Magnesium iodide promoted reactions of nitrones with cyclopropanes: A synthesis of tetrahydro-1,2-oxazines

Article abstract of DOI:10.1021/jo048768f

Anhydrous magnesium iodide (MgI2) is shown to be an effective promoter of the "homo 3+2" dipolar cycloaddition of nitrones with 1,1-cyclopropane diesters. In almost all cases the products tetrahydro-1,2-oxazines are formed in excellent yields. The reactions are highly diastereoselective for a cis relationship between the substitutents at the 3- and 6-positions on the tetrahydrooxazine ring. As an alternative to using a preformed nitrone, the reaction may be performed in a 3-component sense by combining an aldehyde, an hydroxylamine, and the cyclopropane in the presence of catalytic MgI2.

Full text of DOI:10.1021/jo048768f

Magnesium Iodide Promoted Reactions of
Nitrones with Cyclopropanes: A Synthesis
of Tetrahydro-1,2-oxazines
Michael D. Ganton and Michael A. Kerr*
Department of Chemistry, The University of Western
Ontario, London, Ontario, Canada, N6A 5B7
FIGURE 1. Tetrahydro-1,2-oxazine-containing compounds
and the parent heterocycle.
makerr@uwo.ca
hydroxylamine with an aldehyde under the Lewis acid
conditions required for the cycloaddition.4 At that time
we screened a number of other Lewis acids in the hopes
of expanding the scope of the reaction but none surpassed
Yb(OTf)3 in efficiency. We were of course aware of the
contributions from the groups of Carriera,5 Lautens,6 and
Olsson7 in which they each activated the ring opening of
cyclopropanes using MgI2.8 We then turned to our system
and applied anhydrous MgI2 as a catalyst in our “homo
3+2” dopolar cycloaddition of nitrones with 1,1-cyclopro-
panediesters. Carriera postulates a discrete magne-
sioenolate/alkyl iodide intermediate upon treatment of
his spirocyclopropyl oxindole with magnesium iodide. In
the ytterbium triflate-catalyzed reactions, it is possible
that the reactions are proceeding via a concerted (or near
concerted) mechanism and we had hopes that with
magnesium iodide the reaction would be stepwise and
might result in a differing stereoselection as well as a
better mechanistic understanding of the reaction. Herein
we report the results of this research which describe MgI2
as an effective and useful catalyst for this reaction.
Table 1 shows the reaction of a variety of nitrones9 with
1,1-cyclopropanediesters10 under the influence of MgI2
catalysis. In a typical experiment the nitrone (1.5 equiv),
the cyclopropane (1 equiv), and anhydrous MgI2 (10 mol
%) were combined in dry THF and stirred for between
16 and 24 h. The reaction rates for MgI2 are somewhat
shorter than those observed when Yb(OTf)3 was used as
the catalyst. In many cases the yields are superior to
those obtained by us using Yb(OTf)3 as a catalyst and in
other cases the yields are lower. In this sense the
catalysts are complementary. Most notable about the
Received July 19, 2004
Abstract: Anhydrous magnesium iodide (MgI2) is shown
to be an effective promoter of the “homo 3+2” dipolar
cycloaddition of nitrones with 1,1-cyclopropane diesters. In
almost all cases the products tetrahydro-1,2-oxazines are
formed in excellent yields. The reactions are highly diaste-
reoselective for a cis relationship between the substitutents
at the 3- and 6-positions on the tetrahydrooxazine ring. As
an alternative to using a preformed nitrone, the reaction
may be performed in a 3-component sense by combining an
aldehyde, an hydroxylamine, and the cyclopropane in the
presence of catalytic MgI2.
Heterocyclic compounds hold a special place in organic
chemistry. Their role as lead candidates in drug design
cannot be overstated and the appearance of heterocyclic
motifs in natural products is astronomically frequent.
Many heterocycles (e.g. indoles, quinolines, pyridines,
pyrroles, etc) have received an enormous amount of
attention from synthetic chemists from the standpoint
of both their preparation and their reactivity. Other
heterocycles are much rarer and have received very little
attention. One such class is the tetrahydro-1,2-ozazine
(Figure 1). It is not an abundant motif in nature,
appearing in only a few naturally occurring compounds,
most notably FR900482 and related molecules.1 This fact
along with a paucity of methods for its construction2 have
resulted in it being overlooked by the synthetic organic
chemist. Its utility, however, is clear when one considers
that upon cleavage of the N-O bond a synthetically
important 1,4-amino alcohol results, which in turn can
be converted to a variety of useful compounds. In addition
the heterocycle itself may be a useful scaffold for the
development of lead drug candidates. An efficient method
for its construction in a stereodefined manner would be
an important contribution.
(4) Young, I. S.; Kerr, M. A. Org. Lett. 2004, 6, 139-141.
(5) (a) Meyers, C.; Carreira, E. M. Angew. Chem., Int. Ed. 2003, 42,
694-696. (b) Lerchner, A.; Carreira, E. M. J. Am Chem. Soc. 2002,
124, 14826-14827. (c) Fischer, C.; Meyers, C.; Carreira, E. M. Helv.
Chim. Acta 2000, 83, 1175-1181. (d) Alper, P. B.; Meyers, C.; Lerchner,
A.; Siegel, D. R.; Carreira, E. M. Angew. Chem., Int. Ed. 1999, 38,
3186-3189.
(6) (a) Lautens, M.; Han, W.; Liu, J. H.-C. J. Am. Chem. Soc. 2003,
125, 4028-4029. (b) Lautens, M.; Han, W J. Am. Chem. Soc. 2002,
124, 6312-6316.
(7) (a) Bertozzi, F.; Gundersen, B. V.; Gustafsson, M.; Olsson, R.
Org. Lett. 2003, 5, 1551-1554. (b) Bertozzi, F.; Gustafsson, M.; Olsson,
R. Org. Lett. 2002, 4, 3147-3150.
(8) For excellent reviews on activated cyclopropanes in synthesis
see: (a) Reissig, H.-U.; Zimmer, R. Chem. Rev. 2003, 103, 1151-1196.
(b) Wong, H. N. C.; Hon, M.-Y.; Tse, C.-W.; Yip, Y.-C.; Tanko, J.;
Hudlicky, T. Chem. Rev. 1989, 89, 165-198. (c) Danishefsky, S. Acc.
Chem. Res. 1979, 12, 66.
(9) Gautheron-Chapoulaud, V.; Pandya, S. U.; Cividino, P.; Masson,
G.; Py, S.; Vallee, Y. Synlett 2001, 1281-1283.
(10) 7g: commercially available (Aldrich). 7d: Kierstead, R. W.;
Linstead, R. P.; Weedon, B. C. L. J. Chem. Soc. 1952, 3610-3616. 7a-
c,e,f: Corey, E. J.; Chaykovsky, M. J. Am. Chem. Soc. 1965, 87, 1353-
1364.
Recently we described the Yb(OTf)3- catalyzed reac-
tions of nitrones with 1,1-cyclopropanediesters in a
cycloadditive sense to form the product tetrahydro-1,2-
oxazines in good to excellent yields.3 We then reported a
three-component coupling modification in which the
nitrone could be formed in situ by combination of a
(1) (a) Uchida, I.; Shigehiro, T.; Hiroshi, K.; Sumio, K.; Hashimoto,
M.; Tada, T.; Shigetaka, K.; Morimoto, Y. J. Am. Chem. Soc. 1987,
109, 4108-4109. (b) Terano, H.; Takase, S.; Hosoda, J.; Kohsaka, M.
J. Antibiot. 1989, 42, 145-148.
(2) (a) Tsoungas, P. G. Heterocycles 2002, 57, 1149-1178. (b)
Gilchrist, T. L.; Wood, J. E. In Comprehensive Heterocyclic Chemistry
II; Elsevier: Oxford, UK, 1996; Vol. 6, pp 279-299 and 1177-1307.
(3) Young, I. S.; Kerr, M. A. Angew. Chem., Int. Ed. 2003, 26, 3023-
3026.
10.1021/jo048768f CCC: $27.50 © 2004 American Chemical Society
Published on Web 11/05/2004
8554
J. Org. Chem. 2004, 69, 8554-8557

TABLE 1. The MgI2-Catalyzed Reactions of Nitrones
with Cyclopropane Diesters
FIGURE 2. Explanation for the formation of the trans isomer.
noticeable amounts of adducts bearing a trans relation-
ship between the substituents at the 3- and 6-positions.
In all cases the selectivity for the cis isomer is good to
excellent and the cis isomer can be isolated as the sole
compound after a single recrystallization. Also of note is
that the nitrone derived from formaldehyde behaves well
under these conditions (affording compound 8j), some-
thing we have not shown to be true using Yb(OTf)3. This
is important because it leaves this position on the oxazine
ring unsubstituted, which is crucial if this method is to
be used for the total synthesis of FR900482 and related
compounds.
With regards to the diastereoselectivity observed here,
a possible explanation is shown in Figure 2. If one
envisions the ring opening of the cyclopropane 7 by the
nitrone 6 to proceed via an intermediate of type I, the
imminium moiety may exist in a Z (R2 axial) and E (R2
equatorial) conformation. Closure of the magnesioma-
lonate onto the iminium species would give the observed
major product 8-cis. A loss of stereochemical integrity at
the iminium CN bond could yield II in which both R2 and
R3 are equatorial. This intermediate would close to give
the minor 8-trans isomer. Alternatively, I may undergo
a chair flip to give III in which both R2 and R3 are axial.
This intermediate would also produce 8-trans. The
observance of the trans isomer in the case of the MgI2-
promoted reactions and not in the Yb(OTf)3-promoted
reactions may be explained by the fact that the magesi-
omalonate is probably more kinetically stable than its
lanthanide counterpart. Although ytterbium is known for
its oxophilicity and, as a result, a thermodynamically
stable Yb-O bond, it is kinetically labile and undergoes
rapid dissociation from its oxygen ligands.11 Such a
a The reaction was performed in refluxing acetonitrile.
results reported here is the fact that while Yb(OTf)3
results in the formation of almost a single diastereomer,
reactions promoted by MgI2 result in the formation of
(11) Anwander, R. In Lanthanides: Chemistry and Use in Organic
Synthesis; Springer-Verlag: Berlin, Germany, 1999; p 23.
J. Org. Chem, Vol. 69, No. 24, 2004 8555

TABLE 2. The MgI2-Catalyzed Three-Component
Coupling of Aldehydes, Hydroxylamines, and
Cyclopropane Diesters
This is exciting because it allows for the formulation of
a model that explains this effect and provides some
insight into the mechanistic aspects which could in turn
result in the development of a catalyst that is selective
for the trans isomer.
Experimental Section
Typical Experimental Procedure for the Cycloaddition
of Nitrones with 1,1-Cyclopropane Diesters (Two-Com-
ponent Coupling): Synthesis of Tetrahydro-1,2-oxazine
8a. Cyclopropane 7a (0.100 g) and 1.5 equiv of nitrone 6a were
added to a sealed tube with 2 mL of dry THF, the mixture was
thoroughly degassed with argon for 2 min, and the screw cap
was replaced. MgI2 (10 mol %) was added and the mixture was
stirred for approximately 20 h. After this time the contents were
preabsorbed on 500 mg of SiO2 and subjected to flash column
chromatography (elution with a 0-10% gradient of ethyl acetate
to hexanes) to yield pure cycloadduct 8a as a 15:1 mixture of cis
and trans diastereomers. Recrystallization of this compound was
effected in 1 mL of dichloromethane and 4 mL of hexanes in a
small vial with a septum over top for 2 days. The recrystallized
product was found to be the pure cis isomer. The yield after
chromatography was 0.186 g (98% yield). The major diastere-
omer was purified by crystallization as colorless needles. Mp
154-155 °C; 1H NMR (400 MHz, CDCl3) major diastereomer, δ
7.62 (dd, J ) 1.5, 8.4, 2H), 7.59 (d, J ) 7.5, 2H), 7.48 (dd, J )
7.5, 7.5, 2H), 7.43-7.40 (m, 1H), 7.24-7.20 (m, 3 H), 7.03 (d, J
) 8.7, 2H), 6.97 (d, J ) 8.7, 2H), 5.78 (s, 1H), 5.07 (dd, J ) 12.0,
3.0, 1H), 3.95, (s, 3H), 3.49 (s, 3H), 2.91-2.80 (m, 2H), 2.20 (s,
3H), and representative peaks for the minor diastereomer
(note: some aromatic peaks for the minor diastereomer are
indistinguishable from those of the major diastereomer) δ 7.44
(d, J ) 8.7, 2H), 7.38 (dd, J ) 7.5, 7.5, 2H), 7.18 (d, J ) 8.7,
2H), 7.07 (d, J ) 8.7, 2H), 6.69 (dd, J ) 12.0, 3.0, 1H), 5.58 (s,
1H), 3.47 (s, 3H), 3.45 (s, 3H), 3.18 (dd, J ) 18.0, 8.0, 1H), 2.72
(dd, J ) 18.0, 6.0, 1H), 2.28 (s, 3H); 13C NMR (100 MHz, CDCl3)
major diastereomer, δ 170.1, 168.3, 146.2, 139.5, 135.1, 130.9,
130.5, 129.0, 128.6, 128.2, 128.0, 127.9, 126.4, 116.0, 78.7, 66.0,
59.5, 53.4, 52.5, 31.6, 20.5. These data match the data previously
reported by us for this compound (ref 3).
Typical Experimental Procedure for the Three-Com-
ponent Coupling of Aldehydes, Hydroxylamines, and
Cyclopropanes: Synthesis of Tetrahydro-1,2-oxazine 8i.
MgI2 (12 mg, 10 mol %) was added to a solution of the
hydroxylamine 9a (0.070 g, 0.567 mmol, 1.3 equiv) and aldehyde
10a (0.065 g, 0.436 mmol, 1.4 equiv) in THF (2 mL) containing
activated 4 Å molecular sieves. The solution was stirred under
an argon atmosphere for 30 min at room temperature, after
which the cyclopropane (0.100 g, 0.436 mmol) was added. Solid
cyclopropanes were added directly to the solution, while oils were
added as a solution in toluene (2 × 1 mL). After the reaction
was complete as determined by TLC, the contents were preab-
sorbed on 500 mg of SiO2 and subjected to flash column
chromatography (elution with a 0-10% gradient of ethyl acetate
to hexanes) to yield pure cycloadduct 8i as an 8:1 mixture of cis
and trans diastereomers. Samples could be recrystallized from
CH2Cl2/hexanes as above to yield the pure cis diastereomer.
Yield after flash chromatography was 0.166 g, 89%. The major
diastereomer was purified by crystallization as small colorless
prisms. Mp 127-130 °C; 1H NMR (400 MHz, DMSO) major
diastereomer, δ 7.52 (d, J ) 3.6, 2H), 7.49-7.42 (m, 2H), 7.36-
7.33 (m, 3H), 7.30 (d, J ) 4.4, 2H), 7.25 (d, J ) 4.4, 2H), 7.24-
7.19 (m, 2H), 7.15 (d, J ) 6.8, 2H), 6.62 (d, J ) 15.6, 1H), 6.44-
6.38 (dd, J ) 15.6, 6.0, 1H), 4.75 (s, 1H), 4.43-4.39 (m, 1H),
3.77 (s, 3H), 3.68 (d, J ) 14.0, 1H), 3.58 (J ) 14.0, 1H), 3.27 (s,
3H), 2.42 (d, J ) 11.6, 1H), 2.37-2.32 (dd, J ) 11.6, 3.2, 1H);
13C NMR (100 MHz, CDCl3) δ 170.4, 168.8, 137.0, 136.8, 132.1,
131.5, 129.2, 128.8 (2C), 128.5, 128.3 (3C), 128.1, 127.4, 126.8
(2C), 76.5, 59.6, 59.3, 53.3, 52.6, 30.6; IR (thin film) νmax 3061,
3030, 2953, 2894, 1743, 1495, 1453, 1436, 1259, 1198, 1179,
1077, 967, 922, 746, 699; HRMS calcd for C29H29NO5 471.2046,
found 471.2041.
dissociative process in the case of a putative intermediate
like I would leave a more naked malonate anion to
undergo ring closure. The net result would be a faster
ring closure, thereby maintaining a cis relationship. A
magnesiomalonate, on the other hand, would result in a
longer lived acyclic intermediate, allowing for increased
probability of stereochemical leakage to the trans isomer.
It must be noted that the involvement of an intermediate
in which an iodide nucleophilically opens the cyclopro-
pane prior to nitrone addition5 cannot be ruled out.
Attempts to modulate the diastereoselectivity through
variation of the temperature and solvent met with no
success.
In many cases it is desirable to generate the nitrone
in situ, particularly when the nitrone is unstable. To this
end we demonstrated that the MgI2 catalyst protocol is
amenable to a three-component coupling procedure. Table
2 shows a short series of five adducts generated by this
method. In general the yields are comparable to the
method in which a preformed nitrone is used. It is worthy
of note that like our previous 3-component couplings4
using Yb(OTf)3, it is required to premix the hydroxyl-
amine and aldehyde for a short period of time prior to
addition of the cyclopropane to avoid ring opening of the
cyclopropane by the nucleophilic hydroxylamine.
In conclusion, we have presented herein an alternative
catalyst system for a relatively new type of dipolar
cycloaddition reaction. The yields of the cycloadducts
obtained with catalytic MgI2 were, in many cases, supe-
rior to those obtained with Yb(OTf)3 as the catalyst. In
addition we have now been able to employ the less stable
formaldehyde nitrone in the cycloaddition. This will make
the reaction more applicable to molecules such as
FR900482. This new protocol, then, represents a com-
plimentary approach for use in organic synthesis. Most
importantly, however, we now have observed significant
quantities of the trans cycloadduct in some instances.
8556 J. Org. Chem., Vol. 69, No. 24, 2004

Supporting Information Available: Complete experi-
Acknowledgment. We are grateful to the Natural
Sciences and Engineering Research Council, The Ontaro
Ministry of Science and Technology, and MedMira
Laboratories for the financial support of this research.
M.D.G. is grateful to NSERC for funding in the form of
a scholarship (PGS-A and PGS-B).
1
mental procedures as well as H NMR and 13C NMR data for
new compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
JO048768F
J. Org. Chem, Vol. 69, No. 24, 2004 8557