An enantioselective synthesis of the maoecrystal V (1) carbon skeleton is described. The key transformations include arylation of a 1,3-dicarbonyl compound with a triarylbismuth(V)dichloride species, oxidative dearomatization of a phenol, and a subsequent intramolecular Diels-Alder reaction.Maoecrystal V (1, Figure 1) was isolated and characterized in 2004 by Sun and coworkers from the Chinese medicinal herb Isodon Eriocalyx. 1 It possesses an unusual architecture rendering it the most modified naturally occurring ent-kauranoid isolated to date. 2 The structure was confirmed by X-ray crystallographic analysis to contain five highly congested rings and seven stereocenters, two of which are adjacent all carbon quaternary centers located in its interior. One of these all carbon quaternary centers is also the bridgehead carbon of the [2,2,2]-bicyclooctanone moiety. This letter reports an early approach to 1 pursued in our laboratory from

Enantiomerically pure reagents for allyl additions were prepared by [3,3] sigmatropic rearrangements (see scheme) followed by a separation of the diastereoisomers. The highly stable allylboronic esters with a stereogenic center α to a boron atom were used to synthesize Z‐configured homoallylic alcohols with an enantiomeric excess >94 %.

Allylboronates are highly attractive reagents for allyl additions. Enantiomerically pure, stable reagents with a stereogenic centre in alpha-position to boron are especially versatile, albeit often difficult to synthesize. Starting from boron-containing allyl alcohols 6 and 7, which are discussed in detail herein, a set of reagents were obtained via [3,3]-sigmatropic rearrangements and consecutive transformations in the side chain. The configurations could be established first by chemical correlation, but also by X-ray crystallography (16, 18, 34, and 39). Allyl additions were performed resulting in the formation of predominantly (Z)-configured homoallylic alcohols (31, 43-45) with high enantiomeric excess. Detailed investigations on the matched-mismatched interaction between the reagents 15/16 (and ent-15/ent-16, respectively) and isopropylidene glyceraldehyde 42d are presented.

Starting from commercially available 1‐substituted propargylic alcohols 6, 18 and ent‐18, stable enantio‐ and diastereomerically pure 1,3‐disubstituted allylboronic esters 10, 19 and 20 were synthesized. The key step − a Johnson rearrangement − was highly diastereoselective and yielded the products with almost complete transfer of the stereogenic information from the intermediate allyl alcohols. The configurations of all the new derivatives were unequivocally assigned either by chemical correlation (e.g. allylboronic ester 10 and its diastereomer 11 were independently synthesized by cross‐metathesis of the known boronates 2a and 3a with styrene) or by analysis of the NMR data, and additionally by X‐ray structural analysis (of 20). Allyl addition to benzaldehyde in all cases yielded enantiomerically highly enriched homoallylic alcohols 15/ent‐15 and 21/ent‐21 (> 98% ee). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)