Combinatorial Synthesis of Functionalized 1,3-Thiazine Libraries Using a Combined Polymer-Supported Reagent/Catch-and-Release Strategy

Article abstract of DOI:10.1002/anie.200352731

A dual role: The supported sulfonic acid reagent promotes the ring closure of the thiazine products and also binds them in situ as ion pairs (see scheme). The nonbasic by-products and excess reagents can be removed by filtration and the products subsequen

Full text of DOI:10.1002/anie.200352731

Angewandte
Chemie
are mainly dominated by two strategies: solid-phase syn-
thesis[2] and with increasing importance solution-phase pro-
tocols with the aid of scavenger resins and polymer-bound
reagents.[3] A special concept in solution-phase synthesis is the
“catch-and-release” strategy, where the formed target mole-
cule is selectively bound (covalently or by means of an ionic
bond) to a resin. After excess reagents, unreacted starting
materials, catalysts, etc. are removed by simple washings, the
product is released from the polymer support. One could
envision that in an ideal case such catch-and-release methods
could be incorporated in the synthesis step. Appropriately
functionalized polymeric resins could be utilized that not only
mediate a specific chemical reaction but at the same time
selectively remove the desired product from the reaction
mixture. However, examples for such one-pot, in situ syn-
thesis/catch-and-release protocols are extremely rare.[4]
Herein we report on the generation of libraries of densely
functionalized (five diversity points) 1,3-thiazines of type 8,
employing a resin-bound sulfonic acid that acts simultane-
ously as reaction promoter in a cyclocondensation step and as
a selective sequester of the final basic thiazine products
(Scheme 1).
Compound Libraries
Combinatorial Synthesis of Functionalized 1,3-
Thiazine Libraries Using a Combined Polymer-
Supported Reagent/Catch-and-Release
Strategy**
Gernot A. Strohmeier and C. Oliver Kappe*
Combinatorial chemistry has emerged as a highly valuable
and powerful method in medicinal chemistry, catalyst discov-
ery, and materials science over the past few years.[1] Driven by
the force to discover and develop new molecules with tailored
properties more efficiently and in increasingly shorter times,
scientists have developed several highly successful concepts.
Combinatorial methods focusing on small organic molecules
Scheme 1. Synthesis of thiazines 8. a) 10 mol% 3, chlorobenzene,
1158C, 5 h; b) 0.6 equiv 5, 0.5 equiv 6, dioxane, 908C, 18 h; c) wash-
ing, then MeOH/TEA 3:1. TEA=triethylamine.
The 2-amino-1,3-thiazine-5-carboxylate scaffold 8 has
hitherto received scant attention,[5,6] despite its close struc-
tural similarity to the dihydropyrimidinones (DHPMs), which
are privileged structures with heterocyclic cores[7] and well-
documented pharmacological properties.[8] The overall strat-
egy for the generation of a library of thiazines 8 utilizing the
tandem ring-closure/resin-capture approach is outlined in
Scheme 1.
[*] Dr. G. A. Strohmeier, Prof. Dr. C. O. Kappe
Institute of Chemistry, Organic and Bioorganic Chemistry
Karl-Franzens University Graz
Heinrichstrasse 28, A-8010 Graz (Austria)
Fax: ( +43)316-380-9840
E-mail: oliver.kappe@uni-graz.at
[**] We are indebted to BASF AG, Ludwigshafen (Germany), and the
Austrian Science Fund (FWF, P-15582) for financial support.
Our synthesis started with the Knoevenagel condensation
of b-keto esters 1 with aldehydes 2 under open-vessel
conditions to facilitate the removal of water formed during
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 621 –624
DOI: 10.1002/anie.200352731
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621

Communications
Table 1: Solution-phase synthesis of 1,3-thiazines 8.
the reaction. A slight excess of the aldehyde was required to
Entry R1 R2 R3
R4
X
Yield [%][a] Purity [%][b]
ensure good conversion in this step. We found the polymer-
bound piperazine diacetate 3 to be a very effective catalyst for
this reaction.[9] Not only is the catalyst itself easily removed,
this method also removes the basic 2-amino-1,3-cyclohexa-
diene by-products formed by known catalyst-derived side
reactions.[10,11] Importantly, these basic by-products must be
removed so they do not interfere with the subsequent catch-
and-release of the thiazine products (4!7!8). After filtra-
tion from the catalyst the crude enones were used directly in
the subsequent ring-closing reaction. In the second step, the
enones 4 (in excess) were treated with the appropriate
thioureas and the polymer-bound sulfonic acid 6[12,13] at 908C
in inert dioxane, which proved superior to other solvents
tested. Since the formed 1,3-thiazines are the only basic
molecules in the reaction system, they are selectively
captured by the supported sulfonic acid, presumably as tightly
coordinated ion pairs with the strongly basic amidine-like
isothiourea moiety.[14] Here, the polymer-bound sulfonic acid
acts as an acidic mediator to facilitate the thiazine ring-
closure and subsequently as a selective sequester for the
desired basic thiazine products (see Scheme 1). Finally, by
filtration and multiple washing steps, reagents, excess starting
materials, solvents, and by-products (see below) are removed.
The desired products are cleaved from the resin by displace-
ment with triethylamine, which is a significantly stronger base
than the 1,3-thiazines 8.
1
2
Et Me Ph
Et Me Ph
NH2
S
S
S
S
S
S
S
S
S
78
97
72
71
18
10
51
89
45
93
>98
93
96
>98
80
94
>98
94
NHMe
NHPh
cycl[c]
NHMe
NH2
NHBn
NHMe
NH2
NH2
NH2
NHMe
NHBn
NH-Mes
NH2
3
Et Me Ph
4
Et Me Ph
5
6
7
8
Et Me 2-CF3-Ph
Me Et 2-CF3-Ph
Me Et 2-CF3-Ph
Et Me 2,3-Cl-Ph
Et Pr 2,3-Cl-Ph
Et Pr 2,3-Cl-Ph
Et Pr 2-Cl-Ph
Bn Me 2-Cl-Ph
Et Pr 2-Cl-Ph
Bn Me 2-Cl-Ph
Et Me 2-thienyl
Et Me 2-thienyl
Et Me 2-thienyl
Et Pr C5H11
Et Me 3-OH-Ph
Et Me 3-OH-Ph
iPr Me 3-Cl-Ph
iPr Me 3-Cl-Ph
Et Pr 3-NO2-Ph
Et Pr 3-NO2-Ph
Bn Me 3-NO2-Ph
Me Et 4-Cl-Ph
Me Et 4-Cl-Ph
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
Se 11
90
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
35
30
70
42
56
15
15
26
24
36
78
75
22
46
13
32
66
43
67
94
98
98
87
96
>98
90
85
84
90
98
96
89
78
72
97
95
NHMe
cycl[c]
NH2
NH2
NHMe
NH2
NHMe
NH2
NHMe
cycl[c]
NH2
NHPh
iPr Me 2,5-MeO-Ph NH2
iPr Me 2,5-MeO-Ph NHMe
91
92
This approach was used to prepare 28 1,3-thiazines and
one 1,3-selenazine[15] in general good to excellent purities and
overall yields from good to moderate over the two reaction
steps (Table 1). It is evident that aldehyde building blocks 2
without or with little steric hindrance lead to generally higher
yields. For aromatic aldehydes, substituents in ortho position,
in particular if they are electron withdrawing, lower the yields
significantly, although not the purities. Aliphatic substituents
in position 4 of 1,3-thiazines (see R2 in Table 1) are important
for the success of the method. Despite the effects of the
substituents on the efficiency of the thiazine ring-closure, it is
important to note that even in cases of low yields (10–15%)
the purity of products is still high (72–98%). This clearly
demonstrates the success of the tandem ring-closure/catch-
and-release strategy, since incomplete conversions—presum-
ably as a result of the reduced reactivity of some building
blocks—do not result in the propagation of impurities. The
efficacy of the concept is clearly illustrated by HPLC
monitoring at different stages of the synthesis.[16]
[a] Yields calculated on the basis of the experimentally determined
loading of over two reaction steps. [b] Purity
(4.18 mmolgꢀ1
determined by LC-MS. [c] Imidazolidine-2-thione (see Scheme 1).
6
)
In order to synthesize more diverse derivatives of the
parent scaffold, we envisioned a selective and flexible
functionalization of the amino group in position 2 of the
1,3-thiazines 8. This is of particular importance because the
number of thioureas successfully applied in the ring-closure
sequence 4 + 5!8 is limited. Therefore, we developed a
protocol for the selective alkylation of the 2-amino group on
the thiazine ring utilizing the Mitsunobu reaction as the
diversity-generating method and again employing polymer-
supported sequesteration reagents (Scheme 2).[17]
Scheme 2. Scaffold decoration of thiazine libraries by Mitsunobu alkyl-
ation. a) 2 equiv TFAA, dichloromethane, RT, 30 min; b) 4 equiv DIAD,
4 equiv TPP, 4 equiv R5OH, THF, RT, 12 h; c) 5 equiv 6, RT, 10 min;
d) 2 equiv 11, RT, 12 h; e) aq NH3, RT, 3 h; f) 2 equiv 6, RT, 10 min;
g) MeOH/TEA 3:1. DIAD=diisopropyl azodicarboxylate, RT=room
temperature, TEA=triethylamine, TFAA=trifluoroacetic anhydride,
TPP=triphenylphosphane.
This intermediate (9) was easy to generate, and the trifluoro-
acetate was easy to remove at the end of the synthesis by
treatment with aqueous ammonia. The alkylation sequence
started with the standard acetylation of 2-aminothiazines 8
We found that activation of the amino group as a
trifluoroacetamide was an effective method to provide an
acidic nitrogen prone to undergo Mitsunobu alkylation.[18]
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Chemie
(R4 = H) with trifluoroacetic anhydride and subsequent
concentration to dryness to produce pure nonbasic trifluoro-
acetamides 9. This change of basicity upon acylation also
plays a critical role in the subsequent purification. Mitsunobu
alkylation with primary alcohols was performed under
classical conditions using a combination of diisopropyl
azodicarboxylate (DIAD) and triphenylphosphane as well
as the appropriate alcohol.[18] It should be noted that the
application of a polymer-bound Mitsunobu reagent[19] did not
result in useful conversions. Since the alkylated thiazine 10
remains nonbasic, it is possible to remove excess Mitsunobu
reagent and the reduced N,N’-diacylhydrazide by-product by
treating the reaction mixture with polymer-supported sulfonic
acid 6. In order to completely sequester all of the Mitsunobu
reagent we also added a resin-bound amine base (11) that
effectively sequestered the azodicarboxylate by forming
Table 2: Solution-phase N-alkylation of 1,3-thiazines under Mitsunobu
conditions to give products 12.
Entry R1 R2 R3
R5
Yield [%] Purity [%][a]
1
2
3
4
5
6
Et
Et
Et
Et
Et
Me Ph
Me 2,3-Cl-Ph 2-EtO-ethyl 41
Me 2-thienyl
Me C5H11
Me 3-Cl-Ph
3-Me-butyl
38
95
80
94
75
96
94
2-EtO-ethyl 52
3-F-benzyl
hexyl
19
24
19
Me Et
4-Cl-Ph
propyl
[a] Purity determined by LC-MS.
addressed in the synthesis and subsequent scaffold decora-
tion, with yields up to 97% and good to excellent purities.
either the polymer-bound triazene or monoamide Experimental Section
(Scheme 3).[20,21] To the best of our knowledge this propensity
Typical procedure for the Knoevenagel condensation (outlined for
the enone leading to thiazine 8, entry 1 in Table 1: Benzaldehyde
(110 mg, 1.04 mmol), ethyl acetoacetate (118 mg, 0.91 mmol), and
polymer-supported piperazine (as the diacetate) (90 mg, 10 mol%)
were placed in a glass vial containing chlorobenzene (1 mL) and
heated at 1158C for 5 h under open-vessel conditions. After filtration
and washing with dry dioxane (2 ” 0.8 mL) the combined filtrates
(approximately 2 mL) were directly subjected to the subsequent
reaction.
A solution of the appropriate enone and thiourea (38 mg,
500 mmol) were added to dry DOWEX50X2 6 (102 mg, 426 mmol,
4.18 mmolgꢀ1 as experimentally determined) in a Teflon frit (ACT
Synthesizer PLS 6x4) and heated at 908C for 18 h. After cooling, the
resin was washed (dioxane, MeOH, water, MeOH, dichloromethane),
and the product was released by addition of triethylamine (500 mL)
and methanol (1.5 mL). After the cocktail had been shaken for
20 min, it was filtered and the resin washed twice with 10%
triethylamine in methanol (1.5 mL). The combined filtrates were
evaporated to dryness, redissolved in dichloromethane, and filtered
through a 1-cm plug of silica gel (eluent: ethyl acetate/petroleum
ether 3:1) yielding 92.4 mg (334 mmol, 78% based on experimentally
determined loading of the ion-exchange resin) of compound 8
(entry 1 in Table 1).
Scheme 3. Polymer-supported scavenging mechanisms for DIAD.[22]
of polymer-supported amines to react with azodicarboxylates
constitutes a new strategy for the removal of Mitsunobu
reagents. Procedures previously published applied tagged or
polymer-supported phosphanes,[22,23] acid-labile ester groups
(tert-butyl) for the selective degradation of Mitsunobu
reagents,[23] and ROMP-based sequestration.[24] Finally the
trifluoroacetate is cleaved with aqueous ammonia to yield
thiazine 12 in its basic form. This recovery of basicity now
allows for a catch-and-release strategy at the end using
sulfonic acid resin 6 to produce pure monoalkylated thiazine
products. The final purification effectively removes the excess
triphenylphosphane and the formed triphenylphosphane
oxide. It must be pointed out that the manipulations required
for the sequence 8!12 consist solely of evaporation and
polymer-assisted steps, and the sequence is therefore appli-
cable to parallel synthesis.
Selective N-monoalkylation was accomplished in accept-
able yield and products the were obtained with good purity
over three reaction steps and subsequent catch-and-release
purification (Table 2). In all cases the Mitsunobu reactions
went to completion; the moderate yields were a consequence
of incomplete sequestration of the product from the complex
reaction mixture.
Received: August 28, 2003[Z52731]
Keywords: combinatorial chemistry · heterocycles · molecular
.
recognition · polymer-bound reagents · scavenger resins
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In summary, we have presented a concept for the
construction of diverse libraries of 1,3-thiazines based on
the dual action of a polymer-bound sulfonic acid as the
mediator of a ring-closure reaction and the concomitant
scavenger of the desired product. Five diversity points are
Angew. Chem. Int. Ed. 2004, 43, 621 –624
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Communications
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624
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