ChemInform Abstract: Face‐Selective and endo‐Selective Cycloadditions with Enantiomerically Pure Cyclopentadienes.

Abstract

A general synthetic route to the enantiopure bicyclic cyclopentadienes (S)-3 and (R)-3 of the hydrindane series, starting from the Hajos-Wiechert ketone 4 or its enantiomer is described. Reasons for the excellent face and endo selectivity of cycloadditions and the resulting consequences for chiral recognition are discussed.Encouraged by the high p-face and endo selectivity reported for the cycloadditions of the steroid-derived cyclopentadiene l a by Solo ['] and by we prepared the phenyl derivative l b with the intention to perform highly diastereoselective transformations with Diels-Alder adducts to this 47t system, in order to release at a later stage enantiopure building blocks in a thermal retro reaction. Additionally, we could easily predict chiral discrimination['*] resulting in kinetic resolution of racemic dienophiles having their stereogenic center in close neighborhood of the 2n system (see 10).It was with these aims in mind that we replaced the 0-acetate of the Solo-Bull diene by a phenyl ring or an aromatic system (see below), since a threefold task was assigned to this substituent.First of all, to realize regioselectivity of Diels-Alder cycloadditions, it had to take the function of an inert donor substituent, similar to the acetoxy groups, but as hydride reductions as well as organometallic transformations were also planned to be performed with the later cycloadducts a carbonyl group was clearly undesirable. Second, this aromatic ring system was expected to shield functional groups in the cycloadducts to be formed, thus improving the chances for diastereoselective transformations at this stage. Finally, as a third function of the aromatic ring we hoped for a much lower activation barrier for the retro Diels-Alder process since Czarnik and his collaborators reported on a substantial enhancement of rates for retro reactions with phenyl-substituted cycloadd~cts[~-~1, an observation which may find its explanation in recent results communicated by Hall and co-workers[61.We have already reported on the synthesis of enantiopure lbL71. All these expectations were satisfied with this diene and its cycloadducts and an additional observation was made with adducts formed from symmetric dienophiles like maleic anhydride, e.g. 2.Although the transformations with 2 proved of course to be highly diastereoselective we observed with these cyclic anhydrides also a quite interesting regioselectivity. Nucleophilic attack on this moiety took place preferentially at the carbonyl group neighboring the phenyl ring (see arrow!) which is located nearly perpendicular to the steroid ring systernr71. As an explanation for this result we assumed the C-7-sp3 center of ring B to exercise a stronger influence on the carbonyl attack than the phenyl group at C-17.Since with anhydrides of this type regioselectivity in carbony1 transformations is translated directly into enantioselectivity, this observation called for further inspection.The easiest way to check the explanation given above would certainly be the investigation of cycload...

Synthesis, Molecular Structure, and Chiroptical Properties of Dibenzylidene Derivatives of Bicyclo[3.3.1]nonane and Brexane

“…Next, we attempted deoxygenation of enone 2 via ionic reduction using sodium borohydride in combination with trifluoroacetic acid. 39 Reduction of 2 with sodium borohydride-TFA in a mixture of dichloromethane-acetonitrile 40 afforded endo,endo-diol 8 (vide infra) exclusively in high yields (Scheme 2), whereas treatment of 2 (or 8) with sodium borohydride in neat trifluoroacetic acid led to intractable mixture of products. Standard acetylation of diol 8 with acetic anhydride in pyridine furnished corresponding allylic bis(acetate) 9, which was found to be unreactive in Pd (PPh 3 ) 4 catalyzed reduction with sodium cyanoborohydride.…”