Ring Expansion into Five-Membered Heterocycles

Ring Expansion into Five-Membered Heterocycles 

Benzofuran derivatives have been prepared through phenolate neighboring-group participation (Eq. 190).  

Another reaction giving rise to benzofuran derivatives is seen in Eq. 191. 

On the action of BF3, a steroid oxirane undergoes rearrangement to a tetrahydro-furan. 7-Lactones are formed from oxirane by nucleophilic ring-opening (Eqs. 192-194). °  

Rearrangement of a,(3-epoxydiazoketones takes place on catalysis with BF3 this is followed by ring-closure (Eq. 195).  

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C. Ring Expansion into Five-Membered Heterocycles... 

Preparations of cinnolines by expansion of five-membered heterocycle rings include the oxidation of /V-aminooxindoles (241 — 242), the treatment of isatogens with ammonia (Scheme 30) and the base-catalyzed conversion of 1-aminodioxindoles (243) into cinnolin-3-ones. 

Ring expansion of five- to six-membered rings such as oxazole —> pyridine derivatives via a Diels-Alder reaction is a well-established procedure. However, the conversion of a six-membered heterocycle into a five-membered ring system has not been exploited to any great extent, and those systems that have been studied usually involve a cationic species. 

Key to method (A) exchange reaction with tin heterocycle (B) hydride addition to diyne (C) oxidation of saturated ketone (D) bromination-dehydrobromination by pyrolysis (E) reaction of RLi or ArLi with exocyclic M-Cl of preformed diene (F) ring expansion reaction from cyclopentadiene derivative (G) LiAlHi reduction of exocyclic M-Cl (H) carbene insertion into five-membered cyclo-pentadiene derivative. Doering-Hoffman method (I) 1,6-cycloaddition of GeCU. 

Five-membered heterocyclic -oxides undergo analogous ring-expansions. Thus, the nitrone (94) is converted into the oxazine (95).74 Support for the... 

Reactions of carbenes other than cyclopropanation can also be performed, and recent examples include the ring expansion of five-ring heterocycles, such as the indoles (18), to their six-membered counterparts (19), and the production of formamides from secondary amines (Scheme 7), both with dichlorocarbene. The latter method is of interest because of its relation to the catalysis of dichlorocarbene generation by tertiary amines in two-phase systems. Recent work indicates that such catalysis is possible because the carbene, after generation at the phase boundary, is transferred into the organic phase (to undergo reaction) in the form of the N-ylid adduct (20). 

Five-membered fluorine-containing heterocyclic compounds with two or three sulfur atoms are more extended. A ring expansion reaction of thietanes into 1,2-dithiolanes involving heating with sulfur was used. ... 

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