Allylic alcohols 316 Chain initiating step

At present, we can say that copolymerization initiated by various salts proceeds by an anionic mechanism, after dissociation of the initiators in the reaction medium. The primary step is the addition of the initiator anion to the epoxide. In the initiation by Lewis bases, i.e. by tertiary amines, initiation involves formation of a primary active centre of an anionic character. This active centre is probably generated by interaction of the tertiary amine with the anhydride and an allyl alcohol. The allyl alcohol can be formed by a base-catalyzed isomerization of the epoxide. In the presence of a proton donor, the formation of active centres is possible through interaction of tertiary amine, anhydride and proton donor without epoxide isomerization. Another way of initiation consists in a direct reaction of epoxide with tertiary amine yielding an anionic primary active centre. We believe that in both kinds of initiation in the strict absence of proton donors, the growing chain end has the character of a living polymer. The presence of proton donors, however, gives rise to transfer reactions. 

The first step is the formation of an alkoxide anion by the initiating alcohol (allyl alcohol is the initiator most commonly used, although other initiators have been suggested). The appropriate oxide(s) is (are) then added to the alcohol initiator. This causes the opening of the oxirane ring in the oxide and propagates the chain growth of the alkylene oxide on the initiator. The last step is the neutralization of the alkoxide anion to terminate the polymerization. 

A related synthesis of 12-acetoxysinularene (122) differs in that the magnesium-ene unit is part of the noibomene skeleton which carries the enophilic chain at C-2 (Scheme 26). Hence, by reversing the relative position of the reaction partners the bond between C-5 and C-6 was closed in the key step (119) —> (120) with simultaneous formation of both the C-7A -15 methylene group and the C-12 Mg functionality. Accordingly, one synthetic operation provided alcohol (121) in 62% yield from allyl chloride (118) (again the initial cis relation of C-12/C-7 was altered thereafter by an epimerization at C-5). 

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