other hand, could fold
to an (M)-helix, with axial disposition [25] of an F-atom in each residue. For the
hexapeptide 26b, with two geminal F-atoms in each amino acid moiety of the (bhVal-
bhAla-bhLeu)2 chain, a weaker tendency for folding to the 314-helix must be envisaged
[6].
Synthesis. The assembly of the three F-substituted b-hexapeptides 24b – 26b was
performed as outlined in Scheme 4, by using a classical solution-phase Boc strategy for
the coupling (with NMM/HOBt/EDC or NMM/HATU15), and a C-terminal benzyl
ester group. After attaching the bhAla and bhVal units to the bhLeu-benzyl esters (i.e.,
1cac, 2cac, 3cac), the tripeptide derivatives 25abb – 26abb were divided into two
portions, one to be Boc-deprotected (TFA in CH2Cl2), the other one to be
debenzylated (H2/Pd-C in MeOH).
Fragment coupling of the two trimers gave the terminally protected hexapeptides
24bbb – 26bbb, which were investigated as such or after deprotection to 24baa – 26baa,
respectively. Most intermediates and final products of these syntheses were charac-
1
terized by melting points, [a]D values, H-, 13C-, 19F-NMR, MS, IR, CD spectroscopy,
and elemental analyses (see Exper. Part).
CD and NMR Spectra. In our work on b-peptides, we have preferentially used
MeOH as solvent for CD and NMR recordings; it turned out that some of the fluoro-
hexapeptide derivatives are poorly soluble in this solvent. For the high dilution (0.2 nm)
of solutions for recording CD spectra, the solubility in MeOH was sufficient, but for
NMR recordings, we had to turn to more polar solvents such as (D6)DMSO (H-bond
destroying) or CF3CD2OD (TFE, favoring secondary-structure formation).
The CD spectra of the hexapeptide 24bbb, consisting of like-fluoro-b-amino acid
residues in hexafluoro-isopropanol ((CF3)2CHOH; HFiPrOH), of the unlike-isomer
25bbb and its deprotected form 25baa (in MeOH, TFE, and HFiPrOH), and of the
hexapeptide 26bbb with geminal difluoro groups and its unprotected form 26baa (in the
same solvents) are shown in Fig. 3, a, b, and c, respectively. The peptide consisting of
(S,S)-building blocks exhibits two strong Cotton effects (q ca. 80,000) at 213 (positive)
and 195 nm (negative) in HFiPrOH; due to lack of comparison, it is impossible to
interpret this spectrum. If we compare the CD spectra of the hexapeptide derivatives
with one F-atom per amino acid unit and (2R,3S)-configuration (Fig. 3,b) and with two
F-atoms per amino acid unit (Fig. 3,c) in the standard solvent MeOH, we would

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Scheme 4. Synthesis of the b-Tri- and b-Hexapeptide Derivatives 24a – 26a and 24b – 26b, Respectively,
with One or Two F-Sustituents in Each Amino Acid Residue. The syntheses were carried out by coupling
in solution using the Boc strategy. For details, see the Exper. Part.
conclude that there is no secondary structure in the first case (weak extrema) and an
unknown structure in the second case (strong maxima of q up to 100,000 near 220 and
215 nm)17). There are two surprises in the CD spectra.
i) In the fluorinated alcohols as solvents, the unprotected b-hexapeptide 25baa with
(R,S)-building blocks exhibits a CD pattern, which ꢁthe experts in the areaꢂ would
associate with a left-handed 314-helix (minimum, albeit weak, near 215, and intensive
maximum near 195 nm; Fig. 3,b); such a helix would have all F-atoms in the
thermodynamically more stable axial disposition (with antiperiplanar arrangement of
F- and carbonyl O-atom, i.e., FꢀCꢀC¼O dihedral angle of 1808 [6]). The terminally
protected form 25bbb has a more or less flat CD pattern near zero in all three solvents
(Fig.3,b).
17
)
A typical CD spectrum of a b-peptide folding to a 314-helix shows a strong negative Cotton effect
near 215 nm [25].

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Fig. 3. CD Spectra of the b-hexapeptide derivatives with one or two F substituents in each residue

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ii) The b-hexapeptides with geminal difluoro substitution in each residue give rise
to CD spectra, which are almost superimposable with those for the protected form
26bbb (Fig. 3,c), and quite similar to those for the unprotected form 26baa in all three
solvents (Fig. 3,c), meaning that – in first approximation – the secondary-structure-
inducing fluorinated solvents do not change the backbone conformation, as compared
to MeOH.
Caveat: Suggestions, made in this section, about possible structures of the
fluorinated hexapeptides must be taken with due care: we have reiterated that the
CD spectra of b-peptides can be deceiving, to say the least [25][26].
NMR Spectra of the hexafluoro- and dodecafluoro-b-hexapeptides with and
without terminal protection (N-Boc, OBn) were recorded with solutions in
CF3CD2OD/TFA, CD3OH, or (D6)DMSO (see Fig. 4); they are described in the
Exper. Part; an interpretation will have to wait for our detailed NMR-structural
analyses with the parameters determined from the cyclic compounds.
6. Conclusions and Outlook. – Configurationally uniform mono- and difluoro-b-
amino acid derivatives and peptides with and without N- and/or C-terminal protections
have been successfully prepared by known methods. The intermediacy of N,N-dibenzyl-
aziridinium ions in the DAST reaction of vicinal amino-hydroxy-substituted C-chains
has been confirmed. The regioselectivity of SN2-type ring opening of these three-
membered rings has been shown to be subject to intriguing, subtle substitution effects.
The expected crystallinity of cyclic derivatives of the fluorinated b-amino acids has
come true. The tetrahydropyrimidin-4(1H)-ones readily form suitable single crystals,
for X-ray structure determination. The cyclic b-tri- and b-tetrapeptides could be
purified to give correct elemental analyses, but are poorly soluble, so that we have to
resort to – ongoing – powder X-ray diffraction measurements in a synchroton beam.
The results of the X-ray-structural investigations will be reported separately, together
with full NMR-structural analyses and computational results. The multiply F-
substituted b-peptides have, so far, resisted all attempts of structure determination,
due to poor solubility in common solvents and to lack of reliable Karplus parameters.
With the data collected from the cyclic derivatives, we are now in the process of making
another attempt. The CD spectra of these linear b-hexapeptides, as reported herein,
can be merely considered as ꢂfingerprintsꢃ, and the discussed similarities of CD patterns
with those of b-peptides of known structure may be accidental.
We thank the NMR (B. Brandenberg, P. Zumbrunnen, Dr. M.-O. Ebert, and Prof. B. Jaun), the MS
(Dr. W. Amrein, R. Hꢃfliger, O. Greter, and L. Bertschi), the elementary-analyses (P. Kꢃlin and M.
Schneider), and the X-ray (Dr. W. B. Schweizer and M. Solar) services of the Laboratorium fꢀr
Organische Chemie (ETH Zꢀrich) for their assistance. We also acknowledge the financial support by the
Swiss National Foundation (SNF) and Novartis Pharma AG.
Experimental Part
1. General. Abbreviations: b-hAa: b-Homoamino acid, Bn: benzyl, Boc: (tert-butoxy)carbonyl,
Boc2O: di(tert-butyl) dicarbonate, DAST: (diethylamino)sulfur trifluoride, EtNiPr2 (Hꢀnig base):
ethyl(diisopropyl)amine, EDC: 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride, FC:
flash chromatography, HATU: O-(7-azabenzotriazol-1-yl)-N,N,N’,N’-tetramethyluronium hexafluoro-
phosphate, HFiPrOH: 1,1,1,3,3,3-hexafluoropropan-2-ol, HBTU: O-(benzotriazol-1-yl)-N,N,N’,N’-tetra-

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Helvetica Chimica Acta – Vol. 94 (2011)
Fig. 4. 1H- and 13C-NMR Spectra of the hexapeptide derivatives 25bbb, 25baa, and 26baa. For 1H- and 19F-
NMR spectra of 24bbb and for the 19F-NMR spectra of 26bbb, see the Exper. Part. For 25bbbb, 1H-, 13C-,
and 19F-NMR spectra are described in the Exper. Part.
methyluronium hexafluorophosphate, HOBt: 1-hydroxy-1H-benzotriazole, h.v.: high vacuum (0.01 –
0.1 Torr), NMM: 4-methylmorpholine, NFSI: N-fluorobenzene-sulfonimide, TFA: trifluoroacetic acid,
TFE: 2,2,2-trifluoroethanol.
DMSO and Et3N were distilled over CaH2 and stored over 4-ꢄ molecular sieves. Solvents for FC and
workup procedures were distilled over Sikkon (anh. CaSO4; Fluka). a-Amino acids were purchased from

Helvetica Chimica Acta – Vol. 94 (2011)
1923
Bachem, Senn, or Degussa. Dry THF was distilled from sodium and benzophenone; dry CH2Cl2 was
distilled from CaH2. All moisture-sensitive reactions were carried out under a positive pressure of N2 or
Ar in oven-dried glassware (1408). All other reagents and solvents were used as received from Fluka or
Aldrich. Sat. HCl/MeOH soln. was prepared by bubbling anh. HCl gas into MeOH at 08 (ice bath).
Aldehydes 4b and 4c [11], cyanohydrins 5b and 5c [12], alanine derivatives 1aba, 2aba, 3aba, 6a, epi-6a,
and 7a [4] were synthesized according to published procedures.
TLC: Merck silica gel 60 F254 plates; detection with UV light or by dipping into a soln. of ninhydrin
(0.6 g), AcOH (2 ml), H2O (13 ml), and BuOH (285 ml), or a soln. of phosphomolybdic acid (25 g),
Ce(SO4)2 · H2O (10 g), conc. H2SO4 (60 ml), and H2O (940 ml), followed by heating. FC: Fluka silica gel
60 (40 – 63 mm); at ca. 0.3 bar. Anal. RP-HPLC: Knauer HPLC system K 1000, pump type 64,
EuroChrom 2000 integration package, degaser, UV detector K 2000 (variable-wavelength monitor),
Macherey-Nagel C8 column (Nucleosil 100-5 C8 (250 ꢁ 4 mm)); at 220 nm. Prep. RP-HPLC: Knauer
HPLC system, pump type 64, programmer 50, UV detector (variable-wavelength monitor), Macherey-
Nagel C8 column (Nucleosil 100-7 C8 (250 ꢁ 21 mm)); at 220 nm. M.p.: Bꢀchi 510 apparatus; uncorrected.
Optical rotations ([a]rD:t: ): Perkin-Elmer 241 polarimeter (10 cm, 1-ml cell) at r.t.; the solvent and the
concentration (g/100 ml) are given in the procedures. CD Spectra: Jasco J-710 spectrophotometer,
recording from 190 to 250 nm at 208; 1-mm rectangular cell; average of five scans, corrected for the
baseline; peptide concentration, 0.2 mm; band width, 1.0 nm; resolution, 0.2 nm; sensitivity, 100 mdeg;
response, 0.5 s; speed, 20 nm/min.; molar ellipticity [q] in deg cm2 molꢀ1 (l in nm); smoothing by Jasco
softwares). IR Spectra: Perkin-Elmer 782 spectrophotometer. NMR spectra: Bruker AMX 600 (1H: 600
and 13C: 150.9 MHz), AMX 500 (1H: 500 and 13C: 125 MHz), AMX 400 (1H: 400 and 13C: 100 MHz), or
AV-400 (1H: 400, 13C: 100, and 19F: at 376 MHz), or Varian Gemini 300 (1H: 300, 13C: 75, and 19F:
282 MHz); chemical shifts d in ppm and coupling constants J in Hz. HR-MS: IonSpec Ultima 4.7 (HR-
ESI-MS and HR-MALDI-MS) or Bruker Reflex (MALDI-TOF-MS) spectrometer; in m/z (% of basis
peak). Elemental analyses: performed in the Microanalytical Laboratory of the Laboratorium fꢀr
Organische Chemie, ETH Zꢀrich.
2. General Procedures 1 – 14 (GP 1 – 14). Fluorination of b-Homoalanine Methyl Esters: GP 1. A
soln. of BuLi (1.6m hexane, 2.2 equiv.) was added to a soln. of iPr2NH (2.2 equiv.) in THF (0.5m) at ꢀ 788
under Ar. The mixture was stirred for 1 h at ꢀ 788, then a soln. of the N-Boc- or N-Cbz-amino acid ester
(1 equiv.) in THF (0.5m) was added. The mixture was stirred for 1 h at ꢀ 788 and then a soln. of NFSI (2.5
equiv.) in THF (1m) was added dropwise via syringe. The resulting yellow mixture was stirred for 2.5 h at
ꢀ 788 and for 2 h at 08. Sat. NH4Cl was added, and the aq. layer was extracted with CH2Cl2 (3ꢁ). The
combined org. phases were dried (MgSO4), and the solvent was removed in vacuo. The crude product
was purified by FC.
Fluorination Reactions with DAST: GP 2. All reactions were performed in PET flasks under N2. To a
soln. of the appropriate starting material (1 equiv.) in CH2Cl2 (2 ml/mmol), DAST (1.5 – 3 equiv.) was
slowly added at 08 (ice bath) or r.t. The mixture was stirred at 08 or r.t. for 3 – 6 h, poured into H2O,
cautiously neutralized by the addition of solid K2CO3, and extracted with Et2O (2ꢁ). The combined org.
layers were dried (MgSO4), filtered, and evaporated. The crude product was purified by FC.
Methyl Ester Hydrolysis: GP 3. A mixture of the methyl ester (1 equiv.) and LiOH · H2O (3 equiv.) in
EtOH/H2O 2 :1 (4 ml/mmol) was well stirred for 1 h at r.t., cooled to 08 (ice bath), and neutralized by the
addition of 1m HCl. The mixture was diluted with H2O and extracted with AcOEt or CH2Cl2 (3ꢁ). The
combined org. layers were dried (MgSO4), filtered, and evaporated. The crude product was used without
further purification.
Benzyl Hydrogenolysis: GP 4. To a soln. of the Bn-protected compound in MeOH (10 ml/mmol), a
cat. amount of Pd/C (10%, 30 mg per equiv. of Bn group) was added. The apparatus was evacuated and
flushed three times with H2, and the mixture was vigourously stirred for 12 – 24 h. The catalyst was
filtered off through Celite, washed several times with MeOH, and the combined org. layers were
evaporated. The crude product was used without further purification.
Boc Protection: GP 5. To a soln. of the amine or the TFA salt (1 equiv.) in MeOH (3 ml/mmol),
Boc2O (1.5 equiv.) and Et3N (3 – 5 equiv.) were added. The mixture was stirred at r.t. for 12 h,
concentrated under reduced pressure, dissolved in AcOEt and washed successively with 0.5m HCl, sat.

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Helvetica Chimica Acta – Vol. 94 (2011)
K2CO3, and brine. The org. layer was dried (MgSO4), filtered, and evaporated, and the crude product was
purified by FC.
Benzyl Ester Formation: GP 6. According to the procedure described in [27], a mixture of the
carboxylic acid (1 equiv.) and dry Cs2CO3 (1 equiv.) in DMF (3 ml/mmol) was stirred for 10 min at r.t.,
and treated with BnBr (1.2 equiv.). After stirring for 12 h at r.t., the mixture was diluted with AcOEt and
washed successively with 1m HCl (2ꢁ), sat. K2CO3, and brine. The org. layer was dried (MgSO4),
filtered, and evaporated. The crude product was purified by FC.
Non-Stereoselective Preparation of Cyanohydrins: GP 7. In analogy to the procedure described in
[13], a vigorously stirred, biphasic soln. of the dibenzylamino aldehyde (1 equiv.) in hexane/H2O 3 :1
(1.5 ml/mmol) was treated with acetone cyanohydrin (¼2-hydroxy-2-methylpropanenitrile; 1.5 equiv.) at
r.t. After stirring for 5 min, cat. amounts of KCN (0.03 equiv.) and Bu4NI (0.01 equiv.) were added. The
mixture was stirred at r.t. for 2 h, poured into H2O, and extracted with Et2O (3ꢁ). The combined org.
phases were washed with brine, dried (MgSO4), filtered, and evaporated. The crude epimeric mixture of
cyanohydrins was used without further purification.
Methanolysis of Cyanohydrins: GP 8. A soln. of the cyanohydrin (1 equiv.) in sat. HCl/MeOH (5 –
10 ml/mmol) was stirred at r.t. for 12 h, concentrated under reduced pressure, poured into H2O,
cautiously neutralized by the addition of solid K2CO3, and extracted with AcOEt (3ꢁ). The combined
org. layers were dried (MgSO4), filtered, and evaporated. The crude product was purified by FC.
Oxidation of a-Hydroxy Esters to a-Keto Esters: GP 9. A dry three-necked round-bottom flask,
equipped with a magnetic stirrer and a dropping funnel, was charged with anh. CH2Cl2 (7.5 ml/mmol)
under N2. After cooling to ꢀ 788 (dry ice/acetone bath), oxalyl chloride (1.2 equiv.) and anh. DMSO (2
equiv.) were added dropwise, so that the temp. did not exceed ꢀ 658. The mixture was stirred at ꢀ 788 for
10 min, treated with a soln. of the appropriate methyl ester (1 equiv.) in CH2Cl2 (1 ml/mmol), and stirred
for additional 1.5 h at ꢀ 788. After addition of dry Et3N (4 equiv.), the mixture was allowed to warm to
r.t. over 0.5 h, whereupon H2O (4 ml/mmol) was added. The phases were separated, and the aq. phase
was extracted with CH2Cl2 (3ꢁ). The combined org. phases were washed with 1% HCl, 5% NaHCO3,
and brine, dried (MgSO4), filtered, and evaporated. The crude keto esters are immedially used for the
DAST reactions without prior purification.
Preparation of 6-Alkyl-2-(tert-butyl)tetrahydropyrimidin-4(1H)-ones: GP 10. At ꢀ 208, 1 equiv. of
the N-Cbz-protected b-amino acid was dissolved in THF, and 1.2 equiv. of Et3N and ethyl chloroformate
were added. The resulting colorless suspension was cooled below ꢀ 508, and via a needle NH3 gas was
bubbled in for 1 h. After another 3 h, the solvent was removed by rotary evaporation. To the colorless
solid, H2O was added, and the resulting suspension was filtered and washed with H2O and Et2O. The
isolated powder was dried for 12 h under h.v., then suspended in MeOH, and 10% Pd/C was added. After
12 h under H2 (balloon), the Pd/C was filtered off (Celite), MeOH was evaporated, and the obtained b-
amino acid amide was dissolved in CH2Cl2, 2 equiv. of pivalaldehyde was added, and the mixture was
heated under reflux in an inverse DeanꢀStark trap for 12 h. The solvent was removed, and the isolated
crude product was purified by FC and/or recrystallization.
Cbz Protection of Tetrahydropyrimidin-4(1H)-ones: GP 11. N,O-Bis(trimethylsilyl)acetamide (1.5
equiv.) was added to a soln. of the corresponding tetrahydropyrimidin-4(1H)-one, 11 – 13, in CH2Cl2 at
r.t. After 1 h, the mixture was cooled to 08 and Cbz chloride (1.3 equiv.) was added. After 20 h at 08, sat.
NaHCO3 was added, and the aq. layer was extracted with CH2Cl2 (3ꢁ). The combined org. layers were
dried (MgSO4), filtered, and evaporated. The crude products were purified by FC.
Peptide Coupling: GP 12. GP 12a. With HBTU. A soln. of the ammonium salt of the amino acid ester
or peptide ester (1 equiv.) in CH2Cl2 was cooled to 08, treated successively with the appropriate
carboxylic acid (1 equiv.), NMM (3 equiv.), and HBTU (1.2 equiv.), and stirred for 12 h at r.t. The
mixture was diluted with AcOH, and washed with aq. HCl (1m), sat. K2CO3, and brine. The org. phase
was dried (MgSO4), filtered, evaporated, and the obtained crude product was purified by FC.
GP 12b. With EDC/HOBt. A soln. of the amino acid or peptide ester, or its TFA salt (1 equiv.) in
CH2Cl2 (3 ml/mmol) was cooled to ꢀ 108, treated successively with a soln. of the appropriate acid (1
equiv.) in CH2Cl2 or THF (3 ml/mmol), NMM (3 – 5 equiv.), HOBt (1.2 equiv.) and EDC · HCl (1.2
equiv.), and stirred at ꢀ 108 for 12 h. The mixture was diluted with AcOEt, washed with 1m HCl (3ꢁ),

Helvetica Chimica Acta – Vol. 94 (2011)
1925
sat. K2CO3 (3ꢁ), and brine. The org. layer was dried (MgSO4), filtered, and evaporated, and the crude
product was purified by FC.
GP 12c. With HATU. A soln. of the amino acid or peptide ester, or its TFA salt (1 equiv.) in CH2Cl2
or DMF (6 ml/mmol) was cooled to 08, treated successively with the appropriate acid (1 equiv.), NMM
(3 – 5 equiv.), and HATU (1.2 equiv.), and stirred at r.t. for 12 h. The mixture was diluted with AcOEt,
washed with 1m HCl (3ꢁ), sat. K2CO3 (3ꢁ), and brine. The org. layer was dried (MgSO4), filtered, and
evaporated, and the crude product was purified by FC.
GP 12d. With HATU, Forming an Insoluble Peptide. The peptide-coupling reaction was performed
according to GP 12c, but during the reaction the formed peptide precipitated. The mixture was
evaporated, and the residue was stirred in AcOEt for 10 min. The resulting suspension was centrifuged,
and the solid was stirred successively in AcOEt (2ꢁ) and MeOH/H2O 1:1 (3ꢁ) for 10 min each. After
the final centrifugation, the product was dried under h.v. for 12 h.
Cyclization of Oligopeptides: GP 13. A soln. of the unprotected peptide in DMF (0.4m) was treated
at r.t. with pentafluorophenol (1 equiv.) and EDCI (1 equiv.). After 16 h, the mixture was evaporated,
and the residue was dissolved in CHCl3. The resulting soln. was washed successively with 1m aq. HCl and
brine. The org. phase was dried (MgSO4) and evaporated. The residue was dissolved in CH2Cl2 (0.5m)
and an equal volume of TFA was added at 08. The mixture was stirred for 1 h at 08 and for 1 h at r.t. The
solvent was evaporated, and the residue was dissolved in toluene and evaporated twice. The obtained
residue was dissolved in MeCN (0.025m) and slowly added (over 4 h) to a soln. of Hꢀnigꢃs base (EtNiPr2)
in MeCN (3.3 mm) at 708 (bath temp.) with a syringe pump. The resulting precipitate was filtered,
washed, and dried under h.v.
Boc Deprotection: GP 14. GP 14a. A soln. of the N-Boc-protected compound in CH2Cl2 (3 ml/
mmol) was cooled to 08 (ice bath), treated with TFA (3 ml/mmol), and stirred at 08 for 1.5 h. After
concentration under reduced pressure, the TFA salt was dried under h.v. for 2 h and used without further
purification.
GP 14b. After Boc deprotection according to GP 14a, the TFA salt was dissolved in AcOEt, washed
with sat. K2CO3 (2ꢁ), dried (MgSO4), filtered, and evaporated. The resulting amine was used without
further purification.
3. Preparation of the Fluorinated b-Amino Acid Derivatives 1 – 3, 7, and 8. 3.1. By Enolate
Fluorination (Scheme 2, b). Methyl (2S,3S)-3-{[(tert-Butoxy)carbonyl]amino}-2-fluorobutanoate
(1abb). Fluorination of Boc-hAla-methyl ester (1.87 g, 8.63 mmol; prepared from Boc-Ala [28]) was
performed according to GP 1. Purification by FC (CH2Cl2/AcOEt 98 :2) afforded 1abb (1.38 g, 68%).
Pale yellow oil, which crystallized by scratching. Recrystallization gave colorless needles. M.p. 66 – 688
(hexane/Et2O). [a]D28 ¼ ꢀ1.9 (c ¼ 1.0, MeOH). IR: 3369m, 3252w, 2983w, 2944w, 1758s, 1686s, 1510s,
1440m, 1388m, 1366m, 1338m, 1275m, 1247m, 1225s, 1164s, 1135m, 1112s, 1061s, 1008s, 980m, 938w, 921w,
877m, 849m, 783m, 750m, 696w, 675m. 1H-NMR (400 MHz, CDCl3): 1.16 (dd, J ¼ 7.0, 0.7, Me); 1.46 (s, 3
Me); 3.82 (s, Me); 4.12 – 4.40 (m, NCH); 4.79 (br. s, NH); 5.05 (dd, J ¼ 49.4, 2.2, CFH). 13C-NMR
(100 MHz, CDCl3): 14.2 (d, J ¼ 5.2, Me); 22.3 (3 Me); 47.8 (d, J ¼ 19.7, NCH); 52.5 (Me); 80.0 (CMe3);
90.2 (d, J ¼ 188.2, FCH); 154.9 (CO); 168.2 (d, J ¼ 24.0, FCCO). 19F-NMR (280 MHz, CDCl3): 40.6 (dd,
J ¼ 49.0, 26.9, 1 F). Anal. calc. for C10H18FNO4 (235.25): C 51.06, H 7.71, N 5.95; found: C 51.04, H 7.54, N
6.15.
Methyl (2S,3S)-3-{[(Benzyloxy)carbonyl]amino}-2-fluorobutanoate (1acb). Fluorination of Cbz-
hAla methyl ester (5.03 g, 20.0 mmol, prepared from Cbz-Ala [28]) was performed according to GP 1.
Purification by FC (CH2Cl2/AcOEt 98 :2) afforded 1acb (3.83 g, 71%). Pale yellow oil, which crystallized
by scratching. Recrystallization gave colorless needles. M.p. 52 – 538 (hexane/Et2O). [a]2D8 ¼ þ3.7 (c ¼
1.0, MeOH). IR: 3330m, 3068w, 3038w, 2992w, 2961w, 2896w, 2852w, 1759s, 1682s, 1652m, 1527s,

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Helvetica Chimica Acta – Vol. 94 (2011)
1466m, 1454m, 1437m, 1390w, 1381w, 1334m, 1276s, 1258m, 1228s, 1140m, 1113s, 1068m, 1013s, 983m,
941m, 912m, 860w, 840m, 785m, 751s, 730m, 696s, 671m, 655m. 1H-NMR (300 MHz, CDCl3): 1.17 (d, J ¼
6.9, Me); 3.81 (s, Me); 4.12 – 4.40 (m, NCH); 4.92 – 5.17 (m, NH, FCH, PhCH2); 7.27 – 7.38 (m, 5 arom.
H). 13C-NMR (100 MHz, CDCl3): 14.3 (d, J ¼ 5.3, Me); 48.3 (d, J ¼ 20.4, NCH); 52.6 (Me); 67.1
(PhCH2); 90.0 (d, J ¼ 188.5, FCH); 128.2 (CH); 128.3 (CH); 128.6 (CH); 136.2 (CH); 155.4 (CO); 168.0
(d, J ¼ 23.7, FCCO). 19F-NMR (282 MHz, CDCl3): 41.0 (dd, J ¼ 49.0, 26.7, 1 F). Anal. calc. for
C13H16FNO4 (269.27): C 66.84, H 8.56, N 4.10; found: C 66.57, H 8.46, N 4.11.
3.2. By the Cyanohydrin Route (Scheme 1). 3.2.1. Monofluorinated Ala-Derived Compounds. Methyl
(2R,3S)-3-(Dibenzylamino)-2-fluorobutanoate (epi-7a) and Methyl (2R,3R)-2-(Dibenzylamino)-3-fluo-
robutanoate (iso-epi-7a). The methyl ester epi-6a [4] (2.04 g, 6.51 mmol) was dissolved in CH2Cl2 (13 ml)
and fluorinated with DAST (1.3 ml, 8.70 mmol) at 08 for 3 h according to GP 2. FC (pentane/Et2O 9 :1)
yielded epi-7a (753 mg, 37%) and iso-epi-7a (690 mg, 34%).
Data of epi-7a. Light yellow oil. Rf (pentane/Et2O 5 :1) 0.41. [a]rD:t: ¼ þ27.5 (c¼ 1.0, CHCl3). IR
(CHCl3): 3570w, 3064w, 3032m, 2954w, 2841w, 2810w, 1762s, 1602w, 1495m, 1453m, 1439m, 1382m, 1358m,
1298m, 1171s, 1137w, 1106m, 1074w, 1025s, 948w, 911w, 832w. 1H-NMR (400 MHz, CDCl3): 1.27 (dd, J¼ 0.6,
7.0, Me); 3.29 (ddq, J ¼ 3.8, 7.0, 31.8, NCH); 3.33 (d, J¼ 13.4, 2 PhCHH’); 3.63 (s, MeO); 3.90 (d, J¼ 13.4,
2 PhCHH’); 4.84 (dd, J¼ 3.8, 49.1, CHF); 7.19–7.32 (m, 10 arom. H). 13C-NMR (100 MHz, CDCl3): 8.0 (d,
J¼ 4.4), 51.9 (Me); 53.9 (d, J¼ 18.4, CH); 55.0 (CH2); 94.2 (d, J¼ 189.2), 127.0, 128.1, 129.1 (CH); 139.6,
169.0 (d, J¼ 25.6) (C). 19F-NMR (282 MHz, CDCl3): ꢀ 200.4 (dd, J¼ 32.0, 49.1, CHF). HR-MALDI-MS:
338.2 (11, [Mþ Na]þ), 316.2 (100, [M þ H]þ), 314.2 (5), 296.2 (4), 268.2 (5), 224.1 (9), 158.1 (8). Anal.
calc. for C19H22FNO2 (315.39): C 72.36, H 7.03, N 4.44; found: C 72.51, H 7.08, N 4.35.
Data of iso-epi-7a. Light yellow oil. Rf (pentane/Et2O 5 :1) 0.52. [a]rD:t: ¼ ꢀ104.1 (c ¼ 1.0, CHCl3). IR
(CHCl3): 3063w, 3008m, 2953m, 2845w, 1729s, 1602w, 1495m, 1454m, 1435w, 1383w, 1359w, 1277w, 1162s,
1115w, 1074m, 1024m, 990w, 939w, 877w, 843w. 1H-NMR (400 MHz, CDCl3): 1.35 (dd, J ¼ 6.3, 24.0, Me);
3.37 (dd, J ¼ 5.7, 23.7, NCH); 3.77 (d, J ¼ 14.0, 2 PhCHH’); 3.78 (s, MeO); 4.06 (d, J ¼ 14.0, 2 PhCHH’);
5.12 (ddq, J ¼ 5.8, 6.3, 47.9, CHF); 7.21 – 7.40 (m, 10 arom. H). 13C-NMR (100 MHz, CDCl3): 18.3 (d, J ¼
22.4), 51.4 (Me); 55.6 (CH2); 64.9 (d, J ¼ 19.6), 89.7 (d, J ¼ 173.2), 127.1, 128.3, 128.8 (CH); 139.5, 171.0
(d, J ¼ 6.7) (C). 19F-NMR (282 MHz, CDCl3): ꢀ 181.8 (dquint., J ¼ 23.5, 48.0, CHF). HR-MALDI-MS:
338.2 (13, [M þ Na]þ), 316.2 (100, [M þ H]þ), 314.2 (15), 296.2 (7), 238.2 (14), 224.1 (28), 158.1 (8).
Anal. calc. for C19H22FNO2 (315.39): C 72.36, H 7.03, N 4.44; found: C 72.43, H 7.12, N 4.45
3.2.2. Monofluorinated Val-Derived Compounds. Methyl (2S,3S)-3-(Dibenzylamino)-2-hydroxy-4-
methylpentanoate (6b). The cyanohydrin 5b [12] (12.74 g, 41.3 mmol) was treated with a sat. HCl/MeOH
soln. (210 ml) according to GP 8. FC (AcOEt/hexane 1:9 ! 3 :7) yielded 6b (8.61 g, 61%). Light yellow
oil. Rf (CH2Cl2) 0.19. [a]rD:t: ¼ ꢀ0.5 (c ¼ 1.0, CHCl3). IR (CHCl3): 3532w, 3064w, 2955m, 2873w, 2802w,
1728s, 1602w, 1494m, 1453m, 1366w, 1272m, 1137m, 1089m, 1028w, 990w, 967w, 913w. 1H-NMR
(400 MHz, CDCl3): 0.71 (d, J ¼ 6.6, Me); 1.02 (d, J ¼ 6.7, Me); 2.18 – 2.31 (m, Me2CH); 2.74 (dd, J ¼ 1.5,
10.2, NCH); 2.99 (d, J ¼ 4.6, OH); 3.40 (d, J ¼ 13.9, 2 PhCHH’); 3.72 (s, MeO); 3.95 (d, J ¼ 13.9,
2 PhCHH’); 4.59 (dd, J ¼ 1.5, 4.6, CHOH); 7.20 – 7.38 (m, 10 arom. H). 13C-NMR (100 MHz, CDCl3):
19.8, 21.5 (Me); 26.3 (CH); 52.5 (Me); 54.8 (CH2); 66.5, 67.4, 126.9, 128.1, 129.2 (CH); 139.9, 176.7 (C).
HR-MALDI-MS: 364.2 (9, [M þ Na]þ), 342.2 (100, [M þ H]þ), 252.2 (23). Anal. calc. for C21H27NO3
(341.44): C 73.87, H 7.97, N 4.10; found: C 74.08, H 7.81, N 4.30.
Methyl (2S,3S)-3-(Dibenzylamino)-2-fluoro-4-methylpentanoate (7b). Compound 6b (3.84 g,
11.25 mmol) was dissolved in CH2Cl2 (22 ml) and fluorinated with DAST (2.2 ml, 16.9 mmol) at 08 for
3 h, according to GP 2. FC (pentane/Et2O 10 :1) yielded 7b (2.78 g, 72%). Yellow oil. Rf (pentane/Et2O
10 :1) 0.35. [a]rD:t: ¼ ꢀ15.4 (c ¼ 1.0, CHCl3). IR (CHCl3): 3066w, 3032m, 2956m, 2803w, 1758s, 1602w,

Helvetica Chimica Acta – Vol. 94 (2011)
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1494m, 1476w, 1453m, 1438m, 1366w, 1287s, 1136m, 1116m, 1084s, 1016m, 992w, 968w, 912w, 867w.
1H-NMR (400 MHz, CDCl3): 0.80 (d, J ¼ 6.6, Me); 1.00 (d, J ¼ 6.7, Me); 2.13 – 2.26 (m, Me2CH); 2.89
(ddd, J ¼ 1.0, 9.9, 29.2, NCH); 3.34 (d, J ¼ 13.8, 2 PhCHH’); 3.75 (s, MeO); 3.96 (d, J ¼ 13.8, 2 PhCHH’);
5.37 (d, J ¼ 48.3, CHF); 7.22 – 7.38 (m, 10 arom. H). 13C-NMR (100 MHz, CDCl3): 19.3, 21.3 (Me); 26.3
(d, J ¼ 5.0, CH); 52.3 (Me); 54.6 (CH2); 66.0 (d, J ¼ 18.9), 86.3 (d, J ¼ 195.3), 127.1, 128.2, 129.2 (CH);
139.2, 171.3 (d, J ¼ 23.1) (C). 19F-NMR (282 MHz, CDCl3): ꢀ 201.9 (dd, J ¼ 28.8, 48.0, CHF). HR-
MALDI-MS: 366.2 (1, [M þ Na]þ), 344.2 (18, [M þ H]þ), 300.1 (6), 252.2 (27). Anal. calc. for
C21H26FNO2 (343.44): C 73.44, H 7.63, N 4.08; found: C 73.30, H 7.47, N 4.01.
(2S,3S)-3-{[(tert-Butoxy)carbonyl]amino}-2-fluoro-4-methylpentanoic Acid (1bba). Compound 7b
(2.77 g, 8.06 mmol) was hydrolyzed with LiOH · H2O (1.02 g, 24.2 mmol) in EtOH/H2O (30 ml, 2 :1)
according to GP 3, the carboxylic acid was dissolved in MeOH (80 ml) and hydrogenolyzed according to
GP 4, and Boc-protected with Boc2O (2.0 g, 9.17 mmol) and Et3N (3.2 ml, 22.9 mmol) in MeOH (28 ml)
according to GP 5. FC (CH2Cl2/MeOH/AcOH 100 :3 :0.1 ! 100 :5 :0.2) yielded the carboxylic acid 1bba
(1.62 g, 80% over 3 steps). Colorless solid. M.p. 133 – 1348. Rf (CH2Cl2/MeOH/AcOH 100 :5 :1) 0.25.
[a]Dr:t: ¼ ꢀ5.8 (c ¼ 1.0, CHCl3). IR (CHCl3): 3447m, 3011w, 2980m, 2933w, 1749m, 1712s, 1503s, 1456w,
1
1393w, 1369m, 1161s, 1128w, 1098w, 1041w, 999w, 868w. H-NMR (400 MHz, CD3OD): 0.94 (d, J ¼ 6.8,
Me); 0.96 (d, J ¼ 6.8, Me); 1.44 (s, t-Bu); 1.93 – 2.02 (m, Me2CH); 3.91 (td, J ¼ 5.5, 21.5, NCH); 4.89 (dd,
J ¼ 5.1, 48.7, CHF). 13C-NMR (100 MHz, CD3OD): 18.2, 20.5, 28.8 (Me); 30.0 (d, J ¼ 2.6), 57.9 (d, J ¼
21.2) (CH); 80.4 (C); 91.2 (d, J ¼ 186.9) (CH); 158.3, 172.1 (d, J ¼ 23.1) (C). 19F-NMR (282 MHz,
CD3OD): ꢀ 199.7 (dd, J ¼ 21.3, 49.1, CHF). HR-ESI-MS: 565.2 (23), 543.2 (90, [2M þ 2 Na]þ), 521.3
(59, [2M þ Na]þ), 294.1 (23), 272.1 (100, [M þ Na]þ), 216.1 (14). Anal. calc. for C11H20FNO4 (249.28): C
53.00, H 8.09, N 5.62; found: C 53.04, H 7.86, N 5.44.
Methyl (2S/R,3S)-3-(Dibenzylamino)-2-hydroxy-4-methylpentanoate (6b/epi-6b). A soln. of freshly
prepared 4b [11] (16.2 g, 57.5 mmol) in hexane/H2O (115 ml, 3 :1) was treated with acetone cyanohydrin
(7.85 ml, 86.3 mmol), KCN (112.2 mg, 1.7 mmol), and Bu4NI (148.7 mg, 0.4 mmol) according to GP 7.
The crude epimeric mixture of cyanohydrins was treated with a sat. HCl/MeOH soln. (290 ml) according
to GP 8. FC (hexane/AcOEt 98 :2 ! 7:3) yielded 6b/epi-6b (15.5 g, 79% over 2 steps). Yellow oil. Rf
(hexane/AcOEt 7:3) 0.46. The mixture could be separated by a further FC (hexane/AcOEt 95 :5) to
yielding pure 6b and epi-6b. The 1H- and 13C-NMR data for 6b ((2R,3S)) were in accordance with those
described above.
Data of epi-6b ((2S,3S)). Light yellow oil. Rf (pentane/Et2O 3 :2) 0.42. [a]rD:t: ¼ ꢀ15.6 (c ¼ 1.0,
CHCl3). IR (CHCl3): 3600w, 3527w, 3067w, 3008m, 2961m, 2872w, 1949w, 1887w, 1815w, 1731s, 1494w,
1453m, 1389w, 1269s, 1146m, 1076m. 1H-NMR (300 MHz, CDCl3): 1.06 (d, J ¼ 6.5, Me); 1.16 (d, J ¼ 6.5,
Me); 2.35 – 2.47 (m, Me2CH); 2.82 (dd, J ¼ 3.3, 8.6, NCH); 3.22 (br. s, OH); 3.44 (s, MeO); 3.75 (d, J ¼
13.4, 2 PhCHH’); 3.99 (d, J ¼ 13.7, 2 PhCHH’); 4.33 (d, J ¼ 3.1, CHOH); 7.20 – 7.38 (m, 10 arom. H).
13C-NMR (75 MHz, CDCl3): 21.5, 22.9 (Me); 28.1 (CH); 52.3 (Me); 56.4 (CH2); 65.4, 72.6, 126.7, 128.0,
129.1 (CH); 140.1, 175.4 (C). HR-MALDI-MS: 364.2 (16, [M þ Na]þ), 342.2 (100, [M þ H]þ), 328.2 (11),

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Helvetica Chimica Acta – Vol. 94 (2011)
321.2 (26), 291.2 (19), 266.2 (70), 252.2 (49). Anal. calc. for C21H27NO3 (341.44): C 73.87, H 7.97, N 4.10;
found: C 73.85, H 7.84, N 4.20.
Methyl (2R,3S)-3-(Dibenzylamino)-2-fluoro-4-methylpentanoate (epi-7b). Compound epi-6b
(911 mg, 2.67 mmol) was dissolved in CH2Cl2 (6 ml) and fluorinated with DAST (525 ml, 4.00 mmol)
at 08 for 3 h, according to GP 2. FC (pentane/Et2O 99 :1 ! 97:3) yielded epi-7b (551 mg, 60%). Yellow
oil. Rf (pentane/Et2O 10 :1) 0.21. [a]rD:t: ¼ ꢀ22.8 (c ¼ 1.0, CHCl3). IR (CHCl3): 3065w, 3032m, 2955s,
2851w, 1760s, 1602w, 1494m, 1476w, 1453s, 1390w, 1359m, 1294m, 1144s, 1116m, 1073s, 1016m, 979w, 949w,
911w, 835w. 1H-NMR (500 MHz, CDCl3): 1.02 (d, J ¼ 6.6, Me); 1.12 (d, J ¼ 6.8, Me); 2.29 – 2.37 (m,
Me2CH); 2.88 (ddd, J ¼ 2.6, 9.0, 34.0, NCH); 3.63 (s, MeO); 3.81 (d, J ¼ 13.6, 2 PhCHH’); 3.85 (d, J ¼
13.6, 2 PhCHH’); 5.15 (dd, J ¼ 2.6, 47.9, CHF); 7.19 – 7.30 (m, 10 arom. H). 13C-NMR (125 MHz,
CDCl3): 21.2, 22.1 (Me); 27.8 (d, J ¼ 1.8, CH); 52.0 (Me); 55.6 (CH2); 64.5 (d, J ¼ 18.0), 91.1 (d, J ¼
189.6), 126.9, 128.1, 129.4 (CH); 140.0, 170.2 (d, J ¼ 25.3) (C). 19F-NMR (282 MHz, CDCl3): ꢀ 202.1
(dd, J ¼ 34.2, 48.0, CHF). HR-MALDI-MS: 366.2 (5, [M þ Na]þ), 344.2 (71, [M þ H]þ), 324.2 (12), 252.2
(100), 126.1 (7). Anal. calc. for C21H26FNO2 (343.44): C 73.44, H 7.63, N 4.08; found: C 73.49, H 7.59, N
4.20.
(2R,3S)-3-{[(tert-Butoxy)carbonyl]amino}-2-fluoro-4-methylpentanoic Acid (2bba). Compound
epi-7b (472 mg, 1.37 mmol) was hydrolyzed with LiOH · H2O (173 mg, 4.12 mmol) in EtOH/H2O
(7.5 ml, 2 :1) according to GP 3, the carboxylic acid was dissolved in MeOH (15 ml), hydrogenolyzed
according to GP 4, and Boc-protected with Boc2O (354 mg, 1.64 mmol) and Et3N (570 ml, 4.1 mmol) in
MeOH (5 ml) according to GP 5. FC (hexane/AcOEt 2 :1 þ 2% AcOH) yielded 2bba (293 mg, 85% over
3 steps). Light yellow solid. M.p. 70 – 738. Rf (CH2Cl2/MeOH/AcOH 100 :5 :1) 0.27. [a]rD:t: ¼ ꢀ35.8 (c ¼
1.1, CHCl3). IR (CHCl3): 3440m, 3026w, 2979m, 2923w, 1754w, 1713s, 1505s, 1456w, 1393w, 1369m,
1310w, 1161s, 1077w, 1045w, 904w, 861w. 1H-NMR (300 MHz, CD3OD): 0.97 (d, J ¼ 6.8, Me); 1.03 (d, J ¼
6.5, Me); 1.42 (s, t-Bu); 1.83 – 1.95 (m, Me2CH); 3.80 (ddd, J ¼ 1.9, 8.7, 30.2, NCH); 5.13 (dd, J ¼ 1.9, 48.2,
CHF). 13C-NMR (75 MHz, CD3OD): 18.4, 18.8, 27.5 (Me); 30.1, 57.8 (d, J ¼ 18.9) (CH); 79.0 (C); 88.5 (d,
J ¼ 185.6, CH); 156.9, 177.9 (C). 19F-NMR (282 MHz, CD3OD): ꢀ 203.2 (dd, J ¼ 29.9, 48.0, CHF). HR-
ESI-MS: 565.2 (27), 543.2 (14, [2M þ 2 Na]þ), 294.1 (100), 272.1 (33, [M þ Na]þ). Anal. calc. for
C11H20FNO4 (249.28): C 53.00, H 8.09, N 5.62; found: C 52.96, H 7.96, N 5.43.
3.2.3. Monofluorinated Leu-Derived Compounds. Methyl (2S,3S)-3-(Dibenzylamino)-2-hydroxy-5-
methylhexanoate (6c). The cyanohydrin 5c [12] (4.43 g, 13.8 mmol) was treated with a sat. HCl/MeOH
soln. (70 ml) according to GP 8. FC (AcOEt/hexane 1:9 ! 3 :7) yielded 6c (4.20 g, 86%). Light yellow
oil. Rf (pentane/Et2O 4 :1) 0.23. [a]rD:t: ¼ þ10.5 (c ¼ 1.0, CHCl3). IR (CHCl3): 3528w, 3032w, 2956m,
2869w, 2810w, 1729s, 1602w, 1494m, 1453m, 1366w, 1268m, 1137w, 1089m, 1071m, 966w, 905w. 1H-NMR
(400 MHz, CDCl3): 0.52 (d, J ¼ 6.5, Me); 0.84 (d, J ¼ 6.7, Me); 0.92 (ddd, J ¼ 4.9, 8.4, 13.7, 1 H, CH2CH);
1.67 (ddd, J ¼ 5.0, 8.8, 14.0, 1 H, CH2CH); 1.74 – 1.80 (m, Me2CH); 3.04 – 3.08 (m, NCH); 3.05 (d, J ¼ 6.8,
OH); 3.50 (d, J ¼ 13.8, 2 PhCHH’); 3.71 (s, MeO); 3.86 (d, J ¼ 13.8, 2 PhCHH’); 4.61 (dd, J ¼ 2.4, 6.7,

Helvetica Chimica Acta – Vol. 94 (2011)
1929
CHOH); 7.20 – 7.35 (m, 10 arom. H). 13C-NMR (100 MHz, CDCl3): 21.7, 23.4 (Me); 24.2 (CH); 35.0
(CH2); 52.4 (Me); 54.5 (CH2); 57.9, 69.3, 127.0, 128.2, 129.1 (CH); 140.0, 175.6 (C). HR-MALDI-MS:
378.2 (6, [M þ Na]þ), 356.2 (100, [M þ H]þ), 266.2 (50). Anal. calc. for C22H29NO3 (355.48): C 74.33, H
8.22, N 3.94; found: C 74.15, H 8.08, N 4.02.
Methyl (2S,3S)-3-(Dibenzylamino)-2-fluoro-5-methylhexanoate (7c). Compound 6c (325 mg,
0.91 mmol) was dissolved in CH2Cl2 (2 ml) and fluorinated with DAST (180 ml, 1.37 mmol) at 08 for
3 h, according to GP 2. FC (pentane/Et2O 99 :1 ! 97:3) yielded 7c (277 mg, 85%). Light yellow oil. Rf
(pentane/Et2O 10 :1) 0.24. [a]rD:t: ¼ ꢀ6.6 (c ¼ 1.0, CHCl3). IR (CHCl3): 3065w, 3032w, 2956m, 2869w,
2806w, 1756s, 1603w, 1495m, 1454m, 1439m, 1367m, 1285s, 1162w, 1138w, 1086w, 1070m, 1017w, 967w,
907w, 868w. 1H-NMR (400 MHz, CDCl3): 0.44 (d, J ¼ 6.4, Me); 0.84 (d, J ¼ 6.6, Me); 0.96 – 1.03 (m, 1 H,
CH2CH); 1.74 – 1.84 (m, Me2CH, 1 H of CH2CH); 3.16 (dddd, J ¼ 1.8, 3.8, 9.7, 31.3, NCH); 3.44 (d,
J ¼ 13.8, 2 PhCHH’), 3.74 (s, MeO); 3.94 (d, J ¼ 13.8, 2 PhCHH’); 5.42 (dd, J ¼ 1.8, 50.3, CHF); 7.21 – 7.35
(m, 10 arom. H). 13C-NMR (100 MHz, CDCl3): 21.1, 23.6 (Me); 24.0, 35.2 (d, J ¼ 4.0) (CH); 52.3 (Me);
54.2, 54.3 (CH2); 57.3 (d, J ¼ 19.1), 88.0 (d, J ¼ 191.7), 127.1, 128.2, 129.1 (CH); 139.5, 170.2 (d, J ¼ 24.0)
(C). 19F-NMR (282 MHz, CDCl3): ꢀ 201.6 (dd, J ¼ 30.9, 50.2, CHF). HR-MALDI-MS: 397.0 (7, [M þ
K]þ), 380.2 (12, [M þ Na]þ), 358.2 (100, [M þ H]þ), 354.1 (26), 338.2 (17), 280.2 (45), 266.2 (60).
Anal. calc. for C22H28FNO2 (357.47): C 73.92, H 7.89, N 3.92; found: C 73.80, H 7.73, N 3.96.
Benzyl (2S,3S)-3-{[(tert-Butoxy)carbonyl]amino}-2-fluoro-5-methylhexanoate (1cbc). Compound
7c (4.13 g, 11.55 mmol) was hydrolyzed with LiOH · H2O (1.45 g, 34.65 mmol) in EtOH/H2O (45 ml, 2 :1)
according to GP 3; the obtained carboxylic acid was dissolved in MeOH (115 ml) and hydrogenolyzed
according to GP 4, and Boc-protected with Boc2O (3.02 g, 13.86 mmol) and Et3N (4.87 ml, 34.65 mmol)
in MeOH (35 ml) according to GP 5. The resulting acid 1cba was dissolved in DMF (35 ml) and treated
with Cs2CO3 (3.78 g, 11.6 mmol) and BnBr (1.66 ml, 13.9 mmol) according to GP 6. FC (pentane/Et2O
7:1) yielded 1cbc (3.43 g, 84% over 4 steps). Colorless oil. Rf (pentane/AcOEt 9 :1) 0.44. [a]rD:t: ¼ ꢀ18.2
(c ¼ 1.0, CHCl3). IR (CHCl3): 3446w, 3036w, 2964m, 2872w, 1759m, 1708s, 1503s, 1456w, 1390w, 1369m,
1
1164s, 1128w, 1103w, 1046w, 908w. H-NMR (400 MHz, CDCl3): 0.75 (d, J ¼ 6.5, Me); 0.83 (d, J ¼ 6.7,
Me); 0.96 (ddd, J ¼ 3.1, 10.0, 13.3, 1 H, CH2CH); 1.39 – 1.46 (m, 1 H, CH2CH); 1.44 (s, t-Bu); 1.54 – 1.59
(m, Me2CH); 4.09 – 4.22 (m, NCH); 4.59 (d, J ¼ 9.1, BocNH); 5.06 (dd, J ¼ 2.6, 49.2, CHF); 5.15 (d, J ¼
11.9, 1 H, PhCH2); 5.32 (d, J ¼ 11.9, 1 H, PhCH2); 7.33 – 7.39 (m, 5 arom. H). 13C-NMR (100 MHz,
CDCl3): 21.1, 23.4 (Me); 24.4 (CH); 28.3 (Me); 37.4 (d, J ¼ 3.3, CH2); 50.3 (d, J ¼ 19.7, CH); 67.3 (CH2);
79.9 (C); 90.6 (d, J ¼ 187.5), 128.7, 128.8, 128.9 (CH); 134.9, 155.2, 167.6 (d, J ¼ 24.2) (C). 19F-NMR
(282 MHz, CDCl3): ꢀ 199.0 (dd, J ¼ 26.7, 48.0, CHF). HR-MALDI-MS: 392.2 (7, [M þ K]þ), 376.2 (100,
[M þ Na]þ), 344.2 (12), 320.1 (60), 300.1 (18), 254.2 (48). Anal. calc. for C19H28FNO4 (353.43): C 64.57, H
7.98, N 3.96; found: C 64.51, H 7.82, N 3.91.
Methyl (2S/R,3S)-3-(Dibenzylamino)-2-hydroxy-5-methylhexanoate (6c/epi-6c). A soln. of freshly
prepared 4c [11] (5.6 g, 19.1 mmol) in hexane/H2O (40 ml, 3 :1) was treated with acetone cyanohydrin
(2.6 ml, 28.7 mmol), KCN (40.0 mg, 0.6 mmol), and Bu4NI (47.0 mg, 0.2 mmol) according to GP 7. The
crude epimeric mixture of cyanohydrins was treated with a sat. HCl/MeOH soln. (100 ml) according to
GP 8. FC (pentane/Et2O 4 :1) yielded 6c/epi-6c (7.12 g, 78% over 2 steps). Yellow oil. Rf (pentane/Et2O
4 :1) 0.23. Both epimers could be separated by a further FC (hexane/AcOEt 95 :5) yielding pure 6c and
1
epi-6c. The H- and 13C-NMR data for 6c ((2R,3S)) were in accordance with those described above.

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Helvetica Chimica Acta – Vol. 94 (2011)
Data of epi-6c ((2S,3S)). Light yellow oil. Rf (pentane/Et2O 3 :2) 0.43. [a]rD:t: ¼ þ43.7 (c ¼ 1.0,
CHCl3). IR (CHCl3): 3520w, 3008w, 2957s, 1731s, 1495m, 1454m, 1366w, 1269m, 1155m, 1092m, 1028w,
973w. 1H-NMR (300 MHz, CDCl3): 0.94 (d, J ¼ 6.5, Me); 1.00 (d, J ¼ 6.2, Me); 1.48 – 1.58 (m, 1 H,
CH2CH); 1.60 – 1.69 (m, 1 H, CH2CH); 1.77 – 1.86 (m, Me2CH); 3.00 – 3.08 (m, NCH); 3.36 (d, J ¼ 13.7,
2 PhCHH’); 3.48 (s, MeO); 3.95 (d, J ¼ 13.4, 2 PhCHH’); 4.18 (d, J ¼ 4.1, CHOH); 7.19 – 7.39 (m, 10 arom.
H). 13C-NMR (75 MHz, CDCl3): 22.1, 24.1 (Me); 25.5 (CH); 32.8 (CH2); 52.4 (Me); 55.6 (CH2); 57.2,
73.2, 126.8, 128.1, 129.1 (CH); 139.9, 174.9 (C). HR-MALDI-MS: 378.2 (9, [M þ Na]þ), 356.2 (100, [M þ
H]þ), 266.2 (60), 178.1 (9), 176.1 (8).
Methyl (2R,3S)-3-(Dibenzylamino)-2-fluoro-5-methylhexanoate (epi-7c). Compound epi-6c (2.42 g,
6.80 mmol) was dissolved in CH2Cl2 (14 ml) and fluorinated with DAST (1.34 ml, 10.21 mmol) at 08 for
3 h, according to GP 2. FC (pentane/Et2O 99 :1 ! 96 :4) yielded epi-7c (1.09 g, 45%). Light yellow solid.
M.p. 62 – 638. Rf (pentane/Et2O 10 :1) 0.31. [a]rD:t: ¼ þ31.7 (c ¼ 1.1, CHCl3). IR (CHCl3): 3064w, 3032w,
2957s, 2870w, 2810w, 1762s, 1603w, 1495m, 1454m, 1439w, 1361w, 1304m, 1158m, 1076m, 1029w, 1001w,
972w, 938w, 911w, 887w, 831w. 1H-NMR (400 MHz, CDCl3): 0.89 (d, J ¼ 6.4, Me); 0.92 (d, J ¼ 6.4, Me);
1.50 – 1.76 (m, Me2CH2CH); 3.15 (tdd, J ¼ 4.0, 10.2, 31.4, NCH); 3.40 (d, J ¼ 13.4, 2 PhCHH’); 3.61 (s,
MeO); 3.87 (d, J ¼ 10.3, 2 PhCHH’); 4.97 (dd, J ¼ 3.6, 49.0, CHF); 7.19 – 7.31 (m, 10 arom. H). 13C-NMR
(100 MHz, CDCl3): 22.0, 23.7 (Me); 25.0 (CH); 31.9 (d, J ¼ 3.3, CH2); 51.9 (Me); 55.0, 55.1 (CH2); 56.5
(d, J ¼ 18.1), 91.9 (d, J ¼ 188.8), 126.9, 128.1, 129.3 (CH); 139.7, 169.4 (d, J ¼ 25.6) (C). 19F-NMR
(282 MHz, CDCl3): ꢀ 202.6 (dd, J ¼ 31.0, 48.0, CHF). HR-MALDI-MS: 358.2 (22, [M þ H]þ), 338.2
(62), 278.2 (19), 266.2 (100). Anal. calc. for C22H28FNO2 (357.47): C 73.92, H 7.89, N 3.92; found: C 73.87,
H 8.13, N 4.08.
Benzyl (2R,3S)-3-{[(tert-Butoxy)carbonyl]amino}-2-fluoro-5-methylhexanoate (2cbc). Compound
epi-7c (940 mg, 2.63 mmol) was hydrolyzed with LiOH · H2O (331 mg, 7.89 mmol) in EtOH/H2O 2 :1
(10 ml) according to GP 3, the carboxylic acid was dissolved in MeOH (26 ml) and hydrogenolyzed
according to GP 4, and Boc-protected with Boc2O (707 mg, 3.24 mmol) and Et3N (1.13 ml, 8.10 mmol) in
MeOH (15 ml) according to GP 5. The resulting acid 2cba was dissolved in DMF (8 ml), and treated with
Cs2CO3 (968 mg, 3.0 mmol) and BnBr (385 ml, 3.24 mmol) according to GP 6. FC (pentane/Et2O 9 :1)
yielded 2cbc (861 mg, 92% over 4 steps). Colorless solid. M.p. 80 – 828. Rf (pentane/AcOEt 9 :1) 0.42.
[a]Dr:t: ¼ ꢀ37.1 (c ¼ 1.0, CHCl3). IR (CHCl3): 3439m, 3009w, 2962m, 2923w, 2872w, 1759s, 1709s, 1503s,
1456w, 1392w, 1368m, 1331w, 1164s, 1135w, 1083m, 1052w, 949w, 913w, 867w, 850w. 1H-NMR (400 MHz,
CDCl3): 0.94 (d, J ¼ 6.5, 2 Me); 1.36 – 1.50 (m, CH2CH); 1.42 (s, t-Bu); 1.62 – 1.72 (m, Me2CH); 4.20 – 4.33
(m, NCH); 4.66 (d, J ¼ 10.0, BocNH); 4.89 (dd, J ¼ 1.9, 47.9, CHF); 5.11 (d, J ¼ 12.0, 1 H, PhCH2); 5.27
(d, J ¼ 12.0, 1 H, PhCH2); 7.32 – 7.42 (m, 5 arom. H). 13C-NMR (100 MHz, CDCl3): 22.0, 22.9 (Me); 24.7
(CH); 28.3 (Me); 40.3 (CH2); 50.3 (d, J ¼ 20.4, CH); 67.5 (CH2); 79.7 (C); 89.7 (d, J ¼ 187.4), 128.6, 128.7,
128.7 (CH); 135.0, 155.2, 168.1 (d, J ¼ 25.2) (C). 19F-NMR (282 MHz, CDCl3): ꢀ 204.1 (dd, J ¼ 26.7, 48.0,

Helvetica Chimica Acta – Vol. 94 (2011)
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CHF). HR-MALDI-MS: 376.2 (51, [M þ Na]þ), 320.1 (34). Anal. calc. for C19H28FNO4 (353.43): C
64.57, H 7.98, N 3.96, F 5.38; found: C 64.77, H 7.81, N 3.90, F 5.37.
3.2.4. Difluoro-Substituted Val- and Leu-Derived Compounds. Methyl (3S)-3-(Dibenzylamino)-2,2-
difluoro-4-methylpentanoate (8b). The mixture 6b/epi-6b (5.1 g, 14.9 mmol) was oxidized with oxalyl
chloride (1.5 ml, 17.9 mmol) and DMSO (2.1 ml, 29.8 mmol) according to GP 9. The resulting crude keto
ester was dissolved in CH2Cl2 (26 ml) and fluorinated with DAST (5.5 ml, 44.7 mmol) at r.t. for 6 h,
according to GP 2. FC (pentane/Et2O 95 :5) yielded 8b (3.4 g, 64% over 2 steps). Light yellow oil. Rf
(pentane/Et2O 95 :5) 0.31. [a]rD:t: ¼ ꢀ2.1 (c ¼ 1.0, CHCl3). IR (CHCl3): 3005m, 2954m, 2841w, 1959w,
1769s, 1708s, 1600w, 1492w, 1451m, 1440w, 1415w, 1359s, 1303m, 1281w, 1164w, 1118s, 1087m, 1067s,
1041m, 979w, 913w, 841w. 1H-NMR (500 MHz, CDCl3): 0.96 (d, J ¼ 6.7, Me); 1.06 (d, J ¼ 6.9, Me); 2.13 –
2.19 (m, Me2CH); 3.17 – 3.25 (ddd, J ¼ 7.0, 14.6, 16.9, NCH); 3.69 (s, MeO); 3.81 (d, J ¼ 13.4,
2 PhCHH’); 3.89 (d, J ¼ 13.4, 2 PhCHH’); 7.21 – 7.31 (m, 10 arom. H). 13C-NMR (125 MHz, CDCl3):
19.9, 22.5 (Me); 26.7 (d, J ¼ 2.0, CH); 53.1 (Me); 55.2 (CH2); 63.6 (dd, J ¼ 18.4, 22.8, CH); 119.4 (dd, J ¼
256.7, 262.1, C); 127.1, 128.2, 129.5 (CH); 139.3, 165.1 (t, J ¼ 32.8) (C). 19F-NMR (282 MHz, CDCl3):
ꢀ 105.3 (dd, J ¼ 17.1, 265.7, 1 F, CF2); ꢀ 108.9 (dd, J ¼ 14.9, 264.6, 1 F, CF2). HR-MALDI-MS: 397.0
(15), 375.0 (21, [M þ Na]þ), 362.2 (69, [M þ H]þ), 360.2 (34), 266.2 (70), 252.2 (38). Anal. calc. for
C21H25F2NO2 (361.43): C 69.79, H 6.97, N 3.88, F 10.51; found: C 69.81, H 7.01, N 3.84, F 10.39.
(3S)-3-{[(tert-Butoxy)carbonyl]amino}-2,2-difluoro-4-methylpentanoic Acid (3bba). Compound 8b
(847 mg, 3.0 mmol) was hydrolyzed with LiOH · H2O (379 mg, 9.0 mmol) in EtOH/H2O 2 :1 (12 ml)
according to GP 3. The resulting acid 3bba (790 mg, 98%) was used without further purification.
Colorless solid. M.p. 115 – 1168. [a]rD:t: ¼ ꢀ49.4 (c ¼ 1.0, CHCl3). IR (CHCl3): 3446m, 3026w, 2982m,
2933w, 1760m, 1717s, 1506s, 1394w, 1369m, 1310w, 1160s, 1092w, 1041w, 1005w, 873m. 1H-NMR
(300 MHz, CD3OD): 0.94 (d, J ¼ 6.8, Me); 0.98 (d, J ¼ 6.8, Me); 1.44 (s, t-Bu); 2.01 – 2.09 (m, Me2CH);
4.11 (ddd, J ¼ 5.0, 13.1, 17.4, BocNHCH). 13C-NMR (75 MHz, CD3OD): 17.6, 20.6, 28.4 (Me); 28.7, 57.8 (t,
J ¼ 22.0) (CH); 80.2, 116.5 (t, J ¼ 253.9), 157.8, 166.0 (t, J ¼ 31.7) (C). 19F-NMR (282 MHz, CD3OD):
ꢀ 111.1 (dd, J ¼ 12.8, 254.0, 1 F, CF2); ꢀ 112.6 (dd, J ¼ 17.1, 255.0, 1 F, CF2). HR-ESI-MS: 613.3 (7,
[2M þ 2 K]þ), 579.2 (12, [2M þ 2 Na]þ), 557.2 (100, [2M þ Na]þ), 370.1 (15), 290.1 (64, [M þ Na]þ),
234.1 (7), 212.1 (13).
Methyl (3S)-3-{[(tert-Butoxy)carbonyl]amino}-2,2-difluoro-4-methylpentanoate (3bbb). The hydro-
genolysis of 8b (2.0 g, 5.5 mmol) was performed in presence of Boc2O (1.8 g, 8.2 mmol) in MeOH (90 ml)
according to GP 4. FC (pentane/Et2O 95 :5) yielded 3bbb (1.3 g, 83%). Colorless solid. Rf (pentane/Et2O
95 :5) 0.16. M.p. 48 – 498. [a]rD:t: ¼ ꢀ19.8 (c ¼ 1.0, CHCl3). IR (CHCl3): 3446m, 2974m, 2882w, 1769s, 1713s,
1497s, 1446w, 1395w, 1369s, 1308m, 1159s, 1092w, 1056m, 1005w, 979w, 867w, 836w. 1H-NMR (500 MHz,
CDCl3): 0.95 (d, J ¼ 6.8, Me); 1.01 (d, J ¼ 6.8, Me); 1.44 (s, t-Bu); 2.09 – 2.16 (m, Me2CH); 3.86 (s, MeO);
4.14 – 4.23 (m, NCH); 4.68 (d, J ¼ 10.5, BocNH). 13C-NMR (125 MHz, CDCl3): 17.1, 20.5 (Me); 27.1
(CH); 28.2, 53.4 (Me); 56.5 (dd, J ¼ 21.7, 26.4, CH); 80.2, 115.2 (t, J ¼ 257.1), 155.4, 164.1 (t, J ¼ 31.2) (C).
19F-NMR (282 MHz, CDCl3): ꢀ 111.8 (dd, J ¼ 8.5, 257.2, 1 F, CF2); ꢀ 116.8 (dd, J ¼ 21.3, 257.2, 1 F, CF2).
HR-ESI-MS: 585.3 (39, [2M þ Na]þ), 320.1 (9, [M þ K]þ), 304.1 (100, [M þ Na]þ). Anal. calc. for
C12H21F2NO4 (281.30): C 51.24, H 7.52, N 4.98, F 13.51; found: C 51.41, H 7.46, N 5.14, F 13.51.
Methyl (3S)-3-(Dibenzylamino)-2,2-difluoro-5-methylhexanoate (8c). Mixture 6c/epi-6c (5.3 g,
15.0 mmol) was oxidized with oxalyl chloride (1.6 ml, 18.0 mmol) and DMSO (2.1 ml, 30.0 mmol)
according to GP 9. The resulting crude keto ester was dissolved in CH2Cl2 (30 ml) and fluorinated with
DAST (5.5 ml, 45.0 mmol) at r.t. for 6 h, according to GP 2. FC (pentane/CH2Cl2 95 :5) yielded 8c (3.7 g,
65% over 2 steps). Colorless solid. Rf (pentane/CH2Cl2 95 :5) 0.16. M.p. 608. [a]rD:t: ¼ þ14.1 (c ¼ 1.0,
CHCl3). IR (CHCl3): 3087w, 3032w, 2958m, 2869m, 1767s, 1603w, 1496w, 1454m, 1440w, 1380w, 1313m,
1
1117s, 1062s, 1028w, 966w, 917w. H-NMR (400 MHz, CDCl3): 0.75 (d, J ¼ 6.5, Me); 0.93 (d, J ¼ 6.6,

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Helvetica Chimica Acta – Vol. 94 (2011)
Me); 1.45 – 1.63 (m, CH2CH); 1.79 – 1.88 (m, Me2CH); 3.31 – 3.42 (m, NCH); 3.56 (d, J ¼ 13.4,
2 PhCHH’); 3.68 (s, MeO); 3.82 (d, J ¼ 13.4, 2 PhCHH’); 7.20 – 7.32 (m, 10 arom. H). 13C-NMR
(100 MHz, CDCl3): 22.6, 22.8 (Me); 24.9 (CH); 32.6 (d, J ¼ 2.5, CH2); 52.9 (Me); 54.6 (CH2); 57.0 (dd,
J ¼ 20.4, 25.0, CH); 118.7 (t, J ¼ 258.0, C); 127.1, 128.2, 129.4 (CH); 139.2, 164.8 (dd, J ¼ 30.9, 34.0) (C).
19F-NMR (282 MHz, CDCl3): ꢀ 103.7 (dd, J ¼ 11.7, 255.0, 1 F, CF2); ꢀ 114.2 (dd, J ¼ 23.5, 254.0, 1 F,
CF2). HR-MALDI-MS: 413.3 (10, [M þ K]þ), 398.2 (22, [M þ Na]þ), 376.2 (100, [M þ H]þ), 284.2 (11),
266.2 (15), 181.1 (17). Anal. calc. for C22H27F2NO2 (375.46): C 70.38, H 7.25, N 3.73; found: C 70.43, H
7.40, N 3.72.
Methyl (3S)-3-{[(tert-Butoxy)carbonyl]amino}-2,2-difluoro-5-methylhexanoate (3cbb). The hydro-
genolysis of 8c (1.6 g, 4.2 mmol) was performed in presence of Boc2O (1.3 g, 6.2 mmol) in MeOH (70 ml)
according to GP 4. FC (pentane/Et2O 19 :1) yielded 3cbb (1.3 g, 95%). Colorless solid. Rf (pentane/Et2O
19 :1) 0.17. M.p. 778. [a]rD:t: ¼ ꢀ39.6 (c ¼ 1.0, CHCl3). IR (CHCl3): 3436w, 2964m, 2872w, 1769s, 1713s,
1503s, 1441w, 1390w, 1369m, 1308w, 1159s, 1072m, 1046w, 1021w, 959w, 877w, 846w, 826w. 1H-NMR
(500 MHz, CDCl3): 0.93 (d, J ¼ 6.5, Me); 0.96 (d, J ¼ 6.7, Me); 1.42 (s, t-Bu); 1.36 – 1.46 (m, CH2CH);
1.67 – 1.74 (m, Me2CH); 3.86 (s, MeO); 4.25 – 4.35 (m, NCH); 4.51 (d, J ¼ 10.0, BocNH). 13C-NMR
(125 MHz, CDCl3): 21.2, 23.5 (Me); 24.3 (CH); 28.2 (Me); 36.3 (CH2); 51.2 (dd, J ¼ 23.4, 27.4, CH); 53.4
(Me); 80.2, 114.7 (t, J ¼ 254.3), 155.1, 163.9 (dd, J ¼ 31.3, 33.5) (C). 19F-NMR (282 MHz, CDCl3): ꢀ 103.7
(dd, J ¼ 11.7, 254.0, 1 F, CF2); ꢀ 114.2 (dd, J ¼ 18.1, 254.0, 1 F, CF2). HR-ESI-MS: 660.9 (4), 613.3 (100,
[2M þ Na]þ), 318.1 (53, [M þ Na]þ), 306.6 (5), 214.8 (3), 209.5 (4), 156.2 (4). Anal. calc. for
C13H23F2NO4 (295.33): C 52.87, H 7.85, N 4.74; found: C 52.86, H 7.65, N 4.79.
Benzyl (3S)-3-{[(tert-Butoxy)carbonyl]amino}-2,2-difluoro-5-methylhexanoate (3cbc). Compound
3cbb (1.13 g, 4.02 mmol) was hydrolyzed with LiOH · H2O (506 mg, 12.6 mmol) in EtOH/H2O 2 :1
(15 ml) according to GP 3. The resulting carboxylic acid 3cba was dissolved in DMF (12 ml) and treated
with Cs2CO3 (1.31 g, 4.02 mmol) and BnBr (570 ml, 4.82 mmol) according to GP 6. FC (pentane/Et2O
19 :1) yielded 3cbc (1.40 g, 96% over 2 steps). Colorless solid. M.p. 478. Rf (pentane/Et2O 19 :1) 0.20.
[a]Dr:t: ¼ ꢀ18.6 (c ¼ 1.0, CHCl3). IR (CHCl3): 3436m, 3026w, 2954m, 2861w, 1764s, 1713s, 1503s, 1456m,
1
1390w, 1369s, 1303w, 1267w, 1159s, 1072s, 1046m, 1026w, 954w, 908w, 877w, 846w. H-NMR (500 MHz,
CDCl3): 0.87 (d, J ¼ 6.5, Me); 0.91 (d, J ¼ 6.7, Me); 1.31 – 1.37 (m, CH2CH); 1.43 (s, t-Bu); 1.64 – 1.72 (m,
Me2CH); 4.23 – 4.39 (m, NCH); 4.51 (d, J ¼ 10.3, BocNH); 5.19 (d, J ¼ 12.0, 1 H, PhCH2); 5.32 (d, J ¼
12.0, 1 H, PhCH2); 7.34 – 7.47 (m, 5 arom. H). 13C-NMR (125 MHz, CDCl3): 21.2, 23.4 (Me); 24.3
(CH); 28.2 (Me); 36.6 (CH2); 51.2 (t, J ¼ 26.3, CH); 68.5 (CH2); 80.2, 114.7 (t, J ¼ 255.9) (C); 128.7, 128.8
(CH); 134.2, 155.1, 163.3 (t, J ¼ 31.8) (C). 19F-NMR (282 MHz, CDCl3): ꢀ 116.7 (dd, J ¼ 11.7, 254.0, 1 F,
CF2); ꢀ 118.8 (dd, J ¼ 14.9, 252.9, 1 F, CF2). HR-ESI-MS: 765.4 (18, [2M þ Na]þ), 410.2 (21, [M þ K]þ),
394.2 (100, [M þ Na]þ). Anal. calc. for C19H27F2NO4 (371.42): C 61.44, H 7.33, N 3.77, F 10.23; found: C
61.36, H 7.16, N 3.69, F 10.26.
4. Preparation of the Fluorinated Tetrahydropyrimidin-4(1H)-ones 11 – 13 (Fig. 2). (2S,5S,6S)-2-
(tert-Butyl)-5-fluorotetrahydro-6-methylpyrimidin-4(1H)-ones (11). According to GP 10, aminobutanoic
acid 1aca (1.82 g, 7.13 mmol) was converted to the corresponding Cbz-protected amino acid amide 1acd
(1.11 g, 61% yield), and subsequent hydrogenation (1.00 g, 3.93 mmol) gave the b-amino acid amide 1aad
(470 mg, quant.). Amide 1aad (60.0 mg, 0.50 mmol) was treated with pivalaldehyde to afford crude 11 as
colorless solid. Recrystallization of this crude product from hexane/AcOEt afforded 11 (81.1 mg, 86%
yield). Colorless solid. A second recrystallization gave colorless prisms. M.p. 131 – 1328 (hexane/AcOEt).
[a]2D6 ¼ þ21.0 (c ¼ 1.0, CHCl3). IR: 3318w, 3243m, 3107w, 2967m, 2954m, 2914m, 2871m, 1681s, 1660s,
1478s, 1454m, 1438m, 1417m, 1403m, 1381w, 1365m, 1330m, 1300m, 1284m, 1250m, 1207w, 1152m, 1134m,
1100m, 1056s, 1039s, 1017m, 965m, 937w, 904w, 874w, 805m, 763s, 686s, 635m. 1H-NMR (400 MHz,

Helvetica Chimica Acta – Vol. 94 (2011)
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CDCl3): 0.96 (s, 3 Me); 1.33 (d, J ¼ 6.3, Me); 3.07 – 3.19 (m, NCH); 4.08 (d, J ¼ 7.0, NCHN); 4.26 (dd,
J ¼ 48.3, 10.0, CFH); 6.17 (br. s, NH). 13C-NMR (100 MHz, CDCl3): 18.6 (Me); 24.8 (3 Me); 34.2 (t-Bu);
51.7 (d, J ¼ 21.9, NCH); 75.5 (NCHN); 88.9 (d, J ¼ 189.9, FCH); 168.9 (d, J ¼ 20.5, CFCO). 19F-NMR
(376 MHz, CDCl3): ꢀ 198.5 (dd, J ¼ 48.4, 5.5, F). Anal. calc. for C9H17FN2O (188.24): C 57.42, H 9.10, N
14.88; found: C 57.40, H 9.28, N 14.87.
Benzyl (2S/R,5S,6S)-2-(tert-Butyl)-5-fluorotetrahydro-6-methyl-4-oxopyrimidine-1(2H)-carboxylate
(Cbz-11). According to GP 11, 11 (100 mg, 0.53 mmol) was dissolved in CH2Cl2 (1.0 ml), and treated with
N,O-bis(trimethylsilyl)acetamide (0.20 ml, 0.80 mmol) and Cbz-Cl (0.10 ml, 0.69 mmol). FC (CH2Cl2/
AcOEt 9 :1 ! 3 :1) yielded Cbz-11 (66.7 mg, 39%). Colorless oil. [a]2D4 ¼ þ35.6 (c ¼ 0.77, CHCl3). IR:
3228w, 2961w, 2911w, 2879w, 1688s, 1483w, 1456w, 1391m, 1318s, 1291s, 1198m, 1160w, 1120w, 1076m,
1038s, 1019m, 967w, 898w, 774m, 736m, 697s. 1H-NMR (300 MHz, CDCl3): 1.00 (s, 3 Me); 1.56 (d, J ¼ 6.3,
Me); 4.20 – 4.36 (m, NCH); 4.90 (dd, J ¼ 48.2, 8.6, CFH); 5.18 (s, PhCH2); 5.38 – 5.40 (s, NCHN); 7.10 (s,
NH); 7.31 – 7.41 (m, 5 arom. H). 13C-NMR (100 MHz, CDCl3): 20.4 (Me); 26.5 (3 Me); 38.6 (t-Bu); 53.6
(d, J ¼ 29.6, NCH); 68.5 (PhCH2), 72.9 (NCHN); 86.4 (d, J ¼ 180.1, FCH); 128.2 (arom. C); 128.4
(arom. C); 128.7 (arom. C); 135.6 (arom. C); 152.0 (CO); 167.2 (d, J ¼ 20.2, CFCO). 19F-NMR
(280 MHz, CDCl3): ꢀ 195.5 (dd, J ¼ 47.7, 22.6, F). ESI-MS: 323.1758 ([M þ H]þ, C17H24FN2O3þ ; calc.
323.1765 (err. 2.3 ppm)). Anal. calc. for C17H23FN2O3: C 63.34, H 7.19, N 8.69; found: C 63.08, H 7.28, N
8.40.
(2S,5R,6S)-2-(tert-Butyl)-5-fluorotetrahydro-6-methylpyrimidin-4(1H)-ones (12). According to
GP 10, aminobutanoic acid 2aca (1.80 g, 7.05 mmol) was converted to the corresponding Cbz-protected
amino acid amide 2acd (1.16 g, 65% yield), and subsequent hydrogenation (1.00 g, 3.93 mmol) gave the
b-amino acid amide 2aad (467 mg, 99% yield). Amide 2aad (240 mg, 2.00 mmol) was treated with
pivalaldehyde to give crude 12 as white solid. Purification by FC (CH2Cl2/AcOEt 1:1) afforded 12
(251 mg, 67% yield). Colorless solid. Crystallization gave colorless prisms. M.p. 141 – 1428 (hexane/
AcOEt). [a]2D6 ¼ þ116.6 (c ¼ 1.0, CHCl3). IR: 3321w, 3294w, 3252w, 3193w, 3069w, 2982w, 2950w, 2910w,
2876w, 1671s, 1483m, 1454m, 1411w, 1374m, 1340m, 1287w, 1278w, 1261w, 1234w, 1214w, 1172w, 1144m,
1103w, 1078m, 1049w, 1014w, 993m, 982m, 952m, 938m, 904w, 884w, 869w, 834m, 819s, 798m, 711m, 688m,
661m. 1H-NMR (400 MHz, CDCl3): 0.99 (s, 3 Me); 1.29 (dd, J ¼ 6.8, 1.2, Me); 1.42 (br. s, NH); 2.94 – 3.12
(m, NCH); 4.01 (d, J ¼ 7.0, NCHN); 4.52 (ddd, J ¼ 48.4, 1.8, 0.7, CFH); 6.63 (br. s, NH). 13C-NMR
(100 MHz, CDCl3): 15.9 (d, J ¼ 7.9, Me); 25.0 (3 Me); 34.2 (Me3C); 51.6 (d, J ¼ 21.0, NCH); 76.2
(NCHN); 87.8 (d, J ¼ 174.5, FCH); 166.9 (d, J ¼ 19.1, CFCO). 19F-NMR (376 MHz, CDCl3): 46.7 (ddd,
J ¼ 48.5, 30.1, 4.3, 1 F). Anal. calc. for C9H17FN2O (188.24): C 57.42, H 9.10, N 14.88; found: C 57.16, H
9.07, N 14.68.
Benzyl (2S/R,5R,6S)-2-(tert-Butyl)-5-fluorotetrahydro-6-methyl-4-oxopyrimidine-1(2H)-carboxy-
late (Cbz-12). According to GP 11, 12 (100 mg, 0.53 mmol) was dissolved in CH2Cl2 (1.0 ml), and
treated with N,O-bis(trimethylsilyl)acetamide (0.20 ml, 0.80 mmol) and Cbz-Cl (0.10 ml, 0.69 mmol). FC
(CH2Cl2/AcOEt 3 :1) yielded Cbz-12 (58.1 mg, 34% yield). Crystallization gave colorless prisms. M.p.
124 – 1258 (Et2O). [a]2D4 ¼ ꢀ62.9 (c ¼ 1.0, CHCl3). IR: 3197w, 3121w, 3086w, 2989w, 2961w, 2900w, 1704m,
1677s, 1475w, 1466w, 1458w, 1417m, 1399m, 1390m, 1329m, 1311s, 1299s, 1282s, 1216w, 1193w, 1084s,
1063m, 1046s, 970w, 942w, 899w, 870w, 814w, 774m, 750s, 700s, 630w, 611w. 1H-NMR (400 MHz, CDCl3):
1.01 (s, 3 Me); 1.35 (dd, J ¼ 7.2, 2.5, Me); 4.72 (dd, J ¼ 48.1, 6.4, CFH); 5.05 – 5.15 (m, NCH); 5.20 – 5.21
(m, PhCH2); 5.39 (s, NCHN); 6.41 (s, NH); 7.35 – 7.42 (m, 5 arom. H). 13C-NMR (100 MHz, CDCl3): 15.9
(Me); 26.6 (3 Me); 37.3 (Me3C); 51.7 (d, J ¼ 23.5, NCH); 68.6 (PhCH2), 72.5 (NCHN); 84.0 (d, J ¼
192.9, FCH); 128.3 (arom. C); 128.6 (arom. C); 128.7 (arom. C); 135.5 (arom. C); 156.6 (CO); 166.8
(d, J ¼ 20.8, CFCO). 19F-NMR (376 MHz, CDCl3): ꢀ 198.8 (d, J ¼ 48.5, 1 F). ESI-MS: 323.1766 ([M þ
H]þ, C17H24FN2O3þ ; calc. 323.1765 (err. ꢀ 0.2 ppm)); 345.1585 ([M þ Na]þ, C17H23FN2NaOþ3 ; calc.
345.1585 (err., 0.0 ppm)). Anal. calc. for C17H23FN2O3: C 63.34, H 7.19, N 8.69; found: C 63.09, H 7.12, N
8.65.
(2R/S,6S)-2-(tert-Butyl)-5,5-difluorotetrahydro-6-methylpyrimidin-4(1H)-ones (13a/13b). Accord-
ing to GP 10 aminobutanoic acid 3aca (1.40 g, 5.12 mmol) was converted to the corresponding Cbz-
protected amino acid amide 3acd (930 mg, 67% yield), and subsequent hydrogenation (800 mg,
2.94 mmol) gave the b-amino acid amide 3aad (405 mg, quant.). Amide 3aad (200 mg, 1.45 mmol) was
treated with pivalaldehyde to give crude 13a/13b as white solid. Purification by FC (hexane/AcOEt 7:3,

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hexane/acetone 85 :15) afforded 13a/13b (73.1 mg, 24%; dr 3 :1). Colorless solid. Crystallization gave
colorless prisms (1 :1 diastereoisomeric co-crystal). M.p. 110 – 1138 (hexane/AcOEt). [a]2D0 ¼ ꢀ3.1 (c ¼ 1,
CHCl3). IR: 3321w, 3294w, 3252w, 3193w, 3069w, 2982w, 2950w, 2910w, 2876w, 1671s, 1483m, 1454m,
1411w, 1374m, 1340m, 1287w, 1278w, 1261w, 1234w, 1214w, 1172w, 1144m, 1103w, 1078m, 1049w, 1014w,
993m, 982m, 952m, 938m, 904w, 884w, 869w, 834m, 819s, 798m, 711m, 688m, 661m. 1H-NMR (400 MHz,
CDCl3; *: minor isomer): 0.98 (s, 3 Me); 0.99 (s, 3 Me*); 1.29 (d, J ¼ 5.4, Me*); 1.30 (d, J ¼ 6.6, Me);
1.59 – 1.65 (m, NH); 2.16 – 2.24 (m, NH*); 3.10 – 3.26 (m, NCH); 3.44 – 3.54 (m, NCH*); 4.08 – 4.14 (m,
NCHN, NCHN*); 6.13 (br. s, NH); 6.24 (br. s, NH). 19F-NMR (280 MHz, CDCl3): ꢀ 104.3 (ddd, J ¼
277.5, 13.9, 3.9, 1 F*); ꢀ 120.2 (dd, J ¼ 280.4, 17.9, 1 F); ꢀ 121.4 (ddd, J ¼ 280.3, 6.6, 2.1, 1 F); ꢀ 123.8
(d, J ¼ 275.9, 1 F*).
Benzyl (2S/R,6S)-2-(tert-Butyl)-5,5-difluorotetrahydro-6-methyl-4-oxopyrimidine-1(2H)-carboxy-
late (Cbz-13). According to GP 11, 13 (150 mg, 0.73 mmol) was dissolved in CH2Cl2 (1.4 ml), and
treated with N,O-bis(trimethylsilyl)acetamide (0.27 ml, 1.09 mmol) and Cbz-Cl (0.13 ml, 0.95 mmol). FC
(CH2Cl2/Et2O 97:3) yielded Cbz-13 (15.1 mg, 6%). Colorless oil. IR: 3236w, 2960w, 2875w, 1760w, 1693s,
1483m, 1455w, 1403w, 1373w, 1331w, 1296w, 1284w, 1199s, 1152m, 1104m, 1071m, 1029w, 1003m, 937w,
910w, 805m, 774m, 747m, 697s, 653m. 1H-NMR (400 MHz, CDCl3): 1.00 (s, 3 Me); 1.32 (d, J ¼ 6.6, Me);
3.21 (ddq, J ¼ 19.1, 12.7, 6.4, NCH); 4.13 (dd, J ¼ 11.9, 3.4, NCHN); 5.20 (s, PhCH2); 7.35 – 7.44 (m, 5
arom. H). 13C-NMR (100 MHz, CDCl3): 12.1 (Me); 24.9 (3 Me); 34.2 (Me3C); 53.3 (t, J ¼ 24.5, NCH);
67.9 (PhCH2), 75.7 (NCHN); 110.8 (t, J ¼ 247.0, CF2); 128.1 (arom. C); 128.4 (arom. C); 128.7 (arom.
C); 134.9 (arom. C); 151.8 (CO); 163.6 (t, J ¼ 29.8, CFCO). 19F-NMR (280 MHz, CDCl3): ꢀ 120.5 (dd,
J ¼ 282.2, 17.8, 1 F); ꢀ 121.7 (ddd, J ¼ 282.2, 6.9, 1.9, 1 F). ESI-MS: 341.1684 ([M þ H]þ, C17H23F2N2O3þ ;
calc. 341.1671 (err., ꢀ 3.7 ppm)).
5. Preparation of the Fluorinated Cyclo-b-tripeptides 17 – 19 (Scheme 3, left). 5.1. (S,S)-Isomer 17
from 16a. Boc-(2S,3S)-b2,3-hAla(a-F)-hGly-hGly-OMe (16a). According to GP 12a amino fluoro acid
1aba (700 mg, 3.16 mmol) was converted to the corresponding tripeptide. Purification by FC (hexane/
acetone 7:3 ! 1:1) afforded 16a (821 mg, 69%). Colorless solid. Crystallization gave colorless prisms.
M.p. 155 – 1568 (CH2Cl2/Et2O). [a]2D0 ¼ ꢀ22.6 (c ¼ 1.0, MeOH). IR: 3350m, 3282m, 3086w, 2977w, 2951w,
1734m, 1689s, 1655s, 1640s, 1554m, 1530s, 1442m, 1387m, 1367m, 1337m, 1311m, 1271m, 1250m, 1171s,
1111m, 1062m, 990m, 926w, 883m, 850w, 806w, 781w, 749m, 708m, 643m, 616m. 1H-NMR (400 MHz,
CD3OD): 1.11 (dd, J ¼ 7.0, 0.5, Me); 1.46 (s, 3 Me); 2.43 (td, J ¼ 6.7, 0.5, CH2); 2.55 (t, J ¼ 6.6, CH2); 3.45
(t, J ¼ 6.6, NCH2); 3.47 – 3.56 (m, NCH2); 3.70 (s, Me); 4.12 (dqd, J ¼ 27.3, 7.0, 3.0, NCH); 4.90 (dd,
J ¼ 49.7, 3.0, FCH). 13C-NMR (100 MHz, CD3OD): 12.6 (d, J ¼ 5.7, Me); 27.3 (3 Me); 33.3 (CH2); 34.89
(CH2); 34.93 (CH2); 35.3 (CH2); 49.2 (d, J ¼ 20.5, NCH); 50.8 (Me); 79.0 (Me3C); 92.3 (d, J ¼ 190.6,
FCH); 156.0 (CO); 168.8 (d, J ¼ 20.0, CFCO); 172.3 (CO); 172.4 (CO). 19F-NMR (280 MHz, CD3OD):
ꢀ 203.7 (dd, J ¼ 49.5, 27.2, 1 F). Anal. calc. for C16H28FN3O6 (377.41): C 50.92, H 7.48, N 11.13; found: C
50.96, H 7.44, N 10.99.
Cyclo[(2S,3S)-b2,3-hAla(a-F)-hGly-hGly] (17). According to GP 3, 16a (480 mg, 1.27 mmol) was
converted to the corresponding carboxylic acid (443 mg, 96% yield). According to GP 13, the carboxylic
acid (363 mg, 1.00 mmol) was converted to the pentafluorophenyl ester (471 mg, 89% yield), subsequent
removal of the Boc group (400 mg, 0.756 mmol) afforded the TFA salt, which was converted (by
i
treatment with Pr2NEt in MeCN) to 17 (148 mg, 80%). Colorless solid. M.p. 299 – 3008 (dec., MeCN).
IR: 3388m, 3102w, 2973w, 2949w, 2876w, 1662s, 1646s, 1565s, 1544s, 1454m, 1446m, 1377m, 1351w, 1312w,
1270m, 1237m, 1200m, 1157w, 1133m, 1098m, 1069s, 1029s, 1009m, 988w, 923w, 880w, 864w, 736m, 713m,
671s, 609m. 1H-NMR (300 MHz, CDCl3 þ TFA): 1.49 (d, J ¼ 7.4, Me); 2.52 – 2.64 (m, 2 CHH); 2.72 (dt,
J ¼ 14.0, 3.6, 1 H, CH2); 3.06 (ddd, J ¼ 14.6, 11.8, 5.6, 1 H, CH2); 3.29 – 3.44 (m, 2 NCHH); 3.76 (ddd,
J ¼ 14.1, 5.4, 2.6, 1 H, NCH2); 4.20 – 4.27 (m, 1 H, NCH2); 4.58 (dq, J ¼ 21.6, 7.3, NCH); 4.98 (d, J ¼ 48.6,
FCH). 19F-NMR (280 MHz, CDCl3 þ TFA): ꢀ 179.3 (dd, J ¼ 48.2, 21.2, 1 F). ESI-MS: 246.1241 ([M þ
H]þ, C10H16FN3NaO3þ ; calc. 246.1248 (err., 3.0 ppm)).
5.2. (R,S)-Isomer 18 from 16b. Boc-(2R,3S)-b2,3-hAla(a-F)-hGly-hGly-OMe (16b). According to
GP 12a, amino fluoro acid 2aba (1.19 g, 5.40 mmol) was converted to the corresponding tripeptide.
Purification by FC (hexane/acetone 7 :3 ! 1 :1) afforded 16b (1.51 g, 74%) as colorless solid.
Recrystallization gave colorless solid. M.p. 117 – 1188 (CH2Cl2/Et2O). [a]2D0 ¼ þ22.2 (c ¼ 1.0, MeOH).
IR: 3336m, 3312m, 3089w, 2973w, 2936w, 1732m, 1686s, 1656s, 1640s, 1526s, 1439m, 1366m, 1354m, 1326m,

Helvetica Chimica Acta – Vol. 94 (2011)
1935
1284m, 1247m, 1200m, 1164s, 1104m, 1056m, 1028m, 993m, 921w, 889w, 851w, 782w, 653m. 1H-NMR
(400 MHz, CD3OD): 1.24 (d, J ¼ 7.0, Me); 1.44 (s, 3 Me); 2.34 – 2.48 (m, CH2); 2.56 (t, J ¼ 6.7, CH2);
3.42 – 3.56 (m, 2 NCH2); 3.70 (s, Me); 4.08 – 4.22 (m, NCH); 4.80 (dd, J ¼ 47.7, 3.0, FCH). 13C-NMR
(100 MHz, CD3OD): 15.8 (Me); 27.3 (3 Me); 33.3 (CH2); 35.0 (CH2); 35.1 (CH2); 35.4 (CH2); 47.5 (d,
J ¼ 20.4, NCH); 50.8 (Me); 79.0 (Me3C); 92.5 (d, J ¼ 189.4, FCH); 156.0 (CO); 169.1 (d, J ¼ 21.1,
CFCO); 172.3 (CO); 172.5 (CO). 19F-NMR (376 MHz, CD3OD): 46.7 (dd, J ¼ 47.7, 24.9, 1 F). Anal.
calc. for C16H28FN3O6 (377.41): C 50.92, H 7.48, N 11.13; found: C 51.01, H 7.47, N 10.94.
Cyclo[(2R,3S)-b2,3-hAla(a-F)-hGly-hGly] (18). According to GP 3, 16b (1.00 g, 2.65 mmol) was
converted to the corresponding carboxylic acid (955 mg, 99%). According to GP 13, the carboxylic acid
(850 mg, 2.34 mmol) was converted to the pentafluorophenyl ester (1.09 g, 88%), subsequent removal of
the Boc group (250 mg, 0.472 mmol) afforded the TFA salt, which was converted (by treatment with
EtNiPr2 in MeCN) to 18 (74.8 mg, 75%). Colorless solid. M.p. 3058 (dec., MeCN). IR: 3329m, 3284m,
3098w, 2971w, 2929w, 2871w, 1670s, 1663s, 1648s, 1550s, 1452m, 1435m, 1371w, 1347m, 1303m, 1275m,
1243m, 1199m, 1187m, 1164w, 1119m, 1094m, 1083m, 1066w, 1056w, 1013m, 991m, 960w, 924w, 886w,
1
866w, 788w, 738m, 686s, 670m, 632m, 616m. H-NMR (300 MHz, CDCl3 þ TFA): 1.38 (d, J ¼ 7.0, Me);
2.54 (ddd, J ¼ 13.8, 5.3, 1.8, 1 H, CH2); 2.64 (dt, J ¼ 15.2, 6.2, 1 H, CH2); 2.82 (ddd, J ¼ 13.8, 12.1, 6.1,
1 H, CH2); 3.06 (ddd, J ¼ 15.2, 8.1, 5.6, 1 H, CH2); 3.30 – 3.47 (m, 2 NCHH); 3.70 – 3.79 (m, 1 H, NCH2);
3.89 – 4.00 (m, 1 H, NCH2); 4.67 – 4.89 (m, NCH); 5.01 (dd, J ¼ 47.5, 1.6, FCH); 6.81 (d, J ¼ 9.3, NH);
7.35 (br. s, NH); 7.78 (br. s, NH). 19F-NMR (280 MHz, CDCl3 þ TFA): ꢀ 204.4 (ddd, J ¼ 47.3, 32.2, 3.7,
1 F). ESI-MS: 268.1067 ([M þ H]þ, C10H16FN3NaOþ3 ; calc. 268.1068 (err., 0.5 ppm)).
5.3. Difluoro Derivative 19 from 16c. Boc-(3S)-b2,2,3-hAla(a,a-F2)-hGly-hGly-OMe (16c). Accord-
ing to GP 12a, amino fluoro acid 3aba (1.00 g, 4.18 mmol) was converted to the corresponding tripeptide.
Purification by FC (CH2Cl2/acetone 8 :2) afforded 16c (1.21 g, 73%) as colorless solid. Recrystallization
gave colorless prisms. M.p. 151 – 1528 (CH2Cl2/Et2O). [a]2D8 ¼ þ16.3 (c ¼ 1.0, MeOH). IR: 3345m, 3289m,
3092w, 2975w, 2951w, 1735m, 1693s, 1677m, 1641m, 1533s, 1442m, 1388w, 1367m, 1341m, 1314m, 1269m,
1253m, 1198m, 1172s, 1152m, 1128w, 1105m, 1078m, 1059m, 1044m, 1009m, 997m, 925w, 887w, 870w,
851m, 787w, 754w, 709m, 642m. 1H-NMR (400 MHz, CD3OD): 1.21 (d, J ¼ 7.0, Me); 1.45 (s, 3 Me); 2.37 –
2.50 (m, CH2); 2.56 (t, J ¼ 6.7, CH2); 3.44 – 3.58 (m, 2 NCH2); 3.69 (s, Me); 4.23 – 4.35 (m, NCH).
13C-NMR (100 MHz, CD3OD): 12.6 (Me); 27.3 (3 Me); 33.2 (CH2); 34.7 (CH2); 34.9 (CH2); 35.8 (CH2);
48.3 (t, J ¼ 26.1, NCH); 50.8 (Me); 79.3 (Me3C); 116.0 (t, J ¼ 255.7, CF2); 156.0 (CO); 164.2 (t, J ¼ 28.8,
CFCO); 172.1 (CO); 172.4 (CO). 19F-NMR (376 MHz, CD3OD): ꢀ 117.5 (dd, J ¼ 251.1, 11.0, 1 F);
ꢀ 119.9 (dd, J ¼ 251.1, 15.1, 1 F). Anal. calc. for C16H27F2N3O6 (395.40): C 48.60, H 6.88, N 10.63; found:
C 48.71, H 6.89, N 10.53.
Cyclo[(3S)-b2,2,3-hAla(a,a-F2)-hGly-hGly] (19). According to GP 3, 16c (800 mg, 1.84 mmol) was
converted to the corresponding carboxylic acid (770 mg, quant.). According to GP 13, the carboxylic
acid (700 mg, 1.84 mmol) was converted to the pentafluorophenyl ester (880 mg, 87%), subsequent
removal of the Boc group (660 mg, 1.21 mmol) afforded the TFA salt, which was converted (by treatment
with EtNiPr2 in MeCN) to 19 (194 mg, 61%). Colorless solid. M.p. 297 – 2998 (dec., MeCN). IR: 3307m,
3264m, 3098w, 2986w, 2945w, 1685w, 1664s, 1640s, 1556s, 1538s, 1446m, 1435m, 1372m, 1346w, 1317w,
1278m, 1249m, 1200s, 1185s, 1161m, 1146s, 1118s, 1092m, 1070s, 1023m, 1011m, 977m, 976w, 919w, 890w,
861w, 834w, 778w, 704m, 678s, 654s. 1H-NMR (300 MHz, CDCl3 þ TFA): 1.36 (d, J ¼ 6.9, Me); 2.56 (ddd,
J ¼ 14.2, 5.2, 2.0, 1 H, CH2); 2.62 – 2.87 (m, 2 CHH); 3.03 (ddd, J ¼ 15.3, 7.2, 5.4, 1 H, CH2); 3.40 – 3.57
(m, 2 NCHH); 3.68 – 3.77 (m, 1 H, NCH2); 3.90 – 4.02 (m, 1 H, NCH2); 4.76 – 4.96 (m, NCH); 6.90 (d,
J ¼ 10.2, NH); 7.53 (br. s, NH); 7.82 (br. s, NH). 19F-NMR (280 MHz, CDCl3 þ TFA): ꢀ 109.9 (d, J ¼
256.2, 1 F); ꢀ 126.5 (d, J ¼ 256.2, 21.9, 1 F). ESI-MS: 264.1153 ([M þ H]þ, C10H16F2N3O3þ ; calc.
264.1154 (err. 0.6 ppm)).
6. Preparation of the Di- and Tetrafluoro-cyclo-b-tetrapeptides 21 – 23 (Scheme 3, right). 6.1.
(S,S,S,S)-Isomer 21 from Boc-Dipeptide Ester 20a. Boc-(2S,3S)-b2,3-hAla(a-F)-hGly-OMe (20a).
According to GP 12a, amino fluoro acid 1aba (2.21 g, 10.0 mmol) was converted to the corresponding
dipeptide 20a. Purification by FC (hexane/AcOEt 65 :35 ! 6 :4) afforded 20a (2.36 g, 77%). Colorless
solid. Recrystallization gave colorless prisms. M.p. 114 – 1158 (CH2Cl2/Et2O). [a]2D0 ¼ ꢀ33.0 (c ¼ 1.0,
MeOH). IR: 3355m, 2990w, 2942w, 1733s, 1687s, 1657s, 1550m, 1521s, 1446m, 1427w, 1396w, 1367w,
1333m, 1299w, 1268m, 1250s, 1203m, 1182s, 1162s, 1110m, 1084m, 1061s, 993m, 964w, 934w, 880m, 847w,

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Helvetica Chimica Acta – Vol. 94 (2011)
807w, 779m, 748m, 703m, 639m, 611m. 1H-NMR (400 MHz, CDCl3): 1.11 (d, J ¼ 7.0, Me); 1.45 (s, 3 Me);
2.58 (t, J ¼ 6.1, CH2); 3.54 – 3.65 (m, NCH2); 3.71 (s, Me); 4.20 – 4.40 (m, NCH); 4.76 (br. s, NH); 4.97
(dd, J ¼ 50.1, 2.2, FCH); 6.88 (br. s, NH). 13C-NMR (100 MHz, CDCl3): 14.2 (d, J ¼ 5.3, Me); 28.3
(3 Me); 33.7 (CH2); 34.4 (CH2); 47.7 (d, J ¼ 20.4, NCH); 51.9 (Me); 79.8 (Me3C); 93.0 (d, J ¼ 190.6,
FCH); 154.8 (CO); 167.4 (d, J ¼ 19.1, CFCO); 172.6 (CO). 19F-NMR (376 MHz, CDCl3): ꢀ 196.2 (dd,
J ¼ 46.5, 18.0, 1 F). Anal. calc. for C13H23FN2O5 (306.33): C 50.97, H 7.57, N 9.14; found: C 50.94, H 7.51,
N 9.06.
Cyclo[(2S,3S)-b2,3-hAla(a-F)-hGly]2 (21). According to GP 3, 20a (1.60 g, 5.22 mmol) was
converted to the corresponding carboxylic acid (1.46 g, 95%). According to GP 13, the carboxylic acid
(1.00 g, 3.42 mmol) was converted to the pentafluorophenyl ester (1.43 mg, 91% yield), subsequent
removal of the Boc group (458 mg, 1.00 mmol) afforded the TFA salt, which was converted (by treatment
with iPr2NEt in MeCN) to 21 (203 mg, 58%). Colorless solid. M.p. 2498 (dec., MeCN). IR: 3299m, 3093w,
3059w, 2971w, 2961w, 2941w, 1658s, 1561m, 1541s, 1473w, 1449w, 1425m, 1394w, 1380m, 1353w, 1336m,
1276w, 1242w, 1223m, 1203m, 1159m, 1121m, 1083m, 1036m, 999w, 963w, 941w, 923w, 887w, 811w, 769w,
635m. 1H-NMR (300 MHz, CDCl3 þ TFA): 1.44 (d, J ¼ 7.3, Me); 2.30 (ddd, J ¼ 16.7, 12.0, 3.4, 1 H, CH2);
2.63 (dt, J ¼ 17.0, 3.1, 1 H, CH2); 3.42 – 3.53 (m, 1 H, NCH2); 3.81 – 3.90 (m, 1 H, NCH2); 4.57 – 4.74 (m,
NCH); 4.89 (dd, J ¼ 46.8, 2.1, FCH); 7.00 (dd, J ¼ 9.5, 0.3, NH); 7.75 (br. s, NH). 19F-NMR (376 MHz,
CDCl3 þ TFA): ꢀ 182.7 (ddd, J ¼ 46.5, 19.5, 3.9, 1 F). ESI-MS: 349.1687 ([M þ H]þ, C14H23F2N4O4þ ;
calc. 349.1682 (err. ꢀ 1.3 ppm)).
6.2. (R,S,R,S)-Isomer 22 from 20b. Boc-(2R,3S)-b2,3-hAla(a-F)-hGly-OMe (20b). According to
GP 12a, amino fluoro acid 2aba (1.00 g, 4.52 mmol) was converted to 20b. Purification by FC (hexane/
AcOEt 65 :35 ! 6 :4) afforded 20b (1.08 g, 78%). Colorless solid. Recrystallization gave colorless
needles. M.p. 82 – 838 (CH2Cl2/Et2O). [a]2D8 ¼ þ13.9 (c ¼ 1.0, MeOH). IR: 3336m, 3330m, 2981w, 2940w,
1739s, 1682s, 1658s, 1548m, 1522s, 1440m, 1390w, 1367m, 1351m, 1320m, 1267m, 1250s, 1195s, 1164s,
1121w, 1096m, 1073w, 1055m, 1033m, 987m, 942w, 887m, 846m, 814m, 781w, 758w, 742w. 1H-NMR
(400 MHz, CDCl3): 1.18 (d, J ¼ 6.7, Me); 1.44 (s, 3 Me); 2.58 (td, J ¼ 6.1, 1.7, CH2); 3.59 (q, J ¼ 6.0,
NCH2); 3.72 (s, Me); 4.20 – 4.33 (m, NCH); 4.83 (dd, J ¼ 47.1, 1.8, FCH); 5.17 (d, J ¼ 8.6, NH); 6.90 (br.
s, NH). 13C-NMR (100 MHz, CDCl3): 16.0 (Me); 28.3 (3 Me); 33.7 (CH2); 34.4 (CH2); 47.0 (d, J ¼ 22.3,
NCH); 51.9 (Me); 79.6 (Me3C); 91.6 (d, J ¼ 190.8, FCH); 154.9 (CO); 168.4 (d, J ¼ 19.9, CFCO); 172.6
(CO). 19F-NMR (376 MHz, CDCl3): ꢀ 196.1 (dd, J ¼ 47.1, 18.0, 1 F). Anal. calc. for C13H23FN2O5
(306.33): C 50.97, H 7.57, N 9.14; found: C 50.89, H 7.49, N 9.05.
Cyclo[(2S,3S)-b2,3-hAla(a-F)-hGly]2 (22). According to GP 3, 20b (1.70 g, 5.55 mmol) was
converted to the corresponding carboxylic acid (1.57 g, 97%). According to GP 13, the carboxylic acid
(1.07 g, 3.66 mmol) was converted to the pentafluorophenyl ester (1.45 g, 86% yield), subsequent
removal of the Boc group (458 mg, 1.00 mmol) afforded the TFA salt, which was cyclized by treatment
i
with Pr2NEt in MeCN to give 22 (206 mg, 59%). Colorless solid. M.p. 295 – 2968 (dec., MeCN). IR:
3394w, 3325m, 3298m, 3049w, 2974w, 2950w, 2879w, 1654s, 1567m, 1539s, 1444m, 1416m, 1382w, 1349w,
1316m, 1299w, 1283w, 1254m, 1211m, 1166w, 1112m, 1091m, 1079m, 1052w, 1037w, 1024w, 986m, 964m,
1
924w, 868w, 831w, 771w, 750w, 690m, 667m, 632m, 611m. H-NMR (300 MHz, CDCl3 þ TFA): 1.31 (d,
J ¼ 6.9, Me); 2.46 (ddd, J ¼ 16.2, 8.4, 3.2, 1 H, CH2); 2.77 (ddd, J ¼ 16.2, 8.2, 3.1, 1 H, CH2); 3.24 – 3.37
(m, 1 H, NCH2); 3.60 – 3.74 (m, 1 H, NCH2); 4.13 – 4.32 (m, NCH); 5.05 (dd, J ¼ 46.7, 1.9, FCH); 7.91 (br.
s, NH); 8.12 (d, J ¼ 8.0, NH). 19F-NMR (280 MHz, CDCl3 þ TFA): 40.1 (dd, J ¼ 46.6, 31.8, 1 F). ESI-
MS: 349.1672 ([M þ H]þ, C14H23F2N4Oþ4 ; calc. 349.1682 (err. ꢀ 1.3 ppm)).
6.3. Tetrafluoro-cyclo-b-tetrapeptide 23 from 20c. Boc-(3S)-b2,2,3-hAla(a,a-F2)-hGly-OMe (20c).
According to GP 12a, amino fluoro acid 3aba (1.00 g, 4.18 mmol) was converted to 20c. Purification by
FC (CH2Cl2/acetone 8 :2) afforded 20c (952 mg, 71%). Colorless solid. Recrystallization gave colorless
prisms. M.p. 115 – 1168 (CH2Cl2/Et2O). [a]2D8 ¼ þ8.7 (c ¼ 1.0, MeOH). IR: 3354m, 2987m, 2955w, 1732m,
1688s, 1671m, 1550m, 1523s, 1446m, 1428w, 1397m, 1368m, 1335m, 1314m, 1253m, 1204m, 1180s, 1157s,
1092w, 1076m, 1056m, 1026m, 1010m, 994m, 927w, 892m, 862w, 845m, 782w, 774w, 740w, 710m, 632m.
1H-NMR (400 MHz, CD3OD): 1.21 (d, J ¼ 7.1, Me); 1.45 (s, 3 Me); 2.54 – 2.66 (m, CH2); 3.46 – 3.58 (m,
NCH2); 3.70 (s, Me); 4.30 (ddq, J ¼ 14.6, 11.5, 7.2, FCH). 13C-NMR (100 MHz, CD3OD): 12.6 (Me); 27.3
(3 Me); 32.8 (CH2); 35.1 (CH2); 48.2 (t, J ¼ 25.6, NCH); 50.9 (Me); 79.2 (Me3C); 116.1 (t, J ¼ 255.5,
FCH); 156.0 (CO); 164.2 (t, J ¼ 28.8, CFCO); 172.1 (CO). 19F-NMR (376 MHz, CD3OD): ꢀ 117.5 (dd,

Helvetica Chimica Acta – Vol. 94 (2011)
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J ¼ 251.5, 11.3, 1 F); ꢀ 119.9 (dd, J ¼ 251.5, 15.0, 1 F). ESI-MS: 349.1672 ([M þ H]þ, C14H23F2N4O4þ ;
calc. 349.1682 (err. þ 0.7 ppm)). Anal. calc. for C13H22F2N2O5 (324.32): C 48.14, H 6.84, N 8.64; found: C
48.18, H 6.63, N 8.44.
Cyclo[(3S)-b2,2,3-hAla(a,a-F2)-hGly]2 (23). According to GP 3, 20c (430 mg, 1.33 mmol) was
converted to the corresponding carboxylic acid (406 mg, 97%). According to GP 13, the carboxylic acid
(400 mg, 1.29 mmol) was converted to the pentafluorophenyl ester (570 mg, 93% yield), subsequent
removal of the Boc group (570 mg, 1.20 mmol) afforded the TFA salt, which was cyclized by treatment
with EtNiPr2 in MeCN to give 23 (218 mg, 47%). Colorless solid. M.p. 3008 (dec., MeCN). IR: 3292m,
3077w, 2989w, 2950w, 1679s, 1651s, 1539s, 1451m, 1415w, 1385w, 1351w, 1305w, 1267w, 1242w, 1198m,
1
1169m, 1145s, 1112m, 1074m, 1006m, 914w, 871w, 840w, 801w, 720m, 705m, 689m. H-NMR (300 MHz,
CDCl3 þ TFA): 1.31 (d, J ¼ 6.9, Me); 2.46 (ddd, J ¼ 16.2, 8.4, 3.2, 1 H, CH2); 2.77 (ddd, J ¼ 16.2, 8.2, 3.1,
1 H, CH2); 3.24 – 3.37 (m, 1 H, NCH2); 3.60 – 3.74 (m, 1 H, NCH2); 4.13 – 4.32 (m, NCH); 5.05 (dd, J ¼
46.7, 1.9, FCH); 7.91 (br. s, NH); 8.12 (d, J ¼ 8.0, NH). 19F-NMR (280 MHz, CDCl3 þ TFA): 40.1 (dd,
J ¼ 46.6, 31.8, 1 F). ESI-MS: 385.1499 ([M þ H]þ, C14H21F4N4Oþ4 ; calc. 385.1493 (err., ꢀ 1.3 ppm)).
7. Synthesis of the Hexapeptide Derivatives 24 – 26 by Coupling in Solution (Scheme 4). 7.1. The Boc-
Hexapeptide Benzyl Ester 24bbb, Consisting of (S,S)-Residues 1. Boc-(2S,3S)-b2,3-hAla(a-F)-(2S,3S)-b2,3
-
hLeu(a-F)-OBn (dipeptide derivative from 1cac and 1aba). The benzyl ester 1cbc (198 mg, 0.56 mmol)
was Boc-deprotected according to GP 14a. The resulting TFA salt (1cac · TFA) was dissolved in CH2Cl2
(5 ml) and treated with the acid 1aba (124 mg, 0.56 mmol), NMM (310 ml, 2.8 mmol), HOBt (91 mg,
0.67 mmol), and EDC · HCl (128 mg, 0.67 mmol) according to GP 12b. FC (CH2Cl2/MeOH 100 :1)
yielded the corresponding dipeptide (173 mg, 68%). Colorless solid. M.p. 148 – 1508. [a]rD:t: ¼ ꢀ40.2 (c ¼
1.0, CHCl3). IR (CHCl3): 3436m, 2964m, 2923w, 2872w, 1759m, 1713s, 1528w, 1503s, 1456m, 1369m,
1333w, 1277w, 1164s, 1128w, 1082w, 1062w, 1031w. 1H-NMR (400 MHz, CDCl3): 0.73 (d, J ¼ 6.5, Me); 0.83
(d, J ¼ 6.6, Me); 1.00 (ddd, J ¼ 3.2, 10.2, 13.8, 1 H, CH2CH); 1.12 (d, J ¼ 6.9, Me); 1.45 (s, t-Bu); 1.45 – 1.58
(m, CHH’CH); 4.28 – 4.36 (m, NCH); 4.48 – 4.60 (m, NCH); 4.64 (br. s, BocNH); 4.99 (dd, J ¼ 2.3, 50.0,
CHF); 5.02 (dd, J ¼ 2.9, 49.1, CHF); 5.16 (d, J ¼ 11.9, 1 H, PhCH2); 5.35 (d, J ¼ 11.9, 1 H, PhCH2); 6.36
(br. s, NH); 7.33 – 7.41 (m, 5 arom. H). 13C-NMR (100 MHz, CDCl3): 14.5, 20.7, 23.4 (Me); 24.4 (CH);
28.3 (Me); 36.7 (d, J ¼ 3.3, CH2); 47.7 (d, J ¼ 22.1), 48.3 (d, J ¼ 19.9) (CH); 67.5 (CH2); 79.9 (C); 89.9 (d,
J ¼ 188.8), 93.1 (d, J ¼ 190.7), 128.7, 128.9, 129.0 (CH); 134.7, 154.7, 167.0 (d, J ¼ 24.1), 167.3 (d, J ¼ 19.2)
(C). 19F-NMR (282 MHz, CDCl3): ꢀ 202.8 (dd, J ¼ 28.8, 49.1, CHF); ꢀ 204.6 (dd, J ¼ 26.7, 49.1,
CHF). HR-MALDI-MS: 495.2 (2, [M þ K]þ), 479.2 (61, [M þ Na]þ), 423.2 (8), 403.2 (11), 379.2 (32,
[M þ Na ꢀ Boc]þ), 357.2 (100, [M þ H ꢀ Boc]þ), 339.2 (6). Anal. calc. for C23H34F2N2O5 (456.53): C
60.51, H 7.51, N 6.14; found: C 60.62, H 7.60, N 6.18.
Boc-(2S,3S)-b2,3-hVal(a-F)-(2S,3S)-b2,3-hAla(a-F)-(2S,3S)-b2,3-hLeu(a-F)-OBn (24abb). The dipep-
tide derivative (363 mg, 0.79 mmol) described above was Boc-deprotected according to GP 14a. The
resulting TFA salt was dissolved in CH2Cl2 (4 ml) and treated with the acid 1bba (200 mg, 0.80 mmol),
NMM (440 ml, 4.0 mmol), HOBt (129 mg, 0.95 mmol), and EDC · HCl (183 mg, 0.95 mmol) according to
GP 12b. FC (CH2Cl2/MeOH 100 :5) yielded 24abb (407 mg, 84%). Colorless solid. Rf (CH2Cl2/MeOH
200 :3) 0.36. M.p. 189 – 1908. [a]rD:t: ¼ ꢀ49.9 (c ¼ 1.0, CHCl3). IR (CHCl3): 3432s, 3008w, 2966s, 2874w,
1760m, 1694s, 1522s, 1497s, 1456w, 1392w, 1368m, 1292w, 1161s, 1127m, 1082w, 1030w, 995w, 968w, 903w,
868w, 826w. 1H-NMR (500 MHz, CDCl3): 0.73 (d, J ¼ 6.5, Me); 0.84 (d, J ¼ 6.6, Me); 0.93 (dd, J ¼ 0.9, 6.8,
Me); 1.00 (d, J ¼ 6.7, Me); 0.99 – 1.06 (m, 1 H, CH2); 1.19 (d, J ¼ 7.0, Me); 1.44 (s, t-Bu); 1.44 – 1.57 (m,
1 H, CH2CH); 1.98 – 2.01 (m, Me2CH); 3.94 – 4.03 (m, BocNHCH); 4.51 – 4.71 (m, 2 NCH, BocNH); 4.94
(dd, J ¼ 4.1, 48.9, CHF); 4.99 (dd, J ¼ 2.5, 49.8, CHF); 5.02 (dd, J ¼ 3.0, 49.0, CHF); 5.17 (d, J ¼ 11.9, 1 H,
PhCH2); 5.34 (d, J ¼ 11.9, 1 H, PhCH2); 6.42 (br. d, J ¼ 5.8, NH); 6.47 (br. d, J ¼ 4.9, NH); 7.33 – 7.41 (m, 5
arom. H). 13C-NMR (125 MHz, CDCl3): 14.1, 17.9, 20.2, 20.8 (Me); 23.3 (CH); 24.5, 28.3 (Me); 29.6 (d,
J ¼ 3.2, CH); 36.8 (CH2); 46.0 (d, J ¼ 19.5), 48.5 (d, J ¼ 20.0), 56.9 (d, J ¼ 20.9) (CH); 67.5 (CH2); 79.8
(C); 89.9 (d, J ¼ 188.8), 92.4 (d, J ¼ 191.3), 128.8, 128.9, 129.0 (CH); 134.7, 155.7, 166.9 (d, J ¼ 19.3), 167.0
(d, J ¼ 23.8), 167.3 (d, J ¼ 19.5) (C). 19F-NMR (282 MHz, CDCl3): ꢀ 193.0 (dd, J ¼ 25.6, 49.1, CHF);
ꢀ 202.7 (dd, J ¼ 28.8, 49.1, CHF); ꢀ 204.2 (dd, J ¼ 26.7, 49.1, CHF). HR-MALDI-MS: 626.3 (3, [M þ
K]þ), 610.3 (32, [M þ Na]þ), 554.2 (8), 534.2 (13), 510.3 (8, [M þ Na ꢀ Boc]þ), 488.3 (100, [M þ H ꢀ
Boc]þ), 470.2 (8). Anal. calc. for C29H44F3N3O6 (587.68): C 59.27, H 7.55, N 7.15; found: C 59.17, H
7.34, N 7.16.