Lithiobetaines

Lithium salts effect, that the intermediate betaines do not react further. These lithiobetaines which are quite stable may be deprotonated. Deprotonation is equal to the lost of one stereogenic centre. Use of a steric hindered proton donator then leads to the trans lithiobetaine. The reaction takes normal course, if lithium is exchanged by potassium. 

Beware of thinking that the occurrence of the lithiobetaines A and B must have stereochemical implications. Until fairly recently, lithium /ree betaines were incorrectly considered intermediates in the Wittig reaction. Today, it is known that lithima-containing betaines are formed in a dead-end side reaction. They must revert back to an oxaphosphetane—which occurs with retention of the configuration—before the actual Wittig reaction can continue. 

A different reaction mode of lithiobetaines is used in the Schlosser variant of the Wittig reaction. Here, too, one starts from a nonstabilized ylide and works under non-salt-free conditions. However, the Schlosser variant is an olefination which gives a pure frans-alkene rather than a trans.cis mixture. The experimental procedure looks like magic at first ... 

The asymmetric total synthesis of ISP-I (myriocin, thermozymocidin) was accomplished by utilizing the Schlosser modified Wittig reaction as one of the key steps in the laboratory of Y. Nagao. The phosphonium bromide fragment was treated with PhLi at 0 °C to generate the phosphorous ylide which was reacted with the aldehyde at -78 °C. The resulting mixture of lithiobetaines was treated with PhLi at 0 °C to afford the desired ( j-alkene with excellent stereoselectivity. 

Wittig reaction-Schlosser modification One-pot multistep preparation of (E)-alkenes from "nonstabilized" phosphorous ylides and carbonyl compounds by the equilibration of the intermediate lithiobetaines. 488... 

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