Semipinacol ring expansion

Dake et al. have developed a siloxy-epoxide semipinacol ring expansion process that yields spirocyclic cycloalk-anones (e.g., Scheme 14) <2004JOC5676>. 

A thiol leaving group can also be employed in semipinacol ring-expansion reactions. When dithiophenoxymethyl anion is added to cyclopentanone, cyclohexanone 30 is produced. Treatment of 30 with copper(II) triflate and diisopropylethylamine results in cyclohexanone 31. ... 

Many additional applications of semipinacol rearrangements on a-hydroxy epoxides have been developed,and these reactions have found use in complex molecule synthesis. For example, a-hydroxy epoxide 47 underwent a Lewis acid-mediated semipinacol ring expansion to set the challenging in-out intrabridgehead stereochemistry of the ingenol core. ... 

A Pd(II)/Br0nsted acid-catalysed migratory ring expansion for the synthesis of indene derivatives possessing a stereogenic spirocyclic carbon centre has been reported to proceed via enantioselective allylic C-H activation with concomitant semipinacol ring expansion to the nascent r-allylpalladium species (Scheme 139). ... 

The total synthesis of (+)-furoscrobiculin B, a lactarane sesquiterpene isolated from basidiomycetes of mushrooms, was accomplished in the laboratory of H. Suemune and K. Kanematsu using a furan ring transfer reaction and a semipinacol rearrangement as key steps. The secondary hydroxyl group of the tricyclic c/s-vicinal diol substrate was converted to the corresponding tosylate that in situ underwent a ring-expansion reaction to afford an azulenofuran in good yield. 

The semipinacolic rearrangements of 2-hydroxy selenides are closely related to those of 2-hydroxy sulfides, but they have not been studied as extensively, and the use of selenium does not seem to offer any advantages over sulfur. The rearrangement has been applied mainly to the ring expansion of cyclic ketones via the addition of a-selenoalkyl anions. 

The semipinacol reaction may be modified so that a ring expansion takes place. For example, draw the mechanism for the ring expansion that occurs when 1-aminomethylcyclopentanol is treated with nitrous acid. 

In the pinacol-pinacolone rearrangement a 1,2-diol rearranges to give a carbonyl group that is adjacent to a quaternary carbon. There are many variations of this reaction for example, the semipinacol rearrangement, in which a (P-amino alcohol is treated with nitrous acid. This reaction may be modified so that a ring expansion takes place, in which case it is called the Tiffeneu-Demyanov ring expansion. 

The chiral phosphoric acids (127, 132, 137, 154, 198, 199) catalyzed an asymmetric ring expansion-type, semipinacol rearrangement reaction affording chiral spiroethers (197) from (198) in up to 98% ee and good to high chemical yields under mild conditions, has been discovered by Tu and co-workers (Scheme 55)7 ... 

The semipinacol reaction has been used in ring expansion processes, particularly as an approach to medium rings, which are not easy to prepare by ring closure reactions. In Figure 18.9, cyclohexanone is converted to its cyanohydrin, and this is then reduced to the corresponding amine. A semipinacol rearrangement gives cycloheptanone. 

FIGURE 18.9 Ring expansion using the semipinacol reaction. 

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