No abstract

The first synthesis of a highly functionalized B-homo-C-nor grayanotoxin closely related to kalmanol is reported. An enantiocontrolled route to the diquinane sector was first developed from (4R)-(+)-tert-butyldimethylsiloxycyclopentenone by taking advantage of the Michael acceptor properties of this enone and an α,β-unsaturated ester subsequently derived from it, viz., 4 → 7 → 8. These experiments formed the basis for more advanced substitution of the bicyclo[3.3.0]octane core. In fact, ready access was gained to the α-hydroxy esters 24−27. In these advanced intermediates, it is imperative that the acetyl and carbomethoxy groups bear a trans 1,3-relationship. The neighboring OR substituent should preferably be larger than methoxy in order to guarantee 100% facial selectivity during the ensuing capture by 1 (as its lithiated derivative). This condensation leads unidirectionally to tricyclic lactones represented by 30 and 31 and sets the stage for implementation of sequential Tebbe olefination and Claisen rearrangement. This pivotal two-step process gives rise directly to the targeted tetracyclic framework. The further oxygenation of 35 can be accomplished in a highly stereoselective manner to give 3. The characteristics of the [3.3] sigmatropic event that results in ring expansion plays a significant role in defining absolute configuration at key carbon centers which would otherwise be difficult to establish unequivocally. A total of 25 synthetic steps was involved.

The two antipodes of l-bromo-3,3-dimethyl-4-[(tert-butyldimethylsilyl)oxy]cyclopentene, the dextrorotatory form of which ( 1) is regarded as a potential synthetic precursor to kalmanol, have been prepared in a state of high enantiomeric purity from propargyl alcohol. The key steps in the abbreviated synthetic pathway involve the bromination-dehydrobromination of aldehyde 12 to give 7, the conversion of alcohol 13 to the hydroxyl-substituted bromocyclopentene 18 by a novel tandem Claisen-Sakurai reaction sequence, and efficient enzymatic resolution of 18 via its chloroacetate ester. The absolute configurational assignments are based on 1H NMR analyses of the (R)-and (S)-MTPA esters of (-)-20.

No abstract

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.