Photocycloadditions of electron poor olefins to 2-aryl-1-pyrrolinium salts (original) (raw)

1980, Tetrahedron Letters

Photocycloadditionsofthe electron poor olefins, acrylonitrile and methyl acrylate to a series of para-substituted 2-phenyl-1-pyrrolinium pet-chlorates have been explored as part of studies designed to test new synthetic methods. Studies in our laboratory3 have uncovered interesting olefin photoadditions to iminium salts occuring through electron-transfer or 2+2-cycloaddition pathways. Additions of electron poor olefins to 2-phenyl-1-pyrrolinium perchlorate (l_H+zH) appear to involve initial 2+2-cycloaddition across the C-l C-Z n-bond of the aromatic ring (/H-*?H) followed by ring expansion and aromatization.3b This reaction appears potentially applicable to the synthesis of spirocyclic amines, exemplified by the proposed route to serratinine4 4 outlined retrosynthetically in Chart 1. The success of this approach would depend upon the effect of aromatic ring substitution, needed for spiroannulation and A-ring functional group introduction, on photoaddition. In order to investigate this point, we have studied photocycloadditions of electron poor olefins and a series of Z-p-substituted-phenyl pyrrolinium perchlorates 1. The 1-pyrroline precursors of l_F, lC1, 1CH31 l,Br and 101ZH3,~ were prepared by use of the method of Bielawski 6. involving addition of the corresponding aryl lithium to N-vinylpyrrolidone followed by hydrolysis. Irradiations7 were conducted on methanolic acrylonitrile or methyl acrylate solutions (1 M) of the salts (ca. 10m3-M), formed in situ from the pyrrolines and 70%-HC104. Crude photolysates were neutralized, concentrated, and products purified by molecular distillation and/or chromatography. Yields of the benzospirocyclic amines produced are recorded in Table 1. 9 Product structures were assigned on the basis of spectroscopic data and comparisons with that for the known parent amines, 3CN-H and 3C0 CH-H. 3b-2 3 Chemical correlations of ZCN-Cl and 3CN-Br with ?CN-H (LiA1H4 THF, 6O"C, 12h-reduction to identical mixtures of the stereoisomerit diamines 6) provided unambiguous structural assignments to these materials. The results indicate that additions of electron poor olefins to photo-excited Z-aryl-lpyrrolinium perchlorates, proceeding by initial 2+2 cycloaddition to the aromatic ring, serve as useful methods for construction of the reasonably complex benzospirocyclic amines. An exception to this generality was found in studies of the Z-anisyl-1-pyrrolinium salt, lOCH3. The spirocyclic amines obtained by cycloaddition to p-alkoxy substituted salts would be ideal precursors to the phenoxides 5, useful in Spiro-annulation procedures (Chart 1). Irradiation of methanolic solutions of lOCH3 and acrylonitrile leads to generation of an abnormal photoadduct [. Spectroscopic data indicate that this material is a 1:l adduct containing a e-disubstituted aromatic ring and a 1,2,2-trisubstituted pyrrolidine unit and suggest that it possesses the bicyclic azetidine structure, 7. 1 The characteristic ABM pattern observed in the H-NMR spectrum lo is that expected for a conformationally fixed CH2CH(NR2)CN moiety.