Bromomethoxylation

Tatsuya Shono, Shigenori Kashimura and Naoki Kise TABLE 3. Bromomethoxylation of 1,3-dienes... 

Gopalakrishnan, G., Kasinath, V., Singh, N.D.P., Krishnan, V.P.S., Solomon, K.A. etal, Microwave assisted regioselective bromomethoxylation of alkenes using polymer supported bromine resins, Molecules, 2002, 7,412. 

Bromomethoxylation of the leucoacetate (61) at —10° gave the spiroketalacetal (63), which is nicely crystalline. The structure of compound 63 contains the remarkable sequence ... 

The complementarity of the method to base-catalyzed addition is further exemplified by the enoate 4. When the two-step protocol involving bromomethoxylation/radical reduction was applied, both the cis- and trim-products O.v-5 and tram-5 were obtained17. As shown previously (Section 4.7.1.1.1.2.1.), base-catalyzed addition of methanol to structurally closely related enoates afforded exclusively trim-addition products. 

Addition of bromine in methanol gives exclusive trans addition (to 2-deoxy-2-bromomethyl glycosides) in approximately equal amounts, except for tribenzylgalactal, which, as expected from the steric restrictions imposed by 04, gives tribenzyl-2-bromo-2-deoxymethyl p-galactoside and tribenzyl-2-bromo-2-deoxymethyl a-L-talopyranoside in a 2.3 1 ratio. Only in the case of the bromomethoxylation of triacetylglucal was it possible to intercept the oxocarbenium ion with azide ion. The Jencks clock, using a value of 7 x 10 s ... 

A full report of the chloromethoxylation, bromomethoxylation, and iodomethoxylation of tri-O-acetyl-n-glucal and -n-galactal has now appeared, and has shown that, particularly with chloromethoxylation, a-cis adducts are produced in significant proportions. 1,2-Halonium ions are, therefore, not the sole reactive intermediates oxocarbonium ions play significant roles. 

Scheme 16.74. Bromomethoxylations utilizing polymer-supported brominechloride resin.

Silver(I) salts are often utilized as catalysts for addition reactions. Kozmin and Sun have recently shown that AgNTf2 is a catalyst of choice for the hydroamination of siloxy alkynes with either secondary amides or carbamates to give silyl ketene am-inals [34]. The addition occurs in a syn selective manner, for instance, the reaction of siloxy alkyne (24) with carbamate (25) produces silyl ketene aminal (26) in 86% yield at room temperature under the influence of 1 mol% of AgNTf2 (Scheme 18.9). A six-membered chelated transition state is proposed to explain the high syn selectivity. Diastereoselective bromohydroxylation and bromomethoxylation reactions of cinnamoyl compounds possessing a chiral auxiliary are also effectively promoted by silver(I) salts such as AgNOs [35]. The asymmetric halohydrin reaction has been successfully applied into stereoselective syntheses of (-)-chloramphenicol and (+)-thiamphenicol. Csp-H iodination [36], hydrosilylation of aldehydes [37], and deprotection of TMS-alkynes [38] are also catalyzed by silver (I) salts. 

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