Reductive iodination, tributyltin

The stereoselectivity observed in the cyclization of 4-unsaturated amides is dependent on the electrophilic halogen species. With /V-chlorosuccinimide the trans/cis ratio is reduced to 3 2, while with A -bromosuccinimide the fra .v-isomer is obtained in >98% purity. The structures of the iodolactones 4 were confirmed by reduction with tributyltin hydride to the trans- and rw-2.4-dimethy]-y-lactones. The high 1,3-trans selectivity strongly contrasts with the results obtained when 2-methyl-4-pentenoic acid is cyclized under thermodynamic conditions with iodine in acetonitrile. In this case a 92% yield and a 32 68 mixture of the f/ww/m-com pounds was obtained. 

The reaction of the iodoalkene (178) with Pd(dppe)2 in HMPA followed by reduction with tributyltin hydride gave a mixture of the 6-exo trans product (179), the 6-exo cis product (180), and the 1-endo product (181) in a ratio of 1 1 2 in combined yield of 76% (Equation (18)), identical to the result of using hexalkylditin in benzene <90TL933>. This has led Curran and Chang to propose that the low valent palladium ene-halogenocyclization should be classed as a radical chain reaction that proceeds by way of an atom transfer mechanism and that palladium(O) may be a useful substitute for ditin in other iodine atom transfer reactions. 

Zinc in acetic anhydride has been used to dimerize l,2-diehloro-l,1.2-trifluoro-2-iodoethane (13) to afford the dimerized product 14 in 51 % yield by reductive elimination of iodine. A modified procedure gives a 70% yield of 14, which is reduced selectively by tributyltin hydride... 

The iodocyclization of 4-methyl-5-hexenoic acid performed under conditions of thermodynamic control (iodine in acetonitrile in the absence of a base) leads to the preferential formation of the /raw.v-diequatorial 5-lactones (94 6) in 77% yield1. Similar results were observed starting from (S)-4-methyl-5-hexenoic acid (1), obtained from citronellol, with iodine in acetonitrile, followed by tributyltin hydride reduction. The 5,6-dimethylvalerolactone 2 was obtained in 40%) yield and 93.5 6.5 (irans/cis) diastereoselcctivity 2. 

In order to remove the primary hydroxy group, the protected triol 24 was acet-ylated and treated with pyridinium tosylate to yield the monoester 25. Selective tosylation of the primary hydroxy group followed by Finkelstein exchange for iodine yielded the iodo derivative 26 which was transformed into the protected cyclohexene diol 27 by reductive removal of the iodine with tributyltin hydride followed by protection of the tertiary hydroxy group with dihydopyran. After removal of the benzoyl group by alkaline saponification the resulting allylic alcohol was oxidised to the desired enone 28 with pyridinium chlorochromate in buffered solution (ref. 16). 

Other reactions studied include reduction of the triple bond of a-acetylenic esters and nitriles by tributyltin hydride in methanol electroreductive cyclization of acetylenic halides at a mercury cathode the trimerization and tetramerization of cyclo-octyne in the presence of various transition metals the kinetics of bromination of alka-l,3-diynes, of permanganate oxidation of acetylenedicarboxylic acid, and of iodination of propiolic acid the participation of the triple bond in reactions of various acetylenes of the general formula (225) and the trimerization of but-2-yne with tolyl-palladium chloride to give a [Pg.48]

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