(±)-Allocyathin B2 (2) and (+)-erinacine A (4), the 1-β-d-xyloside of (+)-allocyathin B2, the first
cyathin diterpenes to be prepared, have been synthesized using a carbonyl ene reaction of 14a to
construct an appropriately functionalized seven-membered ring and palladium-catalyzed carbonylation of dienyl triflates 10 and 39 as key steps. The entire cyathin carbon skeleton is constructed
in seven steps, and allocyathin B2 is synthesized in only 17 steps (>5% overall yield) from readily
available enone 9.
.2-Cyclohexenone rings that form part of a larger structural assembly are amenable to peracid oxidation with formation of an epoxy lactone. These intermediates are readily transformed under acidic, basic, or neutral conditions to ringcontracted aldehydo lactones, which are then subjected to condensation with a slight excess of Tebbe reagent. These conditions result in methylenation of carbonyl groups and set the stage for operation of a Claisen rearrangement. When the latter is catalyzed by Tribal, the sigmatropic process occurs at room temperature. With systems typified by 17 and 24, the isomerization is complete within 15 min. The presence of a proximate angular methyl group as in 9b, 29, and 37 exerts a retarding kinetic effect. In such examples, a period of 6 h is required to achieve completion. Swem oxidation completes the conversion to the 4-cyclooctenones, where the two carbons stemming from the Tebbe reagent are inserted between the original carbonyl and a-olefinic carbons. The overall process is tolerant of ether functionality and additional sites of unsaturation. olefination (3) of the related lactones obtained, in turn, by means of various highly stereoselective pathways. The fundamental issue of stereochemical transmission during the [3,3] sigmatropic event in these systems is customarily controlled by thermodynamic factors, chair-chair transition states related to 1 being adopted to the exclusion of boat-chair options (e.g., 2) to skirt production of unstable tratzs-4-cyclooctenone products ( 1, 4).The present study demonstrates the workability of a new cyclohexenone to give epoxy lactone 3, with ensuing isovariant of this ring expansion strategy within the context of merization to aldehydo lactone 4 (Scheme 1). Conversions a synthetically useful two-carbon insertion ( 5 , preliminary of this type are, in fact, known (6) and have been effected communication). We envisioned that a short three-stage sein a single reaction vessel. As will be discussed, efficiency quence could be implemented to considerable advantage. The is improved if 3 is first isolated. Subsequent condensation first step necessitates exhaustive peracid oxidation of a 2-4 [bis(cyclopentadieny~)tita~iu~]-(CL-chl~~~ methylidene)dimethylaluminum (3a, 3b) was viewed as a '~u t h o r to whom correspondence may be addressed.ready means of performing dual olefination at both carCan. J. Chem. Downloaded from www.nrcresearchpress.com by 34.214.212.150 on 05/11/18For personal use only.
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