Enol acetates iodination

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

SO2CI2) or with I2 and silver acetate. Enol acetates have been regioselec-tively iodinated with I2 and either thallium(I) acetateor copper(II) acetate. ot,P-... 

As mentioned earlier, metal complexation not only allows isolation of the QM derivatives but can also dramatically modify their reactivity patterns.29o-QMs are important intermediates in numerous synthetic and biological processes, in which the exocyclic carbon exhibits an electrophilic character.30-33 In contrast, a metal-stabilized o-QM can react as a base or nucleophile (Scheme 3.16).29 For instance, protonation of the Ir-T 4-QM complex 24 by one equivalent of HBF4 gave the initial oxo-dienyl complex 25, while in the presence of an excess of acid the dicationic complex 26 was obtained. Reaction of 24 with I2 led to the formation of new oxo-dienyl complex 27, instead of the expected oxidation of the complex and elimination of the free o-QM. Such reactivity of the exocyclic methylene group can be compared with the reactivity of electron-rich enol acetates or enol silyl ethers, which undergo electrophilic iodination.34... 

N-Iodosuccinimide reacts with enol acetates derived from ketones to give a-iodoketones, and the reaction has found application in the steroid field/ The iodination of the enol acetates seems to proceed by an ionic mechanism, and preliminary work indicates that N-iodosuccinimide is not capable of at least some of the radical-chain iodinations analogous to radical-chain brominations brought about by N-bromosuccinimide. ... 

Chemistry of the Tricyclic Diterpenoids.— The conversion of virescenol A (66 R = OH) into virescenol B (66 R = H) was complicated by the ease of 2,19-ether formation. In the event the transformation was achieved through hydro-genolysis of the keto-furan (67). The chirality of nucleophilic reactions at the C-4 axial aldehydes and methyl ketones has been examined in the podocarpane series. The reaction of some ring B enol-acetates with thallium(l) acetate and iodine has been studied.lodination at the 6a-position is followed by elimination to form a/8-unsaturated ketones. 

OL-Iodo ketones, a-lodo ketones are usually prepared by reaction of enol acetates with NIS. A new method involves reaction of an alkene with silver chromate and iodine in the presence of pyridine. The actual reagent is presumably a mixed anhydride of hypoiodous acid and chromic acid. Dichloromethane is the most useful solvent. ... 

Also, the ketonic isocarbostyryl 4 was converted by Onda et al. into a mixture of Z- and E-enol acetates by reaction with acetic anhydride and potassium acetate. Furthermore, the Z-isomer could be isomerized to the E-isomer by irradiation. The E-isomer was desired since its further irradiation in the presence of iodine was found to yield aromatic benzophenanthridines 5 and 5a " ... 

A new facile synthesis of a-iodo-ketones involves oxidation of alkenes using silver chromate and iodine [equation (48)], while thallium(iii) acetate and iodine are effective for the conversion of enol acetates into a-iodo-ketones. 

Other typical examples of the reactivity of enol acetates are (a) the copper-catalyzed a-iodination (Scheme 21a) and (b) the reductive ozonolysis (Scheme 21b). These methodologies have been recently used for the synthesis of several natural products of the axane family [35] and chiral diols, respectively [59]. 

A number of other methods exist for the a halogenation of carboxylic acids or their derivatives. Acyl halides can be a brominated or chlorinated by use of NBS or NCS and HBr or HCl. The latter is an ionic, not a free-radical halogenation (see 14-2). Direct iodination of carboxylic acids has been achieved with I2—Cu acetate in HOAc. " ° Acyl chlorides can be a iodinated with I2 and a trace of HI. Carboxylic esters can be a halogenated by conversion to their enolate ions with lithium A-isopropylcyclohexylamide in THF and treatment of this solution at -78°C with... 

The preparation of Pans-1,2-cyclohexanediol by oxidation of cyclohexene with peroxyformic acid and subsequent hydrolysis of the diol monoformate has been described, and other methods for the preparation of both cis- and trans-l,2-cyclohexanediols were cited. Subsequently the trans diol has been prepared by oxidation of cyclohexene with various peroxy acids, with hydrogen peroxide and selenium dioxide, and with iodine and silver acetate by the Prevost reaction. Alternative methods for preparing the trans isomer are hydroboration of various enol derivatives of cyclohexanone and reduction of Pans-2-cyclohexen-l-ol epoxide with lithium aluminum hydride. cis-1,2-Cyclohexanediol has been prepared by cis hydroxylation of cyclohexene with various reagents or catalysts derived from osmium tetroxide, by solvolysis of Pans-2-halocyclohexanol esters in a manner similar to the Woodward-Prevost reaction, by reduction of cis-2-cyclohexen-l-ol epoxide with lithium aluminum hydride, and by oxymercuration of 2-cyclohexen-l-ol with mercury(II) trifluoro-acetate in the presence of ehloral and subsequent reduction. ... 

It was envisioned that hydrindanone 83 and cyclopentene 85 could be used as intermediates in the synthesis of e f-retigeranic acid A (1) and e f-retigeranic acid B (2), respectively. To prepare the building block 90, cyclopentene 85 was reduced with diimide (93 %) in order to prevent isomerization and subsequently deprotected with PPTS to yield hydrindanone 90 (quant.), which could provide access to <77/-retigeranic acid B (2) (Scheme 10.7). Hydrindanone 83 was reduced via an enol triflate and then subjected to Pd-catalyzed reduction to provide cyclopentene 91 (87 % from 83). Upon hydrogenation of 91 with Pd/C and cleavage of the acetal with iodine, protected hydrindanone 92 (95 % from 91) was obtained. The deprotection of 92 provided ent-60, whose enantiomer was used in previous syntheses of retigeranic acid A (1) by Corey [14] and Hudlicky [46, 47]. 

The kinetics of the acid-catalyzed esterification reaction of 2,4,6-trimethylbenzoic acid in i-PrOH under microwave irradiation have been investigated [84], A simple and practical technique for MW-assisted synthesis of esters has been reported wherein the reactions are conducted either on solid mineral supports or by using a phase transfer catalyst (PTC) in the absence of organic solvents [85], The esterification of enols with acetic anhydride and iodine has also been recorded [86],... 

The use of TMOF as a solvent provides strong acetalizing conditions (323 330). This allows the generation of enol ether 331, which on electrophilic attack of hypervalent iodine species [PhI(OMe)2] (83IC1563) gives intermediate 332. Nucleophilic attack of the solvent at the C(4)-position of 332, followed by migration of ring A, results in the formation of 326. The minor product 327 is resulted by a Sn2 attack of methanol at the C(3)-position of 333 (Scheme 85). 

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