Benzohydroxamic acid addition to propiolate esters - A reinvestigation

Article abstract of DOI:10.1016/S0040-4039(00)01164-3

Addition of benzohydroxamic acid to propiolate esters gives 1,4,2-diozaxoles which originate, by base-catalysed ring opening with NaH, the isomeric O-vinyl hydroxamic acids. These compounds on thermolysis afford 1,3-oxazoles. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01164-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 7433–7435
Benzohydroxamic acid addition to propiolate esters—
a reinvestigation
Mariana P. Duarte, Ana M. Lobo* and Sundaresan Prabhakar*
Secc¸a˜o de Qu´ımica Orgaˆnica Aplicada, Departamento de Qu´ımica,
Centro de Qu´ımica Fina e Biotecnologia and SINTOR-UNINOVA, campus Faculdade de Cieˆncias e Tecnologia,
Universidade Nova de Lisboa, Quinta da Torre, 2825-114 Monte de Caparica, Portugal
Received 7 June 2000; accepted 9 July 2000
Abstract
Addition of benzohydroxamic acid to propiolate esters gives 1,4,2-diozaxoles which originate, by
base-catalysed ring opening with NaH, the isomeric O-vinyl hydroxamic acids. These compounds on
thermolysis afford 1,3-oxazoles. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: hydroxamic acids and derivatives; Michael reactions; dioxazoles; oxazoles; ring transformations.
As part of our program to explore the utility of enehydroxylamines in 3,3%-sigmatropic
rearrangements,1,2 it was decided to study the base catalysed ring opening of N-
acyloxaziridines3,4 of type 1 (cf. Scheme 1). That prompted us to prepare 1 (R=Ph, R%=Me) by
a recently reported general method,5 i.e. a Michael addition of benzohydroxamic acid to methyl
propiolate catalysed by N-methyl morpholine in CH3CN at rt. The product obtained as a
1
colourless oil6 (yield >90%) possessed a H NMR spectrum identical to that reported.7 However
it failed to release iodine from an iodide solution in acid,3,8 and was thermally stable (110°C/3
h), a behaviour which is inconsistent with the proposed oxaziridine structure8 1. Its molecular
formula (C11H11NO4), and mass spectrum suggested that the product could be the thermody-
namically more stable isomeric 1,4,2-dioxazole 2a9 (Scheme 1) formed by ring closure of the
initially generated O-vinylcarbomethoxy benzohydroxamic acid 3b/4b.
A careful literature search revealed that the ethyl ester 2b had in fact been obtained via a
similar Michael addition of benzohydroxamic acid to ethyl propiolate in DMSO containing
NaCH2SOCH3.10 Compound 2b, thus secured, was found to be identical in all respects (1H, 13C
NMR, IR, TLC) with the product derived from the same reaction partners and N-methyl
morpholine in CH3CN at rt.
* Corresponding authors. E-mail: aml@mail.fct.unl.pt; sp@dq.fct.unl.pt
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01164-3

7434
Scheme 1.
Methyl a-(3-phenyl-1,4,2-dioxazol-5-yl)-propionate (2a) in THF at 0–5°C, on treatment with
NaH, led to a 5.5:1 mixture of sodium salts 3a and 4a, which on careful acidification afforded
3b and 4b.11 These tended to revert slowly to 2a either on standing at room temperature or
during silica gel chromatography. The methyl ethers 3c and 4c,12 obtained almost quantitatively
by methylation of 3b and 4b with CH2N2, on thermolysis (220°C; 15 min) yielded the
1,3-oxazole13 5 in ca. 20% yield.14 Similar thermal and photochemical15,16 fragmentation
reactions have been described and found to be of synthetic value.2
In conclusion, Michael addition of benzohydroxamic acid to propiolate esters does not afford,
as claimed, oxaziridines, but instead leads to the isomeric 1,4,2-dioxazoles. These in turn, under
aprotic basic conditions, suffer ring opening to the corresponding O-vinyl hydroxamic acid
derivatives.

7435
Acknowledgements
Our thanks are due to Fundac¸a˜o para a Cieˆncia e a Tecnologia (Lisbon, Portugal) for partial
financial support through the PRAXIS program and for the award of a PRAXIS doctoral
fellowship (to M.P.D.), and to Dr. S. N. Swami (Pfizer, UK) for the interest shown.
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1
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1
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1
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