Recebido em 17/1/08; aceito em 7/4/08; publicado na web em 30/4/08 NATURAL OCCURRENCE, BIOLOGICAL ACTIVITIES AND SYNTHESIS OF EIGHT-, NINE-, AND ELEVEN-MEMBERED RING LACTONES. The natural occurrence, biological activities and synthetic approaches to natural eight-, nine-, and elevenmembered lactones is reviewed. These medium ring lactones are grouped according to ring size, and their syntheses are discussed. The structures of some natural products early identified as medium-ring lactones were revised after total synthesis.

In this work we describe the cyclofunctionalization of eleven differently substituted alkenyl-beta-dicarbonyl compounds, employing three electrophilic reagents, namely, iodine, p-methoxyphenyltellurium trichloride, and phenylselenenyl bromide. The reactions occur through the enolic form of the substrates, to afford the corresponding iodo-, telluro-, or selenocyclic enol ethers. Substrates bearing trisubstituted double bonds failed in reacting with the selenium and tellurium reagents. In general, beta-diketones reacted faster than beta-keto esters with the three studied electrophiles.

This work describes the use of aryltellurium trichloride and iodine as suitable cyclization reagents for alkenyl substituted β-keto esters. The reaction takes place via the enolic form of the dicarbonyl compounds, giving the corresponding five-membered cyclic ethers in good yields.

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Este trabalho descreve a reação de desidroiodação, promovida por base, de uma série de β-iodoenol éteres cíclicos. Os produtos são tetraidrofuranos contendo duas ligações duplas exocíclicas, die tetraidrobenzofuranos e um derivado de diidropirano, todos obtidos em ótimos rendimentos. This work describes the base-promoted dehydroiodination of differently substituted cyclic β-iodo-enol ethers. The products are tetrahydrofuran derivatives bearing two exocyclic double bonds, di-and tetrahydrobenzofurans and a dihydropyran derivative, which were obtained in very good yields.

Tellurium and Iodine Promoted Cyclofunctionalization of Alkenyl Substituted β-Keto Esters.-Aryltellurium trichloride and iodine are two useful cyclization reagents for alkenyl substituted β-keto esters giving five-membered cyclic esters in good yields. -(FERRAZ, H. M. C.; SANO, M. K.; SCALFO, A. C.; Synlett (1999) 5, 567-568; Inst. Quim., Univ. Sao Paulo, 05599 Sao Paulo, Brazil; EN) 1

Synthesis of Cyclic Enol Ethers from Alkenyl-β-dicarbonyl Compounds.-Electrophile-mediated cyclization of alkenyl-substituted β-keto esters and ketones offers a useful method to prepare five-and six-membered ring enol ethers. I 2 is the most potent agent: even trisubstituted alkenes undergo cyclization. -(FERRAZ, HELENA M. C.; SANO, MYRIAN K.; NUNES, MARTA R. S.; BIANCO, GRAZIELA G.; J. Org. Chem. 67 (2002) 12, 4122-4126; Inst. Quim., Univ. Sao Paulo, 05513 Sao Paulo, Brazil; EN)

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