Organoytterbium ate complexes extend the value of cyclobutenediones as isoprene equivalents

Abstract

3-Methyl-4-alkoxycyclobuten-1,2-diones have long been recognised as valuable isoprene equivalents. Being readily introduced to a substrate as the electrophilic component in an organolithium addition reaction, the thermochemical rearrangement of the resulting adducts give access to quinones, benzoquinones and a host of heteroaromatic ring systems. Importantly, the differential reactivity of the carbonyl groups in 3-methyl-4-alkoxycyclobuten- 1,2-diones provides a reliable means of introducing the isoprene unit in a regiocontrolled manner. However, this feature also imposes a severe limitation, as it allows access to just one regioisomeric series of the products. Herein we detail a simple solution to this longstanding problem that makes use of a hitherto unknown facet of organoytterbium reactivity. In essence, while organolithium reagents favour addition to the C-1 carbonyl of 3-methyl-4 methoxycyclobuten-1,2-dione, the corresponding organoytterbium reagents give near exclusive addition to the C-2 carbonyl of 3-methyl-4-tbutoxycyclobuten- 1,2-dione. The dichotomy extends to a broad spectrum of organolithium and ytterbium reagents (Me, 1°-, 2°- and 3° alkyl, vinyl, aryl, hetaryl and alkynyl), providing access to both regioisomeric series of quinones and benzoquinones without recourse to a cumbersome protecting group strategy. The method has been exemplified with the first total synthesis of the quinone-based natural product (–) mansonone B.

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ORCID for David C. Harrowven: orcid.org/0000-0001-6730-3573

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