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27.19.1.1.3.4 Variation 4: Reaction of N-Benzylhydrazide with a (Hemi)acetal

DOI: 10.1055/sos-SD-027-00644

Schantl, J. G.Science of Synthesis, (200427753.

One of the key steps in a reported total synthesis of (±)-saxitoxin involves the intramolecular [3+2] cycloaddition of the in situ generated azomethine imine functionality to the C=C bond of an imidazolin-2-one moiety. The reaction of the hydrazide group in 85 with benzaldehyde fails to form the azomethine imine 86, but succeeds in giving 87 (R1=Ph) with aromatic and heteroaromatic aldehyde diethyl acetals in dimethylformamide containing a catalytic amount of 4-toluenesulfonic acid.[‌24‌] Likewise, 88 (R1=CO2Me) is formed with the methyl hemiacetal of methyl glyoxalate in the presence of boron trifluoridediethyl ether complex[‌25‌] (Scheme 21).

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