Abstract
The aza-Diels–Alder cycloaddition of 1,2,4-triazines with alkynes offers a rapid and convenient method for the synthesis of highly substituted pyridines, but often requires harsh conditions and long reaction times. The present study offers a solution to these limitations by use of a temporary tether established by a Lewis acid–base complexation of in situ generated alkynylboranes and triazines bearing a Lewis basic donor. The cycloaddition reactions take place within 20 min at 40 °C and provide direct access to a broad range of pyridines with complete and predictable regiocontrol. The carbon—boron bond can be further functionalised by cross-coupling allowing further functionality to be introduced after cycloaddition.
Highlights
Pyridines are a fundamentally important class of aromatic molecules.[1]
The inverse electron demand aza-Diels–Alder reaction of triazines constitutes a useful and much studied method, this process has largely focused on the use of enamine dienophiles as alkyne surrogates because alkynes themselves only participate in [4 + 2] cycloadditions with triazines under very harsh conditions
We found that triazine 1 a was reluctant to undergo efficient reaction with phenylacetylene, providing the corresponding product in low yield after prolonged heating, albeit with high regiocontrol.[6]
Summary
Pyridines are a fundamentally important class of aromatic molecules.[1]. They are present in many bioactive compounds and they play a key role in a number of biological processes. An alternative approach to pyridines is by means of ring synthesis and a number of approaches are established.[2] In this regard, the inverse electron demand aza-Diels–Alder reaction of triazines constitutes a useful and much studied method, this process has largely focused on the use of enamine dienophiles as alkyne surrogates because alkynes themselves only participate in [4 + 2] cycloadditions with triazines under very harsh conditions. Such processes are often poorly regioselective and are relatively low yielding.[3]
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