Methyl groups, neighboring group participation

Cyclization of hydrazone 38 with mercuric oxide and EDTA gave dihy-drotriazine 39 (87AP198). On the other hand, methyl hydrazone 38, under 4-electron withdrawal and neighboring group participation reacts with the same reagent to give lactam 40, a useful precursor for the synthesis of the pyrrolo[2,l-c]triazinium salt 41 by cyclization with perchloric acid (87AP258) (Scheme 12). 

Neighboring-group participation reactions by sulfur and nitrogen functions have also been observed for deoxyhalogeno sugars. An example is the formation of the anhydride 146, which possesses an oxathiabicyclo[2.2.2]octane ring-system constrained in a boat-like conformation, on treatment of methyl 4-0-benzoyl-2-S-benzoyl-6-bromo-6-deoxy-2-thio-Q -D-altropyranoside (145) with methanolic... 

Neighboring-group participation by the vicinal, trans-acetoxyl group (see p. 125) serves to explain the abnormal behavior of methyl 4-0-acetyl-2,3-anhydro-6-0-benzyl- or -trityl-a-D-gulopyranoside with hydrogen chloride in acetone, or with 80% aqueous acetic acid, which give D-galactose, instead of the D-idose, derivatives.67 In the same way, 2-0-acetyl-3,4-anhydro-D-altropyranosides yield D-man-nosides, not D-idosides.9 6z(see p. 125). 

The cation radicals depicted in Scheme 3-24 form upon oxidation ([which, according to Asmus (1990), takes place at the sulfur atom]) of m(/o2-(2-hydroxy-2-methyl-ethyl)-e (/o-6-(methylthio)-bicyclo[2.2.l]heptane and e (/o-2-(carboxyl)-e (/<9-6-(methylthio)-bicyclo[2.2.1]heptane, respectively. When geometric constraints preclude neighboring-group participation, the three-electron bond is not formed. Scheme 3-25 gives one such example, namely, (exo-2-(carboxyl)-e (/o-6-(methylthio)-bicyclo[2.2.1]heptane). 

An interesting work on neighboring group participation in the gas phase is the pyrolysis kinetics of l-chloro-4-(methylthio)butane which forms quantitatively tetrahydrothio-phene and methyl chloride179. In addition to the formation of the cyclic product, the rate of decomposition was also rather high as shown in Table 29. 

Neighboring group participation involving acylamino or acyloxy groups is common in nucleophilic substitution. For example, in the reaction of methyl 4,6-0-benzylidene-2-deoxy-2-benzoylamino-3-O-mesyl-a-D-altro-pyranoside 8 with NaOEt, no 3-0-ethoxy-mannoside derivative was obtained. Instead, oxazoline 9 and epimine 10 were identified in this reaction (O Scheme 5) [12]. This results from the l,2-tra 5 -diaxial relationship between the leaving... 

Neighboring-group participation by the acyloxy group has been especially favorable with certain furanose sugars. In model experiments, Goodman and coworkers showed that methyl 2-0-benzoyl-5-deoxy-3-... 

By contrast, phenyl a-D-mannopyranoside under the same conditions gives only methyl a-D-mannopyranoside. It was suggested that neighboring group participation from the 2-oxygen atom produces 1,2-anhydro-D-mannopyranose, which then reacts with methoxide ion to give the methyl mannoside.219... 

A novel and highly successful oxidizing agent is phenyl iodosoacetate [PhI(OAc)2]. In benzene solution, this easily prepared reagent oxidizes 2-formyl-, 2-acetyl-, and 2-benzoylaniline to anthranil and its 3-methyl and 3-phenyl derivatives in 21, 71, and 81% yield, respectively.170 Evidence has been accumulated to show that the first stage in the oxidation consists of nucleophilic displacement of acetate followed by neighboring group participation of the acyl function as outlined in Eq. (7). 

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