Acetals, favored ring structures

VIII. Favored Ring Structures in Acetals and Ketals. 177... 

Besides the thermodynamic feasibility, there should also be a kinetie pathway for the ring to open, faeilitating polymerization. Cycloalkanes, for example, have no bond in the ring structure that is prone to attaek and thus laek a kinetic pathway. This is in marked contrast to the cyclic monomers sueh as laetones, lactams, cyclic ethers, acetals, and many other cyclic monomers that have a heteroatom in the ring where a nucleophilic or electrophilic attack by an initiator species can take plaee to open the ring and initiate polymerization. Both thermodynamic and kinetic factors are thus favorable for these monomers to polymerize (Odian, 1991). 

The formation and the hydrolysis of acyclic and cyclic acetals have been studied in rather great detail [91]. Several reviews on this topic are available [92] and some comments have been made [13] concerning the carbohydrate series. We have shown in Schemes 1,2, and 3 that a common feature of this reaction seems to be the intermediacy of an oxocarbenium ion. However, the cyclization of such an intermediate has been questioned more recently [93] in the light of the Baldwin s rules for ring closure [94]. At least for the five-membered ring, an SN2-type displacement mechanism far the protonated form (B) of die hemiacetal (A) (favorable 5-exo-tet cyclization) has been proposed rather than the unfavorable 5-endo-trig cyclization of the oxocarbenium ion (C) (Scheme 5). Except when the formation of the enol ether (D) is structurally impossible, the intermediacy of such a compound remains feasible. 

Lupanine is comparatively easily dehydrogenated with mercuric acetate 62, 69), a property not usual for cis-quinolizidine structures. The conformation, with ring C in boat form, appears to be the favored conformation equilibrium (5, 35, 70). 

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