This is some cool cycloaddition chemistry, specifically, the [4+3] cycloaddition of oxyallyl cation attaching to a chiral auxiliary to induce stereoselectivity. The 4-pi diene partner being studied is a pyrrole derivative, which is generally more difficult to undergo cycloaddition as the resulting cycloadduct tends to undergo retro-cycloaddition to restore its aromaticity.
The cycloaddition is performed in a one-pot fashion by a regioselective epoxidation of a chiral allenamide using DMDO (syringe pump addition), followed by addition of pyrrole. The reaction is conducted at -45 degrees C as this is believed to be optimal for epoxidation.
One of the synthetic cycloadduct (13), which was obtained as a single diastereomer in excellent yield (93%; d.r. = 95:5) using chiral allenamide derived from Seebach’s auxiliary was further used in subsequent transformations and has been shown susceptible as an approach in the synthesis of the natural product parvineostemonine (see scheme below, taken from their paper).
The key steps include their ability to selectively allylate an alpha carbon of the cyclic ketone using LHMDS in THF and HMPA, followed by RCM using Grubbs’ first generation catalyst. Another synthetic tactic worth mentioning is that when the deprotection of the Boc group was performed on compound 13 under acidic conditions, retro-Mannich type reaction resulted (see below, taken from their paper). But when either the ketone was reduced or in the case of the above scheme the C=C double bond was hydrogenated before the deprotection, the Boc removal proceeded uneventfully.
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