Conclusions
The present study when compared with our earlier report,4
clearly indicates that LPNM is a better medium than LPDE for
aldol condensation of silyl enol ethers and ketene silyl acetals
with aldehydes. The enhanced reactivity towards aldol con-
densation of aldehydes in LPNM in comparison with LPDE
is attributed to the higher Lewis acidity of lithium ions in
nitromethane than in ether and hence better activation of the
carbonyl group. High chemoselectivity has been observed in
these reactions in that only aldehydes and aliphatic cyclic
ketones reacted while aliphatic and aromatic ketones remained
inert.
Experimental
Materials
Fig. 1 400 MHz 1H NMR Spectrum of compound 17.
Analytical grade nitromethane (1 litre) was stirred with concen-
trated sulfuric acid (150 cm3) overnight at room temperature
and subsequently washed with 10% aqueous sodium carbonate
solution followed by water. It was dried over anhydrous calcium
chloride and fractionally distilled. It was stored over activated
molecular sieves (4 Å type) under nitrogen in an amber col-
oured bottle. Anhydrous lithium perchlorate was dissolved in
dry nitromethane under nitrogen atmosphere with adequate
cooling to obtain a 5 mol dmϪ3 solution. Preparation of 5 mol
dmϪ3 LPDE has been reported earlier.3 CAUTION: Although
lithium perchlorate is stable up to its melting point, preparation of
the anhydrous salt and the solutions in organic solvents should be
done carefully, preferably in the fume cupboard behind a shield.
Starting materials 1–316 and 417 were prepared according
to literature methods. Except for compound 17 all the other
products from the aldol reactions are literature known com-
pounds and they were characterised by IR, high resolution
1H- and 13C-NMR and mass spectroscopic data.
Fig. 2 400 MHz 1H NMR Spectrum (expanded) of compound 17.
General procedure for aldol reaction in 5 mol dm؊3 LPNM
The carbonyl compound (2 mmol) taken up in 2 cm3 of 5 mol
dmϪ3 LPNM under nitrogen was treated with 2.4 mmol of the
silyl enol ether and the mixture was stirred at room temperature
until the disappearance of the starting materials as indicated by
TLC. The mixture was diluted with dichloromethane (25 cm3)
and washed with water (10 cm3). The organic layer was dried
over anhydrous sodium sulfate and the solvent was removed
to obtain the crude product which was purified by column
chromatography over silica gel to obtain the pure products. The
products were generally obtained as a pair of diastereoisomers
and the ratio of the isomers was calculated from the peak
integration of high resolution (400 MHz) 1H-NMR spectra.
Fig. 3 100 MHz 13C NMR Spectrum of compound 17.
Spectroscopic characterisation of products
2-[(4-Acetylphenyl)trimethylsilyloxymethyl]cyclohexanone
Project (S. S.) is gratefully acknowledged. We thank the
Regional Sophisticated Instrumentation Centre, IIT, Madras,
for the high resolution NMR and mass spectral data.
17. Yield 90%, ratio of diastereoisomers 1:1.1; νmax(CCl4)/cmϪ1
2944, 1712 s (C᎐O), 1680 s (C᎐O), 1603 s; δH (CDCl3) isomer I:
᎐
᎐
7.9 (2H, d, J 6.3), 7.4 (2H, d, J 6.3), 5.36 (1H, d, J 4.4), 2.6–1.4
(12H, m), 0.04 (9H, s); isomer II: 7.9 (2H, d, J 6.3), 7.42 (2H,
d, J 6.3), 5.16 (1H, d, J 7.32), 2.6–1.4 (12H, m), 0.03 (9H, s);
δC(CDCl3) isomer I: 210.3 (s), 197.6 (s), 148.2 (s), 135.8 (s),
128.0 (d), 126.4 (d), 71.3 (d), 58.1 (d), 41.8 (t), 29.9 (t), 26.9 (t),
26.5 (q), 24.4 (t), 0.0 (q); isomer II: 210.7 (s), 197.6 (s), 149.9 (s),
136.2 (s), 128.0 (d), 127.1 (d), 72.5 (d), 58.9 (d), 42.2 (t), 28.0 (t),
26.8 (t), 26.5 (q), 24.3 (t), 0.03 (q); m/z (EI, 70 eV) 318 (Mϩ,
25%), 221 (75), 170 (25), 155 (30), 115 (15), 84 (20), 73 (100);
HRMS: calculated for C18H26SiO3 318.16511, found 318.1649.
(Figs. 1, 2 and 3)
References
1 (a) P. A. Grieco, Aldrichimica Acta, 1991, 24, 61; (b) H. Waldmann,
Angew. Chem., Int. Ed. Engl., 1991, 30, 1306; (c) P. A. Grieco, in
Organic Chemistry; Its Language and its State of Art, ed.
V. Kisakürek, VCH, Basel, 1993, p. 133; (d) A. Flohr and
H. Waldmann, J. Prakt. Chem., 1995, 337, 609.
2 P. A. Grieco, J. J. Nunes and M. D. Gaul, J. Am. Chem. Soc., 1990,
112, 4595.
3 V. Geetha Saraswathy and S. Sankararaman, J. Org. Chem., 1994,
59, 4665.
4 V. Geetha Saraswathy and S. Sankararaman, J. Chem. Soc., Perkin
Trans. 2, 1996, 29.
5 (a) M. T. Reetz, B. Raguse, C. F. Marth, H. M. Hügel, T. Bach and
D. N. A. Fox, Tetrahedron, 1992, 48, 5731; (b) J. Ipaktschi and
A. Heydari, Chem. Ber., 1993, 126, 1905.
Acknowledgements
Financial support from CSIR, New Delhi, in the form of a
Senior Research Fellowship (R. S.) and a Sponsored Research
J. Chem. Soc., Perkin Trans. 1, 1999, 383–386
385