Cyclopentadienones via a Tandem C‐Cyclopropylnitrone Cyclization‐Cycloreversion Sequence

Abstract

Aldonitrones derived from spiro[2.4]hepta-4,6-diene-1-carbaldehyde and its benzo analog undergo a tandem uncatalyzed intramolecular cyclopropane-nitrone cyclization-5,6-dihydro-1,2-oxazine cycloreversion to give cyclopentadienones. Similarly, the NH-nitrone generated in situ from spiro[cyclopropane-1,1′-indene]carbaldehyde oxime leads to benzocyclopentadienone (1H-inden-1-one) by the same mechanism. DFT calculations are in favor of a concerted yet highly asynchronous pathway for the cyclizations. Control experiments with the dihydro and tetrahydro derivatives show that the spirocyclopentadiene unit is essential for the success of the reaction, invoking spiroconjugative effects for increased cyclopropane reactivity.

Organocatalytic (3+3)-cycloaddition of ortho-substituted phenyl nitrones with aryl cyclopropane carbaldehydes: a facile access to enantioenriched 1,2-oxazinanes

“…To our delight, heteroatom-containing aryl cyclopropane carbaldehydes ( 1g–1h ) gave the corresponding products ( 4ga–4ha ) in low to moderate yields with low to moderate stereoselectivities. It is worth mentioning that, in all these cases, the corresponding benzaldehydes ( 1a′–1h′ ) were also obtained as the side products via a rearrangement reaction 13 (Fig. S2–S4, ESI†) with considerable yields in the range of 25–40% which can be attributed to the lower yield of the desired cycloadducts ( 4aa–4ha ).…”