Tin-Mediated Free-Radical Cyclization of β-Allenylbenzoyloximes

Abstract

A set of allene‐tethered benzoyloximes (5) has been treated with nBu3SnH. Depending on their substitution pattern, a wide range of compounds has been obtained. If the stannyl radical adds on the allene, the C‐centred radical thus formed undergoes either a 5‐exo ring closure to give the cyclopentene derivatives 7 or a 6‐endo ring closure onto the N atom to give the dihydropyridines 8. If the stannyl radical adds on the benzoyl moiety, an iminyl radical is formed which leads to the 3H‐pyrroles 9 and the alkylidene‐pyrrolines 10. Steric effects as well as polar effects are the factors governing the reaction course.

3H-Pyrroles, Alkylidene-Pyrrolines and Functionalized Pyrrolidines by Radical Cyclization of β-Allenyliminyl Radicals

“…Products 5j؊l and 6j؊l have already been described in our preceding paper. [2] Compound 4i gave a mixture of 5i and 6i. General Procedure for the Preparation of Amides 10a؊h by Reduction and Benzoylation from 5a؊h and 6a؊h: Bu 3 SnH (1.2 mmol) and AIBN (0.2 mmol) were added to a 0.02  cyclohexane solution (50 mL) of the compounds 4a؊h (1 mmol).…”

“…The behaviour of these allene-tethered dithiosemicarbazides towards the tributyltin radical contrasts with that of the allenylbenzoyloximes which we had early studied. [2] …”

“…[1] Recently, we tried to obtain nitrogen heterocycles from the tin hydride-mediated free radical cyclization of allene-tethered benzoyloximes. As already reported, [2] this pathway allowed us to produce highly unsaturated nitrogen heterocycles in fairly good yield but only with strongly hindered allenic precursors. In fact, in this case, six-membered heterocycles and five-membered heterocycles have been obtained as a mixture.…”

Recent developments in the chemistry of dihydropyridines

“…Depending on the substitution pattern of the starting α-allenylbenzoyloximes 52, products from a 5-exo ring closure are also formed (Scheme 16). 88 An aryl radical underwent an intramolecular [1,7] addition in a 3aminosulfonylpyridine to give rise to the 1,4-dihydropyridine, and its oxidized analogue. 89 Fischer alkynylcarbene complexes react with conjugated imines 90 or with 4-amino-1-azadienes 91 to form 1,4-and 3,4dihydropyridines respectively.…”

Mechanistic Aspects and Synthetic Applications of Radical Additions to Allenes

“…Compared with intramolecular radical cyclizations of allenes induced by carbon-centered radicals, instances of reactions initiated by nitrogen-centered radicals are scarce. Nonetheless, two examples have been reported by Hatem, one of which is discussed in more detail in section . In this report, the chemoselectivity of an in-situ-generated stannyl radical was complicated by attack at both the central allene carbon and the oxime benzoyl group.…”