Synthesis of pyrrolizines by intramolecular capture of 1,4-dipolar intermediates in reactions of enamines with dimethyl acetylenedicarboxylate

Abstract

Solvent polarity and reaction temperature strongly influence the reactions of dimethyl acetylenedicarboxylate (DMAD) with I-pyrrolidinyl enamines of acyclic and cyclic ketones. Whereas DMAD and I-[l-phenylt-@henylthio)ethenyl]pyrrolidiae (3) give only a mixture of the isomeric I$-butadiencs (5) in apolar solvents, in methanol the main product is the pyrrolizine 7, together with 5. Again in methanol, DMAD reacts at 0-Y with 8.9 and 10 to give exclusively I : I adducts, tbe pyrrolizines 11, 12 and 13, respectively, whereas at-50" 8 and 9 give I : 2 (enamine : DMAD) adducts. the pyrrolixines 14 and 15, respectively; a siqle crystal X-ray analysis of 14 gave the structure of the I : 2 adducta. In the same solvent methyl propiolate and 8 give only the linear Michael adduct 17. The enamine-ketone 18 reacts with DMAD in propylene carbonate at W' to give, via (2 t Zj-cycloaddition and ring expansion, 19, and the linear Michael adduct 20. The mechanism of (2 t Zj-cycloaddition and pyrrolizine formation is discussed in terms of a common tied-ion pair intermediate formed in the first. ratedetermining step,

N-Methyl-N-(tert-butyldimethylsilyl)trifluoroacetamide and related N-tert-butyldimethylsilyl amides as protective silyl donors

“…In the NMR spectrum, the characteristic NCH absorption at 6 4.74 (dd, J = 5 and 12 Hz) was still present. X-ray diffraction showed that the compound had the methyl l,2,5,6,7,7a,8,8a-octahydro-8-(methoxycarbonyl)-3-(trifluoroacetyl)cyclopenta[b]pyrrolizine-8-acetate (3) structure (Figure 1). We assume that this reaction of 1 proceeds via its tautomeric form 2 (Scheme I) in which trifluoroacetylation takes place at the /3-enamine carbon atom.…”

Synthesis, Molecular Structure and Reactivity of 5-Methylidene-1,2,3,5-tetrahydroimidazo[2,1-b]quinazolines

“…Enamines are known to act as 1,3-dipolarophiles [8,9] and the 1,3-dipolar cycloaddition of nitrones to enamines resulting in the formation of isoxazolidine derivatives is well established [10][11][12] [13,14] or Michael addition [13,15] products. To identify reactive sites both at the alkene and nitrogen atom incorporated into guanidine moiety, the electronic structure of 2 was studied using ab initio 6-31G** calculations [7].…”

Can HCCH/HBNH Break B═N/C═C Bonds of Single-Wall BN/Carbon Nanotubes at Their Surface?

“…Experimental studies , by Reinhoudt and coresearchers indicate that electron-deficient acetylenes (ynamines) undergo [2 + 2]­cycloaddition (under mild condition in apolar solvents) with electron-rich alkenes (enamines) to form cyclobutenes. However, the MP2/6-31G* barrier height of 75 kcal/mol of the reaction between unsubstituted acetylene and ethylene is not supportive of being observed experimentally.…”