C O M M U N I C A T I O N S
Figure 2. Conformer A of the enecarbamate 2c(3S) assigned by NOE.
cycloaddition. Substantiating this inference is the X-ray structure
of the enecarbamate 2c(3S), which shows a dihedral angle of only
34° between the CO double bond in the oxazolidinone and the CC
double bond of the enecarbamate (Figure 1). This is due to the
conjugation of the nitrogen atom with both the carbonyl group and
the CC double bond. Consequently, the carbonyl group may be
distorted only moderately from the planar arrangement and cannot
engage in steric or electronic or both effects with the attacking 1O2.
Mechanistically significant are the NOE effects of the enecar-
bamate 2c(3S), which show that the conformer A is preferred in
solution (Figure 2). In this conformer, the lower face of the
enecarbamate double bond is shielded by the isopropyl substituent
Figure 1. Crystal structures of the enecarbamate 2c and diol 5.
Scheme 2. Chemical Correlation for the Configurational
Assignment of the Dioxetane 3c
1
of the oxazolidinone ring. Consequently, O2 attack occurs from
the upper face [see Scheme 3, shown for 3c(3R)],10 as is observed
experimentally.
Our unprecedented results for the reaction of oxazolidinone-
substituted enecarbamates with 1O2 demonstrate that an appropriate
choice of the chiral auxiliary enables complete diastereofacial
Scheme 3. Preferred π-Facial Attack of 1O2, Controlled by Steric
Shielding of the R Substituent in the Enecarbamate 2
1
control in the [2 + 2] cycloaddition even for O2, the smallest of
all cyclophiles. This high diastereoselectivity is rationalized in terms
of effective π-facial control through steric shielding by the
substituent at the chirality center of the oxazolidinone auxiliary.
Two new stereogenic centers are introduced with complete control
of diastereoselectivity; the resulting dioxetane may be derivatized
to synthetically useful, enantiomerically pure hydroxylated products.
Acknowledgment. The work at Wu¨rzburg was generously
supported by the Deutsche Forschungs-Gemeinschaft (Schwer-
punktprogramm “Peroxidchemie”) and the Fonds der Chemischen
Industrie; the work at New York was financed by the National
Science Foundation through Grant CHE-01-10655. We thank Dr.
M. Schwarm (Degussa-Hu¨ls AG) for a generous gift of the
oxazolidinones and alaninol.
bamate diastereomer 2c(3S) (Figure 1). The diol 5 was prepared
independently in three steps from desoxybenzoin (Scheme 2). The
two diastereomers of the diol 5 were separated by silica gel
chromatography and the NMR spectra of the unlike diastereomer
[the configuration was determined by X-ray analysis (Figure 1)]
were identical with those of the diol 5, obtained from dioxetane
3c(3R) with dr > 95:5. Since in the 2,3-diphenyl-1,2-butanediol
(5) derived from the dioxetane 2c(3R), the C3 stereogenic center
is already known to possess the R configuration, it follows that the
C2 site possesses the S configuration. Thus, since the oxazolidinone
stereogenic center is R-configured, the (1S,2S)-dioxetane was
obtained in the photooxygenation (entry 3, Table 1), which
Supporting Information Available: Experimental details (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
1
establishes that O2 attacks exclusively from above (see Scheme
3) in the [2 + 2] cycloaddition to the enecarbamate 2c, and that
with complete π-facial control.
(1) (1) Jones, S. J. Chem. Soc., Perkin Trans. 1 2002, 95, 1-21.
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The exclusive diastereoselectivity of the 1O2 reaction is already
expressed in the case of the (R)-4-methyl-substituted enecarbamate
2b(3S), since the other possible diastereomer could not be observed
within the experimental uncertainty of 5%; thus, even a methyl
group in the oxazolidinone suffices for perfect π-facial control (entry
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1) and in CD3COCD3:CDCl3 (2.4:1) solvent mixtures (entries 6
and 7, Table 1) resulted in the same selectivity as in CDCl3.
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solvent polarity, which suggests that the carbonyl group plays no
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(10) Note that in the chemical correlation (Scheme 2), the 3c(3R) dioxetane
was used, whereas the NOE effects (Figure 2) were determined on the
2c(3S) enecarbamate; fortunately, the stereogenic center at the C3 position
does not influence the π-facial attack (Scheme 3).
JA026815K
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J. AM. CHEM. SOC. VOL. 124, NO. 30, 2002 8815