Propargyl bromide ions

Diterpenoids related to lambertianic acid were prepared by intramolecular cyclization of either an alkene or an alkyne with a furan ring <2005RJ01145>. On heating amine 101 with allyl bromide, the intermediate ammonium ion 102 was formed which then underwent [4+2] cycloadditions in situ to give the spiroazonium bromides 103 and 104 (Scheme 13). These isomers arose from either endo- or co-transition states. The analogous reaction was also carried out with the same amine 101 and propargyl bromide. The products 105 and 106 contain an additional double bond and were isolated in 58% yield. The product ratios of 103 104 and 105 106 were not presented. 

The selenium counterparts of enolate ions, i.e., eneselenolate ions, can be generated by the deprotonation of enolizable selenoamides [101],selenoesters [102], and selenothioic acid S-esters [103]. The generated eneselenolate ions are reacted with trimethylsilyl chloride, aldehydes, selenoesters, propargyl bromides, and oxiranes to give the products 47-52. 

In the remainder of this chapter, particular reactions are selected for examination of their synthetic potential. Acetylide ions are useful for linking carhon chains, particularly where a double bond is desired with stereoselectivity. Acetylene and 1-alkynes may be deprotonated with strong bases such as LDA and then treated with alkyl halides or carbonyl compounds. Preformed lithium acetylide complexed with ethylenediamine is available as a dry powder. Several alkynes derived from acetylide and carbon dioxide or formaldehyde are available, including propargyl alcohol (HC CCHjOH), propargyl bromide (HC CCH Br), and methyl propio-late (HC=CC02CH3). 

When the alcohol adduct from the allenylzinc reagent and diisopropyl ketone was treated with 80 mol% of allenylzinc bromide in HMPA, a mixture containing 12% of diisopropyl ketone and 88% of recovered alcohol was obtained after 7 days at ambient temperatures (equation 1). Thus, it may be deduced that the allenylzinc additions are reversible. Presumably, the propargyl adducts are intrinsically favored, but steric interactions between the R1 and R2 substituents in the propargyl product favors an increased proportion of allenyl adducts in a reversible process (see Table 1). HMPA would expectedly facilitate reversal of the addition by decreasing the ion pairing between the alkoxide anion and ZnBr cation of the adducts. This expectation was subsequently confirmed by a study of solvent effects. 

Treatment of lithium acetylide with a primary alkyl halide (bromide or iodide) or with aldehydes or ketones produces the corresponding monosubstituted acetylenes or propargylic alcohols. Mercuric ion-catalyzed hydration of these furnishes methyl ketones and methyl a-hydroxy ketones, respectively. 

The ambident zwitterion-carbene intermediate can react with nucleophiles to give products originating from both a and y attack. Hydroxide and ethoxide ions prefer the propargyl to the allenyl position exclusively, while azide prefers it by a factor of 8, thiophenoxide by a factor of 1.6, and bromide attacks the two positions almost equally . The reaction with trimethylamine produces quaternary ammonium halides these have propargylic structure when R = R = CHg or -(CHafs-. When R and R are larger than CH3 the products are allenic. Diethyl sodiomethylmalonate affords propargylic and allenic products in the ratio of 2.6 1 (ref. 59). Under aprotic conditions the zwitterion-carbene adds on to olefins with formation of alkenylidencyclopropanes " (cf. p. 407). 

Synthesis of the functionalized propargyl amine 95 via the addition of (o-nitro)-phenylmagnesium bromide to the iminium ion... 

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