CONFORMATION OF TERPENES. II. PINOCAMPHEOLS AND PINOCARVEOLS¹

Abstract

Pinocampheol (I) should exist as four stereoisomeric dZ-pairs by virtue of its four centers of asymmetry two of which are connected by a methylene bridge. All these isomers are known and Schmidt (1) has derived their configuration partly on the basis of Auwers-Skita rules. Since these rules have been shown to be without validity in the case of several 1,3-disubstituted cyclohexane derivatives(2), it seems desirable to examine the stereochemistry of pinocampheols on the basis of the concept of axial and equatorial bonds. This concept has been successfully applied to the study of the stereochemistry of several types of compounds (3-5) including terpenes like menthols (3) and carvomenthols (6).

The Preparation and Stereochemistry of Pinane Derivatives

“…The methods of conformational analysis applicable to unstrained cyclohexanes (21) have been widely used to assign the relative configurations discussed in sections IV to IX, and such extrapolation to bicyclics usually seems to be justified (27) in terms of pseudo-orientations based on the skeletons XI and XII. The following well-established principles will be applied without, comment to specific structures and reactions in subsequent sections.…”

“…Reduction of (+)+pinene and (+)-cis-and (-)-trans-& pinenes over nickel or platinum oxide largely eliminated rearrangement and led to the isolation of (+)-XVIIb, firmed by conformational arguments based on the interconversions of pinanes, &pinenes, and pinocampheols. It was pointed out that XVIIb would be the more stable, as it could adopt a conformation with an equatorial methyl group (27,124). These preparations are probably impure, as recent glc analysis of products from similar reductions indicate up to 17% of the minor epimer (15), and careful fractionation of a product mixture from (-)-@-pinene gave (-)-XVIIa, [ a ]~ (calcd) -24.2" (58).…”

“…Pinanes substituted at Little is known about the preferred conformations, C-2 or C-10 are referred to these parents: thus III is although these have an important influence on reac- tivities. The skeleton was originally considered to be a bridged chair (27), but this must be an oversimplification, for, while effectively staggering the hydrogen atoms and substituents on C-2, C-3, and C-4, this geometry would eclipse the hydrogen atoms on C-2 and C-4 with those on the bridgehead carbon atoms and cause interactions of the "bowsprit-flagpole" type (a bridged chair is also a bridged boat). Both these latter interactions could be relieved by flattening part of the six-membered ring to give a Y-shaped molecule, shown in Newman projection in X, but this would in turn completely eclipse hydrogens and substituents on C-2, C-3, and C-4.…”

“…The chemistry of these compounds has been reviewed up to 1952 (97,133), and more recently certain stereochemical aspects have been clarified (27,28); but despite extensive study the details of the structure of certain compounds are in dispute and much of the recorded data are inconclusive or inconsistent. In work carried out before the last decade, gas-liquid chromatography (glc) was unavailable for isolation of pure samples and for product analysis; physical methods were rarely used for the elucidation of structure; and the methods and rules now systematized as conformational analysis (53) were seldom applied to elucidate stereochemistry.…”

DÉRIVÉS DU TRIMÉTHYL-1,8,8 BICYCLO[3,2,1]OCTANE: III. SYNTHÈSE DES TRIMÉTHYL-1,8,8 BICYCLO[3,2,1]OCTANOLS-2α ET -2β ET DES TRIMÉTHYL-5,8,8 BICYCLO[3,2,1]OCTANOLS-2α ET -2β

“…Les hydroxyles des cleux alcools au soclium seraient ainsi Cquatoriaux e t leurs configurations a. Les alcools au platiile seraient par consCquent les alcools axiaux, de configuration P , ce qui est conforme A ce que 1'011 sait de I'hydrogCnation catalytique qui conduit, dans des conditions expkrimentales ne favorisant pas une isombrisation, A la fixation de l'hydrog&ne sur le c6tC le plus dCgag6 de la rnolCcule: gCnCralement le groupe-OH est axial. Pour de nombreuses rkfbrences, voir(7,9,10,11,12). Can.…”