Patent ID: 8198474

Claim:
A compound of formula (15): wherein: A is —(CH 2 ) x —O— or a covalent bond; n is 0 or 1; x is 1-6; R 1a is hydrogen, a C 1 -C 6 alkyl group, a C 3 -C 6 alkenyl group, a C 3 -C 6 alkynyl group, or a phosphite oxygen protecting group or a phosphate oxygen protecting group; one of R 2a and R 2b is H and the other is a monovalent nitrogen protecting group; or R 2a and R 2b taken together are a divalent nitrogen protecting group; when A is —(CH 2 ) x —O—, one of R 3a and R 3b is H and the other is a monovalent nitrogen protecting group, or R 3a and R 3b taken together are a divalent nitrogen protecting group; when A is a covalent bond, R 3a and R 3b are a C 1 -C 6 alkyl group, or R 3a and R 3b taken together are —(CH 2 ) 4 —, —(CH 2 ) 5 —, or —(CH 2 ) 2 —O—(CH 2 ) 2 —; R 4 is a C 5 -C 12 alkyl group or a C 5 -C 12 alkenyl group; and R 5 is a C 5 -C 15 alkyl group or a C 5 -C 15 alkenyl group; or a salt thereof.