Ring Expansion of Cyclobutylmethylcarbenium Ions to Cyclopentane or Cyclopentene Derivatives and Metal-Promoted Analogous Rearrangements

Direct Syntheses of Spiro‐ and Fused‐Hydrofurans by a Tunable Tandem Semipinacol Rearrangement/Oxa‐Michael Addition Protocol

“…As shown in Table 2, the substrates with cis ‐ p ‐Me‐phenyl, cis ‐ p ‐Br‐phenyl, diphenyl, and spiro‐cycloalkyl at the C3′‐position gave the products in good yields of 64 ( 2 c / 2 c′ ), 63 ( 3 c / 3 c′ ), 61 ( 4 c / 4 c′ ), and 75 % ( 5 c / 5 c′ ). Furthermore, the reaction exhibited a trend in migratory aptitude similar to that observed in the normal semipinacol rearrangement, in that the tertiary carbon atom migrated preferentially to the secondary carbon atom 7d. 12 Accordingly, substrates bearing different substituents (e.g., di‐ n ‐butyl, di‐ i ‐butyl, spiro‐cycloalkyl, spiro‐pyranyl, and ketospiro‐cycloalkyl) at the C2′‐position tended to undergo C2′ carbon atom migration reactions to give the corresponding spiro‐furans 6 c / 6 c′ , 7 c / 7 c′ , 8 c / 8 c′ , 9 c / 9 c′ , and 10 c / 10 c′ containing two contiguous quaternary centers in reasonable yields.…”

“…As a result, this reaction has been applied to the efficient syntheses of many natural products and bioactive compounds 9. Over the past decade, our own group7c and several other chemists7a, b, d have directed a great deal of effort towards the development of semipinacol rearrangement based synthetic methodologies for the formation of structurally diverse quaternary centers. In connection with our ongoing investigations regarding this rearrangement process and interests in constructing cycloketone spiro‐ and fused‐dihydrofuran units, we hypothesized that these units might be derived from a readily available diene–ketone intermediate10, 11 through a tandem semipinacol rearrangement/oxa‐Michael addition protocol under appropriate conditions (Scheme ).…”

Synthesis of 1,2‐Diols and Their Base‐Initiated Rearrangement to Butyrolactones

“…Another related reaction showing a 1,2‐alkyl shift is the semi‐pinacol rearrangement, which is analogous to the pinacol rearrangement, carried out under basic conditions, with one of the hydroxy functions converted into a good leaving group such as a tosylate 17. Also in this case, an intramolecular alkyl group migration occurs, which has been exploited to promote ring‐expansion reactions 17,18…”

When Alkyne π-Activation Meets Pinacol-Type [1,2]-Rearrangement: On the Invention of Domino Reactions for the Synthesis of Carbocycles and Heterocycles

“…To achieve clean rearrangement in these cases, an unsaturated moiety is present in a position adjacent to the hydroxy group (Scheme 30) and the release of ring strain plays an important role. 63,64 As an archetypical example, Toste and co-workers described gold(I)-catalyzed ring expansion reactions of alkynylcyclopropanols and alkynylcyclobutanols, for example, 97 (Scheme 31). 65,66 The positive charge induced on the alkyne moiety by coordination of the cationic gold(I) complex is responsible for the rearrangement.…”