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

Abstract

Synthetic Application of Epoxynitrones I. Nitrone, a New α‐Methylidene‐γ‐lactone Annelating Reagent The N ‐(2, 3‐epoxypropyliden)‐cyclohexylamine‐ N ‐oxide/CF 3 SO 3 SiR 3 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/AgBF 4 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 CF 3 SO 3 SiMe 3 ( 23 ) and CF 3 SO 3 Si ( t ‐Bu)Me 2 ( 24 ), respectively. Low temperature 1 H‐ and 13 C‐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 ).