A very expedient and efficient new route toward taiwaniaquinoids, bearing the 4a-methyltetrahydrofluorene skeleton, is reported. Key steps are the intramolecular Friedel-Crafts alkylation of an aryldiene and the degradative oxidation of a methylenedioxy group; the latter process could also be utilized for building the 2-hydroxy-1,4-benzoquinone unit, which is frequently found in natural products. Utilizing this new methodology, (+/-)-dichroanone (7) (three steps, 77% overall yield) and (+/-)-taiwaniaquinone H (6) (four steps, 70% overall yield) have been synthesized from commercial alpha- (11a) or beta-cyclocitral (11b).

Raney Nickel is an effective reagent to achieve the chemoselective reduction of conjugated olefins in a,b-unsaturated carbonyl compounds that also contain isolated double bonds. Its use is also compatible with a variety of other functional groups.The selective reduction of conjugated olefins in a,b-unsaturated carbonyl compounds, that also contain isolated carbon-carbon double bonds, constitutes an important challenge in organic synthesis. This can be achieved by treating the carbonyl derivative with triethylsilane, in the presence of catalytic amounts of (tris(triphenylphosphine)rhodium chloride; 2a was converted in 2b following this procedure. 1 Some organic hydrides also allow this transformation to be carried out. Thus, L-, K-selectride and lithium triethylborohydride have been used to reduce a,b-unsaturated esters and ketones; however, this reactivity is not general and depends upon the steric hindrance of the alkene and of the ring size in cyclic ketones, b-alkylation usually completely supresses the 1,4-reduction. For example, treatment of 1a with an equivalent of K-selectride afforded 1b in a quantitative yield; 2 2a gave the dienic alcohol as the only product under similar conditions. 2 Enantioselective reduction of a,b-unsaturated carboxylates with sodium borohydride in the presence of cobalt complexes has been reported. 3 Semmelhack et al have demonstrated that copper salts modify the reactivity of lithium and sodium hydridoaluminate derivatives so that the reagents deliver hydride in a 1,4 fashion to alkenones. 4 Other copper hydrides have also been successfully used for this purpose. 5-8 Keinan et al described that a threecomponent system comprised of a soluble palladium catalyst, hydridosilane and zinc chloride is capable of efficient conjugate reduction of a,b-unsaturated carbonyl compounds. 9 A ruthenium complex combined with cyanoborohydride has also been reported for carrying out this transformation. 10 Very recently the selective 1,4-reduction of a,b-unsaturated carbonyl compounds by combined use of bis(1,3-diketonate)cobalt(II) complex and diisobutylaluminium hydride has been described. 11 On the other hand, conventional catalytic hydrogenation, using platinum or palladium, is not suitable to achieve the conjugate reduction because of its low selectivity, procuring in most cases the complete reduction of carbon-carbon double bonds. 12,13 Following the authors' research into the synthesis of valuable compounds from available natural terpenes we have found that Raney Nickel exhibited a good selectivity to carry out the conjugate reduction of a,b-unsaturated carbonyl compounds bearing other isolated carbon-carbon double bonds. In order to establish the scope and synthetic applications of this reaction, the behaviour of Raney Nickel against different a,b-unsaturated carbonyl compounds has been studied and compared with that of palladium over carbon (Table). In most cases, treatment with Raney Nickel in tetrahydrofuran for 20-30 minutes at room temperature afforded the 1,4-reduction compound in...

Raney nickel is an effective reagent to achieve the chemoselective reduction of aldehydes in the presence of ketones, which takes place in high yield. Only highly hindered aldehydes do not undergo reduction.

A new methodology is described for transforming resinic acids into nor-alcohols and nor-olefins, via the Baeyer-Villiger rearrangement of the derived aldehyde. Based on this methodology 4-hydroxy-18-nor-abieta-8,11,13-trien-7-one and 18-nor-abieta-8,11,13-triene-4,7a-diol, two new terpenoids recently described, have been synthesized from abietic acid. Some interesting terpenoids, due to their biological activity, have a functionalized A-ring structure. Representative compounds are quassinoids, such as bruceantin (1), 1 and the antiherpes nor-diterpenes 2 and 3. 2 Over the last few years the isolation of different natural nor-terpenes, e.g. 4-6, 12c and 20, has been reported . 3,4 Following the authors' research into the synthesis of valuable compounds from available natural diterpenes, 5,6 we are now dealing with the synthesis of A-ring functionalized terpenes, such as quassinoids and other nor-terpenes, from terpenic acids by removing the carboxylic group through the corresponding aldehyde.The most common and widely studied reagent for oxidative decarboxylation, lead tetraacetate, usually produces a mixture of unsaturated hydrocarbons with small admixtures of acetates and the corresponding alcohols. 7 The oxidative decarboxylation by hydrogen peroxide and a mercury (II) salt has also been used to transform carboxylic acids into the corresponding nor-derivatives. 8 The Barton decarboxylation method has become a suitable procedure for preparing olefins, nor-hydroperoxides or nor-alcohols from carboxylic acids. 9,11 2 and 3 were synthesized by applying the Barton decarboxylation protocol. 2 Aldehydes undergo Baeyer-Villiger rearrangement to give carboxylic acids due to preferential migration of hydrogen. Alternative migration of the carbon group to yield formates seldom occurs. Aryl aldehydes having electronreleasing groups are the only exceptions and are converted to aryl formates by the Baeyer-Villiger oxidation. At the present time only a few examples of the preferred migration of a carbon moiety in an aliphatic aldehyde have been reported. 12-14 Recently we have found that aldehyde 7a is transformed with an excellent yield into the formate 7b through peroxyacid treatment. 15 In order to elucidate the scope and synthetic applications of this reaction the behaviour of aldehydes 8a-12a having a variety of carbon skeletons, including mono-, di-, tri-and tetracyclic structures, and different additional functions have been studied. They were efficiently prepared with an 65-70% overall yield from carboxylic acid in a two-step sequence involving reduction with lithium aluminium hydride and further oxidation with pyridinium chlorochromate. Treatment of these aldehydes with peroxyacid afforded the corresponding formates 8b-12b as the only product in all cases. These esters might easily be transformed into the derived alcohols or olefins (Scheme 1). The complete sequence constitutes a simple and efficient way for transforming terpenic acids into the corresponding noralcohols and nor-olefins. Scheme 1Time reactions and...