Preparation of Geometrically Defined Alkenes

HF-induced elimination (5). To a solution of aqueous HF (4 drops, 50%) in MeCN (8 ml) was added a solution of the /5-hydroxysilane (lmmol) in MeCN (2 ml), and the mixture was stirred at room temperature until t.l.c. analysis indicated completion (5-20min). The reaction mixture was then partitioned between pentane (50 ml) and saturated sodium hydrogen carbonate solution (10ml). The aqueous layer was extracted thoroughly with pentane (3 x 50 ml), and the combined organic extracts were washed with brine and dried. Concentration followed by chromatographic purification gave the product alkenes. 

Alternatively, the above process can be performed with pyridine (0.5 ml) also being present in the original HF/MeCN mixture. 

The main utility of Peterson olefination lies in the contrasting stereochemical requirements (6) for elimination, use of base requiring a syn conformation whereas acid conditions demand an anti conformation, with complementary geometrical results  

Given a good diastereoisomeric excess in the initial formation of the /9-hydroxysilane, either geometrical isomer of the alkene can be obtained. 

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