Structure-Activity Relationship Studies of Benzoxazinones and Related Compounds. Phytotoxicity on Echinochloa crus-galli (L.) P. Beauv. FRANCISCO A. MACI ÄAS,* NURIA CHINCHILLA,ROSA M. VARELA, ALBERTO OLIVEROS-BASTIDAS, ² DAVID MARI ÄN, AND JOSE Ä M. G. MOLINILLO Grupo de Alelopatı ´a, Departamento de Quı ´mica Orga ´nica, Facultad de Ciencias, Universidad de Ca ´diz, c/o Repu ´blica Saharaui s/n, 11510 Puerto Real, Ca ´diz, Spain Echinochloa crus-galli (E. crus-galli; barnyardgrass) is a weed widely distributed. It constitutes a serious weed problem in 42 countries and has been found in at least 27 more. It is the world’s main weed of rice affecting up to 36 crops worldwide. Several biotypes of this plant, with resistance to herbicides with different modes of action have evolved. In our ongoing studies regarding the potential application of benzoxazinones and their soil degradation products for weed control, a complete structure-activity relationships (SARs) study was made by using barnyardgrass as the target plant. Compounds used in this study were previously tested on a wide variety of standard target species (STS), and they include natural allelochemicals 2-O--D-glucopyranosyl-4-hydroxy-(2H)-1,4-benzoxazin-3(4H)-one (DIBOA-Glc), 2,4-dihydroxy-7-methoxy-(2H)-1,4-benzoxazin-3(4H)-one (DIMBOA), and 2,4-dihydroxy- (2H)-1,4-benzoxazin-3(4H)-one (DIBOA), together with some degradation derivatives found in wheat crop soil and some synthetic analogues. Their phytotoxicity on E. crus-galli is discussed and compared with the results obtained from previous screening. This work constitutes the next step in the search for natural herbicide models based on benzoxazinones and their degradation products. The most active compounds were the degradation product 2-aminophenol (APH) and the synthetic analogue 4-hydroxy-(2H)-1,4-benzoxazin-3(4H)-one (D-DIBOA). Their activities confirm the direction proposed in our previous SAR study, which establishes D-DIBOA to be the best lead for natural herbicide model development with benzoxazinone structure. KEYWORDS: Rice; Echinochloa crus-galli; barnyardgrass; benzoxazinones; DIMBOA; DIBOA; 4-hydroxy- (2H)-1,4-benzoxazin-3(4H)-one; bioassay; phytotoxicity INTRODUCTION Barnyardgrass (Echinochloa crus-galli (L.) P. Beauv.; E. crus- galli) is a plant originally from Europe and India. It constitutes a serious weed problem in 42 countries and has been found in at least 27 more (1). It is the world’s main weed of rice (2) and, likely, has been for a long time: barnyardgrass is recognizable in Chinese drawings from 1590 (3). Also, barnyardgrass has been detected in 36 crops worldwide. It is particularly problematic for rice crops all over the world. It is among the three worst weeds in cotton in Australia, Russia, and Spain; in corn in Australia and the former Yugoslavia; and in sugarbeets in the United States. Also, it is a main weed in many other crops such as cotton, corn, and potatoes, for example, in the United States (2). Barnyardgrass has ecological preferences similar to rice, and young plants look similar (4); several thousand years of intensive handweeding of rice in Asia may have selected for rice mimicry (5). Many plants infesting rice fields may be transplanted in by accident (2). Heavy infestations may remove 60-80% of the nitrogen from the soil (2), as well as considerable amounts of other macro- nutrients (6). Fertilizer applications favor the weed over rice (2). It reduces rice tillering by 50% and also reduces the number of panicles, height, weight of grains, the and number of grains per panicle; rice yields may be reduced by 2000-4000 kg ha -1 (2). Barnyardgrass has been proven to reduce the yields of potatoes (7), snap beans (8), corn (9), grain sorghum (10), sugarbeets, green peas, and melons (11). Barnyardgrass inter- feres with harvesting of row crops and increases labor costs: the crop must be separated from the weed clumps (6). Barnyardgrass is also a host to many viruses of rice and other grass crops (2), as well as a host of Striga asiatica (L.) Kuntze, which infests sorghum, corn, millet, sugar cane, rice, and tobacco in India, Africa, and the United States (3). The success of barnyardgrass is attributed to prolific seeding, seed dormancy, the ability to grow rapidly, flowering in a wide range of photoperiods, and relative resistance to herbicides (6). * 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. 10.1021/jf0502911 CCC: $30.25 © xxxx American Chemical Society PAGE EST: 7.4 Published on Web 00/00/0000