Diquinane enone

The first total synthesis of kelsoene was achieved by Mehta and Srinivas [7, 8]. The tricyclic scaffold was established by an intermolecular [2+2]-photocycloaddition of diquinane enone rac-6 and 1,2-dichloroethylene (7) as the key step (Scheme 2). As a consequence of the steric hindrance implemented... 

In a succeeding publication, the same authors reported on an enantiose-lective approach to diquinane enones 6 and ent-6 by combining the above-described synthesis with an enzymatic kinetic resolution (Scheme 4) [12]. After lipase-catalyzed enantioselective transesterification of diol rac-12. 

The approach of Schulz et al. to enantiomerically pure diquinane enone ent-6 employed (i )-(+)-pulegone (17) as chiral pool starting material [9, 10] (Scheme 5). Bromination and Favorskii rearrangement of 17 generated a... 

Racemic diquinane enone rac-6 was prepared by Piers and Orellana starting from cyclopentenone (Scheme 6) [11]. After the preparation of the heterocuprate from stannane 20, conjugate addition to cyclopentenone in the presence of BF3 Et20 provided carbonyl compound 21. It was expected that conversion of 21 by intramolecular alkylation and subsequent hydrogenation should provide the desired endo-substituted diquinane rac-13. While other hydrogenation methods proved to be rather unselective, reduction in the presence of Wilkinson s catalyst finally resulted in the formation of rac-13 with good facial diastereoselectivity [11]. 

In a study which was conducted simultaneously to the work in the Mehta group and which also aimed to prove the absolute configuration of natural kelsoene (1), Schulz et al. used a stereoselective approach starting from the enantiomerically pure chiral pool material (i )-pulegone 17 [9, 10] (see above). The final steps of their synthesis of the unnatural enantiomer of kelsoene (ent-l) were similar to the above-described first total synthesis of natural kelsoene (1) (Scheme 8). Taking into account the steric limitations of the system as communicated by Srinivas and Mehta, diquinane enone ent-6... 

Lam [77] has described highly enantioselective Cu(I)-catalyzed borylative aldol cyclizations of enone diones 237 via a chair-like transition state 238, which resulted in densely functionalized decalin-, hydrindane-, and diquinane-based products 239 containing four contiguous stereocenters, two of which are quaternary (Scheme 11.51). 

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