Traceless solid-phase synthesis of [6,7,8 + 5,6,7]-fused molecular frameworks

Article abstract of DOI:10.3390/molecules23051090

We report two synthetic strategies for traceless solid-phase synthesis of molecular scaffolds comprising 6- to 8-membered rings fused with 5- to 7-membered rings. Traceless synthesis facilitated preparation of target molecules without any trace of polymer

Full text of DOI:10.3390/molecules23051090

molecules
Article
Traceless Solid-Phase Synthesis of
[6,7,8 + 5,6,7]-Fused Molecular Frameworks
Vanesa Giménez-Navarro and Viktor Krch nˇ ák 1,2,* ID
1
1
Department of Organic Chemistry, Faculty of Science, Palacky University, 17. Listopadu 12,
7
71 46 Olomouc, Czech Republic; viktor.krchnak@upol.cz
2
Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Center,
University of Notre Dame, Notre Dame, IN 46556, USA
*
Correspondence: vkrchnak@nd.edu; Tel.: +1-574-631-5113
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
Received: 14 April 2018; Accepted: 2 May 2018; Published: 4 May 2018
Abstract: We report two synthetic strategies for traceless solid-phase synthesis of molecular scaffolds
comprising 6- to 8-membered rings fused with 5- to 7-membered rings. Traceless synthesis facilitated
preparation of target molecules without any trace of polymer-supported linkers. The cyclization
proceeded via acid-mediated tandem N-acylium ion formation followed by the nucleophilic addition
of O- and C-nucleophiles. The presented synthetic strategy enabled, through the use of simple
building blocks without any conformational preferences, the evaluation of the predisposition of
different combinations of ring sizes to form fused ring molecular scaffolds. Compounds with any
combination of [6,7 + 5,6,7] ring sizes were accessible with excellent crude purity. The 8-membered
cyclic iminium was successfully fused only with the 5-membered cycle and larger fused ring systems
were not formed, probably due to their instability.
Keywords: bicyclic compounds; heterocycles; iminiums; scaffold; solid-phase synthesis
1
. Introduction
The enormous structural diversity of natural products comprising fused ring systems and,
in particular, their wide range of biological activities, make them attractive synthetic targets. More than
0% of the drugs on the market have structures derived from natural origins [ ]. Therefore, the search
6
1,2
for pharmacologically relevant molecules that mimic natural products has become an integral part
3
of drug discovery. Specific structural features of these molecular scaffolds are the presence of sp
hybridized carbons and stereogenic centers [
products, they are often missing in traditional compound collections for High Throughput Screening
HTS) [4].
Generally, the chemical space occupied by natural products overlays with biological space.
Therefore, the exploitation of core structures derived from bioactive natural products has led to the
design and implementation of biology-oriented synthesis (BIOS) [ ] instead of synthesis focused
3]. Although these characteristics are typical of natural
(
5,6
only on the structural dissimilarity of compounds, i.e., diversity-oriented synthesis (DOS) [6,7].
Natural products comprising fused and bridged carbo- and heterocycles represent particularly
intriguing molecular scaffolds with 3D architecture. Among the various chemical routes used to
synthesize diverse nitrogenous heterocycles, tandem iminium ion cyclization-nucleophilic addition
represents one of the most versatile strategies [
8–11]. Typically, a cyclic iminium is formed from
an amide nitrogen and an aldehyde attached to acyclic intermediate. Then, a cyclic N-acyliminium
ion is transformed into fused or bridged structures by intramolecular nucleophilic addition [12
,
13].
Specifically, oxygen has previously served as an internal nucleophile in the preparation of five- [14
,
15],
six- [16–19], seven- [13,20] and eight-membered lactam rings [13].
Molecules 2018, 23, 1090; doi:10.3390/molecules23051090
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Molecules 2018, 23, 1090
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Incorporation of fused lactam rings was also frequently applied for the synthesis of
peptidomimetics [ 21]. The constrained structures were designed to mimic -turns, the most
8,
β
frequent secondary structure found in proteins. Amino acid motifs constrained within a bicyclic
lactam architecture have been applied to the design of modified peptides and peptidomimetics to
discover new therapeutics and better understand the interactions of ligands with target enzymes and
receptors. Compounds
I [19,22] and II [23] (Figure 1) were designed as β-turn mimetics, and scaffold
7
8
III 24] was synthesized as a conformationally constrained analogue for the Phe -Phe region of
[
substance P. The solution-phase synthesis of constrained dipeptide surrogates comprising [6 + 5]-fused
lactams (structure IV) from 3-aza-1,5-diketoacids and amino alcohols was developed using a one-pot
strategy [17]. Tricyclic constrained scaffold
V was reported as a carbapenem mimetic [25]. Nielsen
et al. synthesized a range of bicyclic dipeptide mimetics forming 5-hydroxylactam/N-acyliminium
ion intermediates [15]. Depending on the amino acid side-chain, the cyclization occurs as side-chain
cyclization VI if a nucleophilic moiety (X = O, S, NH, n = 1–3) is presented in the side-chain and
as backbone cyclization VII in absence of heteroatomic moieties in the side-chain. Their studies
on N-acyliminium Pictet-Spengler reactions [26] were extended to synthesize pyrroloisoquinolines
VIII
tetracyclic scaffolds [28]. Additionally N-carbamyliminium Pictet-Spengler reaction was used to form
tetrahydro- -carbolines and tetrahydroisoquinolines [29]. Synthesis of fused natural product-like
[27] or with an indole moiety and heterocycles such as furans and thiophenes to yield tri- and
β
diketopiperazines comprising different ring combination yielded molecular scaffolds IX [30].
Recently, our group reported a synthetic strategy enabling the incorporation of bicyclic
scaffolds as peptide constraints during traditional peptide synthesis. The synthesis of
bridged 6-oxa-3,8-diazabicyclo[3.2.1]octan-2-ones [31], fused tetrahydropyrazino[2,1-b][1,3]oxazine-
4
X
,7(6H,8H)-diones
(X = NR , n = 1) [32] and hexahydropyrimido[1,2-d][
X
(X = O, n = 1) [32], hexahydro-4H-pyrazino[1,2-a]pyrimidine-4,7(6H)-diones
3
3
1
,4
]diazepine-4,7(1H,6H)-diones
X
(X = NR ,
n = 2) [33] were performed via westbound direction (towards the amino terminus) N-acyliminium
cyclization–nucleophilic addition with Ser, Thr and 2,4-diaminobutyric acid as the sources of
oxygen, sulfur and nitrogen nucleophiles, respectively. The solid-phase synthesis of tetrahydro-2H-
2
oxazolo[3,2-a]pyrazine-5(3H)-ones IV (R = H) [34] was carried out via eastbound direction (towards
the carboxyl terminus) cyclization, and the scope of this synthetic strategy was expanded recently to
yield [7,8,9 + 5]-fused rings from Ser, Cys and C-nucleophiles (structure XI) [13].
R1
R1
H
R2
N
X
R1
N
O
R2
N
N
Ph
H N
2
N
Ph
N
R1
H N
2
O
O
OH
R3
O
OH
O
O
X = O, S
I
O
O
II
III
IV
O
R3
R1
HN
R1
O
N
H
NH
R2
O
O
N
N
O
N
N
R1
H
R2
O
OH
O
n
O
O
X
O
H
X = O, S, NH
VII
VIII
V
VI
O
AA1 OH
H
R1
R1
n
n
N
H
R1
X
X
N
O
N
R2
N
N
R2
m
O
N
R3
O
O
R4
O
O
R2
X = O, S, NR3
O
IX
X
XI
Figure 1. Reported synthesized bicyclic and tricyclic lactams.

Molecules 2018, 23, 1090
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In this report, we describe two synthetic strategies for traceless solid-phase synthesis of bicyclic
scaffolds. Traceless synthesis provided access to compounds without any trace of a linker, a critical
feature for preparation of compounds for structure-activity relationship (SAR). The synthesis was
carried out in a modular fashion that allowed us to test any combination of potential ring sizes, with the
first ring formed by a cyclic N-acyliminium species, and the second one fused by addition of oxygen
and carbon nucleophiles, and to obtain [6,7,8 + 5,6,7]-fused ring systems.
2
. Results
To prepare compounds on solid phase in a traceless manner we selected two potential anchoring
functional groups that enabled immobilization of the first building blocks and polymer-supported
synthesis of the acyclic precursor. The selected functional groups were not involved in any
transformation during solid-phase synthesis, however, they participated in ring-closing reactions
(
C-N and C-O/C-C bond formations) in the final cleavage/cyclization step. The attachment to the resin
was achieved via specific linkers that served as polymer-bound protecting group. Both strategies using
either amide nitrogen or nucleophile for attachment to solid support were used (Figure 2, L stands
for an acid-labile linker). Acid-mediated cleavage released compounds from the resin, demasked the
aldehyde and triggered cyclic N-acyliminium formation followed by nucleophilic addition.
Figure 2. Two ways for immobilization of acyclic intermediates.
To perform the synthesis, we used simple and commercially available building blocks: amino
alcohols (Fmoc-glycinol, Fmoc-
β-alaninol and 4-(Fmoc-amino)butanol), α-bromocarboxylic acids
(
bromoacetic acid and (S)- and (R)-bromopropionic acids), protected amino aldehydes (containing one,
two and three-carbon spacers) and sulfonyl chlorides or aryl fluorides. Because each ring is formed
from different building blocks, this strategy allowed us to use an identical synthetic strategy to prepare
any combination of different ring sizes in a truly combinatorial fashion. The size of the first ring,
the cyclic N-acyliminium, can be controlled by different amino acids (α, β, γ) and by the length of the
carbon spacer bearing the protected aldehyde. In this study, we focused on varying the length of the
aldehyde spacer.
The synthesis of resin-bound acyclic precursors was accomplished using well-documented
transformations. Briefly, the synthesis began with attachment via the oxygen atom of Fmoc-protected
amino alcohols to Wang resin
The Fmoc group was cleaved by piperidine, and the polymer-supported amines were acylated with
-bromocarboxylic acids to afford resin . Nucleophilic substitution of bromine with different protected
amino aldehydes provided resin-bound secondary amines. To obtain resin , the final derivatization
was performed using 4-nitrobenzenesulfonyl chloride (Ns-Cl), 4-methylbenzene-sulfonyl chloride
Tos-Cl) or 4-fluoro-3-nitrobenzotrifluoride. TFA exposure of the polymer-supported acyclic precursors
triggered their release from the resin, the removal of the aldehyde protecting group, and the formation
of the six-, seven- and eight-membered N-acyliminium ions via hemiaminals , followed by internal
nucleophilic attack to provide target molecular scaffolds . We typically used 50% TFA in DCM in the
final step. However, the LC/MS analysis of crude compound {3,2,1,3} revealed partial decomposition,
1 by using trichloroacetimidate activation [35] to yield resin 2 (Scheme 1).
α
3
4
(
5
6
6
i.e., the purity and yield of the crude products were only 38% and 16%, respectively, from the cleavage
cocktail. We therefore used 10% TFA in DCM for cleavage followed by extraction of DCM/TFA with

Molecules 2018, 23, 1090
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aqueous sodium bicarbonate. When the organic layer was separated and DCM evaporated under
a stream of nitrogen, the purity substantially increased to 88%. Individual compounds possess four
6
1
2
points of diversification: m and n refer to the ring sizes and R and R are the substituents at carbon
α
C and amine nitrogen, respectively (Table 1).
Scheme 1. Polymer-supported synthesis of fused bicyclic compounds
6
. Reagents and conditions:
◦
(i) trichloroacetonitrile, anhydrous DCM, 0 C, 30 min, DBU, DCM, rt, 1 h; resin washed and
Fmoc-amino alcohol, BF3 Et O, anhydrous THF added, rt, 30 min; (ii) 50% piperidine/DMF, rt, 15 min;
2
·
(
iii) α-bromocarboxylic acid, DIC, DCM, 5 min, then DIU filtered, DIEA added and the solution
transferred to the reaction vessel with resin, rt, 1 h; (iv) amino-aldehyde dialkyl acetal, DIEA, DMF,
rt, 2 h; (v) Ns-Cl or Tos-Cl, DIEA, DCM, rt, 2 h or 4-fluoro-3-nitrobenzotrifluoride, DIEA, DMSO, rt,
overnight; (vi) TFA/DCM 1:1, rt, 1 h; (vii) MeOH, rt, overnight.
Table 1. Synthesized fused heterocyclic compounds 6.
Yield [%] b
Entry *
Compound
m
n
R1
R2
Rings
Purity [%] a
1
2
3
4
5
6
7
8
9
6{1,2,1,1}
6{1,2,1,2}
6{1,2,1,3}
6{1,3,1,1}
6{1,3,1,2}
6{2,1,1,3}
6{2,2,1,1}
6{2,2,1,2}
6{2,2,1,3}
6{3,1,1,3}
6{3,2,1,2}
6{3,2,1,3}
1
1
1
1
1
2
2
2
2
3
3
3
2
2
2
3
3
1
2
2
2
1
2
2
H
H
H
H
H
H
H
H
H
H
H
H
Ns
Tos
-4-CF
Ns
Tos
-4-CF
Ns
7 + 5
7 + 5
7 + 5
8 + 5
8 + 5
6 + 6
7 + 6
7 + 6
7 + 6
6 + 7
7 + 7
7 + 7
94
93
68
86
53
97
98
97
95
86
72
88
70
70
64
66
21
62
83
90
80
84
54
44
2-NO
2
2
3
-C
6
6
H
H
3
3
2-NO
3
-C
Tos
2-NO
2-NO
2
2
-4-CF
-4-CF
Tos
3
3
-C
6
-C
6
H
H
3
3
1
1
1
0
1
2
2-NO
2
-4-CF
3
-C
6
H
3
*
Entry 6 has previously been reported [34]. a The overall purity of crude final compounds
6
after a five-step
b
synthesis; Yield of the HPLC-purified compounds.
Analysis of the results indicated that all combination of cyclic N-acyliminium 6- and 7-membered
rings (prepared using protected aldehydes with one and two-carbon spacers, n = 1, 2) fused with 5-, 6-
and 7-membered rings and provided the target molecular scaffolds with good to excellent purities and
yields (Figure 3).

Molecules 2018, 23, 1090
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Figure 3. Color-coded structures of fused scaffolds. Green: good to excellent purity of product; blue:
only specific combination was formed; red: no cyclic product detected.
The protected aldehyde with a three-carbon spacer used for the synthesis of [8 + 5,6,7]-fused
rings successfully cyclized only for [8 + 5] scaffolds derivatized as sulfonamides (
{1,3,1,2}) not for those derivatized as aryl compounds. Attempts to prepare [8 + 6,7] fused rings were
unsuccessful; LC/MS indicated the presence of a compound corresponding to cyclic hemiaminal
To evaluate different cyclization procedures, we released hemiaminals {1,3,1,3}, {2,3,1,3} and {3,3,1,3}
from resins using TFA/DCM 1:1. After the evaporation of TFA/DCM under a stream of nitrogen,
the compounds were dissolved in methanol and left overnight at rt to yield {1,3,1,3}, {2,3,1,3}
and {3,3,1,3} in more than 90% crude purity. Compounds {1,3,1,3} and {2,3,1,3} were isolated,
purified by reversed-phase HPLC and characterized by LCMS and NMR (Supplementary Materials).
Then, we screened several Lewis acids, including stannous chloride dihydrate (SnCl2 2H O) [36 37
and scandium triflate (Sc(OTf) ) [38] as well as different reaction conditions (temperature, concentration
6{1,3,1,1} and
6
5.
5
5
5
4
7
7
7
7
7
·
, ]
2
3
of Sc(OTf) , reaction time from 1 to 10 days) and solvents (DCM, MeCN, dioxane) for the cyclization
3
of
amount, was detected.
To address the stereoselectivity of this modular solid-phase synthesis, [7 + 5]-membered fused
compound {1,2,2,2} was synthesized using (S)- -bromopropionic acid. The fused bicyclic compound
7{1,3,1,3}, 7{2,3,1,3} and 7{2,3,1,3} in solution. Disappointingly, no cyclic product, not even a trace
6
α
featuring two asymmetric carbons was obtained as a mixture of diastereoisomers with a 5:1 ratio.
Both diastereoisomers were isolated by reversed-phase HPLC, and their structures were confirmed
from their 1D and 2D NMR spectra. The assignment of the configuration of the new stereogenic center
in both diastereomers of fused bicycle
6{1,2,2,2} was based on the diagnostic NOE effect observed
in the NOESY 1D and ROESY 2D spectra of the compounds in deuterated DMSO. Whereas the

Molecules 2018, 23, 1090
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NOE correlation between 6-H and CH was exhibited in both diastereoisomers, the NOE correlation
3
between CH and 9a-H was exhibited only in the minor isomer and thus was decisive in confirming
3
its configuration as (R,R)-
same experimental procedure and 1D and 2D NMR experiments, we identified the structures of the
major and minor isomers of the enantiomeric compound afforded using (R)- -bromopropionic acid
as (S,S)- {1,2,3,2} and (S,R)- {1,2,3,2}, respectively. Both diastereoisomers were isolated and fully
characterized (Table 2).
6{1,2,2,2} and that of the major isomer as (R,S)-6{1,2,2,2} (Figure 4). Using the
α
6
6
Figure 4. Configurational assignment of compounds (
a
) (R,R)- and (R,S)-
6
{1,2,2,2} and (
b
) (S,S)- and
(
S,R)-6{2,2,2,2}.
Table 2. Synthesized chiral fused heterocycles 6.
Entry
Compound
(R,R)- {1,2,2,2}
(R,S)-6{1,2,2,2}
(S,S)-6{1,2,3,2}
(S,R)-6{1,2,3,2}
(S,S)-6{2,2,3,2}
(S,R)-6{2,2,3,2}
m
n
R1
Rings
Purity [%] a
Yield [%] b
1
2
3
4
5
6
6
1
1
1
1
2
2
2
2
2
2
2
2
(R)-Me
(R)-Me
(S)-Me
(S)-Me
(S)-Me
(S)-Me
7 + 5
7 + 5
7 + 5
7 + 5
7 + 6
7 + 6
85
15
85
15
55
45
b
49
6
44
8
32
17
a
The overall purity of crude final compounds (R and S)-
6
after a five-step synthesis.
Yield of the
HPLC-purified compounds.
The reactions with (R)-α-bromopropionic acid provided [7 + 6]-fused ring model compounds
6
{2,2,3,2} as a 55:45 mixture of diastereomers (Table 2). Both isomers were isolated by reversed-phase
HPLC, and their structures were confirmed by 1D and 2D NMR spectroscopy. The NOESY experiments
were performed in CDCl because of the overlap of the resonances of protons H6 and H10a in
3
DMSO-d . The results indicated that the major isomer prepared using (R)-α-bromopropionic acid was
6
(S,S)-6{2,2,2,2} and the minor isomer was (S,R)-6{2,2,2,2} (Table 2).
NMR spectra of the compounds in CDCl provided interesting information regarding the
3
stability of the diastereomers. We observed that the purified isomer (S,S)-
stable in CDCl solution and was partially converted into the (S,R)- {2,2,3,2} isomer at room
{2,2,3,2}, which was converted
{2,2,3,2}. After 8 days in CDCl solution, the two isomers (S,S)- {2,2,3,2} and (S,R)- {2,2,3,2}
6{2,2,3,2} was not
6
3
temperature. The same effect was also observed for isomer (S,R)-
6
into (S,S)-
6
6
6
3
formed an equilibrium mixture of diastereomers with the same ratio (55:45), likely due to the opening
of the acid-mediated fused ring and the formation of the cyclic iminium followed by ring closure.
The same ratio of diastereomers was observed in the crude product after its TFA-mediated cleavage
from the resin.
The experimental results indicated that 8-membered cyclic N-acyliminiums formed fused systems
with 5-membered rings but not with 6 and 7-membered rings. To address the effect of introducing
a conformational constraint into the acyclic component (3-aminopropanol and 4-aminobutanol)
on cyclization, we prepared resin-bound intermediates using 2-(aminomethyl)phenol (
2-(aminomethyl)phenyl)methanol ( ) (Figure 5). The beneficial effect of conformational constraints on
cyclization was reported in unrelated cyclizations [39 40]. N-Fmoc derivatives of amino alcohols and
, amino aldehydes with three different carbon spacers, bromoacetic acid and Tos-Cl were used for the
8) and
(
9
,
8
9

Molecules 2018, 23, 1090
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synthesis of model compounds 10 according to Scheme 1. Cleaving compounds 10{1,1} and 10{1,2}
was accomplished using a mild cleavage cocktail of 10% TFA in DCM. For compounds 10{2,1} and
1
0{2,2}, the typical cleavage cocktail (50% TFA in DCM) was employed.
The results indicated that good to excellent purities and yields of 10 were successfully obtained
via traceless solid-phase synthesis of combinations of cyclic N-acyliminium 6- and 7-membered rings
prepared from protected aldehydes with one and two-carbon spacers, n = 1, 2) with 6- and 7-membered
rings (Table 3). Neither of the constrained amino alcohol promoted cyclization to form [8 + 6]- or
(
[8 + 7]-fused rings.
Figure 5. Compounds 10 prepared using the constrained amino alcohols
8 and 9 according to
the Scheme 1.
Table 3. The synthesized fused heterocyclic compounds 10.
Yield [%] b
Entry
Compound
m
n
Purity [%] a
1
2
3
4
10{1,1}
10{1,2}
10{2,1}
10{2,2}
0
0
1
1
1
2
1
2
76
81
97
96
41
43
90
81
a
The overall purity of the crude final compounds 10 after a five-step synthesis process; b Yield of the
HPLC-purified compounds.
To demonstrate the second traceless strategy of the synthetic route for molecular scaffolds,
we synthesized model compounds containing aromatic C-nucleophiles to effect the fused ring closure.
In the previous synthesis, we used oxygen nucleophiles from amino alcohols for immobilization,
a scenario that is not applicable for C-nucleophiles. To change our synthetic strategy, we attached
the acyclic precursors to the resin via amide nitrogen. This strategy also allowed for the traceless
synthesis of the target compounds. As shown in Scheme 2, synthesis was carried out on aminomethyl
polystyrene resin 11 equipped with an acid-labile BAL (4-(4-formyl-3-methoxyphenoxy)butyric acid)
linker [41].
Scheme 2. Polymer-supported synthesis of fused bicyclic compounds 15 and 16. Reagents and conditions:
(i) 0.5 M 2-(3-methoxyphenyl)ethan-1-amine in 10% AcOH/DMF, rt, overnight, then NaBH(AcO)3,
rt, 5 h, then a second portion of NaBH(AcO) , rt, 3 h; (ii) bromoacetic acid, DIC, DCM, rt, 1 h;
3
(
(
iii) amino-aldehyde dialkyl acetal, DIEA, DMF, rt, 2 h; (iv) Tos-Cl, DIEA, DCM, rt, 2 h; (v) TFA/DCM
1:1), rt, 1 h.

Molecules 2018, 23, 1090
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After immobilization of the primary amines via reductive amination using 2-(3-methoxyphenyl)
ethan-1-amine, secondary amines resin 12 was acylated with bromopropionic acid to give resin
13. Then, the reaction with the amino-aldehyde dialkyl acetal, followed by N-derivatization with
Tos-Cl, were carried out to yield intermediates 14. Finally, TFA exposure triggered the release of the
polymer-supported acyclic precursors from the resin, unmasking the protecting groups and forming
the six- and seven-membered N-acyliminium ions. Internal nucleophilic attack then provided target
molecules 15{1} and isomeric 16{1} or, in the case of the protected aldehyde with the n = 2 carbon
spacer, only compound 15{2} (Table 4). These compounds were isolated by reversed-phase HPLC, and
NMR spectra confirmed their structures. Disappointingly, formation of the [8 + 6] combination was
not detected.
Table 4. Synthesis of fused heterocyclic compounds 19 and 20.
Yield [%] b
Entry
Compound
n
Purity [%] a
1
2
3
15{1}
16{1}
15{2}
1
1
2
83
17
77
42
6
22
a
The overall purity of the crude products after a five-step synthesis. b Yield of the HPLC-purified compounds.
3
. Materials and Methods
3
.1. General
The solid-phase syntheses were carried out in plastic reaction vessels (syringes equipped with
porous discs) using a manually operated synthesizer [42]. The volume of the wash solvent was 10 mL
per 1 g of resin. The resin slurry was washed by shaking it with fresh solvent for at least 1 min before
the solvent was changed. All reactions were carried out at ambient temperature unless stated otherwise.
Commercially available Wang (100–200 mesh, 1.0 mmol/g) and aminomethyl resins (100-200 mesh,
1.26 mmol/g) were used. Commercial chemicals and solvents were ACS reagent grade. The yields of
the crude products were calculated with respect to the loading of the first building block.
3
.2. Reaction of Wang Resin 1 with Trichloroacetonitrile and Fmoc-amino Alcohol (Resin 2)
Wang resin (1 mmol/g loading, 1 g) was suspended in 10 mL of anhydrous DCM, then 1.5 mL of
trichloroacetonitrile was added, and the resin was left in a freezer for 30 min. Next, a solution of 100
of DBU in 2 mL of anhydrous DCM was added, and the slurry was shaken for 1 h. The resin was
washed with anhydrous DCM (3 ) and anhydrous THF (3 ). The solution of Fmoc-amino alcohol
3 mmol) in 10 mL of anhydrous THF was added to the resin, followed by the dropwise addition of
L of a solution of BF3 Et O, and the slurry was shaken for 30 min. Resin was washed THF (3 ),
MeOH (3 ), and DCM (5 ). A sample of the resin was washed with MeOH (3 ) and dried, and
0 mg was cleaved with 50% TFA for 30 min, and the product quantified (Fmoc absorbance at 300 nm).
Typical loading was between 0.35–0.50 mmol/g.
1
µL
×
×
(
6
3
µ
·
2
×
2
×
×
×
1
3
.3. Acylation with α-Bromocarboxylic Acids (Resins 3 And 13)
Resin (1 g) was washed with DCM and DMF, Fmoc was deprotected with 50% piperidine in
DMF for 15 min, and the resin was washed with DMF (3 ) and DCM (5 ). Reaction of the resin
with a solution of -bromocarboxylic acid (5 mmol) in 10 mL of DCM was carried out in a syringe
with a frit, and DIC (2.5 mmol, 386 L) was added. After 5 min, DIU was filtered, and the solution
was transferred to the syringe with resin and shaken for 1 h. When bromoacetic acid was used,
DIEA (2.5 mmol, 436 µL) was also added. Then, the resin was washed with DCM (3×).
2
×
×
α
µ
1

Molecules 2018, 23, 1090
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3
.4. Reaction with Amino-aldehyde Dialkyl Acetal and N-Derivatization (Resin 4)
Resin (1 g) was washed with DMF (3 ), a solution of amino-aldehyde dialkyl acetal (10 mmol)
3
×
and DIEA (10 mmol, 1.74 mL) in 10 mL of DMF was added, and the slurry was shaken for 2 h.
An analytical sample was taken for analysis, reacted with 0.5 M Fmoc-OSu in DCM for 1 h and cleaved
with 50% TFA for 30 min.
Resin from the previous step was split into four 250 mg portions, which were washed
with reaction solvent: DCM for sulfonamides (Ns-Cl, Tos-Cl) (1 mmol) and DMSO for 4-fluoro-3-
nitrobenzotrifluoride (1 mmol). Then, 3 mL of the reaction solvent and DIEA (1.8 mmol, 174 µL) was
added, and the slurry was shaken for 2 h (in the case of sulfonamides) or overnight (in the case of
4
-trifluoromethyl-2-nitroaryl). Resin
4
was washed with the reaction solvent (3
×
) and then DCM (3 ).
×
A sample for analysis was reacted with 0.5 M Fmoc-OSu in DCM for 1 h and cleaved with 50% TFA for
3
0 min.
3
.5. Acid-Mediated Cleavage, Cyclization and Isolation (Compounds 6 and 7)
Resin
washed with 50% TFA in DCM (3
of nitrogen. When TFA exposure yielded cyclic hemiaminals (
in MeOH and left at rt overnight to yield . Compounds and
methanol and purified by semi-preparative reversed-phase HPLC.
4
was treated with 50% TFA in DCM for 1 h. The TFA solution was collected, the resin was
), and the extracts were combined and evaporated under a stream
), the oily residues were dissolved
were dissolved in acetonitrile or
×
5
7
6
7
3
.6. Reductive Amination of the BAL Resin (Resin 12)
A 20 mL syringe was charged with BAL resin 11 (500 mg) and washed with anhydrous DMF (3
Then, 0.5 M 2-(3-methoxyphenyl)ethan-1-amine and 10% AcOH in anhydrous DMF (5 mL) was added
into the syringe and shaken overnight. Next, sodium triacetoxyborohydride NaBH(AcO) (2.5 mmol,
×
).
3
5
28 mg) was added to the syringe. Because of the evolution of hydrogen gas, the reaction vessel was
vented to relieve pressure by piercing it on the top with a needle on the top, placing it in a horizontal
position and shaking it for 5 h. A second portion of NaBH(AcO) (2.5 mmol, 528 mg) was added and
3
shaken for 3 h. After the reaction, the resin was washed with 5% AcOH in DMF (3
×
) and DMF (3
×
),
neutralized with 5% piperidine in DMF (10 mL) and washed with DMF (5
×
) and DCM (5 ) to yield
×
resin 12.
3
.7. Reaction with Amino-aldehyde Dialkyl Acetal and N-Derivatization (Resin 14) Acid-mediated Cleavage,
Cyclization and Isolation (Compounds 15 and 16)
The protocol was the same as that described for the synthesis of compound 6.
4
. Conclusions
We developed two synthetic strategies for traceless polymer-supported synthesis of natural
product-like molecular scaffolds comprising two fused rings. The linear precursors were cyclized
via acid-mediated tandem N-acylium ion formation followed by nucleophilic addition of O- and
C-nucleophiles. Synthetic scheme facilitated the preparation of any combination of acyclic ring
precursors for the synthesis of different ring sizes and assessed ring-forming limitations with respect
to ring sizes. [6,7 + 5,6,7]-Fused cyclic combinations were obtained with excellent crude purity and
yield. The 8-membered cyclic iminium was successfully fused only with the 5-membered cycle,
whereas larger fused rings were not formed. The syntheses proceeded under mild conditions and
used commercially available building blocks. Further work exploring the potential applicability of this
strategy to larger ring sizes by using building blocks with the predisposition to adopt conformations
favorable for large ring closure is in progress.
Supplementary Materials: The following are available online. 1H, 13C NMR spectral data and figures of
all compounds.

Molecules 2018, 23, 1090
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Author Contributions: V.K. conceived and designed the experiments; V.G.-N. performed the experiments.
Acknowledgments: This research was supported by the Department of Chemistry and Biochemistry, University
of Notre Dame, and by research grant 16-06446S from the Grant Agency of the Czech Republic (GACR).
Conflicts of Interest: The authors declare no conflict of interest.
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2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).