A compound of Formula I: or a pharmaceutically acceptable salt thereof, wherein R 1 is H or CH 3, R 2 is H or OH, (a) when n is 1, 2 or 3, then (a.i) R 3 is H and R 4 is optionally substituted C 1 -C 4 alkyl; or (a.ii) R 4 is H and R 3 is optionally substituted C 1 -C 4 alkyl; or (a.iii) R 3 and R 4 are both optionally substituted C 1 -C 4 alkyl; or (b) when n is 0, then (b.i) R 3 is H and R 4 is optionally substituted C 1 -C 4 alkyl; or (b.ii) R 4 is H and R 3 is optionally substituted C 1 -C 4 alkyl; or (b.iii) R 3 and R 4 are both optionally substituted C 1 -C 4 alkyl; or (b.iv) R 3 and R 4 and the carbon atoms to which they are attached together form a five, six, or seven membered ring, which is optionally mono or disubstituted by C 1 -C 4 alkyl, wherein the carbon atoms labeled * and ** are independently in the R or S configuration, and wherein the carbon atom labeled * and the carbon atom labeled ** are both in the R configuration, or wherein the carbon atom labeled * and the carbon atom labeled ** are both in the S configuration, or the carbon atom labeled * is in the R configuration and the carbon atom labeled ** is in the S configuration, or the carbon atom labeled * is in the S configuration and carbon atom labeled ** is in the R configuration; provided the compound is not R 1 is H or CH 3, R 2 is H or OH, (a) when n is 1, 2 or 3, then (a.i) R 3 is H and R 4 is optionally substituted C 1 -C 4 alkyl; or (a.ii) R 4 is H and R 3 is optionally substituted C 1 -C 4 alkyl; or (a.iii) R 3 and R 4 are both optionally substituted C 1 -C 4 alkyl; or (b) when n is 0, then (b.i) R 3 is H and R 4 is optionally substituted C 1 -C 4 alkyl; or (b.ii) R 4 is H and R 3 is optionally substituted C 1 -C 4 alkyl; or (b.iii) R 3 and R 4 are both optionally substituted C 1 -C 4 alkyl; or (b.iv) R 3 and R 4 and the carbon atoms to which they are attached together form a five, six, or seven membered ring, which is optionally mono or disubstituted by C 1 -C 4 alkyl, (a) when n is 1, 2 or 3, then (a.i) R 3 is H and R 4 is optionally substituted C 1 -C 4 alkyl; or (a.ii) R 4 is H and R 3 is optionally substituted C 1 -C 4 alkyl; or (a.iii) R 3 and R 4 are both optionally substituted C 1 -C 4 alkyl; or (a.i) R 3 is H and R 4 is optionally substituted C 1 -C 4 alkyl; or (a.ii) R 4 is H and R 3 is optionally substituted C 1 -C 4 alkyl; or (a.iii) R 3 and R 4 are both optionally substituted C 1 -C 4 alkyl; or (b) when n is 0, then (b.i) R 3 is H and R 4 is optionally substituted C 1 -C 4 alkyl; or (b.ii) R 4 is H and R 3 is optionally substituted C 1 -C 4 alkyl; or (b.iii) R 3 and R 4 are both optionally substituted C 1 -C 4 alkyl; or (b.iv) R 3 and R 4 and the carbon atoms to which they are attached together form a five, six, or seven membered ring, which is optionally mono or disubstituted by C 1 -C 4 alkyl, (b.i) R 3 is H and R 4 is optionally substituted C 1 -C 4 alkyl; or (b.ii) R 4 is H and R 3 is optionally substituted C 1 -C 4 alkyl; or (b.iii) R 3 and R 4 are both optionally substituted C 1 -C 4 alkyl; or (b.iv) R 3 and R 4 and the carbon atoms to which they are attached together form a five, six, or seven membered ring, which is optionally mono or disubstituted by C 1 -C 4 alkyl, wherein the carbon atoms labeled * and ** are independently in the R or S configuration, and wherein the carbon atom labeled * and the carbon atom labeled ** are both in the R configuration, or wherein the carbon atom labeled * and the carbon atom labeled ** are both in the S configuration, or the carbon atom labeled * is in the R configuration and the carbon atom labeled ** is in the S configuration, or the carbon atom labeled * is in the S configuration and carbon atom labeled ** is in the R configuration; wherein the carbon atom labeled * and the carbon atom labeled ** are both in the R configuration, or wherein the carbon atom labeled * and the carbon atom labeled ** are both in the S configuration, or the carbon atom labeled * is in the R configuration and the carbon atom labeled ** is in the S configuration, or the carbon atom labeled * is in the S configuration and carbon atom labeled ** is in the R configuration; provided the compound is not