Mol Divers DOI 10.1007/s11030-014-9542-6
FULL-LENGTH PAPER
Catalytic enantioselective diversity-oriented synthesis of a small library of polyhydroxylated pyrans inspired from thiomarinol antibiotics Raed M. Al-Zoubi · Dennis G. Hall
Received: 22 May 2014 / Accepted: 30 July 2014 © Springer International Publishing Switzerland 2014
Abstract A small library of 30 thiomarinol analogues was successfully synthesised using as a key step–a catalytic enantioselective tandem oxa[4+2] cycloaddition/aldehyde allylboration methodology. With this method, highly substituted α-hydroxyalkyl dihydropyrans were assembled in a single three-component reaction utilizing three different enol ethers and a wide variety of aldehydes, such as aromatic, heteroaromatic, unsaturated and aliphatic aldehydes. In a second operation, a mild and direct method for reducing an acetal unit in the α-hydroxyalkyl dihydropyrans was optimised without the need for protecting a nearby hydroxyl group. This procedure facilitated the synthetic sequence, which was completed by a dihydroxylation of the residual olefin of α-hydroxyalkyl 2H -pyrans to provide the desired library of dihydroxylated pyran analogues reminiscent of the thiomarinol antibiotics. The relative stereochemistry of the resulting library compounds was demonstrated by X-ray crystallography on one of the analogues.
Keywords Allylboration · ATP mimics · Combinatorial library · Enantioselective catalysis · Hetero-Diels–Alder cycloaddition · Pyrans
Electronic supplementary material The online version of this article (doi:10.1007/s11030-014-9542-6) contains supplementary material, which is available to authorized users. R. M. Al-Zoubi · D. G. Hall (B) Department of Chemistry, University of Alberta, Edmonton, AB T6G 2G2, Canada e-mail: [email protected]
Introduction Mupirocin (pseudomonic acid A, Fig. 1) is one of the world’s leading topical antibiotics initially commercialised by GlaxoSmithKline under the name Bactroban [1]. It is produced by Pseudomonas fluorescens and was reported to possess antibacterial activity as early as 1887 [2]. In the 1960s, the mixture of pseudomonic acids was found to be the active component [3]. The major constituent was identified and characterised in the 1970s, and was named pseudomonic acid A (mupirocin) [4,5]. Mupirocin is active against Grampositive aerobic bacteria and a few Gram-negative strains. It is prescribed for treating skin infections, such as impetigo, candidiasis as well as burn wounds and cuts. Mupirocin inhibits the bacterial isoleucyl tRNA synthetase enzyme responsible for loading the amino acid isoleucine (Ile) onto its cognate tRNA, a process required for ribosomal protein synthesis [6]. It was found to be 10,000 times more potent on the bacterial enzyme than on the human homologue, which ensures its selectivity as a pharmaceutical drug [7,8]. The binding of mupirocin to the enzyme– tRNA complex is competitive with isoleucine and ATP, and also with Ile-AMP derivatives. The X-ray crystal structure of mupirocin bound to the complex between tRNAIle synthetase and its cognate tRNA was solved in 1999 [9]. It confirmed that mupirocin acts mainly as an ATP mimic and occupies the site normally occupied by the ribose ring. Pseudomonic acids have attracted considerable attention for their unique structure and biological activity [10]. Unfortunately, mupirocin displays poor oral absorptivity and low metabolic stability. In the bloodstream, its ester linkage is quickly hydrolyzed to an inactive product, monic acid [11]. Consequently, there has been significant interest in the development of improved analogues that could also be suitable as oral antibiotics. Thiomarinol A [12] and thiomarinol H
123
Mol Divers Fig. 1 Structures of mupirocin (pseudomonic acid A), monic acid and selected members of the thiomarinol family
[13] and others [14,15] (Fig. 1) are rare marine natural products recently isolated from the bacterium Alteromonasrava sp. nov. SANK 73390. These closely related families of naturally occurring antibiotics display minor structural differences. The structures of thiomarinols A and H differ from mupirocin by the notable presence of a C4-hydroxyl, a shorter C1-alkoxy chain, and the replacement of the C10– C11 epoxide with an E alkene unit. These natural substances were found to be equally potent as pseudomonic acid A, and thiomarinol A was found to possess a wider spectrum of activity (against both Gram-positive and -negative bacteria) [12,13]. Our group has recently synthesised thiomarinol H [16] and other thiomarinol analogues [17]. We postulated that the same method could be amenable to the design and synthesis of a diversity-oriented library of simplified analogues. It is believed that mupirocin and the thiomarinols share the same target, bacterial isoleucine tRNA synthetase. A pharmacophore model of mupirocin was developed even before the availability of X-ray crystallographic information [9], which revealed that the dihydroxypyran core is essential for the
123
OH O HN
O
H N
8
5 O O
left-hand side chain (variable)
OH
HO
HO
H
11
O right-hand side chain (somewhat variable, size must be small)
pyran core (conserved)
Fig. 2 Qualitative pharmacophore model for thiomarinol
antibacterial activity and it should be conserved. It was also shown that the left-hand and right-hand side chains are quite variable, as depicted in Fig. 2. Natural products are ideal lead structures in combinatorial library design and can deliver higher hit rates, as exemplified with several convincing examples in a review paper by Waldmann [18]. Numerous enzymes are ATP binders. In order to perform small molecule screening against some of these enzymes, we planned to design a small library of thiomarinol analogues.
Mol Divers
OEt
OEt
OEt
2b
2a
2c
CHO
CHO
CHO CHO
X
X 3a, X = H 3b, X = CN 3c, X = CH 3 3d, X = NO 2 3e, X = CF3 3f, X = F
3l
3j
3g, X = CH3 3h, X = F
CHO
CHO
F CHO
Br 3i
3m
3k CHO
H N O
N
N
CHO 3n
3o
CHO 3p
O
X
CHO
3q, X = O 3r, X = S
CHO CHO
Scheme 1 Three-component hetero [4+2] cycloaddition/allylboration reaction approach to a library of thiomarinol analogues
3s
3t
O
CHO
3u
Fig. 3 Enol ethers 2(a–c) and aldehydes 3(a–u) as diversity reagents
Results and discussion
Three-component synthesis of α-hydroxyalkyl dihydropyran intermediates 5
Design and retrosynthesis of a diversity-oriented library The library retrosynthesis is presented in Scheme 1. All the thiomarinol analogues 8 were prepared from key intermediate 6 through a catalytic enantioselective inverse electron demand hetero [4 + 2] cycloaddition/allylboration tandem reaction between boronoacrolein pinacol ester 1, enol ethers 2, and aldehydes 3 developed in the Hall Laboratory [19]. The cycloaddition step is catalysed by Jacobsen’s Cr(III) complex 4 [20,21] and affords dehydropyranyl boronates 5 which can be trapped with aldehydes (3) to give products 6. This multicomponent reaction functions well with a wide range of enol ethers and aldehydes, and was demonstrated successfully in the total synthesis of thiomarinol H [16]. Acetal reduction on intermediates 6 and a stereoselective dihydroxylation of dihydropyrans 7 would afford the final, desired thiomarinol analogues 8. The set of building blocks depicted in Fig. 3 was chosen based on the objective of displaying a diversity of functional groups and structural elements. Three different enol ethers 2a, 2b and 2c were used for the preparation of the library. Enol ether 2c was made using a known method [22,23], while 2a and 2b were commercially available. The chosen panel of commercially available aldehydes included benzaldehydes substituted with various functional groups, as well as heterocyclic and α, β-unsaturated aldehydes.
The key substrate (E)-3-boronoacrolein pinacolate 1 was made in good yield in two steps according to our published procedure [19,24]. The three-component [4+2] cycloaddition/ allylboration procedure previously optimised in the Hall Group was employed in the first step of library preparation. Towards this end, (E)-3-boronoacrolein pinacol ester 1 was reacted with enol ethers 2(a–c) to form the cycloadducts 5, which then reacted with a representative subset of the aldehydes 3(a–u) (Fig. 3) to provide the desired α-hydroxyalkyl dihydropyran analogues 6 (Table 1). It is noteworthy that the thermal allylboration reaction required longer reaction times and elevated temperatures with substituted enol ether 2c than with enol ethers 2a and 2b. The scope of aldehydes that can be utilised in the threecomponent oxa[4+2] cycloaddition/allylboration reaction is remarkable. As indicated with the yields shown in Table 1, a variety of substituted aromatic, heteroaromatic, α, βunsaturated and aliphatic aldehydes are all suitable substrates for this tandem reaction. All cycloaddition reactions were performed at a temperature between 18 and 20 ◦ C, and quickly purified by flash chromatography (deactivated silica) following the general published procedure from our group [19,24]. To deduce the optical purity of products 6 and validate the high optical purity of the allylboronate precursors 5 expected from our previous reports [19,24], the optical purity
123
Mol Divers Table 1 Synthesis of α-hydroxyalkyl pyrans 6 from (E)-3-boronoacrolein pinacol ester 1, enol ethers 2(a–c) and aldehydes 3(a–u)
O
B
O R1 + OEt
O 1
O
Jacobsen's cat. 4 (1 mol%) BaO, 18-20 oC 1.5 h
B
O R1
R 2CHO (2 equiv) 3(a-u) 45-120
O
2(a-c)
oC
OEt
R1 R2
O
OEt
HO H 6 (>96 de, 95.5% ee)
5
Entry
Enol ether
Aldehyde
α-Hydroxyalkyl dihydropyran
% Yield
1
2a
3a
6aa
67
2
2a
3b
6ab
65
3
2a
3c
6ac
38
4
2a
3d
6ad
62
5
2a
3e
6ae
75
6
2a
3f
6af
73
7
2a
3g
6ag
47
8
2a
3h
6ah
51
9
2a
3i
6ai
71
10
2a
3j
6aj
81
11
2a
3k
6ak
49
12
2a
3l
6al
81
13
2a
3m
6am
79
14
2a
3n
6an
36
15
2a
3o
6ao
69
16
2a
3p
6ap
54
17
2a
3q
6aq
59
18
2a
3r
6ar
65
19
2b
3a
6ba
79
20
2b
3b
6bb
50
21
2b
3c
6bc
65
22
2b
3d
6bd
61
23
2b
3e
6be
63
24
2b
3g
6bg
93
25
2b
3h
6bh
97
26
2b
3i
6bi
59
27
2b
3j
6bj
78
28
2b
3k
6bk
53
29
2b
3l
6bl
74
30
2b
3m
6bm
59
31
2b
3q
6bq
58
32
2b
3s
6bs
84
33
2b
3t
6bt
63
34
2b
3u
6bu
70
35
2c
3a
6ca
79
36
2c
3b
6cb
72
37
2c
3c
6cc
65
38
2c
3g
6cg
67
123
Mol Divers Table 1 continued Entry
Enol ether
Aldehyde
α-Hydroxyalkyl dihydropyran
% Yield
39
2c
3h
6ch
62
40
2c
3l
6cl
72
41
2c
3m
6cm
43
42
2c
3s
6cs
75
43
2c
3t
6ct
57
of product 6a was measured by chiral HPLC as described before [19] and was confirmed to be >90 % ee. This process was repeated each time a new batch of catalyst 4 was prepared and employed in the cycloaddition. The relative stereochemistry of the optically enriched α-hydroxyalkyl dihydropyran products was confirmed by the X-ray crystallographic analysis of one of the library members (vide infra). Mechanistically, the [4+2] cycloaddition of 1-boronoacrolein pinacolate 1 with ethyl vinyl ether is expected to proceed with complete endo selectivity to give the allylboronate intermediate 5 (Scheme 2). In the allylboration step, a boat-like pyran transition state with a pseudo-equatorial ethoxy substituent was proposed [19,24] leading to the desired α-hydroxyalkyl dihydropyran product.
Reduction of the acetal unit of α-hydroxyalkyl dihydropyrans We then turned our attention to the reduction of the acetal unit in the α-hydroxyalkyl dihydropyran intermediates 6. Previous experience in the Hall group regarding this step was performed using TiCl4 /Et3 SiH conditions on a protected secondary alcohol (Eq. 1) [6,24,25].
(1)
Acetal reduction of one of the α-hydroxyalkyl dihydropyran derivatives using TiCl4 /Et3 SiH
Scheme 2 Stereoselectivity of tandem three-component oxa[4+2] cycloaddition/allylboration reaction
This precedent guided us in an attempt with substrate 6aa containing a free secondary alcohol. To simplify the preparation of the library, we aimed to reduce the acetal moiety directly from α-hydroxyalkyl dihydropyrans without protecting the α-hydroxyl group. Unfortunately, the acetal reduction reaction under TiCl4 /Et3 SiH was unsuccessful for the unprotected α-hydroxyl dihydropyrans, providing other reduced products. Nevertheless, many standard conditions for acetal reduction can be found in the literature. For instance, Gray and co-workers published in 1982 a report on the reductive cleavage of glycosides using BF3 · Et2 O/TFA/Et3 SiH conditions to provide the reduced acetal product in good to excellent yields at 0 ◦ C [26]. In 2000, Toone and co-workers reported a regioselective reduction of 4,6-O-benzylidenes using BF3 · Et2 O/Et3 SiH at 0 ◦ C [27]. A number of different reagents have been employed for the reduction of acetal units by other investigators, such as LiAlH4 –AlCl3 [28], NaCNBH3 /HCl [29], Et3 SiH/TFA [30], and EtAlCl2 /Et3 SiH [31]. With this precedent in mind, initial trials were conducted to investigate the most effective conditions to provide the requisite acetal reduced products in our library synthesis without any decomposition and unwanted side products.
123
Mol Divers
(A)
(C)
(B)
(D)
Scheme 3 The effect of acid in Et3 SiH-promoted acetal reduction of α-hydroxyalkyl dihydropyrans
To our satisfaction, the use of 1.2 equivalents of TiCl4 /Et3 SiH at −50 ◦ C provided the acetal ring open product 9aa in 78 % yield (Scheme 3, Eq. A), while the use of TFA/Et3 SiH provided a mixture of acetal ring open product 9aa and bicyclic product 10aa (Scheme 3, Eq. B). To our surprise, it was found that the use of only one equivalent of BF3 · Et2 O/Et3 SiH cleanly provided the bicyclic products 10 (Scheme 3, Eq. C). These complex acetal compounds originate from intramolecular trapping of the free secondary hydroxyl onto the in situ generated oxonium, and are produced in moderate to good yields. A total of seven examples of this interesting scaffold were prepared so as to test the generality of this process. To provide the desired reduced products in good to excellent yield, the most suitable conditions identified with model unprotected substrate 6aa were found to be the use of 2 equivalents of BF3 · Et2 O/Et3 SiH (Scheme 3, Eq. D). Presumably, the use of a second equivalent of Lewis acid leads to coordination of the secondary alcohol and prevents its nucleophilic attack on the oxonium, which can be reduced non-competitively with the silane reagent. These conditions afforded dehydropyran 7aa in 76 % yield, and were employed for the next step in the library preparation (Table 2). With conditions in hand that would provide the desired α-hydroxyalkyl 2H -pyran products and would also be amenable to a streamlined library synthesis, a subset of α-hydroxyalkyl dihydropyran substrates 6 was selected for acetal reduction under the optimised conditions. Towards this end, most of the α-hydroxyalkyl dihydropyrans were suc-
123
cessfully reduced, generating the desired products 7, except those with an allylic alcohol (Table 2, entries 32 and 41 with R2 = alkenyl) and also those bearing a heterocyclic ring (Table 2, entries 15–18, 22, 27, 31–33), which decomposed immediately upon the addition of the boron trifluoride reagent. Successful reactions were run in parallel at −50 ◦ C for 1 h and allowed to warm up to room temperature for 12 h and once reactions were completed, they were basified with bicarbonate, extracted with dichloromethane, concentrated and purified using an automated flash silica chromatography system to provide the desired α-hydroxyalkyl 2H -pyrans 7 in isolated yields generally between 50 and 90 %. Dihydroxylation of α-hydroxyalkyl 2H-pyran derivatives into “Thiomarinol Analogues” Following the successful optimisation of the acetal reduction, which can be accomplished without the need for alcohol protection, the library of thiomarinol analogues was completed by dihydroxylation of the α-hydroxyalkyl 2H pyrans following the standard published reaction conditions [16,19,24,32]. Thus, a subset of α-hydroxyalkyl 2H pyrans were subjected to the optimised reaction conditions and subsequently worked up and purified with an automated flash chromatography system. Towards this end, all α-hydroxyalkyl 2H -pyrans 7 were successfully dihydroxylated to provide 30 derivatives of the requisite thiomarinol analogues 8 (Fig. 4) in isolated yields ranging from 35 to 95 %
Mol Divers Table 2 Acetal reduction of α-hydroxyalkyl dihydropyrans using BF3 · Et2 O/Et3 SiH
Entry
α-Hydroxyalkyl dihydropyrans 6
R1
R2
Products 7
% Yields
1
6aa
H
C6 H5
7aa
76
2
6ab
H
4-CNC6 H4
7ab
31
3
6ac
H
4-MeC6 H4
7ac
13
4
6ad
H
4-NO2 C6 H4
7ad
74
5
6ae
H
4-CF3 C6 H4
7ae
59
6
6af
H
4-FC6 H4
7af
53
7
6ag
H
2-MeC6 H4
7ag
57
8
6ah
H
2-FC6 H4
7ah
91
9
6ai
H
PhCH2 CH2
7ai
45
10
6aj
H
2-Naphthyl
7aj
78
11
6ak
H
2-Br-5-FC6 H3
7ak
88
12
6al
H
C4 H11
7al
59
13
6am
H
C6 H11
7am
73
14
6an
H
4-AcNHC4 H4
7an
0
15
6ao
H
N-Ac-3-indolyl
7ao
0
16
6ap
H
N-CH3 -2-Pyrolyl
7ap
0
17
6aq
H
Furanyl
7aq
0
18
6ar
H
Thiophen-2-yl
7ar
0
19
6ba
CH3
C6 H5
7ba
52
20
6bb
CH3
4-CNC6 H4
7bb
48
21
6bi
CH3
PhCH2 CH2
7bi
52
22
6bc
CH3
4-MeC6 H4
7bc
0
23
6bg
CH3
2-MeC6 H4
7bg
31
24
6be
CH3
4-CF3 C6 H4
7be
61
25
6bf
CH3
4-FC6 H4
7bf
60
26
6bh
CH3
2-FC6 H4
7bh
69
27
6bj
CH3
2-Naphthyl
7bj
0
28
6bk
CH3
2-Br-5-F-C6 H3
7bk
76
29
6bl
CH3
C4 H11
7bl
76
30
6bm
CH3
C6 H11
7bm
81
31
6bq
CH3
Furanyl
7bq
0
32
6bs
CH3
PhCHCH
7bs
0
33
6bu
CH3
2-(5-Methyl furan-2-yl)propyl
7bu
0
34
6ca
C6 H13
C6 H5
7ca
75
35
6cb
C6 H13
4-CNC6 H4
7cb
67
36
6ci
C6 H13
PhCH2 CH2
7ci
77
37
6ce
C6 H13
4-CF3 C6 H4
7ce
71
123
Mol Divers Table 2 continued Entry
α-Hydroxyalkyl dihydropyrans 6
R1
R2
Products 7
% Yields
38
6cf
C6 H13
4-FC6 H4
7cf
51
39
6cl
C6 H13
C4 H11
7cl
97
40
6cm
C6 H13
C6 H11
7cm
67
41
6cs
C6 H13
PhCHCH
7cs
0
(Table 3). The purity of the final products was estimated by 1 H NMR analysis after flash column chromatographic separation. Most diols 8 were obtained in over 95 % purity as indicated by the absence of unaccounted peaks integrating for 5 % or more. The absolute and relative stereochemistry of adducts from the oxa[4+2] cycloaddition/allylboration has been demonstrated previously by our group [19] and others [33,34]. Dihydroxylation of analogous dehydropyrans occurred from the face opposite to the hydroxyalkyl side chain. Here, the relative stereochemistry of the final products of dihydroxylation was unambiguously confirmed by X-ray crystallography on one of the thiomarinol analogues, 8ad. The observed stereochemistry in all thiomarinol analogues can be explained by a preferential dihydroxylation from the least hindered face of the double bond, which occurs on the same side for both conformers as demonstrated in Scheme 4. Conclusion
Fig. 4 Structures of thiomarinol analogues in the final library
123
Through a methodology involving a tandem oxa[4+2] cycloaddition/allylboration reaction of aldehydes, we have successfully generated a small library of 30 analogues with the dihydroxylated pyran core of the thiomarinol antibiotics. This reaction sequence employed three different enol ether dienophiles and a wide variety of aldehydes for the allylboration step, such as aromatic, heteroaromatic, unsaturated and aliphatic aldehydes to allow the generation of highly substituted α-hydroxyalkyl dihydropyrans in a single step. Furthermore, we have developed a method for mild acetal reduction of α-hydroxyalkyl dihydropyrans without the need for hydroxyl group protection, which facilitated the synthesis of this library. Dihydroxylation of the residual olefin of α-hydroxyalkyl 2H -pyrans proceeded smoothly following standard conditions. The resulting relative stereochemistry was elucidated by X-ray crystallographic analysis, and matched the requisite stereochemistry of thiomarinols. The resulting library of simplified thiomarinol analogues could be screened against a variety of ATP-binding enzyme targets.
Mol Divers Table 3 Dihydroxylation of α-hydroxyalkyl 2H -pyrans using OsO4 /NMO conditions
Entry
α-Hydroxyalkyl 2H -pyrans 7
R1
R2
Products 8
%Yields
1
7aa
H
C6 H5
8aa
66
2
7ab
H
4-CNC6 H4
8ab
75
3
7ac
H
4-MeC6 H4
8ac
51
4
7ad
H
4-NO2 C6 H4
8ad
72
5
7ae
H
4-CF3 C6 H4
8ae
85
6
7af
H
4-FC6 H4
8af
52
7
7ag
H
2-MeC6 H4
8ag
51
8
7ah
H
2-FC6 H4
8ah
52
9
7ai
H
PhCH2 CH2
8ai
78
10
7aj
H
2-Naphthyl
8aj
71
11
7ak
H
2-Br-5-FC6 H3
8ak
74
12
7al
H
C4 H11
8al
89
13
7am
H
C6 H11
8am
62
14
7ba
CH3
C6 H5
8ba
77
15
7bb
CH3
4-CNC6 H4
8bb
43
16
7bi
CH3
PhCH2 CH2
8bi
55
17
7bg
CH3
2-MeC6 H4
8bg
72
18
7be
CH3
4-CF3 C6 H4
8be
63
19
7bf
CH3
4-FC6 H4
8bf
80
20
7bh
CH3
2-FC6 H4
8bh
75
21
7bk
CH3
2-Br-5-FC6 H3
8bk
51
22
7bl
CH3
C4 H11
8bl
73
23
7bm
CH3
C6 H11
8bm
84
24
7ca
C6 H13
C6 H5
8ca
72
25
7cb
C6 H13
4-CNC6 H4
8cb
35
26
7ci
C6 H13
PhCH2 CH2
8ci
69
27
7ce
C6 H13
4-CF3 C6 H4
8ce
68
28
7cf
C6 H13
4-FC6 H4
8cf
61
29
7cl
C6 H13
C4 H11
8cl
76
30
7cm
C6 H13
C6 H11
8cm
95
Experimental section General methods Chromium(III) catalyst 4 was prepared according to Jacobsen’s procedure [20,21]. Alkenylboronate 1 was prepared
according to our previously published procedure [19,24] and purified by Kugelrohr distillation (
FULL-LENGTH PAPER
Catalytic enantioselective diversity-oriented synthesis of a small library of polyhydroxylated pyrans inspired from thiomarinol antibiotics Raed M. Al-Zoubi · Dennis G. Hall
Received: 22 May 2014 / Accepted: 30 July 2014 © Springer International Publishing Switzerland 2014
Abstract A small library of 30 thiomarinol analogues was successfully synthesised using as a key step–a catalytic enantioselective tandem oxa[4+2] cycloaddition/aldehyde allylboration methodology. With this method, highly substituted α-hydroxyalkyl dihydropyrans were assembled in a single three-component reaction utilizing three different enol ethers and a wide variety of aldehydes, such as aromatic, heteroaromatic, unsaturated and aliphatic aldehydes. In a second operation, a mild and direct method for reducing an acetal unit in the α-hydroxyalkyl dihydropyrans was optimised without the need for protecting a nearby hydroxyl group. This procedure facilitated the synthetic sequence, which was completed by a dihydroxylation of the residual olefin of α-hydroxyalkyl 2H -pyrans to provide the desired library of dihydroxylated pyran analogues reminiscent of the thiomarinol antibiotics. The relative stereochemistry of the resulting library compounds was demonstrated by X-ray crystallography on one of the analogues.
Keywords Allylboration · ATP mimics · Combinatorial library · Enantioselective catalysis · Hetero-Diels–Alder cycloaddition · Pyrans
Electronic supplementary material The online version of this article (doi:10.1007/s11030-014-9542-6) contains supplementary material, which is available to authorized users. R. M. Al-Zoubi · D. G. Hall (B) Department of Chemistry, University of Alberta, Edmonton, AB T6G 2G2, Canada e-mail: [email protected]
Introduction Mupirocin (pseudomonic acid A, Fig. 1) is one of the world’s leading topical antibiotics initially commercialised by GlaxoSmithKline under the name Bactroban [1]. It is produced by Pseudomonas fluorescens and was reported to possess antibacterial activity as early as 1887 [2]. In the 1960s, the mixture of pseudomonic acids was found to be the active component [3]. The major constituent was identified and characterised in the 1970s, and was named pseudomonic acid A (mupirocin) [4,5]. Mupirocin is active against Grampositive aerobic bacteria and a few Gram-negative strains. It is prescribed for treating skin infections, such as impetigo, candidiasis as well as burn wounds and cuts. Mupirocin inhibits the bacterial isoleucyl tRNA synthetase enzyme responsible for loading the amino acid isoleucine (Ile) onto its cognate tRNA, a process required for ribosomal protein synthesis [6]. It was found to be 10,000 times more potent on the bacterial enzyme than on the human homologue, which ensures its selectivity as a pharmaceutical drug [7,8]. The binding of mupirocin to the enzyme– tRNA complex is competitive with isoleucine and ATP, and also with Ile-AMP derivatives. The X-ray crystal structure of mupirocin bound to the complex between tRNAIle synthetase and its cognate tRNA was solved in 1999 [9]. It confirmed that mupirocin acts mainly as an ATP mimic and occupies the site normally occupied by the ribose ring. Pseudomonic acids have attracted considerable attention for their unique structure and biological activity [10]. Unfortunately, mupirocin displays poor oral absorptivity and low metabolic stability. In the bloodstream, its ester linkage is quickly hydrolyzed to an inactive product, monic acid [11]. Consequently, there has been significant interest in the development of improved analogues that could also be suitable as oral antibiotics. Thiomarinol A [12] and thiomarinol H
123
Mol Divers Fig. 1 Structures of mupirocin (pseudomonic acid A), monic acid and selected members of the thiomarinol family
[13] and others [14,15] (Fig. 1) are rare marine natural products recently isolated from the bacterium Alteromonasrava sp. nov. SANK 73390. These closely related families of naturally occurring antibiotics display minor structural differences. The structures of thiomarinols A and H differ from mupirocin by the notable presence of a C4-hydroxyl, a shorter C1-alkoxy chain, and the replacement of the C10– C11 epoxide with an E alkene unit. These natural substances were found to be equally potent as pseudomonic acid A, and thiomarinol A was found to possess a wider spectrum of activity (against both Gram-positive and -negative bacteria) [12,13]. Our group has recently synthesised thiomarinol H [16] and other thiomarinol analogues [17]. We postulated that the same method could be amenable to the design and synthesis of a diversity-oriented library of simplified analogues. It is believed that mupirocin and the thiomarinols share the same target, bacterial isoleucine tRNA synthetase. A pharmacophore model of mupirocin was developed even before the availability of X-ray crystallographic information [9], which revealed that the dihydroxypyran core is essential for the
123
OH O HN
O
H N
8
5 O O
left-hand side chain (variable)
OH
HO
HO
H
11
O right-hand side chain (somewhat variable, size must be small)
pyran core (conserved)
Fig. 2 Qualitative pharmacophore model for thiomarinol
antibacterial activity and it should be conserved. It was also shown that the left-hand and right-hand side chains are quite variable, as depicted in Fig. 2. Natural products are ideal lead structures in combinatorial library design and can deliver higher hit rates, as exemplified with several convincing examples in a review paper by Waldmann [18]. Numerous enzymes are ATP binders. In order to perform small molecule screening against some of these enzymes, we planned to design a small library of thiomarinol analogues.
Mol Divers
OEt
OEt
OEt
2b
2a
2c
CHO
CHO
CHO CHO
X
X 3a, X = H 3b, X = CN 3c, X = CH 3 3d, X = NO 2 3e, X = CF3 3f, X = F
3l
3j
3g, X = CH3 3h, X = F
CHO
CHO
F CHO
Br 3i
3m
3k CHO
H N O
N
N
CHO 3n
3o
CHO 3p
O
X
CHO
3q, X = O 3r, X = S
CHO CHO
Scheme 1 Three-component hetero [4+2] cycloaddition/allylboration reaction approach to a library of thiomarinol analogues
3s
3t
O
CHO
3u
Fig. 3 Enol ethers 2(a–c) and aldehydes 3(a–u) as diversity reagents
Results and discussion
Three-component synthesis of α-hydroxyalkyl dihydropyran intermediates 5
Design and retrosynthesis of a diversity-oriented library The library retrosynthesis is presented in Scheme 1. All the thiomarinol analogues 8 were prepared from key intermediate 6 through a catalytic enantioselective inverse electron demand hetero [4 + 2] cycloaddition/allylboration tandem reaction between boronoacrolein pinacol ester 1, enol ethers 2, and aldehydes 3 developed in the Hall Laboratory [19]. The cycloaddition step is catalysed by Jacobsen’s Cr(III) complex 4 [20,21] and affords dehydropyranyl boronates 5 which can be trapped with aldehydes (3) to give products 6. This multicomponent reaction functions well with a wide range of enol ethers and aldehydes, and was demonstrated successfully in the total synthesis of thiomarinol H [16]. Acetal reduction on intermediates 6 and a stereoselective dihydroxylation of dihydropyrans 7 would afford the final, desired thiomarinol analogues 8. The set of building blocks depicted in Fig. 3 was chosen based on the objective of displaying a diversity of functional groups and structural elements. Three different enol ethers 2a, 2b and 2c were used for the preparation of the library. Enol ether 2c was made using a known method [22,23], while 2a and 2b were commercially available. The chosen panel of commercially available aldehydes included benzaldehydes substituted with various functional groups, as well as heterocyclic and α, β-unsaturated aldehydes.
The key substrate (E)-3-boronoacrolein pinacolate 1 was made in good yield in two steps according to our published procedure [19,24]. The three-component [4+2] cycloaddition/ allylboration procedure previously optimised in the Hall Group was employed in the first step of library preparation. Towards this end, (E)-3-boronoacrolein pinacol ester 1 was reacted with enol ethers 2(a–c) to form the cycloadducts 5, which then reacted with a representative subset of the aldehydes 3(a–u) (Fig. 3) to provide the desired α-hydroxyalkyl dihydropyran analogues 6 (Table 1). It is noteworthy that the thermal allylboration reaction required longer reaction times and elevated temperatures with substituted enol ether 2c than with enol ethers 2a and 2b. The scope of aldehydes that can be utilised in the threecomponent oxa[4+2] cycloaddition/allylboration reaction is remarkable. As indicated with the yields shown in Table 1, a variety of substituted aromatic, heteroaromatic, α, βunsaturated and aliphatic aldehydes are all suitable substrates for this tandem reaction. All cycloaddition reactions were performed at a temperature between 18 and 20 ◦ C, and quickly purified by flash chromatography (deactivated silica) following the general published procedure from our group [19,24]. To deduce the optical purity of products 6 and validate the high optical purity of the allylboronate precursors 5 expected from our previous reports [19,24], the optical purity
123
Mol Divers Table 1 Synthesis of α-hydroxyalkyl pyrans 6 from (E)-3-boronoacrolein pinacol ester 1, enol ethers 2(a–c) and aldehydes 3(a–u)
O
B
O R1 + OEt
O 1
O
Jacobsen's cat. 4 (1 mol%) BaO, 18-20 oC 1.5 h
B
O R1
R 2CHO (2 equiv) 3(a-u) 45-120
O
2(a-c)
oC
OEt
R1 R2
O
OEt
HO H 6 (>96 de, 95.5% ee)
5
Entry
Enol ether
Aldehyde
α-Hydroxyalkyl dihydropyran
% Yield
1
2a
3a
6aa
67
2
2a
3b
6ab
65
3
2a
3c
6ac
38
4
2a
3d
6ad
62
5
2a
3e
6ae
75
6
2a
3f
6af
73
7
2a
3g
6ag
47
8
2a
3h
6ah
51
9
2a
3i
6ai
71
10
2a
3j
6aj
81
11
2a
3k
6ak
49
12
2a
3l
6al
81
13
2a
3m
6am
79
14
2a
3n
6an
36
15
2a
3o
6ao
69
16
2a
3p
6ap
54
17
2a
3q
6aq
59
18
2a
3r
6ar
65
19
2b
3a
6ba
79
20
2b
3b
6bb
50
21
2b
3c
6bc
65
22
2b
3d
6bd
61
23
2b
3e
6be
63
24
2b
3g
6bg
93
25
2b
3h
6bh
97
26
2b
3i
6bi
59
27
2b
3j
6bj
78
28
2b
3k
6bk
53
29
2b
3l
6bl
74
30
2b
3m
6bm
59
31
2b
3q
6bq
58
32
2b
3s
6bs
84
33
2b
3t
6bt
63
34
2b
3u
6bu
70
35
2c
3a
6ca
79
36
2c
3b
6cb
72
37
2c
3c
6cc
65
38
2c
3g
6cg
67
123
Mol Divers Table 1 continued Entry
Enol ether
Aldehyde
α-Hydroxyalkyl dihydropyran
% Yield
39
2c
3h
6ch
62
40
2c
3l
6cl
72
41
2c
3m
6cm
43
42
2c
3s
6cs
75
43
2c
3t
6ct
57
of product 6a was measured by chiral HPLC as described before [19] and was confirmed to be >90 % ee. This process was repeated each time a new batch of catalyst 4 was prepared and employed in the cycloaddition. The relative stereochemistry of the optically enriched α-hydroxyalkyl dihydropyran products was confirmed by the X-ray crystallographic analysis of one of the library members (vide infra). Mechanistically, the [4+2] cycloaddition of 1-boronoacrolein pinacolate 1 with ethyl vinyl ether is expected to proceed with complete endo selectivity to give the allylboronate intermediate 5 (Scheme 2). In the allylboration step, a boat-like pyran transition state with a pseudo-equatorial ethoxy substituent was proposed [19,24] leading to the desired α-hydroxyalkyl dihydropyran product.
Reduction of the acetal unit of α-hydroxyalkyl dihydropyrans We then turned our attention to the reduction of the acetal unit in the α-hydroxyalkyl dihydropyran intermediates 6. Previous experience in the Hall group regarding this step was performed using TiCl4 /Et3 SiH conditions on a protected secondary alcohol (Eq. 1) [6,24,25].
(1)
Acetal reduction of one of the α-hydroxyalkyl dihydropyran derivatives using TiCl4 /Et3 SiH
Scheme 2 Stereoselectivity of tandem three-component oxa[4+2] cycloaddition/allylboration reaction
This precedent guided us in an attempt with substrate 6aa containing a free secondary alcohol. To simplify the preparation of the library, we aimed to reduce the acetal moiety directly from α-hydroxyalkyl dihydropyrans without protecting the α-hydroxyl group. Unfortunately, the acetal reduction reaction under TiCl4 /Et3 SiH was unsuccessful for the unprotected α-hydroxyl dihydropyrans, providing other reduced products. Nevertheless, many standard conditions for acetal reduction can be found in the literature. For instance, Gray and co-workers published in 1982 a report on the reductive cleavage of glycosides using BF3 · Et2 O/TFA/Et3 SiH conditions to provide the reduced acetal product in good to excellent yields at 0 ◦ C [26]. In 2000, Toone and co-workers reported a regioselective reduction of 4,6-O-benzylidenes using BF3 · Et2 O/Et3 SiH at 0 ◦ C [27]. A number of different reagents have been employed for the reduction of acetal units by other investigators, such as LiAlH4 –AlCl3 [28], NaCNBH3 /HCl [29], Et3 SiH/TFA [30], and EtAlCl2 /Et3 SiH [31]. With this precedent in mind, initial trials were conducted to investigate the most effective conditions to provide the requisite acetal reduced products in our library synthesis without any decomposition and unwanted side products.
123
Mol Divers
(A)
(C)
(B)
(D)
Scheme 3 The effect of acid in Et3 SiH-promoted acetal reduction of α-hydroxyalkyl dihydropyrans
To our satisfaction, the use of 1.2 equivalents of TiCl4 /Et3 SiH at −50 ◦ C provided the acetal ring open product 9aa in 78 % yield (Scheme 3, Eq. A), while the use of TFA/Et3 SiH provided a mixture of acetal ring open product 9aa and bicyclic product 10aa (Scheme 3, Eq. B). To our surprise, it was found that the use of only one equivalent of BF3 · Et2 O/Et3 SiH cleanly provided the bicyclic products 10 (Scheme 3, Eq. C). These complex acetal compounds originate from intramolecular trapping of the free secondary hydroxyl onto the in situ generated oxonium, and are produced in moderate to good yields. A total of seven examples of this interesting scaffold were prepared so as to test the generality of this process. To provide the desired reduced products in good to excellent yield, the most suitable conditions identified with model unprotected substrate 6aa were found to be the use of 2 equivalents of BF3 · Et2 O/Et3 SiH (Scheme 3, Eq. D). Presumably, the use of a second equivalent of Lewis acid leads to coordination of the secondary alcohol and prevents its nucleophilic attack on the oxonium, which can be reduced non-competitively with the silane reagent. These conditions afforded dehydropyran 7aa in 76 % yield, and were employed for the next step in the library preparation (Table 2). With conditions in hand that would provide the desired α-hydroxyalkyl 2H -pyran products and would also be amenable to a streamlined library synthesis, a subset of α-hydroxyalkyl dihydropyran substrates 6 was selected for acetal reduction under the optimised conditions. Towards this end, most of the α-hydroxyalkyl dihydropyrans were suc-
123
cessfully reduced, generating the desired products 7, except those with an allylic alcohol (Table 2, entries 32 and 41 with R2 = alkenyl) and also those bearing a heterocyclic ring (Table 2, entries 15–18, 22, 27, 31–33), which decomposed immediately upon the addition of the boron trifluoride reagent. Successful reactions were run in parallel at −50 ◦ C for 1 h and allowed to warm up to room temperature for 12 h and once reactions were completed, they were basified with bicarbonate, extracted with dichloromethane, concentrated and purified using an automated flash silica chromatography system to provide the desired α-hydroxyalkyl 2H -pyrans 7 in isolated yields generally between 50 and 90 %. Dihydroxylation of α-hydroxyalkyl 2H-pyran derivatives into “Thiomarinol Analogues” Following the successful optimisation of the acetal reduction, which can be accomplished without the need for alcohol protection, the library of thiomarinol analogues was completed by dihydroxylation of the α-hydroxyalkyl 2H pyrans following the standard published reaction conditions [16,19,24,32]. Thus, a subset of α-hydroxyalkyl 2H pyrans were subjected to the optimised reaction conditions and subsequently worked up and purified with an automated flash chromatography system. Towards this end, all α-hydroxyalkyl 2H -pyrans 7 were successfully dihydroxylated to provide 30 derivatives of the requisite thiomarinol analogues 8 (Fig. 4) in isolated yields ranging from 35 to 95 %
Mol Divers Table 2 Acetal reduction of α-hydroxyalkyl dihydropyrans using BF3 · Et2 O/Et3 SiH
Entry
α-Hydroxyalkyl dihydropyrans 6
R1
R2
Products 7
% Yields
1
6aa
H
C6 H5
7aa
76
2
6ab
H
4-CNC6 H4
7ab
31
3
6ac
H
4-MeC6 H4
7ac
13
4
6ad
H
4-NO2 C6 H4
7ad
74
5
6ae
H
4-CF3 C6 H4
7ae
59
6
6af
H
4-FC6 H4
7af
53
7
6ag
H
2-MeC6 H4
7ag
57
8
6ah
H
2-FC6 H4
7ah
91
9
6ai
H
PhCH2 CH2
7ai
45
10
6aj
H
2-Naphthyl
7aj
78
11
6ak
H
2-Br-5-FC6 H3
7ak
88
12
6al
H
C4 H11
7al
59
13
6am
H
C6 H11
7am
73
14
6an
H
4-AcNHC4 H4
7an
0
15
6ao
H
N-Ac-3-indolyl
7ao
0
16
6ap
H
N-CH3 -2-Pyrolyl
7ap
0
17
6aq
H
Furanyl
7aq
0
18
6ar
H
Thiophen-2-yl
7ar
0
19
6ba
CH3
C6 H5
7ba
52
20
6bb
CH3
4-CNC6 H4
7bb
48
21
6bi
CH3
PhCH2 CH2
7bi
52
22
6bc
CH3
4-MeC6 H4
7bc
0
23
6bg
CH3
2-MeC6 H4
7bg
31
24
6be
CH3
4-CF3 C6 H4
7be
61
25
6bf
CH3
4-FC6 H4
7bf
60
26
6bh
CH3
2-FC6 H4
7bh
69
27
6bj
CH3
2-Naphthyl
7bj
0
28
6bk
CH3
2-Br-5-F-C6 H3
7bk
76
29
6bl
CH3
C4 H11
7bl
76
30
6bm
CH3
C6 H11
7bm
81
31
6bq
CH3
Furanyl
7bq
0
32
6bs
CH3
PhCHCH
7bs
0
33
6bu
CH3
2-(5-Methyl furan-2-yl)propyl
7bu
0
34
6ca
C6 H13
C6 H5
7ca
75
35
6cb
C6 H13
4-CNC6 H4
7cb
67
36
6ci
C6 H13
PhCH2 CH2
7ci
77
37
6ce
C6 H13
4-CF3 C6 H4
7ce
71
123
Mol Divers Table 2 continued Entry
α-Hydroxyalkyl dihydropyrans 6
R1
R2
Products 7
% Yields
38
6cf
C6 H13
4-FC6 H4
7cf
51
39
6cl
C6 H13
C4 H11
7cl
97
40
6cm
C6 H13
C6 H11
7cm
67
41
6cs
C6 H13
PhCHCH
7cs
0
(Table 3). The purity of the final products was estimated by 1 H NMR analysis after flash column chromatographic separation. Most diols 8 were obtained in over 95 % purity as indicated by the absence of unaccounted peaks integrating for 5 % or more. The absolute and relative stereochemistry of adducts from the oxa[4+2] cycloaddition/allylboration has been demonstrated previously by our group [19] and others [33,34]. Dihydroxylation of analogous dehydropyrans occurred from the face opposite to the hydroxyalkyl side chain. Here, the relative stereochemistry of the final products of dihydroxylation was unambiguously confirmed by X-ray crystallography on one of the thiomarinol analogues, 8ad. The observed stereochemistry in all thiomarinol analogues can be explained by a preferential dihydroxylation from the least hindered face of the double bond, which occurs on the same side for both conformers as demonstrated in Scheme 4. Conclusion
Fig. 4 Structures of thiomarinol analogues in the final library
123
Through a methodology involving a tandem oxa[4+2] cycloaddition/allylboration reaction of aldehydes, we have successfully generated a small library of 30 analogues with the dihydroxylated pyran core of the thiomarinol antibiotics. This reaction sequence employed three different enol ether dienophiles and a wide variety of aldehydes for the allylboration step, such as aromatic, heteroaromatic, unsaturated and aliphatic aldehydes to allow the generation of highly substituted α-hydroxyalkyl dihydropyrans in a single step. Furthermore, we have developed a method for mild acetal reduction of α-hydroxyalkyl dihydropyrans without the need for hydroxyl group protection, which facilitated the synthesis of this library. Dihydroxylation of the residual olefin of α-hydroxyalkyl 2H -pyrans proceeded smoothly following standard conditions. The resulting relative stereochemistry was elucidated by X-ray crystallographic analysis, and matched the requisite stereochemistry of thiomarinols. The resulting library of simplified thiomarinol analogues could be screened against a variety of ATP-binding enzyme targets.
Mol Divers Table 3 Dihydroxylation of α-hydroxyalkyl 2H -pyrans using OsO4 /NMO conditions
Entry
α-Hydroxyalkyl 2H -pyrans 7
R1
R2
Products 8
%Yields
1
7aa
H
C6 H5
8aa
66
2
7ab
H
4-CNC6 H4
8ab
75
3
7ac
H
4-MeC6 H4
8ac
51
4
7ad
H
4-NO2 C6 H4
8ad
72
5
7ae
H
4-CF3 C6 H4
8ae
85
6
7af
H
4-FC6 H4
8af
52
7
7ag
H
2-MeC6 H4
8ag
51
8
7ah
H
2-FC6 H4
8ah
52
9
7ai
H
PhCH2 CH2
8ai
78
10
7aj
H
2-Naphthyl
8aj
71
11
7ak
H
2-Br-5-FC6 H3
8ak
74
12
7al
H
C4 H11
8al
89
13
7am
H
C6 H11
8am
62
14
7ba
CH3
C6 H5
8ba
77
15
7bb
CH3
4-CNC6 H4
8bb
43
16
7bi
CH3
PhCH2 CH2
8bi
55
17
7bg
CH3
2-MeC6 H4
8bg
72
18
7be
CH3
4-CF3 C6 H4
8be
63
19
7bf
CH3
4-FC6 H4
8bf
80
20
7bh
CH3
2-FC6 H4
8bh
75
21
7bk
CH3
2-Br-5-FC6 H3
8bk
51
22
7bl
CH3
C4 H11
8bl
73
23
7bm
CH3
C6 H11
8bm
84
24
7ca
C6 H13
C6 H5
8ca
72
25
7cb
C6 H13
4-CNC6 H4
8cb
35
26
7ci
C6 H13
PhCH2 CH2
8ci
69
27
7ce
C6 H13
4-CF3 C6 H4
8ce
68
28
7cf
C6 H13
4-FC6 H4
8cf
61
29
7cl
C6 H13
C4 H11
8cl
76
30
7cm
C6 H13
C6 H11
8cm
95
Experimental section General methods Chromium(III) catalyst 4 was prepared according to Jacobsen’s procedure [20,21]. Alkenylboronate 1 was prepared
according to our previously published procedure [19,24] and purified by Kugelrohr distillation (
