2- Aminofurans, rearrangements

Carbene complexes of Fe and Co carbonyls are also prepared. Unlike the Cr carbene complexes, no cyclopropanation of alkenes occurs with these carbene complexes. Furans are formed by the reaction of alkynes involving rearrangement of methoxy group. The 2-aminofuran 323 is formed by the reaction of the dimethylaminocarbene complex 319 of Fe carbonyl, via rearrangement of the amino group. Under CO pressure, pyrone 324 is the main product [97]. In these reactions, the... 

A similar transformation of 2-furylcarbamates (e.g. 42) to A-carboxy-5-hydro-xypyrrolin-2-ones has been reported (78H1603, 80H1073). This type of oxidative rearrangement probably also accounts for the failure to isolate the 2-aminofurans 71 formed from 3-benzylidene-2,4-pentanediones and alkyl isocyanides (Scheme 14) (97MC697, 04TL1413). 

The iV-acyl ureas 157, prepared by amidine rearrangement (Section II.B.3), undergo thermal elimination of phenyl isocyanate with formation of the amides 158 (Eq. (24)) (01JCS(P1)680). This approach provides access to amides of unstable 2-aminofurans that cannot be prepared by acylation of the primary amine. 

Iron and cobalt carbene complexes are capable of much more selective furan synthesis. Reactions of cobalt methoxycarbenes with internal alkynes give good to excellent yields of 2-methoxyfurans and have been applied to a synthesis of a natural product, bovolide [75 b]. Both terminal and internal alkynes are viable substrates in the preparation of 2-aminofurans from iron (dimethylamino)carbenes [78], although the nature of the rearrangement of the heteroatom-containing substituent is also as yet unclear [Eq. (33)]. The inclusion of elevated carbon monoxide pressure can divert this reaction to the production of pyrones [75c, 79]. 

A novel three-component reaction sequence leading to 5-hydroxy-3-pyrrolin-2-ones has been reported by Liang and coworkers (Scheme 108 2007T8987). Mixing arylaldehyde 410, 2,4-pentanedione (361), and cyclohexyl isocyanide (260) with piperidine gives the 3-pyrrolin-2-one 411. The reaction proceeds via the formation of2-aminofuran intermediates followed by a Dimroth-type rearrangement to the corresponding 3-pyrrolin-2-ones. 

Heating an aromatic aldehyde (e.g., 420), DMAD (303), and the N-amino-isocyanide 421 gives N-amino-3-pyrrolin-2-ones (e.g., 422) (Scheme 111). Presumably, this is another example of a multicomponent reaction leading to a 2-aminofuran intermediate, which is converted to a 3-pyrtolin-2-one via a Dimroth rearrangement. 

---