One-pot synthesis of an AZT boranophosphate conjugated with tyrosine: A potential prodrug candidate

Article abstract of DOI:10.1081/NCN-120022613

A one-pot synthesis of P-tyrosinyl(P-O)-5′-P-AZT boranophosphate 7 via a phosphoramidite method is described. The P-boranophosphate diastereomers were separated by RP-HPLC, and their structures were confirmed by NMR and MS.

Full text of DOI:10.1081/NCN-120022613

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One-Pot Synthesis of an AZT Boranophosphate
Conjugated with Tyrosine: A Potential Prodrug
Candidate
Ping Li a & Barbara Ramsay Shaw a b
a Department of Chemistry , Paul M. Gross Chemical Laboratory, Duke University , Durham,
North Carolina, USA
b Department of Chemistry , P.M. Gross Chemical Laboratory, Duke University , Durham, NC,
27708-0346, USA
Published online: 31 Aug 2006.
To cite this article: Ping Li & Barbara Ramsay Shaw (2003) One-Pot Synthesis of an AZT Boranophosphate Conjugated
with Tyrosine: A Potential Prodrug Candidate, Nucleosides, Nucleotides and Nucleic Acids, 22:5-8, 699-701, DOI: 10.1081/
NCN-120022613
To link to this article: http://dx.doi.org/10.1081/NCN-120022613
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NUCLEOSIDES, NUCLEOTIDES & NUCLEIC ACIDS
Vol. 22, Nos. 5–8, pp. 699–701, 2003
One-Pot Synthesis of an AZT Boranophosphate Conjugated
with Tyrosine: A Potential Prodrug Candidate
*
Ping Li and Barbara Ramsay Shaw
Department of Chemistry, Paul M. Gross Chemical Laboratory,
Duke University, Durham, North Carolina, USA
ABSTRACT
A one-pot synthesis of P-tyrosinyl(P-O)-50-P-AZT boranophosphate 7 via a
phosphoramidite method is described. The P-boranophosphate diastereomers
were separated by RP-HPLC, and their structures were confirmed by NMR
and MS.
Key Words: Nucleotide prodrug; Boranophosphate; AZT; Tyrosine.
Most anti-HIV drugs targeted at reverse transcriptase are 20,30-dideoxynucleo-
side (ddN) analogues. For example, 30-azido-30-deoxythymidine (AZT) was the first
drug approved by the FDA for the treatment of HIV infection. In most cases,
conversion of the nucleoside analogues to their 50-monophosphates (ddNMPs)
can be considered as the bottleneck in the overall metabolic pathway leading to the
formation of the active 50-triphosphate metabolites (ddNTP).[1] Therefore, prodrug
approaches have been employed to design 20,30-dideoxynucleoside 50-monophos-
phates that will deliver the 50-monophosphate derivatives into the HIV-affected cells.
*Correspondence: Barbara Ramsay Shaw, Department of Chemistry, P.M. Gross Chemical
Laboratory, Duke University, Durham, NC 27708-0346, USA; Fax: þ1 919 660 1618; E-mail:
brshaw@chem.duke.edu.
699
DOI: 10.1081/NCN-120022613
Copyright # 2003 by Marcel Dekker, Inc.
1525-7770 (Print); 1532-2335 (Online)
www.dekker.com

700
Li and Shaw
This process circumvents the initial nucleoside-kinase dependency step.[1] We pro-
pose to use nucleoside boranophosphate conjugated with tyrosine as a candidate
for such a prodrug.
P-Tyrosinyl(P-O)-50-P-AZT boranophosphate 7 was synthesized according to
the following scheme using a phosphoramidite method. Commercially available
tyrosine 1 was protected with a 4-nitrobenzyl group to give compound 2. Phosphor-
amidite 3 was formed by phosphitylating compound 2 with N,N,N0,N0-tetraisopro-
pylphosphane in the presence of 1H-tetrazole. After the reaction mixture was
stirred for 40 min, AZT and another portion of 1H-tetrazole were added simulta-
neously to the mixture. The coupling reaction was completed in 15 min as monitored
by 31P NMR, in which the singlet at 148 ppm for phosphoramidite 3 disappeared and
two new singlets, corresponding to the diastereomers of phosphotriester 4, appeared
around 135 ppm. Phosphotriester 4 was then boronated with excess dimethyl sulfide-
borane to yield AZT boranotriester 5, which showed a characteristic broad peak cen-
tered at 116 ppm.[2] Compound 5 was then treated with concentrated NH4OH,
hydrolyzing the cyanoethyl and 4-nitrobenzyl groups to yield boranophosphate 6.
Further treatment of 6 with trifluoroacetic acid in acetonitrile afforded the title com-
pound P-tyrosinyl(P-O)-50-P-AZT boranophosphate 7. The boron-modified P-dia-
stereomers of 7 were separated in a 1:1 ratio by RP-HPLC on a C18 column
eluted with a 0.01 M solution of triethylammonium bicarbonate (TEAB) containing
15 to 25% acetonitrile; 7a (19.0 min) and 7b (19.8 min) were the faster and slower
eluting diastereomers, respectively.

AZT Boranophosphate Conjugated with Tyrosine
701
In summary, we synthesized the first nucleoside boranophosphate conjugated
with tyrosine by a one-pot reaction in good yield (from 2 to 7, 42% yield). Introduc-
tion of a BH3 group in such a conjugate is expected to confer on the conjugate good
substrate properties[3] and to increase lipophilicity[3] and stability toward drug
resistance,[4] relative to the unboronated prodrug.
ACKNOWLEDGMENT
This research was supported by NIH grant RO1-AI-52061 to B.R.S.
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