Definition: In this task, you're given a paragraph from the research paper and your task is to generate a suitable title for the research paper based on the given paper. Under 100 words is a good title length.
Input: To study the influence of a linker rigidity and donor-acceptor properties, the P-CH 2 -O-CHR-fragment in acyclic nucleoside phosphonates (e.g., acyclovir, tenofovir) was replaced by the P-CH 2 -HN-C(O)-residue. The respective phosphonates were synthesized in good yields by coupling the straight chain of x-aminophosphonates and nucleobase-derived acetic acids with EDC. Based on the 1 H and 13 C NMR data, the unrestricted rotation within the methylene and 1,2-ethylidene linkers in phosphonates from series a and b was confirmed. For phosphonates containing 1,3-propylidene (series c) fragments, antiperiplanar disposition of the bulky O,O-diethylphosphonate and substituted amidomethyl groups was established. The synthesized ANPs P-X-HNC(O)-CH 2 B (X = CH 2 , CH 2 CH 2 , CH 2-CH 2 CH 2 , CH 2 OCH 2 CH 2 ) appeared inactive in antiviral assays against a wide variety of DNA and RNA viruses at concentrations up to 100 lM while marginal antiproliferative activity (L1210 cells, IC 50 = 89 ± 16 lM and HeLa cells, IC 50 = 194 ± 19 lM) was noticed for the analog derived from (5-fluorouracyl-1-yl)acetic acid and O,O-diethyl (2-aminoethoxy)methylphosphonate. O H N X (EtO) 2 (O)P X = CH 2 , CH 2 CH 2 , CH 2 CH 2 CH 2 , CH 2 OCH 2 CH 2 B = natural or modified nuclebases

Acyclic nucleoside phosphonates containing the amide bond