· Allyltrimethylsilylzinc Chloride
· (MW 214.09)
(allylmetal reagent; useful for synthesis of spiro-g-lactones1)
Preparative Methods: deprotonation of Allyltrimethylsilane with n-Butyllithium in THF at -78 °C or s-Butyllithium in THF/TMEDA, followed by the addition of Zinc Chloride.1,2 The reagent is prepared in situ and used directly.
Spiro-g-lactone Synthesis via Reactions with Carbonyl Compounds.
Although lithiated allyltrimethylsilane adds to simple ketones with good regioselectivity (eq 1),2 its addition to 17-ketosteroids produces a mixture of a and g adducts. Transmetalation of allyltrimethylsilyllithium with ZnCl2 leads to a reagent (allyltrimethylsilylzinc chloride) which adds with complete regioselectivity to this type of hindered ketone (eq 2).1 The resulting silylated homoallylic alcohols can be converted by a simple succession of steps to valuable steroidal g-lactones. It is interesting to note that the 4-hydroxybutenylsilane obtained by the addition of the allyltrimethylsilyl anion to adamantan-2-one undergoes some unusual transformations3 in the presence of N-Bromosuccinimide, leading to the oxetane (1) via a 4-exo-tet
ring opening4 of the intermediate bromonium ion (eq 3). The extent of a and g selectivity can be fine-tuned by changing the metal, and very high a-regioselectivity is observed using magnesium as the counterion.5 Similarly, the addition of Triethylaluminum to the lithium reagent leads to a high a-regioselectivity (eq 4).6 A stereocontrolled synthesis of anti-homoallylic alcohols is possible by preparing the corresponding allylic boronic ester and subsequent reaction with an aldehyde. A stereoselective elimination allows the preparation of either (E)- or (Z)-dienes (eq 5).7 The isomeric reagent (1) is a useful reagent for the preparation of 2-substituted polyfunctional allylic silanes (eq 6).8
- 1. Ehlinger E.; Magnus, P. TL 1980, 21, 11.
- 2. Ayalon-Chass, D.; Ehlinger, E.; Magnus, P. CC 1977, 772.
- 3. Ehlinger, E.; Magnus, P. CC 1979, 421.
- 4. Baldwin, J. E. CC 1976, 734.
- 5. Lau, P. W. K.; Chan, T. H. TL 1978, 2383.
- 6. (a) Yamamoto, H., Saito, Y.; Maruyama, K. CC 1982, 1326; (b) Yamamoto, Y.; Saito, Y.; Maruyama, K. TL 1982, 23, 4597; (c) Yamamoto, Y.; Yatagai, H.; Saito, Y.; Maruyama, K. JOC 1984, 49, 1096.
- 7. (a) Tsai, D. J. S.; Matteson, D. S. TL 1981, 22, 2751; (b) Sato, F.; Suzuki, Y.; Sato, M. TL 1982, 23, 4589.
- 8. Minato, A.; Suzuki, K., Tamao, K.; Kumada, M. TL 1984, 25, 83.
Philipps-Universität Marburg, Germany
Copyright © 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.