Carbonyl dibromide synthesis

The key butenolide needed by Buszek, for his synthesis of (—)-octalactin A, had already been prepared by Godefroi and Chittenden and coworkers some years earlier (Scheme 13.4).9 Their pathway to 10 provides it in excellent overall yield, in three straightforward steps from l-ascorbic acid. The first step entails stereospecific hydrogenation of the double bond to obtain L-gulono-1,4-lactone 13. Reduction occurs exclusively from the sterically less-encumbered ot face of the alkene in this reaction. Tetraol 13 was then converted to the 2,6-dibromide 14 with HBr and acetic anhydride in acetic acid. Selective dehalogenation of 14 with sodium bisulfite finally procured 10. It is likely that the electron-withdrawing effect of the carbonyl in 14 preferentially weakens the adjacent C—Br bond, making this halide more susceptible to reductive elimination under these reaction conditions. 

Fig. 1-30. Polyester synthesis from the carbonylative polycondensation of aromatic dibromides and diols (adapted from [237]).

Some industrially important polymeric materials can be prepared using the basic strategies discussed earlier. A representative example can be found in the synthesis of polyesters using the carbonylative polycondensation of aromatic dibromides and diols (Fig. 1-30) [237]. The underlying principle is no different from the fundamentals of carbonylative coupling presented earlier in Section 1.5.1.3. Replacement of the diols with hydrazides 86 similarly yields poly(acylhydrazide)s 87 [238]. The catalytic... 

Nickel carbonyl cyclization was the key step in a total synthesis of humulene (18) by Corey and Hamanaka.18 When a solution of the dibromide (16) in N-methyl-... 

Corey and Wat17 found that the method of forming cycloolefins from allylic dihalides and nickel carbonyl provides an unusually efficient route for the formation of large rings. Because it leads to cyclic 1,5-dienes, it makes available a variety of cyclic structures not obtainable in a practical way via the acyloin synthesis. Diacetylenic diols (10) were converted by selective reduction into the corresponding cis.cis- and owu,/ra -ethylenic diols followed by reaction with PBr3 to form the diallylic dibromides (12), which were then cyclized with nickel carbonyl. 

Corey and Broger have published a very elegant synthesis of elemol (250) involving, as the key step, the nickel-carbonyl-induced cyclisation of the ester-dibromide (251). According to previous work by Corey and co-workers this reaction could have led to the formation of (252) and/or (253) in addition to other geometric and epimeric isomers. In the event, however, the major product was the elemadiene isomer (253, 7)5-C02Me) and none of the trans,trans-cyclodeca-l,5-diene (252) was formed. The ester (253) was then converted to elemol by treatment with methylmagnesium bromide. 

The recently discovered reaction880 of methylene dibromide or diiodide with carbonyl compounds in the presence of magnesium amalgam provides an extremely simple synthesis of terminal olefins ... 

The ring-forming step in a reported synthesis of cembrene was the nickel tetra-carbonyl-catalysed cyclization of acetoxy-dibromide (37 = H, = OAc) to the... 

Transition metal ketene complexes have been derived from various metals through the study of CO reduction relating to the Fischer-Tropsch synthesis. The old preparative method for the ketene-metal complex is carbonylation of carbene complex, in which gem-dibromide is used as a common source of carbene. The reaction is carried out in the presence of Zn powder as a reducing agent. Among the examined metals such as Pd, Ni, Pt, and Co, Pd catalyst is shown to be inferior to the other metals, especially Co complex. Some typical examples using Pd catalyst are summarized in Table... 

Carbon-heteroatom double bonds can also participate in this reaction. These include both carbonyl compounds (Scheme 11.37) and imines (Scheme 11.38). Addition to aldehydes is co-catalysed by tin(II) or indium(III) salts. Under these conditions, tetrahydrofiirans are obtained. The presence or absence of the co-catalyst can also switch the reaction from one mode to another (Scheme 11.39). An indium cocatalysed cycloaddition to a 7-pyrone aldehyde 11.117 was used in a synthesis of aureothin 11.122 and A-acetylaureothamine 11.123 (Scheme 11.40). Cross-metathesis of the exo-cyc ic alkene 11.118 allowed a subsequent Suzuki coupling with a gem-dibromide 11.120 that showed the expected selectivity (Section 2.1.4.2). This reaction required the use of thallium ethoxide as the Lewis base to suppress the formation of side products. A Negishi coupling completed the synthesis of aureothin 11.122. Reduction and acylation of the nitro group yielded A-acetylaureothamine 11.123. The latter compound is active digainst Helicobacter pylori, a bacterium behind stomach ulcers. 

MacrocycHzation for the synthesis of macrolactones was also achieved by C-C bond formation using a variety of techniques (Scheme 6.5). Oxidative acetylene coupling for exaltolide 12, as reported by Bergelson et al. [61], nickel-carbonyl-mediated coupling of allyhc dibromides by Corey and Kirst [62], palladium-catalyzed... 

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