Chemo‐, Regio‐, and Stereoselective Diels–Alder Reaction of Ambident Dienophilic Monothiomaleimide

Abstract

Keywords: Cycloadditions / C = S Dienophile / N-Arylmonothiomaleimides / Chemo-, regio-, and stereoselectivity / AM 1 ~ calculation Both the C=S and C=C functionalities of N-arylmonothiomaleimides readily undergo a (4 + 21 cycloaddition reaction at ambient temperatures with both electron-donating and -withdrawing dienes. The C=S groups of N-arylmonothiomaleimides are generally more reactive than the C=C groups, especially toward dienes bearing conjugation and electronattracting groups, and provide mixtures of ortho,endo products 3 and orfho,exo products 4 with complete regioselectivity (100% orfho) and with high stereoselectivity (more than 86% endo). The C=C groups, on the other hand, display a high stereoselectivity (100% endo) but a low regioselectivity, providing ortho,endo adducts 5 and meta,endo adducts 6 in ratios ranging from 1: 1 to 6: 1. Compared with N-phenylmonothiomaleimide ( 1 ) , N-p-nitrophenylmonothiomaleimide ( l l a ) is more reactive and displays a higher C=S vs. C=C chemoselectivity in favor of C=S, while N-p-anisyl ( l l b ) and N-benzyl (1 lc) derivatives display quite the reverse reactivity and selectivity. N-Phenylmonothiocitraconimide (1 Id) serves as a C=S specific dienophile and reacts with dienes to furnish ortho,endo adducts 12 with excellent regio-and stereoselectivities (100% ortho,endo). Both hard (BF, . OEtz, TiC1,) and soft (CuBr . SMe2) Lewis acids enhance the reaction rate enormously and increase chemoselectivity in favor of C=S. The chemo-(C=S/C=C), regio-(ortholmeta) and stereochemical outcomes (endolexo) are rationalized by transition structure analysis based on AM1 calculation.Hetero Diels-Alder reactions of C=O121, C=Nl2I, and C=S[2p8] dienophiles and donor-substituted dienes are among the most powerful carbon-carbon and carbonheteroatom bond-forming processes and have long been of great interest to synthetic and theoretical organic chemists. In these reactions, C = O and C = N dienophiles react with donor-substituted unsymmetrical dienes to give [4 + 21 cycloadducts via path a (Scheme l), while C=S dienophiles activated by conjugation with electron-withdrawing groups (acyl, alkoxycarbonyl, CN, etc.) react selectively via path b[3g3h34c3g,5c1. Ordinary thioaldehydes and thioketones also serve as reactive dienophiles; however, they display a reverse regioselectivity, showing a preference for path a[3h3J,4d,bl. This contrasting regioselectivity of thiocarbonyl dienophiles has been addressed theoretically by Vedejs and H o~k [~l . Here, we report that (i) N-phenylmonothiomaleimide (1) serves as a very reactive ambident C=S and C = C dienophile toward variously substituted 1,3-butadienes (Scheme 2); (ii) the C=S group of 1 displays comparable reactivity to the C=C group of 1 toward donor-substituted dienes, while the C=S group is much more reactive than the C = C group towards dienes substituted with conjugating and electron-withdrawing groups; (iii) the C = S group reacts with most dienes with exclusive ortho regioselectivity and with excellent endo stereoselectivity (more t...

Synthesis and photolysis of N-phthalimidoaziridines with electron-withdrawing substituents

“…N-Benzylmaleimide [17] and the monoadduct 6 of cyclopentadiene with benzoquinone [8,9] were obtained by the known methods. Electron impact (EI) mass spectra were obtained on a MX 1303 mass spectrometer.…”