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I am working towards a natural product. Presented are three intermediate compounds. After oxidation of the hydroxyl to a carbonyl, I want to introduce an imino-nitrogen atom into the structure. This way, an imino bond creates a five-membered ring, and the double bond disappears.

What reagent(s) should I choose for this type of reaction, and what does the reaction mechanism look like?

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    $\begingroup$ This is a tricky. You can't go with something like AcONH4/AcOH because you have other centres that can react, including a second enone (though presumably less reactive due to hindrance). I would be inclined to consider a two step process with 1eq of the anion of trifluoroacetamide or t-butylcarbamate followed by a deprotection to unmask the amine. $\endgroup$
    – Waylander
    Apr 2, 2020 at 18:23
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    $\begingroup$ You may find the following paper interesting: Reduction of Oximes and Aliphatic Nitro Compounds to Imines for further in situ Reactions: a Novel Synthesis of Pyrroles and Pyrrolin-2-ones. Barton, D. H. R.; Motherwell, W. B.; Simon, E. S.; Zard, S. Z. J. Chem. Soc., Perkin Trans. I 1986, 2243. $\endgroup$
    – user91807
    Apr 7, 2020 at 6:28

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I suggest two possible approaches. Narasaka, et al. have effected similar cyclizations photochemically. I prefer to start with the allylic alcohol 1 to avoid complications with oxime formation. Formation of the bis acetate 2b is not a problem. However, given the greater acidity of oximes relative to alcohols does not preclude the formation of oxime acetate 2a. In the photolysis the naphthalene serves as a sensitizer while 1,4-cyclohexadiene functions as a hydrogen atom donor. The addition of hydrogen is likely from the convex face as shown. If not, after saponification of acetate 3b and Swern oxidation, base equilibration would likely lead to ketone 4.

Alternatively, a Barton-type radical cyclization (5 --> 3a) might be considered.

ADDENDUM: Direct photolysis of a structure similar to 5 without Bu3SnH has been achieved. See ref 2.

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1) M. Kitamura, Y. Mori and K Narasaka, Tetrahedron Letters, 2005, 46, 2373.
2) F. Gagosz and S. Z. Zard, Synlett., 1999, 1978.

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