Furan 2-amino-, stability

The [4+3] cycloaddition between furan and amino-stabilized allyl cations has not been as actively studied. An intramolecular cycloaddition between a furan and a 2-aminoallyl cation, generated from methyleneaziridine under Lewis acid-promoted conditions, is shown in Equation (50) <2004AGE6517>. An AgBF4-promoted asymmetric intermolecular [4+3] cycloaddition of 2-aminoallyl cations, derived from chiral a-chloroimines, with furan to give cycloadducts of up to 60% ee was also reported <1997TL3353>. 

Since Woodward s work on the synthesis of chlorophyll a (60JA3800) it is known that the intrinsic unstable thioformyl moiety can be stabilized by the delocalization effect of heterocyclic systems. Recently the synthesis of 2-amino- and 3-aminothioformylthiophenes (and furans) and the corresponding benzo derivatives (Scheme 19) has been reported (96S1185). These compounds exist as amino tautomers (91S609 96S1185). 

The amino form is usually much more favored in the equilibrium between amino and imino forms than is the hydroxy form in the corresponding keto-enol equilibrium. Grab and XJtzinger suggest that in the case of a-amino- and a-hydroxy-pyrroles, structure 89 increases the mesomeric stabilization and thus offsets the loss of pyrrole resonance energy, but the increase due to structure 90 is not sufficient to offset this loss. Similar reasoning may apply to furans and... 

Propargyl amines (145) have been used to prepare 3-amino-2,3-dihydro-2-arylmethylidenebenzofurans (146) with the use of Ag (I) catalyst (Scheme 31). At elevated temperatures (100 °C) or with the use of Lewis acids, compound (146) ionizes to the highly stabilized 2-benzofuranmethyl carbocation (147). The resulting salt collapses to provide a good yield (87%) of the 1,3-migration product (148). With the addition of furan or an allylstannane, the cationic intermediate provides products (149) and (150) in 43% and 73% yields, respectively. 

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