Lactol hypoiodites

The Selective P-Fragmentation of Alkoxyl Radicals Generated by the Photolysis of Lactol Hypoiodites... 

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... 

Medium-sized lactones A new method for synthesis of lactones that is particularly useful in the case of medium sized ones involves the hypoiodite reaction with a catacondensed lactol such as 1, which results in cleavage to the ten-membered isomeric iodo lactone 2. Phoracantholide I (3) is obtained on reductive removal of iodine. 

TBS-Protected 7-lactol is obtained through the formation of hypoiodite, followed by the formation of an alkoxyl radical, 1,5-H shift, and then formation of 8-iodoalcohol, and finally polar cyclization. The Fenton system with ROOH and Fe2+ also generates alkoxyl radical, RO. These reactions indicate that the Barton (-type) reactions are remote functionalization of non-activated C-H bonds at the 8- or e-position. 

Ring expansion of cycloalkanones.1 The hypoiodite reaction can also be used for a four-atom ring expansion of cyclic ketones via the corresponding lactols (cf., 13, 150). 

The scission reaction is carried out by irradiation of the hypoiodite of the lactol. The process is formulated for conversion of 7-cholesianone (1) into 7-oxacholestanc (4). 

Reactions with lead tetraacetate and iodine are frequently complicated by bifunctional attack, leading to lactols (9) after hydrolysis of intermediate a -iodo-ethers (8). This does not arise in reactions with lead tetraacetate alone (section 5). In general terms, the hypoiodite reaction sequences appear to be of the type depicted in Fig. 48 [34]. 

Oxidation Reactions. Hypoiodite intermediates may be generated from the reaction of simple alcohols with NIS. When conducted under photochemical irradiation, the products of Barton-type or fragmentation reactions of alkoxyl radical intermediates may be obtained. Aldehydes are oxidized to methyl esters via hemiacetal intermediates by reaction with NIS in methanol at rt. However, such conditions are not effective for the oxidation of simple alcohols. The combination of NIS and Tetrabutylammonium Iodide in dichloromethane has been developed for the oxidation of a variety of alcohols to the corresponding carbonyl corrpounds (eq 8). This reagent system is most widely used for the oxidation of lactols to lactones, in which near-quantitative yields are generally obtained under mild conditions (eq 9). ... 

Among the numerous results reported by the Swiss group, two typical examples of remote functionalization with this reagent are outlined in Schemes 16 and 17. The products of the remote functionalization of steroids by the hypoiodite reaction are either the five-membered oxide (Scheme 16) or the a-lactol acetate (Scheme 17), depending on the relative disposition between the relevant alkoxyl radical and the iodine attached to CIS or C19 of a conformer of the first-formed intermediate monoiodide. Thus, the five-membered oxide is an exclusive product in the example outhned in Scheme 16, because the iodo-methyl group can adopt an orientation that is appropriate for an S 2 or S 2 displacement of the iodine by the 6P-alkoxyl radical or ion, to give the observed oxide, as shown in Scheme 18. On the other hand, the a-lactol acetate is an exclusive product in the example outlined in Scheme 17, because the preferred orientation of the iodomethyl group with respect to the 4P-hydroxyl in the first-formed intermediate... 

According to Suarez and collaborators, the photolysis of the hypoiodites of certain bicyclic carbinolamids in the presence of DIB-iodine leads to fragmentation to give cyclic imides, as outlined in Scheme The cleavage, however, appears not to be as selective as the cleavage of lactols. 

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