The 1-methyl(iso-propyl)-2-benzyl-2,4-dihydro-1H-pyrazino[2,1-b]quinazoline-3,6-diones 8 and 9 could be regio- and diastereoselectively alkylated at C(4) with retention of configuration at both stereocentres. (C) 2002 Elsevier Science Ltd. All rights reserved.