R¹ is

(a) a heteroaryl-C_1-3-alkyl group,

wherein heteroaryl is selected from the group consisting of phenylpyrimidinyl, quinolinyl, isoquinolinyl, quinazolinyl, quinoxalinyl, naphthyridinyl and phenanthridinyl which may be optionally substituted by R¹⁰, R¹¹ and R¹²,

where R¹⁰ is selected from hydrogen, fluorine, chlorine, bromine, methyl, difluoromethyl, trifluoromethyl, phenyl, cyano, methoxy, difluoromethoxy, trifluoromethoxy, amino, methylamino, dimethylamino, pyrrolidin-1-yl, piperidin-1-yl or morpholin-4-yl group;

R¹¹ is selected from hydrogen, methyl, methoxy or cyano; and

R¹² is hydrogen or methyl,

or

(b) a naphthyl-C_1-3-alkyl group, which may be substituted by R¹³ and R¹⁴,

where R¹³ is selected from hydrogen, fluorine, chlorine, bromine methyl, difluoromethyl,

trifluoromethyl, cyano, methoxy, difluoromethoxy or trifluoromethoxy; and

R¹⁴ is selected from hydrogen, methyl, methoxy or cyano;

R² is selected from methyl, ethyl, propyl, isopropyl, cyclopropyl or phenyl;

R³ is selected from 2-butyn-1-yl, 1-buten-1-yl, 2-buten-1-yl or 3-methyl-2-buten-1-yl and

n is 1 or 2;