No abstract

The homofragmentation of 1,4-diol monosulfonate esters was applied as a key step in the synthesis of α-santalanes. The enantiospecific synthesis of (+)-α-santalan-12-one (1d) was achieved via the tricyclic dione 2 as an intermediate that could be obtained from (R)-(−)-carvone. A 1,2-carbonyl transposition in dione 2 followed by functional group transformations afforded the tricyclic 1,4-diol monomesylate 3, which could be homofragmented in 76% yield to the tricyclic aldehyde 4. From this compound the synthesis of several α-santalanes can be achieved; the synthesis of 1d was accomplished in 11 steps from 2 in 8% overall yield.

The kinetics of the desilylation reactions of a range of sulfonated and methoxylated norbomyl silyl ethers 1-10 were investigated to establish the quantitative correlation between the geometry of the -relay and the rate of desilylation. These desilylation rates generally decrease in the order W > sickle-like > U arrangement of the -relay. The methoxylated silyl ethers show a decreased reactivity in comparison with the corresponding sulfonated silyl ethers. The orientation of the silyloxy group does not affect the reaction rate. Conformational mobility and alkyl substitution of the -relay also are of influence on the rate of desilylation. The observed effects are confirmed by similar behavior in the desilylation reactions of the silyl ethers 11-14 derived from transperhydronaphthalene-l,4-diols.

The manifestation of through-five-bond interactions in the reactions of the rigid 1,5-diol monosulfonate esters (1−3), having an ideal all-trans geometry of the σ-relay, with sodium tert-amylate is investigated. It has been shown that the deprotonation of the alcohol group in 2 and 3, which results in an increased electrofugal ability of this group, finds expression in a seven-center fragmentation. This fragmentation also illustrates that effective through-five-bond interactions exist between the alcoholate group and the carbocationic center which is generated during the heterolysis. The reaction outcome of mesylate 1 does not indicate effective through-bond interactions, and only elimination is observed. This difference can be attributed to the alkyl substituents on the γ- and α-positions to the mesylate group in 2 and 3, respectively, which stimulate the seven-center fragmentation. Though a reasonable amount of fragmentation product is obtained from 3, the through-five-bond interaction is not strong enough to dominate the reaction course completely and typical E1-like processes, i.e., elimination and rearrangement, are competitive. As expected, only a 1,2 Me-shift is observed in the reaction of the axial mesylate 4 where a gauche interaction is present in the geometry of the σ-relay. The presence of through-bond interactions in the reactions of 3 and 4 becomes apparent by comparison of the reactivity of 3 and 4 with their O-silylated analogs 5 and 6.

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