2-Chloro-3-ethylbenzoxazolium Tetrafluoroborate1

[63212-53-3]  · C9H9BClF4NO  · 2-Chloro-3-ethylbenzoxazolium Tetrafluoroborate  · (MW 269.44)

(reagent for the conversion of alcohols to alkyl chlorides,2,3 and preparation of isocyanides from formamides,4 ketones from a-hydroxy carboxylic acids,5 alkynes from aryl ketones,6 and 1,2-trans-nucleosides from 1-hydroxy sugars7)

Physical Data: mp 152 °C (dec).

Solubility: sol DMSO; slightly sol CH2Cl2, CHCl3; insol Et2O.

Form Supplied in: white solid.

Preparative Method: from o-aminophenol in three steps.2 Commercially available.

Handling, Storage, and Precautions: hygroscopic and should be handled and stored under an argon atmosphere.

Synthesis of Alkyl Chlorides.

2-Chloro-3-ethylbenzoxazolium tetrafluoroborate (1) reacts with a variety of alcohols in the presence of tetraethylammonium chloride and triethylamine to afford the corresponding alkyl chlorides in high yields and generally with inversion of configuration.2 1-Hydroxy sugars are converted to the corresponding 1-chloro sugars with retention of configuration at the anomeric centers (eq 1). In reactions with alcohols, (1) is more reactive than the related reagents, 2-halopyridinium salts (2)8 and 2-fluorobenzothiazolium tetrafluoroborate (3).9

1,2-Dichloroalkanes are prepared from epoxides under similar reaction conditions (eq 2).3

Synthesis of Isocyanides.

Isocyanides are useful intermediates in organic synthesis, and are frequently prepared via the dehydration of formamides.10 Although several methods for achieving this transformation have been reported, rather drastic conditions, such as reflux in the presence of base, are often required.11 As a useful alternative, dehydration of formamides proceeds smoothly in the presence of (1) and two equiv of triethylamine at room temperature; a variety of isocyanides can be prepared using this mild and simple procedure (eq 3).4

Synthesis of Ketones from a-Hydroxy Carboxylic Acids.

Treatment of a-hydroxy carboxylic acids, readily prepared from the corresponding carboxylic acids, with (1) and two equiv of triethylamine at room temperature affords ketones in good yields (eq 4).5

One-Step Formation of Arylalkynes from Aryl Ketones.

Aryl ketones are converted to the corresponding alkynes by treatment with (1) in the presence of a large excess of triethylamine at room temperature.

Synthesis of 1,2-trans-Nucleosides.

1,2-trans-Nucleosides are prepared from 1-hydroxy sugars and heterocycles by using (1) as a condensing reagent.7

1. Mukaiyama, T. AG(E) 1979, 18, 707.
2. Mukaiyama, T.; Shoda, S.; Watanabe, Y. CL 1977, 383.
3. Echigo, Y.; Watanabe, Y.; Mukaiyama, T. CL 1977, 1013.
4. Echigo, Y.; Watanabe, Y.; Mukaiyama, T. CL 1977, 697.
5. Mukaiyama, T.; Echigo, Y. CL 1978, 49.
6. Tsuji, T.; Watanabe, Y.; Mukaiyama, T. CL 1979, 481.
7. Mukaiyama, T.; Shoda, S.; Nakatsuka, T.; Narasaka, K. CL 1978, 605.
8. (a) Mukaiyama, T.; Ikeda, S.; Kobayashi, S. CL 1975, 1159. (b) Kobayashi, S.; Tsutsui, M.; Mukaiyama, T. CL 1976, 373.
9. Mukaiyama, T.; Hojo, K. CL 1976, 893.
10. Hoffmann, P.; Gokel, G.; Marquarding, D.; Ugi, I. In Isonitrile Chemistry; Ugi, I., Ed.; Academic: New York, 1971; p 10.
11. Sandler, S. R.; Karo, W. Organic Functional Group Preparations; 2nd ed.; Academic: New York, 1989; Vol. 3, p 206.

Shū Kobayashi

Science University of Tokyo, Japan

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