Iodine cycloalkenes

Stereoselective cis-dihydroxylation of the more hindered side of cycloalkenes is achieved with silver(I) or copper(II) acetates and iodine in wet acetic acid (Woodward gly-colization J.B. Siddall, 1966 L. Mangoni, 1973 R. Criegee, 1979) or with thallium(III) acetate via organothallium intermediates (E. Glotter, 1976). In these reactions the intermediate dioxolenium cation is supposed to be opened hydrolytically, not by Sn2 reaction. 

Oxidative reactions frequently represent a convenient preparative route to synthetic intermediates and end products This chapter includes oxidations of alkanes and cycloalkanes, alkenes and cycloalkenes, dienes, aromatic fluorocarbons, alcohols, phenols, ethers, aldehydes and ketones, carboxylic acids, nitrogen compounds, and organophosphorus, -sulfur, -selenium, -iodine, and -boron compounds... 

In a similar manner these hypervalent iodine derivatives promote the addition of the non-nucleophilic 4-nitrofur-azan-3-olate functions to cycloalkenes to give product 210 (Scheme 49) <2001RCB2479>. 

The electrophilic character of iodine in IOB is greatly increased by the addition of boron trifluoride, probably because of the in situ formation of the monomeric dipole Phl + OBF3 . Some cycloalkenes reacted with IOB.BF3 undergoing ring-contraction, for example cyclohexene gave formylcyclopentane (60%), and 3,4-dihydro-27/-pyran gave tetrahydrofurfural [8] ... 

OL-Jodo ketones result from the reaction of alkenes and cycloalkenes, especially the electron-rich ones, with silver chromate and iodine (equations 92 and 93) [<5i6]. 

Cycloalkenes tethered with a y,5- or 5,8-unsaturated acid side chains react with Brj or I2 to furnish the corresponding halolactones. lodolactonization is more commonly used than bromolactonization since iodine is easier to handle (solid) and is more chemoselective (less reactive) than bromine. Halolactonization with aqueous base is kinetically controlled and proceeds via addition of a Br or B atom to the double bond to form a transient halonium ion. In the absence of strong directing steric effects, formation of the halonium ion may occur at either diastereoface of the double bond. However, only the halonium ion intermediate which allows trans-diaxial Sj. 2 opening by the neighboring carboxylate nucleophile leads, if the intramolecular reaction is sterically favorable, to the lactone. 

In electrophilic addition reactions, transannular participation by an oxirane to form an oxonium ion occurs in medium-ring cycloalkenes. Thomas first investigated this transannular addition in epoxide 73 [86]. Treatment with iodine gave iodinated bicyclic ethers 74 and 75, Eq. 58. 

Cycloalkenes may be converted into episulphides by reaction with an ethereal or dichloromethane solution of iodine and thiocyanogen, in equimolar proportions, followed by alkaline hydrolysis. Iodine thiocyanate (ISCN) appears to be present in the solution, and to add diaxially to the olefinic bond hydrolysis then closes the episulphide ring. 5a-Cholest-2-ene (156), for example, gave the 2/ff,3)5-epithio-derivative (157) in acceptable yield. 

Diels-Alder reactions continue to be of value for the interception of highly reactive dienes or dienophiles. Thus, irradiation of 2-cyclo-octen-l-one or of 2-cyclohepten-l-one in the presence of excess cyclopentadiene affords the norbornene derivatives (173 n = 5 or 4 respectively), which undergo retro-[4 -I- 2] addition on distillation. Presumably the trans-2-cycloalken-l-ones are reactive intermediates in these processes. Elimination of acetic acid from bis(trifluoromethyl)cyclopentadienylcarbinyl acetate yields the reactive 6,6-bis(trifluoromethyl)fulvene, which rapidly dimerizes by [4 + 2] cycloaddition the fulvene intermediate can be intercepted by similar addition to cyclopentadiene. The adduct (174) is formed on reaction of cyclopentadiene with the imine produced by iodine-catalysed rearrangement of NN-... 

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