1-acetylvinyl acrylates: new captodative olefins bearing an internal probe for the evaluation of the relative reactivity of captodative against electron-deficient double bonds in Diels-Alder and Friedel-Crafts reactions

Abstract

As olefinas captodativas 3a e 3b derivadas dos ácidos metacrílico e trans-crotônico foram preparadas. A presença de uma segunda ligação dupla na molécula, atuando como marcador interno, permitiu-nos comparar sua reatividade relativa em reações de Diesl-Alder e Friedel-Crafts. A reatividade foi avaliada com ciclopentadieno (6) atuando como dieno em cicloadição Diels-Alder, e com furano (9) e tiofeno (10) como substratos heteroaromáticos Friedel-Crafts. Em ambos os processos, a ligação dupla da enona captodativa mostrou-se mais reativa que a do sistema acrílico. A teoria FMO considera essa quimiosseletividade como consequência da maior contribuição da enona ao orbital LUMO dessas moléculas. A pequena seletividade exo observada na cicloadição com 6 concorda com a maior estabilidade do estado de transição, de acordo com os resultados de cálculos B3LYP/6-311G(d,p).The captodative olefins 1-acetylvinyl esters of methacrylic and trans-crotonic acids, 3a and 3b, have been prepared. The presence of a second double bond in the molecule, acting as an internal probe, allowed us to compare their relative reactivity in Diels-Alder and Friedel-Crafts reactions. The reactivity was evaluated with cyclopentadiene (6) as diene in Diels-Alder cycloadditions, and with furan (9) and thiophene (10) as heteroaromatic Friedel-Crafts substrates. In both processes, the captodative enone double bond proved to be more reactive than that in the acrylic moiety. FMO theory accounted for this chemoselectivity as a consequence of the major contribution of the enone to the LUMO of these molecules. The slight exo stereoselectivity observed in the cycloaddition to 6 parallels the higher stability of the corresponding transition state, according to the results of B3LYP/ 6-311G(d,p) calculations.

A new diastereoselective approach to the synthesis of α-hydroxy-ß-amino acids based on the frame of captodative olefins

“…This result confirms the higher reactivity of the captodative C-2/C-3 double bond in comparison with the captodative C-5/C-6 olefin, as shown in Diels-Alder cycloadditions and Friedel-Crafts reactions for analogous compounds. 18 Considering that the 1,4-addition can be favorably promoted by secondary amines, which in turn can be generated by silylating primary amines, 19 we carried out the reaction mentioned above in the presence of different alkyl-and alkylphenyl silyl chlorides, 8a-8c (Table 3). Consequently, the lithium amide of amine 4a, prepared by deprotonation with n-BuLi, was mixed with 8 at -50 °C in anhydrous THF; after stirring the mixture for 30 min, the substrate 1a was added, and the reaction mixture was maintained at 25 °C for 24 h. Both, the proportion of amide 7a and the ratio of the stereoisomeric adducts 5a/6a, were similar in the three cases, showing a slight dependence on the silyl compound used in the reaction (Table 3, entries 1-3).…”