Semipinacol Rearrangement Induced by Cleavage of Dibromocyclopropane

Abstract

A novel semipinacol rearrangement using dibromocyclopropanes as sources of carbocations was developed. Heating dibromocyclopropanes bearing a 1‐hydroxyalkyl group with silver perchlorate and 2,6‐lutidine induced cleavage of the cyclopropane ring to form allyl cations, which underwent 1,2‐shift of a substituent at the α‐position of the hydroxy group to give β,γ‐unsaturated carbonyl compounds having a quaternary carbon at the α‐position. Because the products have convertible functional groups such as bromo, vinyl, and formyl groups, the method is expected to be applicable to the synthesis of various molecules.

Divergent, Strain‐Release Reactions of Azabicyclo[1.1.0]butyl Carbinols: Semipinacol or Spiroepoxy Azetidine Formation

“…Thec ommonly observed relative migratory aptitude in semipinacol rearrangements [9,17,20] of aryl > alkenyl > hydride > substituted alkyl > less substituted alkyl is mirrored here in reactions using TFAA. Asimilar pattern is seen with the more electron-withdrawing Tf 2 O, but the selectivity is lower since it induces af aster reaction.…”

“…We subsequently compared the migration of aP hg roup over other substituents,i ncluding more substituted alkyl groups and other functional groups.Comparing Ph with H, we found that the Ph group migrated exclusively to give aldehydes 6/7n in 51 %a nd 84 %y ields,r espectively. [9] Comparing Ph/ethyl and Ph/alkynyl, again resulted in exclusive migration of the Ph group,g iving azetidines 6/7o,p in excellent yields.H owever,c omparing Ph/Cy resulted in high selectivity but only in the case of TFAA to give 6q;Tf 2 Ogave a1 :1.5 ratio of products 7q/7q' ',n ow in favour of Cy migration. In both cases,t he yield of the ketone was low.I n fact, the main component of the latter reaction was spirocyclic epoxide 13,formed in 66 %.…”

Divergent, Strain‐Release Reactions of Azabicyclo[1.1.0]butyl Carbinols: Semipinacol or Spiroepoxy Azetidine Formation

“…The commonly observed relative migratory aptitude in semipinacol rearrangements [9, 17, 20] of aryl > alkenyl > hydride > substituted alkyl > less substituted alkyl is mirrored here in reactions using TFAA. A similar pattern is seen with the more electron‐withdrawing Tf 2 O, but the selectivity is lower since it induces a faster reaction.…”

“…We subsequently compared the migration of a Ph group over other substituents, including more substituted alkyl groups and other functional groups. Comparing Ph with H, we found that the Ph group migrated exclusively to give aldehydes 6 / 7 n in 51 % and 84 % yields, respectively [9] . Comparing Ph/ethyl and Ph/alkynyl, again resulted in exclusive migration of the Ph group, giving azetidines 6 / 7 o , p in excellent yields.…”

“…We wanted to further develop the use of 2 as a synthon for 1,3,3‐substituted azetidines by exploring alternative electrophiles which could also trigger strain‐release reactivity and considered the use of ketones and aldehydes (Scheme 1 c). We envisaged that after formation of azabicyclo[1.1.0]butyl carbinols (ABB‐carbinols, 5 ), subsequent N‐activation by a suitable electrophile could trigger a pinacol‐type rearrangement, cleaving the bridging C−N bond and releasing ring strain [9, 10] . Such a pinacol‐type rearrangement is analogous to the semipinacol rearrangement of α‐hydroxy epoxides [11] .…”