Dibromo ketones, alkylation

Longifolene has also been synthesized from ( ) Wieland-Miescher ketone by a series of reactions that feature an intramolecular enolate alkylation and ring expansion, as shown in Scheme 13.26. The starting material was converted to a dibromo ketone via the Mr-silyl enol ether in the first sequence of reactions. This intermediate underwent an intramolecular enolate alkylation to form the C(7)—C(10) bond. The ring expansion was then done by conversion of the ketone to a silyl enol ether, cyclopropanation, and treatment of the siloxycyclopropane with FeCl3. 

Reaction of styrene with the commercially available 2,4-dibromopentan-3-one (52) in the presence of a stoichiometric amount of Fc2(CO)9 gives a 65% yield of cyclopentanone (53) as a mixture of two dia-stereoisomers (equation 44). The cycloadduct is formed in a stepwise fashion via electrophilic attack of the incipient (oxyallyl)Fe complex (54) on the alkene to form the zwitterionic species (55). Ring closure of this intermediate affords the cyclopentanone product (equation 45). In light of this mechanism, it is not surprising tl.at the reaction is highly chemoselective, i.e. only electron-rich alkenes will participate in the cycloaddition. One is also limited to alkyl-substituted oxyallyl systems since both a,a -dibromo- and a,a,a, a -tetrabromo-acetone fail to yield any cycloadduct. Fe2(CO)9 is the most efficient reagent, although Fe(CO)s can also be used. The dibromo ketones are either commercially available or easily prepared by simple procedures. ... 

V-Alkyl-substituted guanidin-2-amines condense with 1,2-dicarbonyl compounds to give A -alkyl-l,2,4-triazin-3-amines 7 and 8.115 6-Substituted 1,2,4-triazin-3-amines 7 can also be prepared by reaction of 2-(hydroxyimino)-l-phenylethan-l-onen5 or dibromo ketones with guanidin-2-amine.141 24 2... 

Reaction of diorganocuprates with a,a -dibromo ketones provides a new method for the a-alkylation of a ketone a nucleophilic attack of the cuprate on the intermediary cyclopropanones could explain the reaction (Posner and Sterling, 1973 Posner et al., 1973). The cyclopropyl group can be introduced at the alpha position of ketones as well (Carlson and Mardis, 1975). 

In practice, the azide decompositions are usually carried out in boiling toluene or xylene and give good yields of 3-alkyl-170,184,187 and 3-aryl-anthranils.185,188 Yields of 3-unsubstituted anthranils from o-azidoben-zaldehydes are generally much lower.170 The method has also been used to prepare 3-(j3-styryl)anthranils (106) from o-azidochalcones,189 3-methyl-naphtho[2,3-c]isoxazoles from 3-acetyl-2-azidonaphthalene,190 and 3-(2-pyridyl)anthranils from o-azidophenyl pyridyl ketones.191 This last reaction is of interest in that 2-(2-azido-3,5-dibromobenzoyl)pyridine in boiling toluene yields almost equal amounts of 5,7-dibromo-3-(2-pyridyl)anthranil (43%) and the zwitterionic pyrido[l,2-f>] cinnolin-6-ium 139 (41%). 

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