Structure-Activity Relationships (SAR) Studies of Benzoxazinones, Their Degradation Products and Analogues. Phytotoxicity on Standard Target Species (STS) FRANCISCO A. MACı ´AS,* DAVID MARı ´N,ALBERTO OLIVEROS-BASTIDAS, ² DIEGO CASTELLANO,ANA M. SIMONET, AND JOSE Ä M. G. MOLINILLO Grupo de Alelopatı ´a, Departamento de Quı ´mica Orga ´nica, Facultad de Ciencias, Universidad de Ca ´diz. C/Repu ´blica Saharaui, s/n, 11510 Puerto Real, Ca ´diz, Spain Benzoxazinones 2,4-dihydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one (DIMBOA) and 2,4-dihy- droxy-(2H)-1,4-benzoxazin-3(4H)-one (DIBOA) have been considered key compounds for understand- ing allelopathic phenomena in Gramineae crop plants such as corn (Zea mays L.), wheat (Triticum aestivum L.), and rye (Secale cereale L.). The degradation processes in the environment observed for these compounds, in which soil microbes are directly involved, could affect potential allelopathic activity of these plants. We present in this work a complete structure-activity relationships study based on the phytotoxic effects observed for DIMBOA, DIBOA, and their main degradation products, in addition to several synthetic analogues of them. Their effects were evaluated on standard target species (STS), which include Triticum aestivum L. (wheat) and Allium cepa L. (onion) as monocots and Lepidium sativum L. (cress), Lactuca sativa L. (lettuce), and Lycopersicon esculentum Will. (tomato) as dicots. This permitted us to elucidate their ecological role and to propose new herbicide models based on their structures. The best phytotoxicity results were shown by the degradation chemical 2-aminophenoxazin-3-one (APO) and several 2-deoxy derivatives of natural benzoxazinones, including 4-acetoxy-(2H)-1,4-benzoxazin-3(4H)-one (ABOA), 4-hydroxy-(2H)-1,4-benzoxazin-3(4H)- one (D-DIBOA), and 4-hydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one (D-DIMBOA). They showed high inhibitory activity over almost all species growth. The fact that APO is a degradation product from DIBOA with high phytotoxicity and stability makes it possible to assign an important ecological role regarding plant defense mechanisms. 2-Deoxy derivatives of natural benzoxazinones display a wide range of activities that allow proposing them as new leads for natural herbicide models with a 1,4-benzoxazine skeleton. KEYWORDS: Allelochemicals, DIMBOA, DIBOA, soil degradation products, wheat, phytotoxicity, SAR, STS. INTRODUCTION Benzoxazinones containing the hydroxamic acid moiety have attracted attention of phytochemistry researchers since the first isolation of 2,4-dihydroxy-(2H)-1,4-benzoxazin-3(4H)-one (DIBOA) (1) in 1959 and 2,4-dihydroxy-7-methoxy-(2H)-1,4- benzoxazin-3(4H)-one (DIMBOA) in 1962 (2)(Table 1, A). Interesting bioactivity was observed for both compounds, some of their degradation products, and also some synthetic analogues, being antimicrobial (3), antifeedant, insecticide (4), and phy- totoxic behaviors (5, 6) widely described. The DIBOA natural glycoside (2-O--D-glucopyranosyl-4-hydroxy-(2H)-1,4-benz- oxazin-3(4H)-one (DIBOA-Glc) (Table 1, A) is the form in which DIBOA is preserved inside the plant prior to its release (7). Benzoxazolin-2-one (BOA) and 6-methoxybenzoxazolin- 2-one (MBOA) (Table 2, A) are the first chemicals in the DIBOA and DIMBOA degradation series, respectively (8-10). There are interesting precedents regarding their bioactivity on different systems. Lactam 2-hydroxy-(2H)-1,4-benzoxazin- 3(4H)-one (HBOA) (Table 1, B) has been proposed as a biosynthetic precursor of DIBOA (11, 12). 2-Hydroxy-7- methoxy-(2H)-1,4-benzoxazin-3(4H)-one (HMBOA) (Table 1, B) has been detected as a degradation product for DIMBOA in wheat crop soil (9). Both compounds have been isolated from crop soils of plants with high prodcution of benzoxazinones. Antifeedant and antiaphid capabilities for DIMBOA and DIBOA, together with their related benzoxazolinones (Table 2, A) benzoxazinoid lactams (Table 1, B), and synthetic analogues (2-deoxybenzoxazinones, Table 1, C), were evaluated by Escobar et al. (4) on aphid Sitobion aVenae. A complete * Author to whom correspondence should be addressed. Phone: +34-956.016.370. Fax: +34-956.016.193. E-mail: famacias@uca.es. ² Permanent address: Laboratorio de Quı ´mica Ecolo ´gica, Departamento de Quı ´mica, Facultad de Ciencias, Universidad de Los Andes, Nu ´cleo Universitario Pedro Rinco ´n Gutie ´rrez, La Hechicera, Me ´rida 5101-A, Venezuela. 538 J. Agric. Food Chem. 2005, 53, 538-548 10.1021/jf0484071 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/19/2005