HSING-JANG LIU, TENG KO NGOOI, and ERIC N. C. BROWNE. Can. J. Chem. 66, 3 143 (1988). Under stannic chloride catalysis, 2-carbomethoxy-2-cyclohexen-I-one (5) was found to undergo Diels-Alder reaction with a variety of dienes to give directly the cis-1-octalone system possessing a functionalized substituent at the angular position. With unsymmetrically substituted dienes, the adducts were those predicted on the basis of the normal rules (orrho and para) governing the orientation of Diels-Alder addition. In cases where differing secondary orbital overlap would affect the stereochemistry of the adducts, the major isomer in each case was that produced by secondary overlap of the diene with the ester carbonyl rather than the ketone carbonyl, presumably due in part to a steric effect. The degree of stereoselectivity was further shown to be dependent upon both the reaction temperature and the diene used and was generally enhanced when the reaction was carried out at lower temperatures or when a bulkier diene was involved.HSING-JANG LIU, TENG KO NGOOI et ERIC N . C. BROWNE. Can. J. Chem. 66, 3143 (1988). On a trouvt que, sous I'influence d'une catalyse par le chlorure stannique, la carbomtthoxy-2 cyclohextne-2 one-1 (5) subit des rtactions de Diels-Alder avec un grand nombre de dienes pour conduire directement au systeme de l'octalone-1-cis posstdant un substituant fonctionnalist en position angulaire. Lorsqu'on effectue la rtaction avec des dienes qui ne sont pas substituts d'une fa~on symttrique, les adduits sont ceux que I'on peut prtvoir en se basant sur les regles normales (ortho et para) rtgissant l'orientation de l'addition de Diels-Alder. Dans les cas oh un recouvrement d'orbitales secondaires difftrent peut affecter la sttrtochimie des adduits, l'isomere principal obtenu est toujours celui provenant du recouvrement secondaire du ditne avec le carbonyle de I'ester plutbt qu'avec le carbonyle de la cttone; ce resultat est vraisemblablement dfi a un effet sttrique. On a dtmontrt que le degrt de sttrtostlectivitt dtpend aussi de la temperature de la rtaction ainsi que du diene utilist; on a remarqut qu'il augmente g~n%ralement lorsqu'on optre des temptraturG plus basses ou lorsqu'un diene plus encombrt est impliqut.[Traduit par la revue]

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HSING-JANG LIU and TENG KO NGOOI. Can. J. Chem. 62, 2676Chem. 62, (1984.The total synthesis of petasitolone (I), a sesquiterpenoid of the eremophilane family, has been accomplished. The key step of this efficient synthesis is the Lewis acid catalyzed Diels-Alder addition of 2-carbomethoxy-2-cyclohexenone (3) to diene 9. The cycloaddition, which gives adduct 16 in good yield, facilitates the rapid construction of the parent ring system of the target molecule and allows an excellent control of the required stereochemistry.HSING-JANG LIu et TENG KO NGOOI. Can. J. Chem. 62, 2676Chem. 62, (1984.On a rialist la synthkse totale de la pitasitolone (I), un sesquiterpkne de la famille de 1'CrCmophilane. L'itape fondamentale de cette synthtse efficace est une addition de Diels-Alder, catalysCe par un acide de Lewis, de la carbomCthoxy-2 cyclohexkne-2 one (3) sur le dikne 9. La cycloaddition, qui conduit ? i I'adduit 16 avec un bon rendement, facilite 1'Cdification rapide du systkme cyclique parent de la molCcule recherchCe et permet un excellent contr6le de la stCrCochimie requise.[Traduit par le journal]We wish to report the first total synthesis, in racemic form, of petasitolone (I), a sesquiterpene of the eremophilane family which has been isolated from Petasites japonicus Maxim (1). The synthesis also demonstrates a new approach to eremophilane sesquiterpenes which are widely distributed in nature (2). These naturally occumng compounds have attracted a great deal of synthetic effort during the past two decades (3). IAlthough a rapid construction of the required decalin framework can conceivably be achieved by an intermolecular Diels-Alder reaction, the difficulty' in controlling the desired cis stereochemistry of the vicinal methyl groups has hampered the efficient use of such an approach to the synthesis of eremophilanes. During a recent study of the dienophilicity of 2-carbomethoxy-2-cyclohexenone (3) (5), it was observed that the Diels-Alder reactions of this enone ester with 1-substituted and 1,3-disubstituted dienes led, in each case, to the preferential formation of the adduct that arose from secondary orbital overlap between the diene and the ester moeity of 3. For example, its reaction with trans-piperylene at -78°C and under stannic chloride catalysis gave a 5 : 3 mixture of adducts 4 and 5. The major adduct 4, which has the all-cis stereochemistry for the three contiguous chiral centers, was formed by endo addition of the diene to the ester group of 3. It was also observed that addition of trans-2-methyl-l,3-pentadiene to enone ester 3 under the same conditions led to a mixture of adducts 6 and 7. In this case, an improved stereoselectivity of 13: 1 in favor of adduct 6 was obtained. The remarkable ability of the ester group to reverse the course of stereoselectivity normally observed for 2-cyclohexenones makes possible the direct control of the eremophilane stereochemistry using a Diels-Alder reaction. This is illustrated with the efficient total synthesis of petasitolone described below.Although the Die...

An unequivocal correlation of keto ester 3 and the adduct obtained from the aluminium chloride catalyzed Diels–Alder reaction of 2-methyl-2-cyclohexenone with isoprene confirms the structure of the adduct to be 2.

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