meso-and dl-Bivalvane (Pentasecododecahedrane). Enantiomer Recognition during Reductive Coupling of Racemic and Chiral2,3-Dihydro-and Hexahydrotriquinacen-2-ones
Abstract: Pinacolic reduction of both optically pure and racemic hexahydrotriquinacen-2-one ( 6 ) proceeds with exo,exo carbon-carbon bond formation. Coupling of enantiomerically pure 6 produces diol 11 exclusively. Formation of comparable amounts of 11 and 12 from ( f ) -6 shows that the diastereomeric transition states involved are of comparable energy. Studies with chiral and racemic 2,3-dihydrotriquinacen-2-one (13) gave analogous results. The various 1,2-glycols are identified by their I3C N M R (symmetry is thereby revealed), 'H NMR (shielding of the endo protons at Cj and C3, by hydroxyl is witnessed), and ir spectra, in tandem with the method of synthesis. Conversion of the four diols to their thionocarbonates, and subsequent treatment with triethyl phosphite at the reflux temperature, provides the structurally related olefins stereospecifically. Catalytic hydrogenation of 17 provides dl-bivalvane (2), whereas reduction of 19 gives rise to the meso-bivalvane isomer 3. Other aspects of these transformations and conformational possibilities for 2 and 3 are discussed. The dodecahedron is one of the five perfect solids (the others are the tetrahedron, cube, octahedron, and icosahedron) and consequently has been a source of fascination for mathematicians since the time of Pythagoras and Plato. Only recently, however, has the dodecahedrane molecule attracted the attention of synthetic chemists. Notwithstanding, this (CH)2o polyhedron, which is endowed with most intriguing geometry, remains an unknown Although several approaches to dodecahedrane are currently under investigation in these laboratories, attention is focused specifically herein on the fundamental aspects of that scheme which involves effective "dimerization" of two triquinacene halves. The essence of,this concept is not new, it having been advanced earlier in singular form by Woodward, Fukunaga, and Kelly2 who viewed dodecahedrane as composed of two triquinacene subunits which when properly arranged might be coaxed into sixfold carbon-carbon bond formation as illustrated in 1. That efforts along this . 1 w line have consistently failed is not surprising, for the requirement of a highly ordered transition state which can maintain strict stereochemical control simultaneously (or nearly so) a t 12 trigonal carbon atoms is certain to be overwhelmingly endothermic. Moreover, the added restriction that all six bonds must form from the heavily encumbered endo direction at both termini will surely be most prohibitive. The further need to impinge any given T bond upon lobes of two different noninteracting' double bonds constitutes a process for which chemical analogy is lacking if concerted or metal catalyzedS8 Despite the disadvantages intrinsic to the above model, it is apparent that initial endo,endo coupling of two monomeric triquinacene units at C2 could go far in setting the stage for the ultimate construction of the desired polyhedron. Such a synthetic plan requires that there exist a reaction sequence which can lead readily to 2 as well as to more highly functionalized derivatives of this hydrocarbon. As we have noted earlier,5 a problem immediately presents itself, for the dissymmetry of the necessarily derivatized triquinacene precursors is such that a pair of coupling products having the structural features of 2 and 3 can arise if the monomer 2 3 --is a racemic mixture. Only when bonding between chiral triquinacenes results can it be guaranteed that unwanted formation of "dimer" 3 will be precluded. These conditions presuppose that reductive coupling of racemic triquinacenes will produce diastereomeric products with comparable facility. However, no information is available with which to formulate a reliable prediction. On a promising note, Touboul and Dana have recently reported that electrolytic reduction of racemic 4 proceeds in rigorously enantio-selective fashion with formation of cis-threocis diol 5 to the exclusion of the cis-erythro-cis and other w possible i~o m e r s .~ For the present study where 2,3-dihydrotriquinacen-2-one and its tetrahydro derivative serve as pivotal compounds, the prevailing steric factors are recognized to be generally inimical to bonding from the endo direction.I0 Because adherence to an exo,exo bonding scheme would necessitate ultimate inversion of configuration at C2 in both triquinacene segments, the synthetic methodology of choice should be one which is capable of achieving this transform readily. With these considerations in mind, convenient preparations of 2,3, and several related unsaturated congeners have presently been realized. In structural terms, 2 is recognized to be one of a number of possible pentasecododecahedranes
