· (2,6-Di-t-butyl-4-methylphenoxy)methylaluminum Trifluoromethanesulfonate
· (MW 410.43)
(designer Lewis acid for allylic ether cleavage and its application to biogenetic-type terpene synthesis1)
Solubility: sol toluene and CH2Cl2; slightly sol hexane.
Form Supplied in: prepared and used in situ.
Preparative Method: treatment of 2,6-di-t-butyl-4-methylphenol in dry CH2Cl2 with a 1-2 M hexane solution of Trimethylaluminum at -78 °C for 10 min and at 0 °C for 30 min affords Dimethylaluminum 2,6-Di-t-butyl-4-methylphenoxide which, after cooling to -78 °C, is reacted with Trifluoromethanesulfonic Acid (TfOH). Upon warming to 0 °C, gas evolution is observed to give a pink solution after further stirring at 0 °C for 30 min.2
Handling, Storage, and Precautions: the dry solid and solutions are flammable and must be handled in the absence of oxygen and moisture. The solution should be used as prepared for best results. Use in a fume hood.
Intramolecular Prenyl Transfer Reaction.
This modified organoaluminum reagent has been developed for effecting the intramolecular version of prenyl transfer reactions.2 Thus reaction of isopentenyl prenyl ether with the highly oxygenophilic organoaluminum reagent in CH2Cl2 at 25 °C enables head-to-tail condensation, resulting in formation of the two C10 alcohols in a 1:3 ratio (eq 1). Conventional strong Lewis acids such as Titanium(IV) Chloride, Tin(IV) Chloride, and Boron Trifluoride Etherate readily attack both ethereal oxygen and alkenes in isopentenyl prenyl ether to furnish a number of side-reaction products. With weaker acids it is difficult to cleave the allylic C-O bond of isopentenyl prenyl ether. Clearly, the present oxygenophilic aluminum reagent is strong enough to coordinate to the ether
oxygen, but not to double bonds. It could thus play a key role in promoting a combined C-O bond fission/rearrangement. In addition, the bulky 2,6-di-t-butyl-4-methylphenoxy group in the aluminum reagent may assist anchimerically the through-space interaction of the p-electron orbital of the prenyl cation via a six-membered transition state.
The organoaluminum reagent can also effect the biogenetic-type head-to-head condensation of C5 units. Thus treatment of diprenyl ether with the reagent at 0 °C for 1.5 h affords the C10 alcohol lavandulol exclusively, in 33% yield (eq 2). This method provides a very simple route to lavandulol, hitherto accessible only by multi-step synthesis.3
Biogenetic-type asymmetric cyclization of (R)-(+)-binaphthol mononeryl ether to optically active limonene can be realized using this type of reagent in CFCl3 at -130 °C (eq 3). Among various oxygenophilic organoaluminum triflates examined, (2,4,6-tri-t-butylphenoxy)isobutylaluminum trifluoromethanesulfonate gives the best result.4
Asymmetric cyclization of (R)-(+)-binaphthol (Z,Z)-monofarnesyl ether can be executed equally well with this aluminum reagent, giving (+)-b-bisabolene in 76% ee (eq 4).
The aluminum reagent possesses high activity in both cationic polymerization and electrophilic reactions.5
- 1. Maruoka, K.; Yamamoto, H. AG(E) 1985, 24, 668.
- 2. Yamamura, Y.; Umeyana, K.; Maruoka, K.; Yamamoto, H. TL 1982, 23, 1933.
- 3. (a) Bertrand, M.; Gil, G.; Viala, J. TL 1977, 1785. (b) Cakleaf, J. A.; Thomas, M. T.; Wu, A.; Snieckus, V. TL 1978, 1645.
- 4. (a) Sakane, S.; Fujiwara, J.; Maruoka, K.; Yamamoto, H. JACS 1983, 105, 6154. (b) Sakane, S.; Fujiwara, J.; Maruoka, K.; Yamamoto, H. T 1986, 42, 2193.
- 5. Collomb, J.; Arlaud, P.; Gandini, A.; Cheradame, H. Cationic Polym. Relat. Processes, Proc. Int. Symp. 1984, 6, 49.
Keiji Maruoka & Hisashi Yamamoto
Nagoya University, Japan
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