First syntheses of natural products with the 2,7-dihydroxy-2H-1,4-benzoxazin-3(4H)-one skeleton

2,7-Dihydroxy-2H-1,4-benzoxazin-3(4H)-one 11 (DHBOA) and 2,4,7-trihydroxy-2H-1,4-benzoxazin-3(4H)-one 14 (TRIBOA) representing aglucones of naturally occurring acetal glucoside type allelo chemicals found in Gramineae have been for the first time synthesized by two pathways both involving selective reductive cyclizations of appropriate 7-benzyloxy-2-nitrophenol derivatives as precursors. TRIBOA 14 and its bioactive naturally occurring 7-methyl ether DIMBOA have been found to undergo a hitherto unknown transformation to the corresponding 2,6-dibromo substituted lactam forms 20 and 21 in the presence of hydrogen bromide in acetic acid, which is of value in a better understanding of the possible mode of bioactivity.