Patent ID: 7253177

Claim:
A compound of the formula I: wherein R 1 is H, CH 3 CH 2 OC═O, (CH 3 ) 2 CHOC═O, (CH 3 ) 3 COC═O, or (CH 3 ) 2 CHCH 2 OC═O R 2 is H, CH 3 CH 2 OC═O, (CH 3 ) 2 CHOC═O, (CH 3 ) 3 COC═O, or (CH 3 ) 2 CHCH 2 OC═O R 3 is H, CH 3 CH 2 OC═O, (CH 3 ) 2 CHOC═O, (CH 3 ) 3 COC═O, (CH 3 ) 2 CHCH 2 OC═O, or R 4 is H, CH 3 CH 2 OC═O, (CH 3 ) 3 COC═O, (CH 3 ) 2 CHCH 2 OC═O, or wherein R 1 , R 2 , R 3 and R 4 are not all simultaneously H.