M. Li et al. / Tetrahedron Letters 48 (2007) 3901–3905
3905
elution was effected with a mobile phase of acetonitrile/
10 mM ammonium acetate (1/1, v/v) at a flow rate of
2 mL/min. The HPLC flow was split at a ratio of 10:1
prior to entering the MS detector; approximately 200 lL/
min of the HPLC flow was directed into the ion source of
the MS detector. Positive electron spray ionization was
used with a spray voltage of 4.5 kV. The capillary
temperature was set at 300 ꢁC and the sheath gas flow
rate 40 units. A full MS scan was obtained in the mass
range of 100–800 Da. Tandem MS/MS experiments were
performed using helium as the collision gas with an
isolation width of 3.0 Da for the parent ion and normal-
ized collision energy of 28.
droxyacetone side chain, would also occur for the 17,21-
diesters of these corticosteroids but under the alkaline
condition. This variation of the original Mattox rear-
rangement proposed in this Letter should facilitate the
understanding of the degradation behavior of relevant
corticosteroid 17,21-diesters, some of which are used
as the active pharmaceutical ingredients in many mar-
keted drug products.
Acknowledgments
9. Acidic conditions for producing enol aldehyde from
betamethasone for analytical purpose: A solution of
1.0 mg of betamethasone in 1.5 mL of acetonitrile was
mixed with 50 lL of concentrated sulfuric acid at room
temperature and allowed to stand at room temperature for
up to 30 min. For isolation of both the E- and Z-isomers
of betamethasone enol aldehyde, 2 mL of concentrated
sulfuric acid was added into a 20 mL, 0.5 mg/mL beta-
methasone solution in acetonitrile/water (1/1, v/v). After
the resulting solution was heated at 66 ꢁC for 20 h, it was
concentrated and portions of the concentrated solution
were separated, through a number of injections, on a
Waters Alliance HPLC system equipped with a Supelcosil
ABZ plus, 250 · 4.6 mm, 5 lm, column and a fraction
collector. A linear gradient with a flow rate of 1.5 mL/min
was generated between mobile phase A (acetonitrile/water,
25/75, v/v) and mobile phase B (acetonitrile/water, 90/10,
v/v); the percentage of solution B was increased from 0%
to 35% in 25 min and continuously increased to 100% in
the next 10 min. Under this condition, the two isomers,
eluting at ꢀ23 and ꢀ24 min with a complete baseline
separation, were collected. The collected fractions of the
E- and Z-isomers from multiple runs were combined,
respectively, and then evaporated in vacuo. The dried
samples of the E- and Z-isomers were analyzed by 1H and
13C NMR; the results and assignment are summarized in
Table 1. Various 2D experiments were performed to
establish the structural connectivity and configurations.
10. The acidic stress conditions used in these experiments to
generate the enol aldehyde from betamethasone and its
derivatives are described in the legend of Figure 3.
We would like to thank Dr. Gilbert Tumambac and Ms.
Rebecca Osterman for technical assistance with some of
the experiments.
References and notes
1. Mattox, V. R. J. Am. Chem. Soc. 1952, 74, 4340–4347.
2. Weiss, G.; Monder, C.; Bradlow, L. J. Clin. Endocrinol.
Metab. 1976, 43, 696–699.
3. Lewbart, M. L.; Mattox, V. R. J. Org. Chem. 1964, 29,
513–521.
4. You, Z.; Khalil, M. A.; Ko, D.-H.; Lee, H. J. Tetrahedron
Lett. 1995, 36, 3303–3306.
5. Herzog, H. L.; Gentles, M. J.; Marshall, H.; Hershberg, E.
B. J. Am. Chem. Soc. 1961, 83, 4073–4076.
6. Lewbart, M. L.; Monder, C.; Boyko, W. J.; Singer, C. J.;
Iohan, F. J. Org. Chem. 1989, 54, 1332–1338.
7. Singer, C. J.; Iohan, F.; Monder, C. Endocrinolgy 1986,
119, 1356–1361.
8. A solution of 1.0 mg of betamethasone 17,21-dipropionate
in 1.5 mL of acetonitrile was mixed with 75 lL of 1 N
NaOH aqueous solution at room temperature. The
resulting mixture was allowed to standard at room
temperature for 20 min. Aliquots of the reaction solution
were injected into a LC–MS system, consisting of a
Thermo Electron Surveyor HPLC system coupled with a
linear ion trap mass spectrometer, for HPLC-PDA/MSn
analyses. The HPLC system was equipped with a Super-
cosil ABZ-Plus C18, 250 · 4.6 mm, 5 lm column. Isocratic