A compound of formula I, ** Formula ** wherein X1 represents -C (R1a) (R1b) -, - N (R1a) - or -O-； the dotted line represents an optional double bond； and in the case that the dotted line does not mean a double bond, X2 and X3 independently represent -C (R1c) (R1d) -, -N (R1c) -, - O -, - C (O) - or - C (R1f) (R1g) -C (R1H) (R1j) - provided that: (i) when X1 represents -N (R1a) -, then both X2 and X3 do not represent both -N (R1e) -； (ii) when X1 represents -O-, then X2 and X3 do not represent both -O-； (iii) when X1 represents -O- and X2 represents -N (R1e) -, then X3 represents -C (O) -； and (iv) when X1 represents -O- and X3 represents -N (R1e) -, then X2 does not represent - C (R1c) (R1D) -, or in the case where the dotted line means a double bond, X2 and X3 independently represent -N- or -C (R1c) -； R1a, R1b, R1c, R1d, R1e, R1f, R1g, R1H and R1j independently represent H, C1-6 alkyl, C1-6 alkoxy, C1-6 alkoxy-C1-6 alkyl, Ar1, Het1, C1-3- alkyl Ar2, C1-3Het2 alkyl, C1-3Ar3 alkoxy, C1-3 alkoxy and -Het3, halo, -C (O) -C1-6 alkyl, -C (O) -AR4 or -C (O) -Het4； Ar1, Ar2, Ar3 and Ar4 independently represent a C6-10 aryl group, whose group is optionally substituted by one or more substituents selected from>； =, - OH, cyano, halogen, nitro, C1-6 alkyl (optionally terminated by -N (H) C (O) OR11a), C1-6 alkoxy, phenyl, - N (R12a) R12b, -C (O) R1c, -C (O) OR12d, -C (O) N (R12e) R12f, - N (R12g) C (O) R12h, -N (R12i) C (O) N (R12j) R12k, -N (R12m) S (O) 2R11b, -S (O) pR11c, -OS (O) 2R11d and -S (O) 2N (R12n) R12p； Het1, Het2, Het3 and Het4 each independently represent a heterocyclic group selected from benzodioxanil, benzodioxepanil, benzodioxolil, benzofuranyl, benzofurazanyl, benzimidazolyl, benzomorpholinyl, benzothiophenyl, chromanyl, canolinide, iminylidamide, iminzole, imanzole imidazole α] pyridinyl, indolyl isoquinolinyl, isoxazolyl, maleimido, morpholinyl, oxazolyl, phthalazinyl, piperazinyl, piperidinyl, purinyl, pyranyl, pyrazinyl, pyrazolyl, pyridinyl, pirimi
