Base-mediated rearrangement

Scheme4.59. Ring opening of epoxides by nucleophilic attack or acid or base-mediated rearrangement. R = carbocation-stabilizing, sterically demanding group.

It has been shown that 4-decyl-l,3,2-dioxathiolane AA-dioxide unexpectedly reacts with BuLi to give dodecanal in 99% yield <1994JOC520>. A few other examples of this base-mediated rearrangement of 1,3,2-dioxathiolane S,S-dioxides (cyclic sulfates) to ketonic products have been described (Table 7). The reaction may start with... 

The well-known base-mediated rearrangement of epoxides into allylic alcohols was first reported as an enantioselective process using a chiral base in 1980. Since then, the reaction has received much attention, mostly due to the significance of chiral allylic alcohols in organic synthesis. Major breakthroughs in the area include the use of a substoichiometric amount of chiral base and the development of chiral bases for a true catalytic reaction protocol. Andersson and co-workers have reviewed this area from 1980 to 2001, with emphasis on the period 1997-2001 <2002CSR223>. 

KHMDS has been studied as a reagent capable of inducing base-mediated rearrangements of 2-benzyloxycyclooctanone and its A -unsaturated congener. Exploring the feasibility of an 0 C 1,2-shift, 2-benzyloxycyclooctanone was treated with bases including NaH in DMF, EDA in THF, and KHMDS in... 

Garratt—Braverman cyclization (base-mediated rearrangement of bis-propargyl sulfones via bis-aUenes generated in situ), a powerful tool for C—C bond formation 12SL2582. 

Base-mediated Rearrangements of the Carhon Skeleton. KHMDS has been utilized to induce the [3,3] sigmatropic anionic oxy-Cope rearrangement with subsequent C-methylation, providing the ring-expanded a-methylated ketone in an excellent yield (81%) in a one step reaction (eq 83). ... 

Asymmetric synthesis of the rocaglamides was accomplished by employing [3+2] photo-cycloaddition mediated by functionalized TADDOL based chiral Brpnsted acids. The synthesis consisted of a [3+2] dipolar cycloaddition, a base-mediated a-ketol rearrangement and a hydroxyl-directed reaction <06JA7754>. Asymmetric synthesis of 1,2-dihydrobenzo /j]furans was achieved by adamantylglycine derived dirhodium tetracarboxylate catalyzed C-H insertion <06OL3437>. 

Other non-traditional preparations of 1,2,3-triazoles have been reported. The rearrangement in dioxane/water of (Z)-arylhydrazones of 5-amino-3-benzoyl-l,2,4-oxadiazole into (2-aryl-5-phenyl-27/-l,2,3-triazol-4-yl)ureas was investigated mechanistically in terms of substituents on different pathways <06JOC5616>. A general and efficient method for the preparation of 2,4-diary 1-1,2,3-triazoles 140 from a-hydroxyacetophenones 139 and arylhydrazines is reported <06SC2461>. 5-Alkylamino-] //-], 2,3-triazoles were obtained by base-mediated cleavage of cycloadducts of azides to cyclic ketene acetals <06S1943>. Oxidation of N-... 

Nevertheless, whereas the base-promoted isomerization of simple linear oxiranes and cyclohexene oxide occurs via a -deprotonation mechanism, recent denterinm-labeUng experiments demonstrate that the LDA-mediated rearrangement of cyclopentene oxide in nonpolar solvents furnishes the corresponding cyclopentenol through an a-deprotonation route (Scheme 7) . 

Care would have to be exercised when exposing the pentapeptide to hydroxide ion-mediated deprotection conditions since the base-catalyzed rearrangement of iso-glutamine to glutamate in peptidoglycan-related structures has been observed. 

Substituted 1,2,3-triazole 1-oxide 448 can be prepared by N-oxidation of 1-substituted 1,2,3-triazoles 457, by N-alkylation of 1-hydroxy-1,2,3-triazoles 443, by cyclization of triazene 1-oxides 460, or by rearrangement of 2-substituted 1,2,3-triazole 1-oxides 445 (R=PMB). Finally, base-mediated ring opening of pyrimidinediones 463 offers a route to 464. 

The hydrolysis of epoxides can occur through neutral, acid-, or base- mediated reactions. Because the add and neutral processes generally dominate over the range of environmental pH, the base-mediated reaction often can be ignored. The products of hydrolysis are usually the corresponding diol and sometimes rearranged products (Equation (12)). 

Since introduction of the Ireland-Claisen rearrangement in 1972, the Ireland variant has become increasingly popular in organic synthesis [66]. Although excess Ic with appropriate base is often employed as selective silylating agent, the role of residual trialkylsilyl triflate and base has not been detailed. Illustrated here are some examples of silyl triflate-mediated rearrangement which can be conducted under milder conditions. 

A synthesis of 2-oxazolidinones has been reported by a base-mediated reaction between epichlorohydrin and primary amines. This reaction proceeded via an intermediate hydroxy-l,3-oxazinan-2-one 300, which rearranged to the more stable 2-oxazolidinone product under the reaction conditions (Scheme 84) <2005JOC5737>. 

A discussion of classical rearrangements and isomerization reactions is a fitting end to this chapter. These types of reactions are extremely useful when considering their ability to convert readily available sugars to both scarce sugars and useful sugar derivatives. In this section, base-mediated isomerization reactions will be addressed, as will the Amadori rearrangement. 

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