Epoxysilanes Preparation

The preparation and applications of a,0-epoxysilane a-anions have been described in detail105, as have the preparation and applications of a,0-epoxysilanes themselves106. 

OL-Alkylidenetetrahydrofuranes. Reaction of epoxysilanes of structure 1 with KH in THF provides a-alkylidenetetrahydrofuranes (2) with >95% retention of stereochemistry. The precursor vinylsilanes can be prepared stcreoselectively from an alkyne by... 

The isomeric epoxysilanes (3) and (4), prepared as shown in Scheme 6, undergo a copper-catalyzed Grignard ring-opening to give P-hydroxysilanes. Oxidative cleavage then completes this selective route to either syn (5) or anti (6) 1,2-diols. Such methodology has been utilized in a synthesis of ( )-cj o-brevi-comin. 

Several procedures are available for the preparation of the requisite p-hydroxysi-lanes such as addition of a-silyl carbanions to aldehydes and ketones, reaction of organocuprates with a,P-epoxysilanes, reduction of P-ketosilanes, and addition of organometallic regents to P-ketosilanes. The selection of a particular procedure is dictated by the structure and stereochemistry of the desired alkene. 

In the P-ketosilane approach, the condensation of a-silyl carbanions with carbonyl compounds produces mixtures of diastereomeric P-hydroxysilanes. Therefore, the preparation of stereodefined alkenes via the Peterson reaction hinges on the availability of just one diastereomer. To overcome this shortcoming, procedures have been developed to prepare stereoselectively P-hydroxyalkylsilanes via the P-ketosilane or the epoxysilane routes, as exemplilRed below. 

Fig. 4. v(CH) range of different epoxysilanes SA-layers on silica prepared under ambient conditions (absorbance given in Kubelka-Munk units). 

An optically active silyl-stabilised lithiated epoxide has been used in the synthesis of (-F)-cerulenin 111 (Scheme 21) [73]. Starting from the epoxysilane 107, obtained in high enantiomeric purity via Sharpless epoxidation of 106, deprotonation by s-BuLi TMEDA was followed by electrophile trapping using (4E,7E)-nonadienal 109 to allow a concise preparation of 110, which is a key intermediate in the (-F)-cerulenin synthesis. 

Organofunctional epoxysilanes may be prepared by hydrosilylating unsaturated epoxides or epoxidized unsaturated silanes. The hydrosilylation of allyl ethers CH2=CHCH20R is commonly used for synthesis (e.g., y-glycidoxytrimethoxysilane) as follows ... 

Epoxysilanes may be prepared by a variant of this approach. Lithiation of chloromethyItrimethyIsilane with 5ec-butyllithium gives an a-chloro carbanion which affords epoxysilanes on reaction with aldehydes and ketones. 

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