Abstract
New shelf-stable pyridylboronic acids have been synthesized:
bromine-lithium exchange followed by reaction with triisopropylborate
(TIPB) yielded 2-fluoro-5-pyridylboronic acid (4), 3-bromo-5-pyridylboronic
acid (5 ) and 2-ethoxy-5-pyridylboronic acid
(6 ); directed lithiation followed by reaction
with trimethylborate (TMB) or TIPB afforded 2-methoxy-3-pyridylboronic
acid (8), 3-bromo-6-methoxy-4-pyridylboronic acid (11 )
and 3-bromo-6-ethoxy-4-pyridylboronic acid (12 ).
Cross-coupling of pyridylboronic acids 4 , 6 , 8 , and 11 with 3-bromoquinoline [Cs2 CO3 , Pd(PPh3 )2 Cl2 ,
1,4-dioxane, 95 °C] gave pyridinylquinoline derivatives 13 , 15 -17 in 50-77% yields:
the analogous reaction of 5 was low yielding
due to further in situ reactions of the product 14 .
Cross-coupling of 12 with 2-bromo-5-nitrothiophene
gave 3-bromo-4-(5-nitro-2-thienyl)-6-ethoxypyridine (18 ).
Key words
pyridine - boronic acids - cross-coupling - Suzuki reaction - heterobiaryl
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