New findings in the alkylation and N-deprotection of (4S)-4-methyl-2-benzyl-2,4-dihydro-1H-pyrazino[2,1-b]quinazoline-3,6-diones

Alkyl halides behave differently to benzyl halides in C-1 alkylation of the title compounds. The syn and anti 1,4-disubstituted diastereomers thus obtained show different regioselectivity by further alkylation leading to the 1,4,4- and 1,1,4-trisubstituted compounds, respectively. Alkylation is always directed anti with respect to the bulkier substituent at C-1 or C-4. Debenzylation attempts on 2-benzyl-derivatives 1b by treatment with HCOOH and C/Pd or H-2/C-Pd/MeOH/H+ led to C-1 oxidised or 7,8,9,10-tetrahydro-derivatives. Deprotection of 2-p-methoxybenzyl- and 2-(2,4-dimethoxybenzyl)-derivatives with CAN and with TFA/anisole, respectively, was successful, but in the latter case epimerization at C-1 occurred. (C) 1998 Elsevier Science Ltd. All rights reserved.