Showing papers in "Journal of Organic Chemistry in 1980"

Synthetic studies toward verrucarol. 2. Synthesis of the AB ring system

Abstract: A synthetic route to the AB ring system of verrucarol is described. After two routes employing intramolecular cyclization failed, the Diels-Alder reaction of methyl coumalate and isoprene afforded bicyclic lactone 13. Transformation of 13 into hydroxy lactone 16 involved cuprate addition, hydroxylation, and oxidation. The conversion of 16 into desired keto alcohol 14 was accomplished by enol ether formation, reduction of the lactone and ester, and hydrolysis of the enol ether. The successful nine-step sequence proceeded in an overall yield of 7.9% from methyl coumalate.

Epoxidation of alkenes by dioxirane intermediates generated in the reaction of potassium caroate with ketones

Abstract: Kinetics and 180-labeling experiments have provided evidence for the involvment of dioxirane intermediates 2a (Scheme I) in the ketone-catalyzed decomposition of potassium peroxomonosulfate (KHS05, hereafter called caroate) .1-3 It has been shown that, under the reaction conditions (in water, pH 7.5, 2-10 “C), the side reaction involving Bayer-Villiger (BV) oxidation of the ketone (path iii) is negligible with most ketones (acetone, dialkyl ketones, acetophenones, etc.), whereas it becomes significant a t lower pH values or with ketones having certain structural features (e.g., cyclobutanone, cy~lopentanone).l-~ In competition with path i, amounting to ketone catalysis of caroate decomposition, the dioxirane intermediate is capable of oxidizing nucleophilic organic and inorganic substrates (S:, path ii).1-3 As an example, oxidation of phenylpropiolic acid, PhC=CC02H, a substrate representative of weakly nucleophilic alkynes, could be achieved by the caroate--ketone s y ~ t e m ; ~ ? ~ this is indicative of the high reactivity of intermediate 2a, as the mentioned alkyne resisted all attempts a t oxidation by peroxoacids (as well as by caroate in absence of ketones) under the given conditions. Similarly, trans-cinammic acid (3a) failed to react with alkaline hydrogen peroxide, m-chloroperoxobenzoic acid (MCPBA) in CHzClz or with caroate alone;3 however, as we have already reported, reaction of 3a with caroateacetone in water affords the trans-epoxide 3b in high yield. As the cis-epoxide 4b could also be obtained in high yield upon reaction of cis-cinammic acid (4a) with caroateacetone, the reaction appears to be highly stereospecific.2 We report here further data which illustrate the versatility of the caroate-acetone system as a generalized epoxidation method. These are shown in Table I. The synthesis of 4,5-epoxy-2-hexenoic acid (5b) in high yield, as the sole reaction product from sorbic acid (5a), shows that epoxidation via dioxirane can be remarkably regioselective. Notice that the sorbic acid 4,5 double bond, which is more nucleophilic, is selectively oxidized. Treatment of 5-hexen-2-one (6a) with caroate and acetone also affords the corresponding epoxide in high yield. It is not surprising, in this case, that caroate in the absence of acetone can also convert the alkene into the epoxide in high yield (although substrate conversion is lower). In fact,

Convenient and inexpensive procedure for oxidation of secondary alcohols to ketones

Abstract: ether became symnietrical in the presence of 5 equiv of 18-crown-6. Comparison of the chemical shift data for the anions of 1 and 2 in the absence and presence of 5 equiv of 18crown-6 reveals a change of 257 Hz for 1 and 144 Hz for 2. The smaller chemical shift change observed for 2 is again consistent with a lesser degree of ion pairing when the counterion is hexafluorophosphate. Central to this argument is the formulation of the crown ether-diazonium salt complex 5 as resulting from insertion of the neck of the arenediazonium cation into a crown ether ~ o l l a r . ~ J ~ J ~ PrevioiJs evidence for this arrangement includes steric effects of substituents in sol~bilization~~'~ and titration cal~rimetric '~ studies and 'H NMR spectral changes of certain macrocyclic polyethers in the presence of arenediazonium salt^.^,'^ Because of insufficient solubility of uncomplexed 2 it was not possible to probe for changes in the 'H NMR spectra of 1 and 2 in 172-dichloroethane upon addition of 18-crown-6. However, such measurements could be made in dimethyl sulfoxide and are summarized in Table V. Although the chemical shifts of the tert-butyl group hydrogens and the 3,5 aromatic ring hydrogens of 1 and 2 are unaffected by the presence of 18-crown-6, the 2,6 aromatic ring hydrogens are shifted upfield by 7-8 Hz in the presence of 18-crown-6. This chemical shift change for only the ortho hydrogens provides further evidence for the proposed diazonium ion-crown ether complex 5.