Fluorinative ring contractions

The thallium trinitrate-mediated ring contraction of frani-decal-2-ones has opened up a new route to the hydrindane system, and fluorinative ring contraction of cyclic alkenes to afford difluorocycloalkanes has been induced by iodotoluene difluoride and EtsN-HF. A possible mechanism is shown in Scheme 78. The double bond of the cyclohexene ring is attacked by iodotoluene difluoride activated by HF from the axial direction, followed by the addition of a fluoride ion from the trans direction. Reductive elimination of iodotoluene from the resulting adduct, ring contraction and the addition of the fluoride ion to the carbocation stabilized by fluorine then take place to give the ring-contracted difluorinated product. 

In quinoline and isoquinoline, the benzene ring is more receptive to fluorma-tion, its double bonds being saturated and the hydrogen atoms replaced in preference to those in the pyridine ring As with pyridine and its homologues, ring contraction takes place during fluorination with cesium tetrafluorocobaltate at... 

It is known66 that the hydrofluoric acid-lead tetraacetate reagent leads, with unsaturated steroids, to cis additions of fluorine. This fact suggests that a cis-difluoro adduct (39b, 42b) is the starting point for the ring contractions observed in the preceding examples. 

Treatment of cholesterol or pregnolone acetate with excess lead(IV) acetate and anhydrous hydrogen fluoride in dry dichloromethane at — 70r C results in controlled cist addition of fluorine without ring contraction to give 5a,6a-difluorocholestan-3j8-ol and 5a,6c<-difluoro-3/J-hy-droxypregnan-20-one in 28 and 27% yield, respectively.25,26... 

A revised version of this synthesis has been subsequently developed that has a higher overall yield and is more easily scalable [42]. Reasoning that the yield and selectivity problems in the previous route were due to the use of ferf-butyl hypochlorite, the transformation of 11-epz-fischerindole G 132 into (-)-fischerindole I 133 was carried out in excellent yield by exposure of 132 to DDQ in the presence of water, presumably through the intermediate unsaturated imine 136. For the final ring contraction step, it was decided to replace the previously employed chlorohydroxylation by a hitherto unknown fluoro-hydroxylation, expecting that the increased hardness of fluorine over chlorine... 

The first stable 1,2-dithiete, 3,4-bis(trifluoromethyl)-1,2-dithiete 527, was reported in 1960, as a result of the reaction of hexafluoro-2-butyne with boiling sulfurother fluorinated derivatives are prepared similarly.The strained acetylene 528 reacts with sulfur to give dithietes. Ring-contraction of 529 with loss of ethylene gives yellow needles of the benzo-1,2-dithiete 526. ° Benzo-dithiete (stable to 180°K) has been obtained by thermolysis of 529a-c or photolysis of 529a. The benzodithiete structure proposed for the product of the oxidation of o-benzenedithiol by iodine is incorrect the material is a polymer. " 3,4-Di-t-butyl-l,2-dithiete is obtained by spontaneous cyclization of the dithione valence tautomer. " ... 

Xu, Y., Wang, Y., Zhu, S. Reactions of fluoroalkanesulfonyl azides with carbocyclic 3-keto esters structural influence of dicarbonyl substrate on distribution of diazo and ring-contraction products. J. Fluorine Chem. 2000,105, 25-30. 

Bicyehe nitrogen-containing heterocyclic compounds were also fluorinated by ECF. ECF of AT-methyl-decahydroquinoline forms a mixture of cis- and tra/j5-isomerie perfluorinated derivatives. Ring contraction and formation of per-fluorocyclohexanes are side processes. Trows-quinolizidine gives perfluoro-tra/w-quinolizidine (yield 16-23%) and a large amount of other compounds with unknown... 

Difluoroiodo)arenes react with aryl-substituted alkenes to afford the rearranged, geminal difluorides, owing to the migration of the aryl group [32,33]. Likewise, the reaction of substituted cyclic alkenes with difluoroiodotoluene and EtsN/SHF results in a fiuorinative ring-contraction with the selective formation of difluoroalkyl substituted cycloalkanes. Thus, the fluorination of 1-methylcyclohexene derivatives 26 affords... 

Work on the production of trifluoroacetyl fluoride via electrochemical fluorination (Simons process) of ethylene glycol, ethylene chlorohydrin, or l,3-dichloro-2-hydroxypropane (the last two also give chlorodifluoroacetyl fluoride) has been reported." Similar electrochemical conversion of benzoyl chloride into perfluoro(fluorocarbonylcyclohexane) and its ring-contracted... 

Further examination of the products arising from the conventional electrochemical fluorination (Simons cell anhydrous HF electrolyte) of benzoyl chloride, which yields cyclo-C,Fn COF (up to 65% yield) as the major product, leads to the conclusion that the acid fluorides (5) (mainly) and (6) are also formed. Some ring-contraction also occurs during similar fluorination of benzenesulphonyl chloride or fluoride, when (7) cis and trans) (ca. 10%) is formed together with cyclo-C Fu-SOjF (up to %). ... 

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