Synthetische Verwendung von Epoxynitronen. I. N‐(2,3‐Epoxypropyliden)‐cyclohexylamin‐oxid, ein neues Reagens zur Synthese von α‐Methyliden‐γ‐lactonen aus Olefinen

Synthetic Application of Epoxynitrones I. Nitrone, a New α‐Methylidene‐γ‐lactone Annelating Reagent The N‐(2, 3‐epoxypropyliden)‐cyclohexylamine‐N‐oxide/CF3SO3SiR3 reagent descried in this communication opens a new and interesting entry to the versatile N‐substituted N‐propenylnitrosonium ions of type b (Scheme 6). One of the uses of this reagent is shown to be the synthesis of α‐methylidene‐γ‐lactones from olefins. This new method shows similar features as the method based on 2, 3‐dichloropropylidenamine‐oxide/AgBF4 originally developed for the same purpose by Petrzilka, Felix and Eschenmoser. Epoxynitrone 18 can be transformed to the positively charged heterodiene of type b (Scheme 5) using the highly electrophilic reagents CF3SO3SiMe3 (23) and CF3SO3Si (t‐Bu)Me2 (24), respectively. Low temperature 1H‐ and 13C‐NMR. spectroscopy at −78° showed the sole formation of the nitrone‐O‐silyl‐ethers a (Scheme 5). Epoxid opening leading to the diene b and subsequent reactions are observed only at about −30°. The diene b prepared in situ, adds to isolated double bonds by way of an inverse Diels‐Alder reaction to afford cycloadducts of type 27 (Scheme 7). Their stable cyanoderivatives, e.g. 28 (Scheme 7), can be isolated and transformed via 31, 44 and 54 into cis annelated α‐methylidene‐γ‐lactones of type 55 (Scheme 11). Using trisubstituted olefins, substitution at the lower substituted olefinic C‐atom competes efficiently with the cycloaddition (e.g. 34, Scheme 8).