Nucleophilic ring-cleavage

Reduction of oxetanes can usually be achieved with lithium aluminum hydride in THF. The reaction is almost general, though polyalkyl substitution may decrease the rate to an impractical level. As is to be expected with a strong nucleophile, ring cleavage occurs generally between the oxygen atom and the least substituted a-carbon atom (equation 46)... 

With the exception of nucleophilic ring cleavage in 1,3,2-dioxathiolane. Y-oxides and. Y,.Y-dioxidcs (Section 6.05.5.3), little new development has been reported on transformations of the heterocycles under consideration since the last review <1996CHEC-II(4)545>. 

The Paquette team proceeded ahead in the expectation that aldehyde 31 would outperform other chiral C10-C17 subunits (Paquette, Pissarnitski, and Barriault 1998). Their routing began with oxirane 22, which is readily available from (i )-(-)-pantolactone (Scheme 15.2). After 0-silylation, 23 was reacted with dilithioacetate in dimethoxyethane to induce nucleophilic ring cleavage with follow-up cyclization. The resulting lactone 24 could be successfully monomethylated under closely... 

When the spiro-activated cyclopropane (479) is heated with aqueous acetone a 9 1 mixture of the lactone (482) and the dicarboxylic acid (480) is obtained. Thus, with water as a nucleophile, ring cleavage is faster than acylal cleavage. Presumably, the spiroacylal functions as an active ester in the cyclization of the initially formed 1,5-adduct (481) (equation 164) °. ... 

Quaternization of isothiazole increases the tendency for nucleophilic ring cleavage. Accordingly, 2-phenylisothiazolium bromides 291 reacted with benzylamine or... 

In contrast to the above examples of nucleophilic ring cleavage which are exclusive to 1,3,5-tri-azine, a reaction of trisubstituted triazines 13 with arylhydrazines, e.g. 14, can be effected by heating at 190°C for half an hour to give the corresponding formazans 15 in about 50% yield.21... 

Asymmetric Epoxidation Reactions. While Ti(0-i-Pr)4 clearly has the capacity to bring about the nucleophilic ring-cleavage of 2,3-epoxy alcohols (see above), it remains the preferred species for the preparation of the titanium tartrate complex central to the Sharpless asymmetric epoxidation process (see, for example, eq 7). Since f-butoxide-mediated ring-opening of 2-substituted 2,3-epoxy alcohols (a subclass of epoxy alcohols particularly sensitive to nucleophilic ring-cleavage) is much slower than by isopropoxide, the use of Ti(0-f-Bu)4 is sometimes recommended in place of Ti(0-i-Pr)4. However, with the reduced amount of catalyst that is now needed for all asymmetric epoxidations, this precaution appears unnecessary in most instances. 

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