Chapter 2. Synthesis and properties of new fluorescent calcium sensors
4.2. Elaboration of a route to phthalic acid derivatives with an azacrown-ether fused to
Fusion of the azacrown ether with the positions 4 and 5 of the aromatic ring in phthalic anhydride (compounds 84a,b) would provide an easy route to the required fluorescent sensors IIa,b (Scheme 36).
Scheme 36. A retrosynthetic consideration of an alternative approach to compounds IIa-c based on the phthalic anhydrides 84a,b.
In order to fuse the azacrown-ether system with the aromatic ring in phthalic anhydride, it was necessary to use its protected form (or a synthetic equivalent), because the multistep procedures for the introduction and cyclization of an azacrown ring involve various highly reactive substances. Methyl groups appeared to be a bad choice, because they require oxidation in the final step, in the presence of the electron-donating azacrown-ether ring, which makes the aromatic nucleus very susceptible to oxidation. Therefore, we chose another synthetic equivalent of the phthalic anhydride, its N-methyl imide 90, which is depicted in Scheme 37.
Scheme 37. Synthesis of the aminophenol 90 as a key intermediate.
To access compound 90, commercially available 4,5-dichlorophthalic acid (85) was transformed into its anhydride 86,[111] and the latter then to the N-methyl imide 87.[112] A twofold nucleophilic substitution of the two chlorine atoms in 87 was performed with KNO2
in DMF at ca. 150 °C to give the ortho-hydroxynitro derivative 88. The formation of the latter may be explained, if we assume that one chlorine atom is replaced with a nitro group, and another one with an O=N–O– group (KNO2 acts as an ambident nucleophile). The nitrite ester group is then hydrolyzed upon aqueous work-up.[111] Catalytic reduction of the potassium salt 89[113] gave 4-amino-5-hydroxy-N-methylphthalimide (90).
NH2 OH 90
O O
O NH O
92 44, CsF, MeCN, reflux, 1 d
O I
I 91 3
1) NaF, DMF, r. t., 30 min 2) DIPEA
91, NaI, DIPEA, DMF, 90 °C, 18 h
(14 %) N
O
O
N O
O 90oC, overnight
3)
Scheme 38. Synthetic approaches to azacrown-N-methylphthalimide 92.
With the required building block 90 at hand, we studied the conditions, under which the azacrown ring can be build-up using the hydroxy and amino groups in the o-position to each other (Scheme 38). For that, it was necessary to use the diiodide 91 prepared from tetraethylene glycol dichloride (50),[114a] because ditosylate 44 failed to provide the required macrocycle 92. Even with the diiodide 91 the highest achieved yield was only 14%.
Therefore, we turned to a step-wise construction of the azacrown ring using tetraethylene glycol derivatives 54-Ac and 54-Z[114b] (Scheme 39).
N
Scheme 39. Synthetic routes to alcohol 93-H.with the linear tetraethyleneglycol chain.
In a first attempt, the potassium salt 89 and the TEG-derivate 54-Ac were used as starting materials. Due to the presence of additional acceptor substituents in compound 89, the nucleophilic substitution proceeded more slowly than for compound 58 (Scheme 17). Even at 100-110°C, it took at least 2 days to achieve an acceptable conversion of compound 89 (in the presence of an equimolar amount of KI). In spite of that, the yield of compound 93-Ac was only moderate (41%). Moreover, it was necessary to remove the acetyl protecting group in a separate step. Though the acetyl group could be removed under mild conditions (K2CO3, MeOH), the yield of the target alcohol 93-H (61%) was far from being quantitative.
Therefore, the overall yield of compound 93-H in the two step sequence was only 25%.
Employment of the alkylating agent of compound 54-Z instead of 54-Ac did not improve the yield of the intermediate 93-H (see Scheme 39).
Cl O
Scheme 40. Synthesis of alkylating reagents 54-Tr, 95, 96
The trityl group was chosen as a new protection for the tetraethylene glycol derivatives (54-Tr, 95, 96) with good leaving groups (Cl, Ms and I) (Scheme 40). The yield of the chloride 54-Tr was low (35%).[115] In the synthesis of compounds 95 and 96, the protecting group was introduced in the first step, by the reaction of compound 52 with chlorotriphenylmethane, which gave the ether 94 in high yield (99%).[116] The remaining free hydroxy group in the ether 94 was converted into mesylate to give compound 95 in 91%
yield;[116] from the latter, the iodide 96 was prepared in 88% yield.[117]
N
Scheme 41. Optimization of the conditions in the preparation of the alcohol 93-H.
In the first experiments, the mesylate 95 was used as an alkylating reagent. The overall yield of the alkylation products (93-Tr and 93-H) was higher than in the reaction of the salt 89 with the chloride 54-Z (44% vs. 34%). However, more by-products were detected, and a mixture of compounds was formed (93-Tr and 93-H). In order to increase the yield of the target product 93-Tr, another alkylating agent, the iodide 96, was used. To suppress the deprotection of the hydroxy group, the reaction temperature was decreased to 70 °C, and a longer reaction time was applied (5 instead of 2 days). The highest conversion of the potassium salt 89 into the ether 93-Tr was found to be 59%; the content of the non-reacted starting material was 25%. In order to prove our assumption that the decomposition of iodide 96 was quicker than its reaction with the potassium-salt 89, the mode of the iodide addition was changed. Half an equivalent of compound 96 was added in four portions (in the beginning and every 12 h) to the reaction mixture. However, the conversion was found to be the same (59% of 93-Tr and 26% of 89). Also, a change of the solvent to HMPTA in this reaction did not help to shift the equilibrium towards the target product 93-Tr. Unfortunately, the ratio became even worse, i. e. 49:37.
For the deprotection of the nitroether 93-Tr, two procedures were applied. Heating the starting material in glacial acetic acid[118] afforded the target product 93-H only in moderate yield (40%). The main by-product in this reaction was the ester 93-Ac. Treatment of the ether
93-Tr with a 2 M methanolic solution of BF3•Et2O[119] gave the pure alcohol 93-H in 98%
Scheme 42. Synthesis of the N-tosylated azacrown ether 99 by a ring-closing reaction sequence.
After that the nitro group in the alcohol 93-H was catalytically reduced, and compound 97 was converted into the bis-tosylate 98 (Scheme 42). Conventional conditions (Cs2CO3, DMF, 90 °C) were used for the ring-closing reaction; unfortunately, the yield of the monotosylate 99 was low (even after the reaction time was increased from 24 to 39 h).
O O
Scheme 43. Attempted N-detosylation of the crown ether 99.
Unfortunately, all approaches to recover the target crown ether 92 from its tosylamide 99 failed. Under mild detosylation conditions (HF•Py[120] or TMSI[121]), only the starting compound 99 was isolated; and with sodium naphthalenide, the reaction mixture did neither contain the starting material 99, nor the target crown ether 92.
O
Figure 64. The intermediate buildings block 100.
In order to be able to efficiently detosylate of compound 99 we were forced to modify the synthesis and circumvent the detosylation step. Towards that, it was necessary to prepare the monotosylate 100. This compound could be produced not only by a selective tosylation of the amino alcohol 97, but also by tosylation of the nitro alcohol 93-H followed by reduction.
O
Scheme 44. Synthesis of the monotosylate 100 and the iodide 102.
Towards that, the nitrotosylate 101 was obtained from the nitroalcohol 93-H in 53% yield according to the standard procedure. Then the nitro compound 101 was quantitatively converted into the amine 100 by catalytic hydrogenation. The iodide 102 was also produced by nucleophilic substitution of the tosyl group in compound 100 with iodide anion.
O
Scheme 45. Initial attempts to perform the cyclization to the crown ether 92.
Initially, two bases (DBU and Cs2CO3) were tried in order to perform the cyclization to the crown ether 92. In both cases, the target product 92 was observed, and in the case of DBU,
this product was isolated. However, in these experiments the yield of the crown ether 92 was inacceptably low (maximum 12%).
O H2N I O
N O
102 O
3
O O O
O RN
92: R = H N O O NaI, DIPEA,
DMF, 90 °C, 30 h
92-Et: R = Et Et2SO4,
140 °C, 1 h conversion 42%
81%
Scheme 46. Successful cyclization of the iodide 102 to the crown ether 92 and the synthesis of the ethylated derivative 92-Et.
To improve the yield of the target product 92, the tosyl leaving group was changed to an iodide (Scheme 46), in addition, sodium iodide was used as a templating agent, and a softer base (DIPEA) was applied. As a result, the yield of compound 92 increased to 42%. Then the ethyl group was introduced into the crown ether 92 by heating it with diethyl sulfate at 140°C for 1 h.[122]