A compound selected from N-[2-(2-methyl-6,7-dihydro-8H-indeno[5,4-d][1,3]oxazol-8-ylidene)ethyl]acetamide, N-[2-(2-methyl-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl]acetamide, N-[2-(2-methyl-6,7-dihydro-8H-indeno[5,4-d][1,3]oxazol-8-ylidene)ethyl]propionamide, N-{2-[2-(4-phenylbutyl)-6,7-dihydro-8H-indeno[5,4-d][1,3]oxazol-8-ylidene]ethyl}acetamide, N-[2-(2-methyl-6,7-dihydro-8H-indeno[5,4-d][1,3]thiazol-8-ylidene)ethyl]acetamide, N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl] acetami de, -N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl] acetami de, -N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl] acetami de, N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl]propionamide, -N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl]propionamide, -N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl]propionamide, N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][1,3]thiazol-8-yl)ethyl] acetami de, -N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][1,3]thiazol-8-yl)ethyl] acetami de, -N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][l,3]thiazol-8-yl)ethyl] acetami de, N-[2-(2-ethyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl]acetamide, -N-[2-(2-ethyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl] acetami de, -N-[2-(2-ethyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl] acetami de, N-[2-(2-methoxy-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl] acetami de, -N-[2-(2-methoxy-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl]acetamide, and -N-[2-(2-methoxy-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl] acetami de or a salt thereof for use in the prophylaxis or treatment of an autism spectrum disorder. N-[2-(2-methyl-6,7-dihydro-8H-indeno[5,4-d][1,3]oxazol-8-ylidene)ethyl]acetamide, N-[2-(2-methyl-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl]acetamide, N-[2-(2-methyl-6,7-dihydro-8H-indeno[5,4-d][1,3]oxazol-8-ylidene)ethyl]propionamide, N-{2-[2-(4-phenylbutyl)-6,7-dihydro-8H-indeno[5,4-d][1,3]oxazol-8-ylidene]ethyl}acetamide, N-[2-(2-methyl-6,7-dihydro-8H-indeno[5,4-d][1,3]thiazol-8-ylidene)ethyl]acetamide, N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl] acetami de, -N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl] acetami de, -N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl] acetami de, N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl]propionamide, -N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl]propionamide, -N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl]propionamide, N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][1,3]thiazol-8-yl)ethyl] acetami de, -N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][1,3]thiazol-8-yl)ethyl] acetami de, -N-[2-(2-methyl-7,8-dihydro-6H-indeno[5,4-d][l,3]thiazol-8-yl)ethyl] acetami de, N-[2-(2-ethyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl]acetamide, -N-[2-(2-ethyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl] acetami de, -N-[2-(2-ethyl-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl] acetami de, N-[2-(2-methoxy-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl] acetami de, -N-[2-(2-methoxy-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl]acetamide, and -N-[2-(2-methoxy-7,8-dihydro-6H-indeno[5,4-d][1,3]oxazol-8-yl)ethyl] acetami de