The fate of the tryptophan stereocenter in the synthesis of 7,10,16,16a-tetrahydro-11H-quinazolino[2′,3′:3,4]pyrazino[1,2-b]β-carboline-5,8-diones

Privileged scaffolds in synthesis: 2,5-piperazinediones as templates for the preparation of structurally diverse heterocycles

“…For example, treatment of 13 with triethyloxonium tetrafluoroborate (Meerwein's salt) afforded lactim ether 14, whose Von Niementowsky-type cyclocondensation with anthranilic acid at high temperature afforded the hexacyclic compound 15, closely related to the ardeemins (Scheme 5a). 14 The step leading to the quinazolidinone ring was subsequently shown to be improved under microwave irradiation conditions. This protocol was employed to generate a variety of heterocyclic frameworks as exemplified in Scheme 5b by the synthesis of pentacyclic compound 17, which arises from a double cyclocondensation.…”

Chemistry of Pyrazino[2,1-b]quinazoline-3,6-diones

“…The conformationally less rigid homologous iminoethers 16 and 18 behaved differently, giving always products with a cis relationship between the H-7 and H-16a protons, as in compounds 17 and 19 in Scheme 6 [23]. The presence of a third stereocenter at C-10 imposes a different control, whereby the H-10 and H-16a protons must be trans in the final product.…”

Chiral Heterocycles by Iminium Ion Cyclization

“…This study showed that the size of the aldehyde has little influence on the diastereoselectivity, although an increase of the steric bulk to cyclohexyl excluded the formation of the cis isomer. The change of methyl for isopropyl ester did not lead to significant improvement, whereas the use of a bulky N -protective group enabled the stereoselective formation of 1,3- trans -carboline, even with acetaldehyde ).…”