Asymmetric Diels–Alder addition of cyclopentadiene to chiral naphthoquinones

Diels-Alder Cycloaddition of Cyclopentadiene to a Bis-naphthoquinone

“…A two proton multiplet at δ 6.24-6.26 was assigned to the vinylic protons H-2 and H-3, whilst a doublet at δ 4.13 with coupling constant J 9a,1 3.9 Hz was assigned to the bridgehead proton H9a. The coupling constant J 9a,1 3.9 Hz is consistent with formation of the bis-endo adduct (2a) [2]. A second product (1 mg) displayed features consistent with endo-exo adduct (2b) formation, most noticeably three vinylic multiplets were observed which integrated for one, two and one proton(s) respectively.…”

“…1 H nmr analysis of the reaction mixture displayed features consistent with formation of adduct (2). The presence of signals resonating at δ 6.0-6.5 were assigned to the vinylic protons whilst a resonance at δ 4.13 was characteristic of the bridgehead proton assigned to H-9a.…”

“…The presence of signals resonating at δ 6.0-6.5 were assigned to the vinylic protons whilst a resonance at δ 4.13 was characteristic of the bridgehead proton assigned to H-9a. When using monomeric quinones as the dienophile exclusive formation of the endo cycloaddition adducts was observed [2]. However, the possibility of forming several diastereomeric adducts arises when bis-dienophile (1) is used.…”

“…These dienes are of particular interest in that the Diels-Alder adducts can undergo selective fragmentation of the C-4/C-4a bond affording an electrophilic site which can then be trapped by a hydroxyl group to give a cyclopentannulated product (Scheme 1). Development of this Diels-Alder/fragmentation reaction in an asymmetric sense has also been the focus of recent investigations [2] using chiral Lewis acid catalysis. As an extension to our earlier work in this area, bis-naphthoquinone (1) was perceived to be a bisdienophile for a double [4+2] cycloaddition reaction with cyclopentadiene (Scheme 2).…”

Enantioselective synthesis of a cyclopentannulated kalafungin analogue

“…In an earlier study 5 of the diastereoselectivity of the Diels-Alder addition of cyclopentadiene to 1,4-naphthoquinones bearing a chiral auxiliary at C-2, it was established that the pantolactone auxiliary afforded the best diastereoselectivity when using zinc(II) chloride as the Lewis acid.…”