Stereo- and regiocontrolled transformations of vinyloxiranes with metal halides

Article abstract of DOI:10.1016/S0040-4039(00)01680-4

Simple procedures for the transformation of vinyl oxiranes to trans, trans-dienes and regio- and stereocontrolled unsaturated bromohydrins are reported. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01680-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 9315–9318
Stereo- and regiocontrolled transformations of vinyloxiranes
with metal halides
Roberto Antonioletti, Paolo Bovicelli, Elettra Fazzolari and Giuliana Righi*
Centro C.N.R. per lo Studio della Chimica delle Sostanze Organiche Naturali, c/o Dipartimento di Chimica,
Universita` ‘La Sapienza’, P.le A. Moro 5, Box 34 Roma 62, 00185 Roma, Italy
Received 2 August 2000; revised 11 September 2000; accepted 27 September 2000
Abstract
Simple procedures for the transformation of vinyl oxiranes to trans, trans-dienes and regio- and
stereocontrolled unsaturated bromohydrins are reported. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: vinyloxiranes; dienes; bromohydrines.
The development of synthetic methodologies to prepare highly functionalised chiral fragments
represents an important challenge in organic chemistry. With this purpose, for a few years our
research group has studied stereo- and regioselective ring-opening of 2-functionalised epoxides;
Scheme 1.
in fact the possibility of obtaining chiral epoxides through recently introduced methodologies
allows access to optically active compounds.1
* Corresponding author. E-mail: giuliana.righi@uniroma1.it
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01680-4

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Scheme 2.
While 2,3-epoxy alcohols and esters reacted with metal halides, affording regiocontrolled C-2
or C-3 ring opening,2 particular reactivity has been shown by a,b-epoxy ketones, which, when
submitted to NaBr or LiBr/Amb. 15 reaction gave the corresponding a-halo-a,b-enones in
nearly quantitative yield3 and with NaI or LiI/Amb. 15, the a,b-enones through deoxygenation4
(Scheme 1).
Since our hypothesis was that the driving-force for the elimination step in both cases was the
restoration of the conjugated system, we decided to explore, under the same reaction conditions,
the reactivity of vinyl epoxides where a CꢀC double bond instead of CꢀO was present.
Vinyl oxiranes were easily prepared from the corresponding allylic alcohols in three efficient
steps: epoxidation of the double bond (by dimethyldioxirane5 for racemic compounds, by
Table 1
Deoxygenation of vinyl epoxides with LiI/Amb. 15a
a A mixture of 1 mmol of substrate, 4 mmol of LiI and 2.5 mmol of Amb. 15 in 8 mL of acetone was stirred for
ꢀ6 h at rt and then filtered on a pad of celite. In the absence of Amb. 15, or with NaI instead of LiI, the reactions
gave the starting material.

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Sharpless AE6 for optically active epoxides), oxidation of the alcohol group and Wittig or
Horner–Emmons olefination (Scheme 2).7
To study the reactivity of these systems we prepared substrates with various R and R% to
investigate the influence of the steric hindrance of R on the oxirane ring and the electronic
effects of R% on the double bond.8
As shown in Tables 1 and 2, while these compounds gave the deoxygenation products as
already found for a,b-epoxyketones with LiI/Amb. 15, with LiBr/Amb. 15 we observed a regio-
and diastereoselective opening of the epoxide to give the corresponding trans-a-bromo-b-
hydroxy olefin derivatives.
Table 2
Ring opening of vinyl epoxides by LiBr/Amb. 15a
a A mixture of 1 mmol of substrate, 4 mmol of LiBr and 1 mmol of Amb. 15 (0.5 when R%=alkyl) in 8 mL of
acetone was stirred for ꢀ2 h at rt and then filtered on a pad of celite. In absence of Amb. 15 the reactions did not
work; with NaBr instead of LiBr chemical yields were lower.
.

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In the case of R%=CO2Et, the reaction with LiI/Amb. 15 worked very well, affording the
corresponding trans, trans-diene in high chemical yield, independently of the steric hindrance of
R, while for R%=alkyl, a complex mixture of products was obtained.
Also the regio- and stereoselectivity of the oxirane ring opening with LiBr/Amb. 15 did not
depend on the size of R (Table 2); the yield of the reaction was very high for R%=CO2Et and
quite good for R%=alkyl. The known high reactivity of the allylic position could be responsible
for the complete regioselectivity observed, particularly when an electron withdrawing substituent
on the double bond makes this position more electron deficient.
In view of the importance of the products obtained, due to the possibility of substituting the
halide, and to functionalise further the double bond, the method appears to be of general value.
The employment of this methodology in the synthesis of polyhydroxylated chains, subunits
present in many naturally occurring compounds, is currently under investigation.
References
1. For a review see: Righi, G.; Bonini, C. Recent Res. Devel. Org. Chem. 1999, 3, 343.
2. Righi, G.; Bonini, C.; Federici, C.; Rossi. L. J. Org. Chem. 1995, 60, 4803; Righi, G.; Bonini, C.; Rumboldt, G.
Tetrahedron 1995, 51, 13401; Righi, G.; Bonini, C.; Rumboldt, G. J. Org. Chem. 1996, 61, 3557.
3. Bovicelli, P.; Righi, G.; Sperandio, A. Tetrahedron Lett. 1999, 40, 5889.
4. Bovicelli, P.; Righi, G.; Sperandio, A. Tetrahedron 2000, 56, 1733.
5. See for instance Curci, R.; Dinoi, A.; Rubino, M. F. Pure Appl. Chem. 1995, 67, 811–821. Adam, W.;
Hadjarapoglou, L. P.; Curci, R.; Mello, R. In Organic Peroxides; Ando, W., Ed.; Wiley: New York; Bovicelli, P.;
Lupattelli, P.; Mincione E. J. Org. Chem. 1994, 59, 4304.
6. Gau, Y.; Hanson, R. M.; Klunder, J. M.; Ko, S. Y.; Masamune, H.; Sharpless, K. B. J. Am. Chem. Soc. 1987,
109, 5765.
7. March, J. Advanced Org. Chem.; John Wiley & Sons: New York, 1985; 3rd ed. p. 845 and references cited therein.
8. Products derived from Wittig reactions were isolated and only the E isomers were utilised.
.