Patent ID: 8063060

Claim:
A compound of formula II: or a pharmaceutically acceptable salt thereof, wherein X 1 is H; R 1 is CH 3 , CH 2 CH 3 , CH 2 CH 2 CH 3 , CH 2 CH 2 ═CH 2 , or cyclopropylmethyl; R 2 is H; and R 3 is methyl, ethyl, n-propyl, isopropyl, cyclopropyl, cyclopropylmethyl, cyclopropylethyl, n-butyl, iso-butyl, sec-butyl, tert-butyl, vinyl, allyl, 2-cyclopropyl-1-ethenyl, 1-propenyl, 1-butenyl, 2-butenyl, 3-butenyl, 2-methyl-1-propenyl, 2-methyl-2-propenyl, ethynyl, 1-propynyl, 2-propynyl, 1-butynyl, 2-butynyl, or 3-butynyl.