Shū
Kobayashi
*,
Keiichi
Hirano
and
Masaharu
Sugiura
Graduate School of Pharmaceutical Sciences, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. E-mail: skobayas@mol.f.u-tokyo.ac.jp; Fax: 81 3 5684 0634; Tel: 81 3 5841 4790
First published on 26th November 2004
α-Aminoallylation of aldehydes in aqueous ammonia has been developed; commercial aqueous ammonia was successfully used, and this method does not require anhydrous conditions thus leading to easy and practical operations.
At the outset, α-aminoallylation of 3-phenylpropanal (1a) as a probe was examined. Reactions were conducted with an aldehyde (0.5 mmol) and allylboronate 2a (0.6 mmol) in 25% aqueous ammonia (1 mL) in the absence or presence of an additive at rt (Table 1). While the chemoselectivity was much lower without any additives (entry 1), it was found that acidic and anionic surfactant molecules (DBSA and lauric acid as well as SDS and SDBS) were effective as additives (entries 2–9).6 Among these additives, DBSA was the best. It was suggested that both the acidic and hydrophobic parts of DBSA played an important role, since toluenesulfonic acid showed much lower activity (entry 10). With DBSA, further optimization of reaction conditions was performed (entries 11–16); the best result was obtained when reactions were carried out for 6–12 h in the presence of 10 mol% of DBSA (entries 15 and 16). It should be noted that the result was further improved compared to that under the reported conditions (NH3/EtOH)4 (entry 17).
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Entry | Additive (mol%) | Yield (%)b | |
3a | 4a | ||
a Reactions were performed using aldehyde 1a (0.5 mmol), allylboronate 2a (0.6 mmol), and an additive (10 mol%) in 25% aqueous ammonia at rt for 2h. b Determined by 1H NMR analysis using 1,2,4,5-tetramethylbenzene as an internal standard. c Dodecylbenzenesulfonic acid. d Sodium dodecyl sulfate. e Sodium dodecylbenzenesulfonate. f Cetyltrimethylammonium bromide. g Polyethylene glycol tert-octylphenyl ether. h Polyethylene glycol (MW ca. 400). i Isolated yield. j Conducted in ammonia-saturated ethanol (ref 4). | |||
1 | None | 34 | 39 |
2 | DBSAc (10) | 72 | 6 |
3 | Lauric acid (10) | 67 | 19 |
4 | SDSd (10) | 56 | 9 |
5 | SDBSe (10) | 72 | 7 |
6 | CTABf (10) | 44 | 21 |
7 | 3-(Dodecyldimethylammonio)propane-sufonate (10) | 32 | 14 |
8 | Triton X-100g (10) | 33 | 12 |
9 | PEG-400#h (10) | 44 | 28 |
10 | TsOH (10) | 25 | 8 |
11 | DBSA (5) | 69 | 7 |
12 | DBSA (20) | 65 | 4 |
13 | DBSA (50) | 42 | 4 |
14 | DBSA (10) (4 h) | 72 | 6 |
15 | DBSA (10) (6 h) | 86 (79i) | 3 |
16 | DBSA (10) (12 h) | 84 | 4 |
17j | – (in NH3/EtOH) | 78 | 3 |
With the conditions using DBSA, α-aminoallylation of various aldehydes in aqueous ammonia was examined (Table 2).† It was found that reactions of benzaldehyde (1b) and its para-substituted derivatives 1c–d showed moderate chemoselectivities (entries 1–3). It was unexpected to find that aromatic aldehydes, which exhibited high selectivity under the reported conditions,4 provided lower chemoselectivity than 3-phenylpropanal (1a). On the other hand, salicylaldehyde (1e), heteroaromatic aldehydes 1f–1l, and cinnamaldehyde (1m) provided the corresponding amines in high yields with high chemoselectivities in most cases (entries 4–12). Interestingly, 3-furaldehyde (1l) or 3-thiophenecarboxaldehyde (1j) showed much better reactivity than their 2-substituted analogues. As for reactions of aliphatic aldehydes, cyclohexanecarboxaldehyde (1n) and n-hexanal (1o) gave amines 3n–o in moderate yields with good chemoselectivity (entries 13 and 14), whereas n-decanal (1p), a highly hydrophobic linear aldehyde, reversed chemoselectivity to give alcohol 4p as the major product. This suggested that hydrophobicity of aldehydes is one of the key factors determining chemoselectivity in this reaction. Although chemoselectivities were not perfect in several cases, it is noted that the simple acid–base extraction enables easy isolation of 3.7
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Entry | R (1) | Time/h | Yield (%) | |
3 b | 4 c | |||
a Reactions were performed using aldehyde 1 (0.5 mmol), allylboronate 2a (0.6 mmol), and DBSA (10 mol%) in 25% aqueous ammonia at rt for 2h. b Isolated yields. c Determined by 1H NMR analysis using 1,2,4,5-tetramethylbenzene as an internal standard. | ||||
1 | Ph (1b) | 6 | 61 (3b) | 15 (4b) |
2 | p-NO2C6H4 (1c) | 6 | 60 (3c) | 8 (4c) |
3 | p-MeOC6H4 (1d) | 6 | 60 (3d) | 17 (4d) |
4 | o-HOC6H4 (1e) | 6 | 75 (3e) | 0 (4e) |
5 | 2-Pyridyl (1f) | 6 | 88 (3f) | 0 (4f) |
6 | 3-Pyridyl (1g) | 2 | 85 (3g) | 0 (4g) |
7 | 4-Pyridyl (1h) | 2 | 83 (3h) | 0 (4h) |
8 | 2-Thienyl (1i) | 6 | 60 (3i) | 15 (4i) |
9 | 3-Thienyl (1j) | 2 | 95 (3j) | 0 (4j) |
10 | 2-Furyl (1k) | 2 | 53 (3k) | 2 (4k) |
11 | 3-Furyl (1l) | 2 | 93 (3l) | 4 (4l) |
12 | (E)-PhCH![]() |
2 | 78 (3m) | 15 (4m) |
13 | c-C6H11 (1n) | 6 | 68 (3n) | trace (4n) |
14 | n-C5H11 (1o) | 2 | 49 (3o) | 6 (4o) |
15 | n-C9H19 (1p) | 6 | 2 (3p) | 52 (4p) |
While DBSA was found to play an important role in the present reaction, the reaction of 1a was found to work well in 25% aqueous ammonia and THF (1/1) without DBSA at rt for 2 h leading to almost the same result as the DBSA system (3a: 79%, 4a: 12%). Two liquid phases were observed in this system, and a turbid mixture similar to that observed in the DBSA system was formed with vigorous stirring. However, these conditions exhibited low substrate generality, indicating a strong dependence on aldehyde structure.
Next, α-aminocrotylation in aqueous ammonia was investigated. Whereas 3-phenylpropanal (1a) and benzaldehyde (1b) afforded the corresponding amines in moderate yields,8 2-pyridinecarboxaldehyde (1f) exhibited high reactivity to give amine 5f in high yield with high chemo- and diastereoselectivity (Scheme 2). In these cases, high stereospecificity (Z to syn and E to anti) was observed.
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Scheme 2 α-Aminocrotylation of aldehyde 1f in aqueous ammonia. |
In conclusion, we have demonstrated that α-aminoallylation of aldehydes with allylboronates proceeded smoothly in aqueous ammonia. Several effective conditions such as use of DBSA as an additive were found. It should be noted that the use of commercial aqueous ammonia makes the reaction easy and practical.
This work was partially supported by CREST, SORST, and ERATO, Japan Science and Technology Agency (JST) and a Grant-in-Aid for Scientific Research from Japan Society of the Promotion of Science.
Footnote |
† General procedure of α-aminoallylation of aldehydes using DBSA in aqueous ammonia: A mixture of allylboronate 2 (0.6 mmol) and dodecylbenzenesulfonic acid (DBSA) (10 mol%) in 25 wt% aqueous ammonia (1 mL, ca. 30 equiv.) was stirred at rt for 30 min. To the suspension was added aldehyde 1 (0.5 mmol), and the mixture was stirred vigorously at rt for the indicated time. The mixture was acidified with 3 N hydrochloric acid and extracted with dichloromethane (3 × 20 mL); the combined organic phases afforded crude alcohol 4. The aqueous layer was basified with 6 N sodium hydroxide and extracted with dichloromethane (3 × 20 mL). The combined organic phases were dried over Na2CO3, filtered, concentrated in vacuo, and purified by preparative TLC (hexane/isopropylamine) to give amine 3. |
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