New Highly Strained Multifunctional Heterocycles by Intramolecular Cycloadditions of Nitrones to Bicyclopropylidene Moieties

Intramolecular cycloadditions of various nitrone functionalities with different substituents (R = Me, Bn, t Bu) at the nitrogen atom tethered to a bicyclopropylidene unit through a two‐carbon chain led to cis ‐fused tricyclic isoxazolidines (3‐alkyl‐3,3a,4,5,5a,6‐hexahydrocyclopropa[2,3]cyclopenta[1,2‐ c ]isoxazolespiro[1,1′]cyclopropanes) 26 in 42–58 % yield with complete regio‐ and diastereoselectivity. The thermal rearrangement of the cycloadducts 26 under neutral conditions afforded the corresponding tricyclic tetrahydropyridones 27 (52–53 %). The analogous starting materials with a three‐carbon tether, the 4‐(bicyclopropyliden‐2‐yl)butylidenenitrones furnished tricyclic isoxazolidines 28 (54–58 %) and tetrahydropyridones 29 (55–64 %) by subsequent thermal rearrangement. Under acidic conditions (TFA), the cycloadducts 26 and 28 underwent fragmentative rearrangements to afford the tricyclic β‐lactams 30 and 32 (50–66 %), respectively, of which the former suffered amide‐bond cleavage in situ to provide the corresponding N ‐trifluoroacetyl β‐amino acid derivatives 31 (68–71 %). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)