Hypoiodites, photolysis

A more recent investigation by Suginome and collaborators has demonstrated that the selective fragmentation of alkoxyl radicals, one of the principal reactions of these species, can be utilized as the key step in the synthesis of a variety of molecules, including several natural products. Subsequently, Suarez and colleagues have also demonstrated the utility of the (diacetoxyiodojbenzene/iodine procedure in the hypoiodite photolysis in a series of articles. 

These results indicate that hypoiodite photolysis with mercury (11) oxide and iodine reagent is a powerful method that is applicable to even remote functionalization involving a many-membered transition state, because alkoxyl radicals are considered to be repeatedly generated from the regenerated hypoiodites in a solution containing an excess of the reagent. 

The application of hypoiodite photolysis for remote functionalization has been mostly in the steroid field. There has been, however, a resurgence of appHcations in the field of spiroacetal-type compounds. The reaction was originally found in 1966 by Mihailovic and collaborators, who described remote functionalization by a thermal reaction of alcohols with lead tetraacetate. ... 

It has been reported that t-butyl hypoiodite adds to alkenes via a bridged iodonium ion intermediate in a trans manner in benzene to give P-iodo-ethers in the dark. Unlike the addition in nitrite photolysis, thermal ionic additions may take place in hypoiodite photolysis. The additions in the steroids, even under photolytic conditions, can be rationahzed in terms of an ionic trans-addition of alkyl hypoiodites via bridged iodonium ions, although a homolytic mechanism cannot be excluded. 

Lactones by ring expansion of lactols. Medium-sized lactones can be obtained in reasonable yield by photolysis of the hypoiodites of catacondensed lactols, which results in regioselective cleavage of the bridging bond. The substrates are available by the general route shown for the 6/6 lactol 1, which is cleaved to the 10-membered lactone 2. This cleavage can be used for preparation of 9-membered... 

For the substitution of the angular methyl groups in steroids five methods are known (a) homolysis of N-chloramines [Loffier-Freytag reaction (only C-18)] (b) oxidation of alcohols with lead tetraacetate (c) photolysis of nitrite esters (d) homolysis of hypochlorites (e) the hypoiodite reaction. ... 

Wittig-Horner reaction (see p. 241). Photolysis of the nitrite of the 21-chloro-20-hydroxy-compound (194) gave the 18-oximino-compound (195) which on Jones oxidation was converted into the chlorolactone (196). Hypoiodite reaction of the 21-chloro-20-hydroxy-compound (194) also gives the chloro-lactone (196) which with modification gives 18-hydroxy-11-deoxycorticosterone (see also ref. 198). 

Only the head-to-tail adducts were obtained in the [2+2] photoaddition of 4-hydroxy-l-phenyl[l,8]naphthyridin-2(l//)-one with various alkenes in methanol (Scheme 3). The photolysis of the hypoiodites generated by the in situ reaction of the cycloadducts with excess mercury(ll) oxide-iodine reagent in benzene induced a regioselective scission of the non-ring junction bond of the alkoxyl radical to give substituted 3,9-dihydro-9-phenylyfuro[2,3- ][l,8]naphthyridin-4(2//)-one and/or 3,5-dihydro-5-phenylfuro[3,2-f][l,8]naphthyridin-4-(2//)-ones <1996T6125>. 

Alkoxyl radicals can be generated by a variety of methods including peroxide reduction, nitrite ester photolysis, hypohalite thermolysis, and fragmentation of epoxyalkyl radicals (for additional examples of alkoxyl radical generation, see Section 4.2.S.2). Hypohalites are excellent halogen atom donors to carbon-centered radicals, and a recent example of this type of cyclization from the work of Kraus is illustrated in Scheme 43.182 Oxidation of the hemiketal (57) presumably forms an intermediate hypoiodite, which spontaneously cyclizes to (58) by an atom transfer mechanism. Unfortunately, the direct application of the Barton method for the generation of alkoxyl radicals fails because the intermediate pyridine-thione carbonates are sensitive to hydrolytic reactions. However, in a very important recent development, Beckwith and Hay have shown that alkoxyl radicals are formed from N-alkoxypyridinethiones.183 Al-... 

Oxygen heterocycles have also been obtained on photolysis of hypo-iodites.441 Irradiation of the hypoiodite (503), for example, gave the tetra-hydropyran (504) by the route shown in Scheme 35.442... 

The photolysis of alkyl hypohalites, especially alkyl hypoiodites [29], can be used in a manner similar to the organic nitrites for functionalization of an unactivated carbon atom. Scheme 25 outlines a general scheme for remote functionalization by the hypoiodite reaction. Readers requiring more information on this subject are advised to refer to review articles [2c, 16, 20],... 

Structures (316) with an oxetan ring, recently proposed250 for two iodo-compounds obtained during the hypoiodite reaction of cholesterol, have now been revised.251 Spectroscopic and X-ray data show the compounds to be iodo-acetals (317), converted by de-iodination and reaction with BF3-acetic anhydride into the dihydropyran (318). Photolysis of the oxime of a l,5-dien-3-one gave mainly rearranged ketonic products, isomers of type (319), which are also obtained by photo-rearrangement of the l,5-dien-3-one.252... 

An alternative method for the formation of acyl hypoiodites, developed by Barton, involves the treatment of the acid with r-butyl hypoiodite. Subsequent white light photolysis in benzene at room temperature gave good yields of it ides from primary, secondary and tertiary acids (equation 21). The method was not applicable in the presence of alcohols. A more recent technique involving hypervalent iodine is due to Suarez primary, secondary or tertiary aliphatic acids are heated to reflux in tetrachloromediane with iodosylbenzene diacetate and iodine resulting in good yields of iodides. The method is mild and, with obvious exceptions such as unprotected alcohols, is tolerant of many functional groups, as illustrated in equation (22). ... 

Examples of the generation and use of oxyl radicals, obtained by photolysis of hypoiodites, have again been described. The conversion of 7-cyano-2,3-benzobicyclo[4.2.0]octa-2,4-dien-6-ol (204) into the homotropone (205) on irradiation in benzene in the presence of HgO-l2 can be rationalized in this way a possible explanation is shown in Scheme 8. 

A minor product of the photolysis of the hypoiodite derived from AT-acetyljervine (19) was originally assigned an oxetan structure (20). The structure of this product has been reassigned by the original authors and is now formulated as the acetal... 

N-Iodoamides and y-lactones. Barton and co-workers found i-butyl hypoiodite to be an excellent reagent for the preparation of N-iodo derivatives of amides (benz-amide, succinimide, n-butyramide, ocladecaneamide). An N-iodoamide of suitable structure (2) is converted on photolysis Into the y-lactone (3). 

Reaction of 6 (3-hydroxy-steroids with ceric ammonium nitrate was reported to give the 6(3, 19-epoxides,123 and fragmentation of 5- and 19-hydroxy-compounds were also reported. A detailed investigation of the hypoiodite reaction and the related Pb(OAc)4 oxidation of a 5a-bromo-6(3-hydroxy-compound indicated that for the former a halocarbon solvent promotes 6(3,19-epoxide formation.124 Photolysis of the 17a-iodobenzoate (148) with PhICI2 gave the 9 -chloro-derivative (149) which on thermolysis gave a good yield of the 9(11), 16-diene (150).125 Photolysis of... 

Photochemically induced nitrogen-halogen bond cleavage is responsible for the conversion of N,N-dihalo-1,1-difluoroamines into the corresponding diazenes, and for the formation of the 1,1-adduct (147) and the 1,2-adduct (148) obtained on photodecomposition of 1,4-dibromo-2,5-piperazinedione (149) in the presence of 3,4-dihydro-2H-pyran. The photolysis of hypoiodites constitutes... 

Photolysis of the hypoiodite of hemiacetal 139 results in carbon-carbon bond cleavage to produce an iodolactone 140, Eq. 96 [83], The iodoalkyl side chain was subsequently homologated to afford the sex pheromone of the rove beetle. 

Photolysis of the hypoiodite derived from N-acetyljervine (27) gave the dimeric compound (28) as the major product, together with small amounts of the iodides (29) and (30), the cyclic ether (31), and lactone (32). In a preliminary communication ( f. Vol. 5, p.263) the authors had misassigned the structures of several of... 

The fourth modification of the Hunsdiecker reaction, pioneered by Barton, is the use of /-butyl hypoiodide.23 Thus, acid 16 was treated with /-butyl hypoiodide to give the acyl hypoiodite, which underwent white-light photolysis at room temperature to give iodide 17. The reaction works for primary, secondary, and tertiary acids. 

In earlier work it has been found that the C-18 methyl group of the c-nor-D-homo-steroid (23) could be functionalized by photolysis in the presence of iodine, lead tetra-acetate, and sodium carbonate, when (24) was produced.The usefulness of the hypoiodite reaction " for the derivatization of c-nor-D-homo-steroids... 

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