Anti selectivity, bromine addition with

The bromoallene (-)-kumausallene (62) was isolated in 1983 from the red alga Laurencia nipponica Yamada [64a], The synthesis of the racemic natural product by Overman and co-workers once again employed the SN2 -substitution of a propargyl mesylate with lithium dibromocuprate (Scheme 18.22) [79]. Thus, starting from the unsymmetrically substituted 2,6-dioxabicyclo[3.3.0]octane derivative 69, the first side chain was introduced by Swern oxidation and subsequent Sakurai reaction with the allylsilane 70. The resulting alcohol 71 was protected and the second side chain was attached via diastereoselective addition of a titanium acetylide. The synthesis was concluded by the introduction of two bromine atoms anti-selective S -substitution of the bulky propargyl mesylate 72 was followed by Appel bromination (tetrabromo-methane-triphenylphosphine) of the alcohol derived from deprotection of the bromoallene 73. 

Room-temperature ionic liquids may be used as green recyclable alternatives to chlorinated solvents for stereoselective halogenation.577 The bromination of alkenes and alkynes in [bmim][Br] is a/m -stereospecific, whereas that of 1,3-dienes gives selectively the 1,4-addition products. The reactions of arylacetylenes, however, are not selective when carried out in [bmim][PF6]. Tetraethylammonium trichloride, a stable crystalline solid may be used in the chlorination of alkenes and alkynes to afford the products with exclusive anti stereoselectivity.578 It has... 

Addition of vinyllithium reagent 636 to 632 occurs largely from the re side (Felkin—Anh selectivity) to give the anti diastereomer 637 (85% de) [193]. Lithium—bromine exchange at — 78 °C followed by protonation affords 638 in 89% yield with complete retention of olefin geometry. Ozonolysis of 638 followed by lithium aluminum hydride reduction of the intermediate aldehyde furnishes protected triol 639 in 78% yield. 

The reaction of an alkene with HBr and peroxide gives the bromide product with the bromine on the less substituted carbon in an anti-Markovnikov addition reaction. This reaction work wells with HBr, but not with HCl or HI because the bromine radical reacts in a selective manner. Differences in the reactivity of halogen radicals are addressed in Chapter 11, Section 11.9. 

While there are several approaches to this chemistry, a cobalt-catalyzed process was successful and selective for the anti-Markovnikov addition product with Z-stereochemistry [144]. A hansition metal-free version of this reaction has been reported using a nucleophUic bromine source [174]... 

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