Synthesis of Enantiomerically Pure 5-Substituted Piperazine-2-Acetic Acid Esters as Intermediates for Library Production

Article abstract of DOI:10.1021/acs.joc.9b00148

The piperazine heterocycle is housed within a large number of FDA-approved drugs and biological probe compounds. Structurally, however, these compounds are mostly confined to substitutions on the two ring nitrogen atoms, rationalizing the expansion of piperazine chemical diversity through carbon substitutions. On the basis of the concept of systematic chemical diversity, a divergent six-step synthesis was developed in which chiral amino acids were transformed, with high diastereoselectivity, into either cis or trans 5-substituted piperazine-2-acetic acid esters that could be chromatographically rendered diastereomerically homogeneous. Starting from six commercially available amino acids or their respective amino alcohols (both antipodes), we obtained a complete set of 24 protected chiral 2,5-disubstituted piperazines, as single stereoisomers in multigram quantities. These diverse and versatile piperazines can be functionalized on either nitrogen atom, allowing them to be used as starting materials for parallel library synthesis and as intermediates for the targeted production of more complex C-substituted piperazine compounds.

Full text of DOI:10.1021/acs.joc.9b00148

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Article
Synthesis of Enantiomerically Pure 5-Substituted-Piperazine-2-
Acetic Acid Esters as Intermediates for Library Production
Prashi Jain, Idris O Raji, Srinivas Chamakuri, Kevin R MacKenzie,
Brandon T Ebright, Conrad Santini, and Damian W Young
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b00148 • Publication Date (Web): 08 Mar 2019
Downloaded from http://pubs.acs.org on March 10, 2019
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Synthesis of Enantiomerically Pure 5-Substituted-Piperazine-2-Acetic
Acid Esters as Intermediates for Library Production
#‡
#‡
#‡
#§‡
†
Prashi Jain , Idris O. Raji , Srinivas Chamakuri , Kevin R. MacKenzie , Brandon T. Ebright , Conrad
Santini#§* and Damian W. Young#§‡*
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#Center for Drug Discovery, Baylor College of Medicine, One Baylor Plaza, Houston, Texas 77030 USA
§Department of Pathology and Immunology, Baylor College of Medicine, One Baylor Plaza, Houston,
Texas 77030 USA
‡Department of Pharmacology and Chemical Biology, Baylor College of Medicine, One Baylor Plaza,
Houston, Texas 77030 USA
†Department of Chemistry, Rice University, 6100 Main Street, Houston, Texas 77005 USA
Abstract
The piperazine heterocycle is housed within a large number of FDA-approved drugs and
biological probe compounds. Structurally however, these compounds are mostly confined to
substitutions on the two ring nitrogen atoms rationalizing the expansion of piperazine chemical
diversity through carbon substitutions. Based on the concept of systematic chemical diversity, a
divergent six-step synthesis was developed in which chiral amino acids were transformed, with
high diastereoselectivity, into either cis or trans 5-substituted-piperazine-2-acetic acid esters that
could be chromatographically rendered diastereomerically homogenous. Starting from 6
commercially available amino acids or their respective amino alcohols, (both antipodes), a
complete set of 24 protected chiral 2,5-disubstituted piperazines was obtained, as single
stereoisomers in multi-gram quantities. These diverse and versatile piperazines can be
functionalized on either nitrogen atom, allowing them to be used as starting materials for parallel
library synthesis and as intermediates for the targeted production of more complex C-substituted
piperazine compounds.
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Introduction
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One of the most critically important steps in early stage small-molecule drug discovery is the
assembly of compounds for screening against disease-implicated biological targets. Compound
collections (“libraries”) are assembled for the purpose of presenting, to the target, a variety of
substances with different chemical structures in the expectation that some of them will act as
ligands to engage the target in a specific manner. Once such a substance is identified, rational
manipulation of its structure can eventually produce derivatives that are capable of eliciting a
beneficial disease-ameliorating clinical effect. Therefore, the composition of a screening collection
used in the earliest part of the drug discovery process is critical for downstream clinical success
and thus constitutes a major intellectual challenge.
For most biological targets, little to no information is initially available to guide a rational approach.
Under this scenario, the prevailing notion is that the screening of compound libraries constructed
based on chemical diversity will provide the most probable access to ligands that can serve as
suitable starting points for lead generation.1,2
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Chemical diversity takes many forms, among them fsp , defined as the fraction of carbon atoms
in a compound that are sp hybridized. Structural elements bearing sp hybridized carbons impart
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3-dimensional (3D) character to a molecule. As compared with 2D structural elements in
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compound libraries that are primarily composed of sp hybridized carbons, the incorporation of
3D character in library design introduces both structural complexity and expanded opportunities
for diversity. A single stereocenter introduces the presence of enantiomers that require either
resolution or enantioselective synthesis, along with structure proof, if the racemic version of the
compound emerges as a substance of interest. If more than one stereocenter is incorporated then
diastereomers are obtained which must be separated and individually characterized, often a non-
trivial endeavor. Each diastereomer, in turn, has two antipodes, creating another layer of
complexity in the library design. Having multiple stereocenters offers the opportunity to create
cohorts of stereoisomers where the same substituent groups are posed in different orientations
in both relative and absolute space. The different substituent orientations can be incrementally
varied in a controlled, systematic manner either by structural alteration or by conformational
biasing. If such a cohort, upon presentation to a biochemical target, affords a specific ligand then
the internally constructed stereochemical matrix will immediately reveal which structural
characteristics are important for engagement with the target. We denote this kind of library design
as systematic chemical diversity (SCD).
Our goal is to demonstrate the value of SCD through the design and production of libraries
composed of complete matrices of different regioisomers and all the stereoisomers of diversifiable
compounds bearing multiple stereocenters. The architecture of such a matrix is schematically
represented in Figure 1. A core structural motif with multiple saturated carbons can bear
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substituents in several relative regioisomeric orientations. From optically pure starting materials,
each regioisomer can be synthetically produced as a pair of optically active diastereomers if the
stereochemistry of the starting material is preserved and the formation of the second stereocenter
can be rendered non-selective (shown as pairs of compounds on the horizontal axis).
Alternatively, if the formation of the second stereocenter is so selective as to afford insufficient
quantities of one diastereomer, an alternative route can be devised to reverse the stereochemical
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outcome such that practical quantities of both diastereomers can be obtained. In either case, the
separation of diastereomers affords its constituent individual absolute stereoisomers. For any
given pair of substituents, represented by the red and blue spheres, there will be twelve possible
stereoisomers.
Figure 1. Systematic Chemical Diversity
The piperazine structural motif was chosen to demonstrate the above described design strategy.
Piperazine may be considered as a “privileged” heterocyclic moiety given that it is ubiquitous in
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both FDA approved clinical agents and numerous other biologically active materials.4,5 However,
within this large cohort of compounds, the structural diversity of piperazines is predominantly
limited to N and N substitutions, eliminating access to a wide chemical space of exploration
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provided by C substitutions. Using SCD as a guiding concept, we envisioned the generation of
the piperazine based “scaffold family” shown in Figure 2. The regioisomeric family branches of
the depicted piperazine-2-acetic acids are denoted by their substitution patterns as 2,3-, 2,5- and
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,6-. The syntheses of the 2,3- and 2,6- family branches have been previously disclosed.6,7 This
report describes the preparation of the remaining 2,5- family branch.
Figure 2. Familial relationship of piperazine scaffolds
Discussion
The scaffold family depicted in Figure 2 was designed to generate 3D diversity for library
production in a combinatorial fashion. Within this family, each atom of the piperazine scaffold can
be functionalized to achieve maximum coverage of chemical space. The ring carbons contribute
both side chain diversity and absolute stereochemical orientation of their covalently bonded
substituents. The inclusion of substituents at two carbon atoms introduces diastereodiversity. The
ring nitrogen atoms serve three purposes; they afford sites for further introduction of substituents,
they modulate the basicity of the fragment and they influence its conformational orientation by
affording alternative states of nitrogen hybridization when diversified with covalently bonded
groups. The reported methodology in this work enables the production of multigram quantities of
the scaffolds under ordinary laboratory conditions, such quantities being essential for compound
library production by parallel synthesis.
Reports of the specific preparation of 2,5-disubstituted piperazines are relatively uncommon. The
synthesis is traditionally carried out from an unsymmetrically substituted 1,2-diamine to which a
bridging moiety of 3 carbons or more is attached. The preparation of 2,5-disubstituted oxo-
piperazines which can serve as precursors for piperazines is the most commonly reported
synthetic strategy.8-14 In all the referenced studies, a fully saturated 2,5-disubstituted piperazine
can only be obtained by subsequent reduction of the corresponding oxo-piperazine. Several
methods have been reported that directly afford limited sets of chiral 2,5-disubstituted
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piperazines. A small set of cis chiral 2,5-disubstituted piperazines using tosyl aziridine chemistry
was described, but with the nitrogen atoms were substituted with methyl and toluenesulfonyl
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groups.16,17 As such, the products are not orthogonally accessible to substitution limiting their
potential for library production. Furthermore, no trans 2,5-disubstituted products are reported.
Another report describes the preparation of a small set of orthogonally protected trans 2,5-
disubstituted piperazines in fair yield on solid support. None of the above studies provide a route
to all the stereoisomers of a given 2,5-disubstituted piperazine.
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Our goal was to prepare all the stereoisomers of the 2,5-disubstituted piperazine branch on
multigram scale, having both nitrogen atoms readily available for diversification. We initially
focused on the precedent set in our own previous work on the preparation of the 2,6-disubstituted
piperazine family branch (see reference 6). The strategy is shown in Scheme 1 using (S)-
phenylalanine 1 as an exemplar. It was envisioned that the orthogonally protected vicinal diamine
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would be regioselectively alkylated on the 2-nosyl protected nitrogen with ethyl 4-
bromocrotonate. Following Boc deprotection, an intramolecular aza-Michael addition would ensue
to furnish the desired piperazine products. If the key intramolecular aza-Micheal reaction was
relatively non-diastereoselctive, then both diastereomers could be obtained from a single reaction
and chromatographically separated to yield the individual stereoisomers. Similar to our previous
routes, the key intermediate 5 could be prepared in several steps from an amino acid by an imide
reduction approach. Accordingly, treatment of (S)-Phe 1 with 2-NO -benzenesulfonyl chloride
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under basic aqueous conditions afforded 219,20 which was exposed to HOBt•NH under amide
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forming conditionsto afford the crude amide 3 (40% by LCMS). Unfortunately, no conditions were
found that would append the Boc group onto the amide nitrogen. Similarly, we were unable to
identify conditions to prepare 4 directly from 2 by reaction with Boc-amine. The inability to reach
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via imide 4 prompted a reformulation of the synthetic strategy away from one centered on amino
acid derived imide reduction to a strategy using amino alcohols as starting materials for the
synthesis.
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Scheme 1. First approach to 5-substituted 2-piperazine acetic acid esters
Starting from (S)-phenylglycinol 6, the second approach to the orthogonally protected diamine 10
is depicted in Scheme 2. (S)-Phenylglycinol was converted to its N-nosylated derivative 7 by
exposure to NsCl in the presence of TEA.21 The N-nosylated amino alcohol 7 was activated by
reaction with MsCl/TEA to afford the corresponding mesylate 8. Rather than employing the
traditional 2-step protocol of azide displacement followed by Staudinger reduction22,23, which
would entail the removal of large amounts of the byproduct TPP-oxide when the reaction was
scaled up, we explored the feasibility of direct mesylate displacement with ammonia. Exposure of
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to a solution of anhydrous ammonia in MeOH (7M) gave, by LCMS, the monoprotected diamine
as a major product. Instead of isolating 9, the reaction was evaporated to a residue and treated
with Boc O/DCM to give the orthogonally protected diamine 10. The yield of 10 from the N-
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nosylated alcohol 7 was modest (35%). The bis-protected diamine 10 was advanced through the
synthesis along the lines previously established in our earlier 2,6-disubstituted piperazine studies.
Regioselective alkylation with ethyl 4-bromocrotonate occurred exclusively at the N-nosyl position
to afford the cyclization precursor 11. Treatment of 11 with TFA unmasked the Boc protected
nitrogen which effected an intramolecular aza-Michael addition to afford a 5:1 mixture of 12 cis
and 13 trans in 61% combined yield.
Scheme 2. Second approach to 5-substituted 2-piperazine acetic acid esters
The overall yield of this approach was deemed unsatisfactory and in searching for a better
alternative, the ammonia displacement reaction offered a clue. Closer examination of ammonia
displacement reaction by LCMS revealed the formation of the N-nosyl aziridine 14 as a major
constituent of the product mixture. The aziridine presumably arises from deprotonation of the N-
nosyl group by ammonia followed by intramolecular mesylate displacement.24-36 Alkyl substituted
N-nosyl aziridines are known to react with nucleophiles to give regioselective ring opened
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products.37,38 Therefore, we rationalized that aziridine generation followed by introduction of the
second nitrogen in protected form would directly lead to orthogonally protected diamines
analogous to 10. Fortunately, a prior example of such a transformation39 had been reported,
encouraging us to investigate this strategy (Scheme 3).
Amino alcohols 15a-h were converted to their N-nosylated derivatives 16a-h by exposure to
NsCl/TEA in DCM at rt.40-44 The crude products were treated with MsCl/TEA in DCM to give the
mesylated intermediates 17a-h by LCMS. Mesylates 17a-h were isolated in crude form by
extractive workup and immediately advanced without further characterization. Treatment with
Cs CO in DCM effectively generated the N-nosyl aziridines 18a-h by LCMS, which were isolated
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as crude solutions given the instability of the N-nosyl aziridine in neat form.45
Introduction of the second nitrogen in Boc protected form was initially pursued (Scheme 3, Boc
pathway). Treatment of aziridines 18a-h with the preformed potassium salt of BocNH in THF at
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78°C directly afforded the regiospecifically ring opened, orthogonally protected diamines 19a-h.
KHMDS/THF was markedly superior to a range of other bases (NaH, KOt-Bu, K CO , NaOTMS)
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and solvents (DMF, DMSO, ACN, dioxane) in this reaction. The use of solid KHMDS, as opposed
to commercially available solutions of this reagent, was critical in rendering the execution of this
transformation manageable on multi-decagram (50g) scale. Using solid KHMDS reduced the
volume of the reactions such that they could be carried out in a conventional laboratory fume
hood.
Orthogonally protected diamines 19a-h were regioselectively alkylated with ethyl 4-
bromocrotonate to obtain the cyclization precursors 20a-h. Nitrogen unmasking with TFA/DCM
followed by bicarbonate neutralization gave a mixture of the piperazines 23a-h cis/24a-h trans
(dr cis/trans = 3.3-23:1). The optically active diastereomers were separated by normal phase
column chromatography to provide the individual scaffolds 23a-h cis and 24a-h trans. While the
success of the synthetic sequence was gratifying, the substantial stereoselectivities rendered in
the cyclization posed a challenge for us. Given that our objective was to obtain multigram
quantities of both diastereomers for library production, we required a means toward generating
the minor trans diastereomer in sufficient quantities. In the previously reported 2,6-piperazine
work the dr upon aza-Michael cyclization was ~3:1, which furnished enough of the minor
diastereomer to serve as a feedstock in combinatorial library synthesis. Using the above
described route, a dr of ≥5:1 in most cases would not enable us to obtain enough of the minor
trans diastereomer to meet our goals. Attempts to alter the cis/trans ratio to afford more trans
isomer using stronger bases (K CO , NaOt-Bu, KHMDS) or prolonged (40 °C, 1 week) exposure
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of the products to Cs CO , which might promote a retro-Michael mediated equilibration, did not
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Scheme 3. General synthetic scheme for 5-alkyl-substituted piperazine-2-acetic acid esters.
We resorted to an alternative mode of aza-Micheal reactivity for piperazine formation in hopes of
providing more of the trans isomer. We surmised that having a substituted nucleophilic nitrogen
would increase the steric demand of the aza-Michael donor, which might alter the
diastereoselectivity of the cyclization. Instead of unmasking the nucleophilic nitrogen under acidic
conditions and effecting the intramolecular aza-Michael addition of the primary amino group to
the acrylate, a base promoted anionic cyclization of a carbamate was pursued (Scheme 4).
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Initially, the base promoted cyclization of the N-Boc precursor 20e was attempted. Treatment of
20e with strong base (NaH, KOt-Bu) afforded only migration of the 2-Ns aryl group from N to C
(see below for a discussion of this rearrangement). Using weaker bases (Cs CO ) led only to
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3
recovered starting material. Reasoning that the N-Boc proton was not acidic enough to be
preferentially removed, the trifluoroacetamide (TFA) N-protecting group was employed, which
would both lower the pK of its attached N atom and could be selectively removed by reduction
a
with NaBH446 in the presence of the ethyl ester.
The base promoted strategy is shown in detail using the (R)-Ala case. Aziridine 18e was used to
alkylate the potassium salt of CF C(O)NH , affording the differentially substituted diamine 21e
3
2
(Scheme 3). Literature precedent indicated that alkylation of 21e should occur on the nosyl
substituted nitrogen instead of the TFA substituted nitrogen given the difference in pK between
a
the two acidic protons (Bordwell pK in DMSOfor CF C(O)NH = 17.2, PhSO NH = 16.1; see also
a
3
2
2
2
references).47,48 Alkylation of 21e with ethyl 4-bromocrotonate proceeded much more cleanly in
ACN than in DMF, although the reaction was noticeably slower (48h instead of 16h). The reaction
afforded a mixture of two products. A sample of the mixture was separated by pTLC and each
1
product was individually characterized by H NMR and LCMS. These were determined to be the
olefin isomers 22e {2,3} and 22e {3,4} in an approximately 1.5:1 ratio. Each alkylation product
was individually advanced through the base promoted cyclization reaction. A limited survey of
bases and solvents revealed that Cs CO was the best choice, but more importantly the reaction
2
3
had to be carried out in a non-polar solvent, either DCM or DCE, in order to minimize the formation
of the Smiles rearrangement product 27 (see the SI for the mechanism of this reaction).49-55 Higher
polarity solvents such as DMF or ACN gave no cyclization product, instead exclusively yielding
27. Even in DCM, yields of piperazines 25e cis/26e trans never exceeded 40%. The
rearrangement product 27, samples of which were isolated for characterization but which was not
isolated for determination of yield, was easily separated from the desired piperazines 25e cis/26e
trans by normal phase chromatography. Removal of the TFA group was accomplished by
reduction with NaBH in absolute EtOH, affording the individual piperazine scaffolds 23e cis and
4
1
24e trans after chromatography. Gratifyingly, structural analysis by 1D H NMR and X-ray
confirmed that the major product of the anionic cyclization was indeed the desired trans isomer.
Once the viability of the above sequence was established the purified 2-nosyl-TFA disubstituted
amines 21a-h were converted to the target piperazine scaffolds 24a-h trans directly from the
mixture of acrylates 22a-h without purification of the intermediates. In general, the amounts of
23a-h cis piperazine scaffolds obtained from the TFA pathway were sufficiently small that their
isolation was not carried out.
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1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
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4
4
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4
4
4
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5
5
5
5
5
5
5
6
Although the quantities of 24e trans produced by the above-described synthesis were sufficient
to meet the immediate needs for library production purposes, there was room for improvement in
the overall yield of the sequence. Recognizing that the most problematic part of the sequence
depicted in Scheme 4 was the propensity for Ns migration from N to C with loss of SO during the
2
base promoted cyclization, several other electron deficient arylsulfonyl amine protecting groups
were employed. To that end the synthesis was modified by using either 4-CF , 2,4-diF and 4-
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
nosyl in place of 2-Ns on small scale in the case of the alanine derived diamine 21e, with all other
steps being carried out as depicted in Scheme 4. In the case of 4-CF -phenylsulfonyl, olefin
3
isomerization took place during the alkylation using ethyl 4-bromocrotonate (3:1 {2,3}:{3,4}), while
the cyclization using Cs CO /DCM provided improved cyclization yield to 25%. The problematic
2
3
56
step was the removal of 4-CF phenylsulfonyl group using Mg/MeOH on our parallel chemistry
3
platform for further N -diversification. In the case of 2,4-diF-phenylsulfonyl no olefin isomerization
4
was observed during the alkylation using ethyl 4-bromocrotonate. However, the Cs CO /ACN
2
3
cyclization was very slow and afforded only minor quantities of cyclized product along with
1
isomerized {3,4} olefin but no observable {2,3} olefin (by H NMR). In the case of 4-nosyl
substitution, the amount of olefin isomerization was reduced to nearly zero (by crude NMR) and
the propensity of the 4-nosyl group to migrate from N to C during the cyclization was not observed
albeit on a smaller scale than performed for the 2-Ns routes. The overall yield of cyclized products
from 21e was improved from 7% to 30% mostly due to the improved outcome of the cyclization
reaction. The reduction in solubility of earlier intermediates in 4-Ns case led to handling problems
on larger scales (50g alaninol). The synthetic sequence was best carried out without purification
of any intermediates between 21e and 24e. At this point, it was decided to move forward with 2-
Ns protecting group because of the demonstrated ease of handling at larger scale as well the
need to immediately obtain sufficient quantities of trans scaffolds to advance our library
generation efforts. The investigation of other sulfonamides that suppress isomerization and
rearrangement that can be scaled is warranted.
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0
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scheme 4. TFA pathway detail
Remarkably, the dr of cyclization was reversed for the TFA pathway, now affording in the case of
(R)-Ala, 24e trans with high selectivity (dr cis/trans = <1:20). Each major product from the
individual pathways using (S)-Ala was crystallized; the N unprotected 23a cis crystallized
1
whereas 24a trans had to be converted back to its TFA precursor 26a trans to obtain crystals for
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5
5
5
5
5
5
5
5
6
X-ray crystallography (Figure 3). The X-ray structures confirmed that the major product from the
Boc pathway was cis while the major product from the TFA pathway was trans.
0
1
2
3
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7
8
9
0
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8
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2
3
4
5
6
7
8
9
0
23a cis (from Boc pathway)
26a trans (from TFA pathway)
Figure 3. X-ray structures of the (S)-Ala derived major products from Boc pathway and TFA pathway.
Consideration of the possible transition states for the two cyclizations provides a rationale for the
different outcomes (Figure 4). Due to resonance stabilization, the N-S bond of the sulfonamide
has considerable double bond character introducing A(1,3) strain with groups on the adjacent
carbon atoms. In a chair-like transition state there is an unfavorable steric interaction between
groups in the pseudoequitorial position of C influencing the larger group to be disposed in a
5
pseudoaxial position in all cases. Thus, we postulate that TS1 is the favored arrangement for the
Boc pathway cyclization whereby the acrylate is equatorial leading to the observed cis isomer as
the major product. For the TFA pathway the C substituent remains pseudoaxial. However, in TS4
5
the pseudo-equatorial orientation for the acrylate leads to a steric clash between it and the
trifluoroacetyl substituent on the incipient N nitrogen. This causes the acrylate to rotate into the
1
pseudo-axial position (TS3) and leads to the observed trans isomer as the major product.
Therefore, through controlling the identity of the nucleophile we were able to achieve
complementary diastereoselective aza-Michael reactions.
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6
7
8
9
0
Figure 4. Proposed alternative transition states for cyclization of (S) configured diamine acrylates 20a-d, 22a-d, 11a,
42a and 44a.
An exception to the synthesis depicted in Scheme 3 was the case of phenylglycine (Scheme 5).
The route shown in Scheme 3 was not employed in this case due to concerns about possible non-
selectivity in the aziridine ring opening.57 The requisite monoprotected diamine precursors 28a
and 28b were obtained commercially and nosylated as described above to give orthogonally
protected diamines 10a/b suitable for use in the Boc pathway. Compound 29a, suitable for the
TFA pathway, was obtained from 10a by Boc removal and reaction of the unmasked nitrogen with
TFAA/TEA. All three intermediates were alkylated with ethyl 4-bromocrotonate as described
above to give 11a,b and the isomeric mixture 30a. Treatment of 11a,b with TFA followed by
neutralization afforded the piperazines 12a cis, 31b cis, 13a trans and 32b trans. By contrast,
treatment of 30a with Cs CO in DCM gave no evidence of the analogous mixture of piperazines
2
3
obtained from the TFA route employed in Scheme 4. Instead two non-cyclized products were
obtained. Proton NMR of the two products showed the phenyl moiety in one and the nosyl moiety
in the other, suggesting that 30a had undergone an unexpected bond scission. This result was
not studied any further.
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0
1
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3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scheme 5. Synthesis of phenylglycine derived scaffolds.
Another exception to the synthesis depicted in Scheme 3 is the case of ornithine (Scheme 6). N-
nosylated amino alcohols 34a/b were prepared from commercially available (S) and (R) N -Cbz
5
Orn 33a/b by reduction with NaBH /I followed by nosylation of the derived amino alcohols (not
4
2
shown). 34a/b were converted to their corresponding mesylates 35a,b as described in Scheme
3. Exposure of mesylate 35a to Cs CO led, by LCMS, to the formation of the aziridine 36a.
2
3
Treatment of aziridine 36a with BocNH /KHMDS afforded, not the desired orthogonally protected
2
acyclic diamine product 37a but instead pyrrolidine 38a. Pyrrolidine 38a would likely arise from
deprotonation of the N-Cbz group by the potassium salt of BocNH followed by cyclization via a
2
58
Baldwin favored “5-exo-tet” aziridine ring-opening mechanism. In order to specifically determine
whether the aziridine 36a is formed as an intermediate in this transformation, and thus provide
further evidence of our structural assignment of 38a, the aziridine forming reaction was carried
out in CD Cl and followed by NMR. The aziridine 36a was observed.
2
2
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1
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5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
Scheme 6. Unsuccessful synthesis of piperazine scaffolds derived from N -Cbz ornithine.
We turned to previously known conditions for the preparation of the ornithine derived piperazine
scaffolds (Scheme 7). Mesylates 35a/b were converted to N-nosyl amino azides 39a,b by reaction
with NaN in DMF. Staudinger reduction afforded the corresponding monoprotected diamines
3
40a,b. These were converted to the orthogonally tris-protected triamines 41a,b which were
alkylated on the nosyl protected nitrogen to give the cyclization precursors 42a,b. Removal of the
Boc group with TFA was followed by neutralization as described above to give the piperazine
scaffolds 45a cis, 45b cis, 46a trans and 46b trans with a dr (10:1 cis/trans), very similar to what
was obtained in Scheme 3.
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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
Scheme 7. Synthesis of piperazine scaffolds derived from N -Cbz ornithine.
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In order to obtain larger quantities of the minor diastereomers of the Boc pathway, 46a,b trans,
the base promoted TFA cyclization pathway was carried out. Triamines 40a,b were treated with
TFAA/TEA to obtain the Cbz-nosyl-TFA tris-protected triamines 43a,b. Alkylation with 4-bromo-
(E)-2-butenylate again proceeded exclusively on the nosyl protected nitrogen to afford the
cyclization precursors 44a,b as mixtures of olefin isomers. Cyclization of 44a,b under basic
conditions afforded the same piperazine products 45a,b and 46a,b with the expected reversal in
dr.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 1 shows the piperazine scaffolds obtained from each amino alcohol and from each synthetic
pathway.
cis/trans derived from (S) amino alcohols
cis/trans derived from (R) amino alcohols
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2
2
2
2
2
2
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5
5
5
6
Amount
produced
Amount
Produced
(g)(trans)
dr
Entry
R
Route
er (%:%)
cis/trans
(g)(cis)
Scheme 3
Boc
Scheme 4
TFA
Scheme 3
Boc
Scheme 4
TFA
Scheme 3
Boc
Scheme 4
TFA
Scheme 3
Boc
Scheme 4
TFA
Scheme 3
Boc
Scheme 4
TFA
Scheme 3
Boc
Scheme 4
TFA
(
2
(
2
S)-CH3
17.4†
4.1†
3.5
2.7†
4.6
2.0
5.5
1.2
4.6
0.3
3.5
0.6
3.4
1
4.2:1
1:15
>99.8:0.2
>99.8:0.2
>99.8:0.2
>99.8:0.2
>99.8:0.2
>99.8:0.2
>99.8:0.2
>99.8:0.2
>99.8:0.2
99.2:0.8
3a cis
S)-CH3
4a trans
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
3
4
5
6
7
8
9
0.23
11.6†
0.3
8.0
--
(R)-CH3
23e cis
4.3:1
1:15
(
2
R)-CH3
4e trans
S)-CH Ph
(
2
4:1
2
3b cis
(
S)-CH Ph
4b trans
2
<1:20
7.5:1
<1:20
23:1
2
(
R)-CH Ph
2
9.0
--
2
3f cis
(
R)-CH Ph
2
24f trans
S)-iBu
3c cis
(
2
7.0
--
(S)-iBu
24c trans
1
0
1
2
<1:20
20:1
(
2
R)-iBu
3g cis
R)-iBu
4g trans
(S)-
1
1
12.0
--
99.6:0.4
(
<1:20
98.8:1.2
2
Scheme 3
Boc
#
13
CH OBz
3.3:1
<1:20
4.7:1
<1:20
4.0
--
1.2
1.0
1.1
1.4
99.6:0.4
99.2:0.8
99.4:0.6
99.7:0.3
2
2
3d cis
S)-
CH OBz
(
Scheme 4
TFA
#
14
15
16
2
2
4d trans
(
R)-
CH OBz
Scheme 3
Boc
#
5.2
--
2
2
3h cis
R)-
CH OBz
(
Scheme 4
TFA
#
2
2
4h trans
(
S)-Ph
12a cis
3a trans
Scheme 5
Boc
>99.8:0.2
>99.8:0.2
1
7
8
4.8:1
5.4:1
3.8
9.7
0.8
1.8
1
(
R)-Ph
31b cis
2b trans
Scheme 5
Boc
>99.8:0.2
>99.8:0.2
1
3
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(
S)-(CH ) -
NHCbz
2
3
Scheme 7
Boc
19
20
21
22
10:1
<1:20
8:1
0.5
--
0.05
0.1
>99.8:0.2
99.3:0.7
99.8:0.2
99.4:0.6
45a cis
(S)-(CH ) -
2
3
Scheme 7
TFA
NHCbz
6a trans
4
(R)-(CH ) -
2
3
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scheme 7
Boc
NHCbz
5b cis
0.4
--
0.05
0.12
4
(
R)-(CH2)3-
Scheme 7
TFA
NHCbz
<1:20
46b trans
Table 1. Scaffolds synthesized by stereochemistry and route. dr was determined either by isolation of the minor
diastereomer or by TLC/crude H analysis. <20:1 = no minor diastereomer detected by TLC/ H NMR and none isolated
by chromatography. the sum of two runs. the stereochemical designation for Ser-O-benzyl ether inverts from (S) to
R) when the amino acid is converted to its corresponding amino alcohol and then to the piperazine. Thus, (S)-Ser-O-
benzyl ether gives (R)-serinol-O-benzyl ether and (R)-23h cis and (R)-24h trans as shown in the structures above and
1
1
†
#
(
vice versa. All other amino acids retain their stereochemical designation.
The use of these piperazine scaffolds, in combination with their related regio- and stereoisomers,
as a demonstration of our SCD strategy depends upon the successful diversification of both of
their nitrogen atoms by parallel synthesis. As described above, the modulation of the chemical
properties and the basicity of the derived diversified fragments can be effected by covalent
attachment of various functional groups. But just as importantly, the judicious choice of
diversification elements offers the possibility of creating conformational diversity by changing the
hybridization state of the nitrogen atoms. Appending a series of functional groups ranging, for
example, from acetyl through methanesulfonyl to methyl changes the bonding geometry on each
nitrogen from trigonal planar through shallow pyramidal to fully tetrahedral. In doing so the library
design enables the compounds to pose the same ring substituent groups in different orientations
by conformational biasing, thus creating an enhanced coverage of chemical space.
Scheme 8 depicts two synthetic sequences that produce final products which illustrate the
2
3
concepts described above using acetyl (for sp hybridization) and methyl (for sp hybridization) as
the diversity elements. Starting with scaffold 23b the alternative pathways, which employ the
same chemistry suitable for parallel synthesis but in different orders, leads to the isomeric
fragments 49 and 52. The N and N atoms are present in alternating hybridization states and the
site of the basic atom is transposed from one region of the molecule to the other. The implications
of these structural modifications will be examined in greater depth in connection with the
production of libraries derived from complete stereochemical matrices of these piperazine
scaffolds.
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Scheme 8. Orthogonal diversification of 23b cis.
Conclusion
In this report we describe the successful completion of a branch of 5-substituted piperazine-2-
acetic acid esters, inspired by the concept of systematic chemical diversity. The piperazines
produced in this study are poised for generating libraries of enantiomerically pure drug-like
piperazine compounds through parallel combinatorial synthesis. The key sequence centered on
the generation of chiral N-nosyl aziridines that could be efficiently converted to orthogonally bis-
protected chiral 1,2-diamines which in short order could be taken forward to the title compounds.
During our previous syntheses of the 2,3 and 2,6 piperazine branches, we capitalized on the
modest stereoselectivity in forming the second stereocenter to generate both cis and trans
piperazines in one pot (see references 6 and 7). Scaling these reactions and efficient
chromatographic separation led to multigram quantities of each diastereomer with ~ 50% fewer
reactions needed to complete these series. In contrast to those studies, our initial 2,5-piperazine
pathway was highly diasteroselective favoring the cis diastereomer. This prompted the
development of a divergent pathway from a common intermediate leading mostly to the trans 2,5-
disubstituted piperazine.
Analogous to natural product total synthesis, systematic chemical diversity is predicated on
achieving a specific predefined outcome. While in total synthesis the natural product serves as
the end goal, SCD requires complete sets of regio- and stereoisomeric products in suitable
quantities for downstream diversification. However, unlike total synthesis the disparate structures
required by SCD may render a single chemical pathway unsuitable toward achieving success.
Indeed, it was our initial plan that all three branches could be synthesized using a common
pathway; however, it became clear early in our studies that a single pathway could not
accommodate the steric and electronic features intrinsic to each scaffold branch. This led to the
development of three unique synthetic plans to complete the scaffold family depicted in Figure 5.
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Figure 5. Summary of the disubstituted piperazine-2-acetic acid ester scaffold family synthesis pathways.
Guided by the concept of systematic chemical diversity, the work described here completes the
third and final branch of the piperazine scaffold family. The deployment of the complete set of
piperazine stereoisomers to generate libraries for fragment-based drug discovery (FBDD) and
DNA-encoded libraries for biological discovery is currently underway and will be reported in due
course.
Experimental
With the exception of Scheme 4, “Ns” refers exclusively to 2-Ns.
General Methods. All starting materials and reagents were purchased from commercial sources
and used without further purification. Solvents were purchased as either anhydrous grade
products in sealed containers or reagent grade and used as received. All reactions were carried
out in dry glassware under a nitrogen atmosphere using standard disposable or gastight syringes,
disposable or stainless steel needles, cannula, and septa. Stirring was achieved with magnetic
stir bars or with an overhead mechanical stirrer. Flash column chromatography was performed
with SiO (230-400 mesh) or by using an automated chromatography instrument with an
2
appropriately sized column. Thin layer chromatography was performed on silica gel 60F254 plates
(E. Merck). Non-UV active compounds were visualized on TLC using one of the following stains:
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6
KMnO , ninhydrin, p-anisaldehyde, PMA, 2,4-DNP or bromocresol green. H and 13C NMR
spectra were recorded on an instrument operating at either 600MHz or 800MHz, and 150MHz or
1
4
2
00MHz respectively. IR spectra were obtained neat on an FT-IR instrument and are expressed
-1
in cm . LCMS data were collected using an HPLC instrument coupled to a low resolution mass
spectrometer with single quadrupole ionization operating in either positive or negative ion mode.
The analytical method utilized a C column (2.1 × 50 mm, 1.8 m) eluting with a linear gradient
18
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of 95%/5% water/CH CN (modified with 0.05% formic acid; T = 0 min flow = 0.35 mL/min) to
3
9
5%/5% CH CN/water (T = 3.5 min flow = 0.5 mL/min) then 95%/5% CH CN/water to T = 5min
3
3
(0.5 mL/min). Peak detection was done at 254 nm and 230 nm for UV active compounds. For
non-UV active compounds total ion count was used. High-resolution mass spectrometry (HRMS)
spectra were obtained on a Thermo Scientific Q Exactive hybrid quadrupole-Orbitrap mass
spectrometer equipped with a HESI source and using lock masses for correction. Samples were
introduced into the HRMS via reversed phase HPLC on an Accucore Vanquish C18+ column (2.1
×
100 mm, 1.5 m) eluting with a linear gradient of 95%/5% water/acetonitrile (modified with 0.1%
formic acid) to 10%/90% water/acetonitrile over 8 min. Chiral HPLC analysis of piperazines was
carried out on an instrument with automated 6-column array (Daicel ChiralPak I-series, IA through
IF, 4.6 × 150 mm, 5 m). Racemates were screened using a heptane (A)/ethanol (B) gradient
(flow = 1 mL/min) as follows: T = 0 min (%A/%B) 95/5, T = 1 min 95/5, T = 11 min 10/90 (linear
gradient), T = 13 min 10/90, T = 13.1min 95/5, T = 15min 95/5. Racemates that gave
unsatisfactory resolution of enantiomers were rescreened using the above gradient with iPrOH
instead of EtOH. Additional editing of the gradient or the use of an isocratic mobile phase to
optimize separation was carried out as needed. Chiral materials were analyzed using optimized
conditions and their traces were compared to the racemic traces for determination of enantiomeric
ratio (er).
To determine our limit of detection (LOD) by chiral HPLC, 5 L aliquots of 2 mg/mL stocks (10 ng
injections) of purified protected scaffolds were subjected to chiral HPLC analysis to resolve and
quantify enantiomers and determine enantiomeric ratios. Scouting solvent conditions across six
4.6 mm x 150 mm chiral columns (ChiralPak IA-IE) identified chromatographic conditions for every
compound that successfully effected baseline resolution of the enantiomers of the racemic
products. Absorbance at 254 nm (nosyl or benzyl) was used to detect and quantify the amount of
scaffold. Serial dilutions of the 2 mg/mL stocks established that our LOD is approximately 0.02
ng/injection, or 0.2% of the total material loaded under these conditions. Therefore the limit of our
detection of enantiomeric ratio is ≥99.8:0.2. All chiral traces of the scaffolds are included in the
Supplementary Information.
Automated preparative reverse-phase HPLC purification was performed using a Mass Directed
Fractionation (MDF) system with UV-DAD detection and a single quadrupole mass spectrometer.
The instrument used a C18 column (21.2 mm i.d. × 150 mm, 5 m, w/19 mm × 10 mm guard
column). The crude product was dissolved in methanol and purified utilizing an elution of water
(modified with 0.05% formic acid) and methanol, with a linear gradient increasing from 5% to
100% methanol over 15 m at a flow rate of 20 mL/min. Purification was carried out by mass trigger
only.
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All NMR chemical shifts are quoted on the  scale and were referenced to residual non-deuterated
solvent as an internal standard. Signal multiplicities are described using the following
abbreviations: s = singlet, d = doublet, t = triplet, b = broad, quar = quartet, quin = quintet, m =
multiplet, v = very; abbreviations are combined, e.g. vbs = very broad singlet. The NMRs of
products isolated as unseparated mixtures of diastereomers are included for reference only.
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Scheme 1
((2-nitrophenyl)sulfonyl)-(S)-phenylalanine (2). (S)-Phenylalanine 1 (1.0 g, 6.05 mmol, 1.0
equiv) was dissolved in a solution of NaOH (363 mg, 9.0 mmol, 1.5 equiv) in water (10 mL) at 0
°
C. Nosyl chloride (2.01 g, 9.05 mmol, 1.5 equiv) was added slowly to the solution, which slowly
becomes homogenous. The resultant mixture was stirred at rt for 3 h and was monitored by TLC
(10% MeOH in DCM) and LCMS. The reaction mixture was extracted with EtOAc to provide the
1
title compound as an off-white solid (mp 92 – 96 °C, 900 mg, 43% yield). H NMR (800 MHz,
CDCl )  7.96 (d, J = 7.7 Hz, 1H), 7.85 (d, J = 7.8 Hz, 1H), 7.70 (t, J = 7.6 Hz, 1H), 7.66 (t, J = 7.8
3
Hz, 1H), 7.28 – 7.19 (m, 3H), 7.15 – 7.14 (m, 2H), 6.00 (d, J = 8.4 Hz, 1H), 4.50 (dt, J = 7.7 Hz,
t
13
1
J = 5.3 Hz, 1H), 3.24 (½ABX, J = 14.1, 7.5 Hz, 1H), 3.08 (½ABX, J = 14.1, 7.5 Hz, 1H). C { H}
d
NMR (201 MHz, CDCl )  175.0, 147.3, 134.5, 134.0, 133.6, 133.1, 130.3, 129.3, 128.7, 127.6,
3
1
25.7, 57.6, 38.7. IR: 3324 (broad), 3096, 2940, 1723, 1536, 1497, 1415, 1347, 1163, 1101, 1058,
+
1015. HRMS (ESI-TOF) m/z: [M + H] Calcd for C H N O S 351.0651; Found 351.0669.
15
15
2
6
Preparation of HOBt•NH : Solid HOBt•H O was dissolved in methanolic ammonia (7N). The
3
2
solution was evaporated to afford a white solid in quantitative yield. The solid was used as a
source of NH in the next reaction.
3
(
S)-2-((2-nitrophenyl)sulfonamido)-3-phenylpropanamide (3). A 25 mL round bottom flask with stir
bar was charged with a solution of 2 (100 mg, 0.28 mmol, 1.0 eq) in dry DMF (5 mL). EDC•HCl
165 mg, 0.85 mmol, 3 eq) and HOBt•NH (345 mg, 2.28 mmol, 8 eq) were added. The reaction
(
3
was stirred at rt for 48 h with monitoring by LCMS. LCMS indicated ≈40% conversion to product.
The reaction was quenched with water (5 mL) and with EtOAc (3 x 10 mL). The combined organic
layers were dried over MgSO , filtered and concentrated in vacuo (80 mg, white viscous oil, 80%
4
crude yield).
Scheme 2
(S)-N-(2-hydroxy-1-phenylethyl)-2-nitrobenzenesulfonamide (7).
Using the method
described to prepare compound 16e using (S)-2-amino-2-phenylethan-1-ol 6 as the starting
1
material, the title compound was obtained (5.5 g, white solid, mp 124 – 127 °C, 78% yield). H
NMR (800 MHz, CDCl )  7.76 (dt, J = 8.9 Hz, J = 1.3 Hz, 2H), 7.59 (dt, J = 7.8 Hz, J = 1.4 Hz,
3
t
d
t
d
1H), 7.46 (dt, J = 7.7, J = 1.3 Hz, 1H), 7.17 – 7.14 (m, 5H), 6.27 (d, J = 8.3 Hz, 1H), 4.68 (ddd,
t
d
J = 8.4, 6.1, 4.4 Hz, 1H), 3.91 (dd, J = 11.4, 4.5 Hz, 1H), 3.84 (dd, J = 11.4, 6.1 Hz, 1H), 1.90
1
(
vbs, 1H). 13C { H} NMR (201 MHz, CDCl )  147.4, 137.0, 134.3, 133.1, 132.5, 130.8, 128.6,
3
128.2, 125.0, 66.2, 60.4, 60.4. IR: 3695, 3679, 3662, 3333 (broad), 2937, 2865, 2843, 1535, 1412,
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+
1
357, 1332, 1161, 1126, 1057, 1032, 1017. HRMS (ESI-TOF) m/z: [M + H] Calcd for C H N O S
14 15 2 5
323.0702; Found 323.0697.
(
S)-2-((2-nitrophenyl)sulfonamido)-2-phenylethyl methanesulfonate (8). Using the method
described to prepare compound 17e using compound 7 as the starting material, the title
compound was obtained (200 mg, off-white semi-solid, 64% yield).
0
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(
S)-N-(2-amino-1-phenylethyl)-2-nitrobenzenesulfonamide (9). A 10 mL microwave vial was
charged with a solution of 8 (200 mg, 0.5 mmol) in methanolic anhydrous ammonia (7M nominal,
.35 mL, 2.5 mmol, 5 equiv) and stirred at rt for 2h. The reaction was monitored by TLC (5%
MeOH/DCM) and LCMS, which showed complete disappearance of sm. All volatiles were
removed and the residue partitioned between DCM and 2M HCl. The aqueous layer was basified
0
to pH 8 with sat aq NaHCO and extracted with EtOAc. The combined organic layers were dried
over MgSO , filtered and concentrated in vacuo to provide the title compound (viscous brown oil,
3
4
60
mg,
38%
yield).
tert-Butyl (S)-(2-((2-nitrophenyl)sulfonamido)-2-phenylethyl)carbamate (10). A 25 mL round
bottom flask was charged with solution of (S)-N-(2-amino-1-phenylethyl)-2-
nitrobenzenesulfonamide 9 (50 mg, 0.15 mmol, 1 equiv) in DCM (2 mL). Boc O (37 mg, 0.17
mmol, 1.1 equiv) was added. The homogeneous reaction mixture was stirred at rt for 16h and
monitored by LCMS (negative ion mode). Water (2 mL) was added and extracted three times with
DCM. The combined DCM layers were dried over MgSO and concentrated in vacuo. The crude
residue was purified by flash chromatography using an EtOAc/hexane gradient (20% to 100%
EtOAc/hex) to provide the title compound as a white viscous oil (55 mg, 83% yield). This product
was identical to compound 10a obtained from compound 28a in Scheme 5. See Scheme 5 for
complete characterization.
a
2
4
Ethyl
(S,E)-4-((N-(2-((tert-butoxycarbonyl)amino)-1-phenylethyl)-2-
Using the method described to prepare
compound 20e using compound 10 as the starting material, the title compound was obtained (30
mg, yellow semi-solid, 48% yield). This product was identical to compound 11a obtained from
compound 10a in Scheme 5. See Scheme 5 for complete characterization.
nitrophenyl)sulfonamido)but-2-enoate (11).
Ethyl 2-((2S,5S)-4-((2-nitrophenyl)sulfonyl)-5-phenylpiperazin-2-yl)acetate (12 cis) and
ethyl 2-((2R,5S)-4-((2-nitrophenyl)sulfonyl)-5-phenylpiperazin-2-yl)acetate (13 trans). Using
the method described to prepare compounds 23e cis/24e trans (Scheme 3), the mixture of the
title compounds was obtained using compound 11 as the starting material (15 mg, tan oil, 61%
yield). See Scheme 5 for the separation of the two diastereomers and for complete
characterization data.
Scheme 3 (Boc pathway, intermediates 16a-h, 17a-h, 18a-h, 19a-h, 20a-h, 23 a-h cis)
(
R)-N-(1-hydroxypropan-2-yl)-2-nitrobenzenesulfonamide (16e). A 2000 mL 3-necked round
bottom flask was fitted with a mechanical stirrer, nitrogen inlet and glass stopper. The flask was
charged with (R)-2-aminopropan-1-ol (15e, 49 g, 650 mmol) followed by dry DCM (500 mL). The
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mixture was stirred until dissolution was complete. With stirring, the reaction solution was cooled
to 0° C and charged with TEA (99.7 mL, 715 mmol, 1.1 equiv). Solid 2-nitrophenyl sulfonyl chloride
(
NsCl, 143 g, 644 mmol, 0.99 equiv) was added in portions. The ice bath was removed and the
reaction stirred at rt for 18h. Analysis was performed by TLC (50% EtOAc/hex) and LCMS. DCM
300 mL) and ice water (250 mL) were added with stirring. The aqueous layer was drawn off by
(
siphon and replaced with brine (200 mL) solution. After stirring, the aqueous was drawn off by
siphon and again replaced with brine (200 mL). After stirring, the brine was drawn off by siphon
1
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0
and the organic layer was recovered by siphon. The organic layer was dried over MgSO , filtered
4
and evaporated by continuous feed to give an off-white semisolid (161 g, 95% crude yield), which
1
was used without further purification. H NMR (600 MHz, CDCl )  8.19 (dd, 1H, J = 9.2, 2.0 Hz),
3
7
.90 (dd, 1H, J = 9.2, 2.0 Hz), 7.78 - 7.76 (m, 2H), 5.60 (bd, J = 6.9 Hz, 1H), 3.63 (bdt, J = 6.5
t
Hz, J = 3.4 Hz, 2H), 3.53 (bquart, J = 4.0 Hz, 1H), 2.24 (t, 1H, J = 5.5 Hz), 1.15 (d, J = 6.8 Hz,
d
13
1
3H). C { H} NMR (151 MHz, CDCl )  147.8, 134.5, 133.6, 133.0, 130.9, 125.4, 66.2, 17.7. IR:
3
3
532, 3331, 3098, 2975, 2936, 2881, 1537, 1413, 1359, 1339, 1165, 1125, 1058. HRMS (ESI-
+
TOF) m/z: [M + H] Calcd for C H N O S 261.0545; Found 261.0541.
9
13
2
5
The following additional intermediates were prepared using the method described above for
compound 16e.
(S)-N-(1-hydroxypropan-2-yl)-2-nitrobenzenesulfonamide (16a). 86 g, off-white semisolid, 100%
1
crude yield. H NMR (600 MHz, CDCl ) Identical to 16e except for  2.24 (t, J = 5.5 Hz, 1H) which
3
13
1
appears as  2.02 – 2.01 (bm, 1H). C { H} NMR (151 MHz, CDCl )  147.9, 134.5, 133.6, 132.9,
3
1
30.9, 125.4, 66.2, 52.5, 17.8. IR: 3346 (broad), 1538, 1440, 1411, 1360, 1340, 1266, 1165, 1124,
+
1086, 1058, 1024. HRMS (ESI-TOF) m/z: [M + H] Calcd for C H N O S 261.0545; Found
9
13
2
5
261.0540.
(
S)-N-(1-hydroxy-3-phenylpropan-2-yl)-2-nitrobenzenesulfonamide (16b). 59 g, light brown oil,
1
96% crude yield. H NMR (600 MHz, CDCl )  7.91 (dd, J = 7.0, 2.2 Hz, 1H), 7.72 (dd, J = 6.3, 2.1
3
Hz, 1H), 7.63 – 7.58 (m, 2H), 7.04 – 6.99 (m, 5H), 3.80 – 3.75 (m, 1H), 3.73 (dd, J = 11.3, 4.3 Hz,
1
H), 3.67 (dd, J = 11.2, 4.9 Hz, 1H), 2.93 (dd, J = 14.0, 5.9 Hz, 1H), 2.75 (dd, J = 14.0, 8.8 Hz,
13
1
1H). C { H} NMR (151 MHz, CDCl )  147.1, 136.9, 134.2, 133.3, 133.1, 130.4, 129.2, 128.4,
3
126.7, 125.5, 64.8, 58.6, 37.8. IR: 3545, 3264, 1536, 1427, 1356, 1324, 1158. HRMS (ESI-TOF)
+
m/z: [M + H] Calcd for C H N O S 337.0858; Found 337.0854.
15
17
2
5
(
S)-N-(1-hydroxy-4-methylpentan-2-yl)-2-nitrobenzenesulfonamide (16c). 66 g, light brown oil,
1
98% crude yield. H NMR (600 MHz, CDCl )  8.12 (dt, J = 9.3 hz, J = 4.0 Hz, 1H), 7.82 (dt, J
3
d
d
d
=
9.3 Hz, J = 3.7 Hz, 1H), 7.75 – 7.70 (m, 2H), 5.70 (vbs, 1H), 3.55 – 3.50 (m, 2H), 3.48 – 3.45
t
(
m, 1H), 2.79 (vbs, 1H), 1.52 (hept, J = 6.5 Hz, 1H), 1.37 – 1.27 (m, 2H), 0.78 (d, J = 6.7 Hz, 3H),
13
1
0
.69 (d, J = 6.6 Hz, 3H). C { H} NMR (151 MHz, CDCl )  147.6, 134.6, 133.7, 133.0, 130.6,
3
125.3, 65.0, 55.0, 40.8, 24.3, 22.8, 21.8. IR: 3336, 2955, 2870, 1538, 1412, 1360, 1338, 1164,
+
1060. HRMS (ESI-TOF) m/z: [M + H] Calcd for C H N O S 303.1015; Found 303.1008.
12
19
2
5
(
S)-N-(1-(benzyloxy)-3-hydroxypropan-2-yl)-2-nitrobenzenesulfonamide (16d). 21 g, off-white
1
semisolid, 99% crude yield. H NMR (600 MHz, CDCl ) identical to 16h with the addition of  6.06
3
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2
2
2
2
2
2
2
3
3
3
3
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3
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4
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6
(
d, J = 7.0 Hz, 1H), 2.22 (t, J = 5.7 Hz, 1H). The singlet signal at  4.33 appears as  4.38 (½AB,
13
1
J = 11.7 Hz, 1H), 4.35 (½AB, J = 11.7 Hz, 1H). C { H} NMR (151 MHz, CDCl )  147.6, 137.2,
134.6, 133.4, 132.9, 130.6, 128.5, 128.0, 127.8, 125.5, 73.5, 70.1, 63.2, 55.6. IR: 3343, 2942,
1538, 1496, 1453, 1412, 1344, 1164, 1058, 1020. HRMS (ESI-TOF) m/z: [M + H] Calcd for
C H N O S 367.0964; Found 367.0958.
3
+
16
19
2
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(R)-N-(1-hydroxy-3-phenylpropan-2-yl)-4-nitrobenzenesulfonamide (16f). 67.8 g, light brown oil,
1
98% crude yield. H NMR (600 MHz, CDCl ) identical to 16b with the addition of  5.83 (d, J = 7.3
Hz), 2.52 (vbs, 1H). C { H} NMR (151 MHz, CDCl )  147.2, 136.7, 134.2, 133.2, 133.1, 130.5,
1
3
13
1
3
29.1, 128.4, 126.7, 125.6, 64.9, 58.6, 37.8. IR: 3569, 3321, 1534, 1451, 1427, 1367, 1338, 1160,
+
1
041. HRMS (ESI-TOF) m/z: [M + H] Calcd for C H N O S 337.0858; Found 337.0857.
15 17
2
5
(
R)-N-(1-hydroxy-4-methylpentan-2-yl)-4-nitrobenzenesulfonamide (16g). 70 g, light brown oil,
1
13
1
9
1
5% crude yield. H NMR (600 MHz, CDCl ) identical to 16c. C NMR { H} (151 MHz, CDCl ) 
3
3
47.6, 134.6, 133.7, 133.0, 130.6, 125.3, 64.9, 55.0, 40.8, 24.3, 22.8, 21.8. IR: 3336, 2955, 1538,
+
1413, 1337, 1164, 1060. HRMS (ESI-TOF) m/z: [M + H] Calcd for C H N O S 303.1015;
found 303.1008.
12
19
2
5
(
R)-N-(1-(benzyloxy)-3-hydroxypropan-2-yl)-2-nitrobenzenesulfonamide (16h). 20.6 g, light
1
brown oil, 97% crude yield. H NMR (600 MHz, CDCl )  8.11 (dd, J = 7.7, 1.6 Hz, 1H), 7.74 (dd,
J = 7.7, 1.6 Hz, 1H), 7.66/7.63 (two overlapping quart, Japparent = 1.4 Hz, 2H), 7.31 – 7.26 (m, 3H),
7
3
.17 – 7.15 (m, 2H), 4.33 (s, 2H), 3.73 (dd, J = 10.4, 4.0 Hz, 1H), 3.67 (m, 2H), 3.57 (dd, J = 9.8,
13
1
5
.0 Hz, 1H), 3.48 (dd, J = 9.8, 4.5 Hz, 1H). C { H} NMR (151 MHz, CDCl )  147.6, 137.3, 134.4,
3
133.5, 132.9, 130.6, 128.4, 127.9, 127.8, 125.4, 73.4, 69.8, 62.9, 55.7. IR: 3329, 2868, 1537,
1411, 1345, 1166, 1059. HRMS (ESI-TOF) m/z: [M + H] Calcd for C H N O S 367.0964; found
3
+
16
19
2
6
67.0957.
(
R)-2-((2-nitrophenyl)sulfonamido)propyl methanesulfonate (17e). A 1000mL round bottom flask
was equipped with a magnetic stirrer, rubber septum and nitrogen inlet. The flask was charged
with a solution of 16e (15 g, 57.6 mmol, 1 equiv) in dry DCM (200 mL). The flask was cooled to
78 °C. TEA (8.84 mL, 63.5 mmol, 1.10 equiv) was added by syringe, followed by MsCl (4.91 mL,
6
3.5 mmol, 1.10 equiv). The reaction was stirred at 78° C for 1h. Complete conversion to the
mesylate was confirmed by TLC (50% EtOAc/hex) and LCMS. The cold reaction was poured into
a separatory funnel followed by water (100 mL). The layers were separated and the organic
washed once with brine (50 mL). The organic layer was dried over MgSO , filtered and evaporated
to give the title compound as an off white solid (19 g, 97% crude yield). The title compound was
immediately used without further purification or characterization. LCMS m/z: Calcd for [M + H]+
C H N O S 339.0; Found 339.0.
4
10
15
2
7
2
The following additional mesylates were prepared using the method described for 17e. In all cases
TLC (50% EtOAc/hex) and LCMS data indicated 100% conversion of alcohols 16a-d, f-h to their
corresponding mesylates. The crude products were immediately used without further
characterization and under the assumption of quantitative conversion. Melting points were not
obtained.
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8
9
(
S)-2-((2-nitrophenyl)sulfonamido)propyl methanesulfonate (17a). 108 g, off-white solid, 96%
+
crude yield. LCMS m/z: Calcd for [M + H] C H N O S 339.0; Found 339.0.
10
15
2
7
2
(S)-2-((2-nitrophenyl)sulfonamido)-3-phenylpropyl methanesulfonate (17b). 61 g, off-white solid,
9
+
5% crude yield. LCMS m/z: Calcd for [M + H] C H N O S 415.1; Found 415.0.
16
19
2
7
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(S)-4-methyl-2-((2-nitrophenyl)sulfonamido)pentyl methanesulfonate (17c). 80 g, white solid, 99%
+
crude yield. LCMS m/z: Calcd for [M + H] C H N O S 381.1; Found 381.1.
13
21
2
7
2
(S)-3-(benzyloxy)-2-((2-nitrophenyl)sulfonamido)propyl methanesulfonate (17d). 30 g, off-white
+
solid, 99% crude yield. LCMS m/z: Calcd for [M + H] C H N O S 445.1; Found 445.0.
17
21
2
8
2
(R)-2-((2-nitrophenyl)sulfonamido)-3-phenylpropyl methanesulfonate (17f). 64 g, off-white solid,
9
+
4% crude yield. LCMS m/z: Calcd for [M + H] C H N O S 415.1; Found 415.1.
16
19
2
7
2
(R)-4-methyl-2-((2-nitrophenyl)sulfonamido)pentyl methanesulfonate (17g). 80 g, white solid,
+
9
9% crude yield. LCMS m/z: Calcd for [M + H] C H N O S 381.1; Found 381.1.
13 21
2
7
2
(R)-3-(benzyloxy)-2-((2-nitrophenyl)sulfonamido)propyl methanesulfonate (17h). 23 g, off-white
+
solid, 90% crude yield. LCMS m/z: Calcd for [M + H] C H N O S 445.1; Found 445.1.
17
21
2
8
2
(R)-2-methyl-1-((2-nitrophenyl)sulfonyl)aziridine (18e). A dry 1000 mL round bottom flask was
equipped with a magnetic stirrer, rubber septum and nitrogen inlet. The flask was charged with a
solution of crude 17e (19 g, 56 mmol, 1.0 equiv) in anhydrous DCM (100 mL). The flask was
cooled to 0 °C and solid Cs CO (20 g, 62 mmol, 1.1 equiv) was added in 2 portions. The ice bath
was removed and the reaction stirred at rt for 18h. Analysis was performed by TLC (50%
EtOAc/hex) and LCMS. Water (50 mL) was added to the reaction mixture and the mixture was
extracted with DCM (3 x 50 mL). The combined organic layers were washed with brine, dried over
2
3
MgSO and partially concentrated in vacuo to provide the crude aziridine as a yellow solution.
4
(Caution: Do not remove all of the solvent since the concentrated aziridine will begin to
polymerize upon standing. The volume of the crude product was approximately 30 mL). The
resulting crude product was diluted with dry THF (30 mL) and used without any further purification
or characterization. LCMS m/z: Calcd for [M + H] C H N O S 243.0; Found 243.1.
+
9
11
2
4
The following additional aziridine intermediates were prepared using the method described for
1
1
THF/DCM, were used without further characterization and under the assumption of complete
conversion of starting mesylate.
8e. In all cases TLC (50% EtOAc/hex) and LCMS data indicated 100% conversion of mesylates
7a-d, f-h to their corresponding aziridines. The crude products, all yellow oils diluted with 1:1
+
(S)-2-methyl-1-((2-nitrophenyl)sulfonyl)aziridine (18a). LCMS m/z: Calcd for [M + H] C H N O S
9
11
2
4
2
43.0; Found 243.0.
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1
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1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
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4
4
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5
5
5
5
5
5
5
5
5
6
+
(
3
S)-2-benzyl-1-((2-nitrophenyl)sulfonyl)aziridine (18b). LCMS m/z: Calcd for [M + H] C H N O S
19.1; Found 319.1.
15 15 2 4
S)-2-isobutyl-1-((2-nitrophenyl)sulfonyl)aziridine (18c). LCMS m/z: Calcd for [M + H]+
(
C H N O S 285.1; Found 285.1.
12
17
2
4
(
S)-2-((benzyloxy)methyl)-1-((2-nitrophenyl)sulfonyl)aziridine (18d). LCMS m/z: Calcd for [M + H]+
C H N O S 349.1; Found 349.0.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
16
17
2
5
+
(
3
R)-2-benzyl-1-((2-nitrophenyl)sulfonyl)aziridine (18f). LCMS m/z: Calcd for [M + H] C H N O S
15 15 2 4
19.1; Found 319.0.
(
R)-2-isobutyl-1-((2-nitrophenyl)sulfonyl)aziridine (18g). LCMS m/z: Calcd for [M + H]+
C H N O S 285.1; Found 285.1.
12 16
2
4
(
R)-2-((benzyloxy)methyl)-1-((2-nitrophenyl)sulfonyl)aziridine (18h). LCMS m/z: Calcd for [M + H]+
C H N O S 349.1; Found 349.0.
16
17
2
5
tert-Butyl (R)-(2-((2-nitrophenyl)sulfonamido)propyl)carbamate (19e): A 1000 mL flask was
equipped with magnetic stirrer, rubber septum and nitrogen inlet. The flask was charged with solid
KHMDS (33.5 g, 168 mmol, 3 equiv) followed by anhydrous THF (200 mL). The resultant slurry
was cooled to 78 °C and a solution of NH Boc (20 g, 168 mmol, 3.0 equiv) in THF (50 mL) was
2
added dropwise to the reaction mixture. The resultant turbid solution was stirred 78 °C for 30 m.
After 30 m the dry ice bath was removed and a solution of aziridine 18e (13.6 g, 56 mmol, 1 equiv)
in THF/DCM (1:1, 60 mL total) was added dropwise over 30m. The resultant dark brown/black
solution was stirred at rt for 18 h. Analysis was performed by TLC (50% EtOAc/hex) and LCMS.
After completion of the reaction, water (200 mL) and EtOAc (200 mL) were added with stirring.
The aqueous layer was drawn off by siphon and replaced with brine (100 mL). After stirring, the
brine layer was drawn off by siphon and the organic layer was recovered by siphon. The organic
was dried over MgSO , filtered and evaporated using continuous feed to afford the title compound
as a yellowish brown oil (18.5 g, 92% crude yield) which was used without any further purification.
4
1
H NMR (600 MHz, CDCl )  8.16 (dt, J = 4.6 Hz, J = 2.0 Hz, 1H), 7.87 (dt, J = 4.4 Hz, J = 2.1
Hz, 1H), 7.77 – 7.74 (m, 2H), 5.59 (bd, J = 7.2 Hz, 1H), 4.94 (t, J = 6.4 Hz, 1H), 3.63 (bhex, J =
.3 Hz, 1H), 3.30 – 3.27 (m, 1H), 3.06 (bquart, J = 6.9 Hz, 1H), 1.42 (s, 9H), 1.10 (d, J = 6.7 Hz,
3
d
t
d
t
4
3
5
13
1
H). C { H} NMR (151 MHz, CDCl )  156.3, 147.9, 134.5, 133.6, 132.9, 130.8, 125.4, 79.7,
1.3, 46.0, 28.3, 19.0. IR: 3343, 2976, 2929, 1689, 1538, 1420, 1364, 1272, 1248, 1162, 1120.
HRMS (ESI-TOF) m/z: [M + H] Calcd for C H N O S 360.1229; Found 360.1223. HRMS (ESI-
TOF) m/z: [M + H - Boc] Calcd for C H N O S 260.0705; Found 260.0701 (base peak).
3
+
14
22
3
6
+
9
14
3
4
The following additional intermediates were prepared using the method described for 19e. In all
cases TLC (50% EtOAc/hex) and LCMS data indicated 100% conversion of aziridines 18a-d, f-h
to their corresponding orthogonally bis-protected diamines.
tert-Butyl (S)-(2-((2-nitrophenyl)sulfonamido)propyl)carbamate (19a). 113 g, brown semi-solid,
1
13
1
1
1
00% yield. H NMR (600 MHz, CDCl ) identical to 19e. C { H} NMR (151 MHz, CDCl )  156.3,
47.9, 134.6, 133.6, 132.9, 130.8, 125.4, 79.8, 51.3, 46.0, 28.3, 19.0. IR: 3339, 2977, 2931, 2359,
3
3
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5
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7
8
9
+
1
693, 1539, 1421, 1365, 1248, 1164, 1121, 1060. HRMS (ESI-TOF) m/z: [M + H - Boc] Calcd for
C H N O S 260.0705; Found 260.0700.
9
14
3
4
tert-Butyl (S)-(2-((2-nitrophenyl)sulfonamido)-3-phenylpropyl)carbamate (19b). 50 g, viscous
1
black oil, 63% crude yield. H NMR (600 MHz, CDCl )  7.92 – 7.90 (m, 1H), 7.75 – 7.74 (m, 1H),
7.65 – 7.60 (m, 2H), 7.01 (bs, 5H), 5.74 (bd, J = 6.8 Hz, 1H), 5.03 (bt, J = 6.5 Hz, 1H), 3.82
(bquart, J = 3.1 Hz, 1H), 3.44 – 3.40 (m, 1H), 3.25 (dt, J = 13.7, 6.7 Hz, 1H), 2.89 (dd, J = 14.2,
3
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
13
1
5
1
.5 Hz, 1H), 2.68 (dd, J = 14.1, 9.0 Hz, 1H), 1.45 (s, 9H). C { H} NMR (151 MHz, CDCl )  156.4,
3
47.2, 136.4, 134.4, 133.1, 133.0, 130.4, 129.1, 128.4, 126.8, 125.5, 79.8, 57.3, 45.1, 39.1, 28.4.
IR: 3349, 2977, 2931, 1691, 1536, 1363, 1161, 1087. HRMS (ESI-TOF) m/z: [M + H - Boc] Calcd
+
for C H N O S 336.1018; Found 336.1015.
15
18
3
4
tert-Butyl (S)-(4-methyl-2-((2-nitrophenyl)sulfonamido)pentyl)carbamate (19c). 73.5 g, dark
1
brown viscous oil, 84% crude yield. H NMR (600 MHz, CDCl )  8.19 – 8.10 (m, 1H), 7.89 – 7.81
m, 1H), 7.78 – 7.67 (m, 2H), 5.62 (d, J = 8.1 Hz, 1H), 4.94 (d, J = 6.4 Hz, 1H), 3.58 (pd, J = 7.1,
.9 Hz, 1H), 3.32 – 3.18 (m, 1H), 3.06 (dt, J = 13.4, 6.3 Hz, 1H), 1.63 – 1.48 (m, 1H), 1.45 (s, 5H),
.40 (s, 9H), 1.29 (t, J = 7.2 Hz, 2H), 0.80 (d, J = 6.7 Hz, 3H), 0.71 (d, J = 6.6 Hz, 3H). C { H}
3
(
3
1
13
1
NMR (151 MHz, CDCl )  156.6, 156.2, 147.8, 134.8, 133.5, 132.9, 130.6, 125.3, 79.6, 79.5, 53.6,
4.9, 42.1, 28.32, 28.25, 24.3, 22.8, 21.9. IR: 3337, 2963, 1691, 1533, 1364, 1272, 1160, 1125.
HRMS (ESI-TOF) m/z: [M + H] Calcd for C H N O S 402.1699; Found 402.1693. HRMS (ESI-
TOF) m/z: [M + H - Boc] Calcd for C H N O S 302.1175; Found 302.1168 (base peak).
3
4
+
17
28
3
6
+
12 20
3
4
tert-Butyl (R)-(3-(benzyloxy)-2-((2-nitrophenyl)sulfonamido)propyl)carbamate (19d). 25 g, dark
1
brown viscous oil, 69% crude yield. H NMR (600 MHz, CDCl )  8.11 (dd, J = 7.7, 1.3 Hz, 1H),
3
7
(
.76 (d, J = 7.8 Hz, 1H), 7.70 – 7.63 (two overlapping quartets, 2H), 7.34 – 7.29 (m, 4H), 7.16
dd, J = 7.5, 2.2 Hz, 2H), 6.05 (d, J = 7.3 Hz, 1H), 4.96 (t, J = 6.2 Hz, 1H), 4.36 (½AB, J = 12.0
Hz, 1H), 4.33 (½AB, J = 12.0 Hz, 1H), 3.72 (bpent, J = 7.1 Hz, 1H), 3.46 (dd, J = 9.8, 5.1 Hz, 1H),
13
1
3
.41 – 3.35 (m, 2H), 3.24 (dt, J = 14.4, 6.2 Hz, 1H), 1.43 (s, 9H). C { H} NMR (151 MHz, CDCl )
3
 156.3, 147.6, 137.2, 134.5, 133.3, 132.8, 130.7, 128.4, 127.9, 127.8, 125.4, 79.7, 73.5, 70.1,
4.6, 42.5, 28.3. IR: 3353, 3164, 2971, 2874, 1686, 1537, 1454, 1434, 1389, 1366, 1293, 1161.
HRMS (ESI-TOF) m/z: [M + H] Calcd for C H N O S 466.1648; Found 466.1645. HRMS (ESI-
TOF) m/z: [M + H - Boc] Calcd for C H N O S 366.1124; Found 366.1120 (base peak).
5
+
21
28
3
7
+
16
20
3
5
tert-Butyl (R)-(2-((2-nitrophenyl)sulfonamido)-3-phenylpropyl)carbamate (19f). 46.2 g, viscous
black oil, 53% crude yield. H NMR (600 MHz, CDCl ) identical to 19b. C { H} NMR (151 MHz,
1
13
1
3
CDCl )  156.4, 147.1, 136.5, 134.4, 133.1, 133.0, 130.4, 129.1, 128.4, 126.8, 125.5, 79.7, 57.4,
5.1, 39.1, 28.4. IR: 3346, 2977, 2929, 1691, 1536, 1454, 1417, 1362, 1248, 1160, 1087. HRMS
ESI-TOF) m/z: [M + H] Calcd for C H N O S 436.1542; Found 436.1544. HRMS (ESI-TOF)
3
4
(
+
20
26
3
6
+
m/z: [M + H - Boc] Calcd for C H N O S 336.1018; Found 336.1018 (base peak).
15
18
3
4
tert-Butyl (R)-(4-methyl-2-((2-nitrophenyl)sulfonamido)pentyl)carbamate (19g). 76 g, dark brown
viscous oil, 85% yield. H NMR (600 MHz, CDCl ) identical to 19c. C NMR { H} (151 MHz,
1
13
1
3
CDCl )  156.2, 147.7, 134.7, 133.6, 132.9, 130.6, 125.4, 79.5, 53.6, 44.9, 42.1, 28.3, 24.3, 22.8,
1.9. IR: 3338, 3293, 1691, 1533, 1446, 1423, 1364, 1336, 1166, 1125. HRMS (ESI-TOF) m/z:
M + H + Na] Calcd for C H N NaO S 424.1518; Found 424.1511. HRMS (ESI-TOF) m/z: [M +
3
2
[
+
17
27
3
6
+
H - Boc] Calcd for C H N O S 302.1175; Found 302.1168 (base peak).
12 20
3
4
29
ACS Paragon Plus Environment