Alkenes oxygen-functional

M-Acyliminium cyclizations of optically active mono- and di-oxygenated hydroxylactam derivatives have been used in the synthesis of a number of natural products. In case of a five-membered lactam the oxygen function adjacent to the iminium carbon directs attack of the internal nucleophile from the least hindered side, opposite to the substituent. In the examples given the size of the newly formed ring is determined by the electronic bias of the alkene substituent. 

Only 3 of the 12 species of Cocculus (Menispermaceae) have been examined for alkaloids and most studies have concerned C. laurifolius, which has yielded the greatest number of alkaloids (see Table I) (57, 55-58, 60, 69-76). The Erythrina-type alkaloids obtained from Cocculus are abnormal in the sense that they contain no oxygen function at C-16, the only exceptions being dihydroerysovine (44), dihydroerysodine (47), and erythroculine (53). Erythroculine is, however, unusual in that it has a methoxycarbonyl group at C-16. The two alkaloids isococculidine (36) and isococculine (37) are of theA2(l)-alkene type rather than the Al(6)-alkene type. 

Among the more interesting reactions involved in making all three of these natural products are the loss of ammonia from phenylalanine to give an alkene and the introduction of extra OH groups around the benzene rings. We know how a para OH of Tyr is introduced directly by the oxidation of prephenic acid before decarboxylation and it is notable that the extra oxygen functionalities appear next to that point. This is a clue to the mechanism of the oxidation. 

Cyclopropanation of Allylic Alcohols. Simmons-Smith type cyclopropanation of the allylic alcohol 22 in the presence of a catalytic amount of the bis-sulfonamide la leads to formation of the corresponding cyclopropane 23 in high yield and selectivity (eq 6, Table 3). The reaction is rapid (< 1 h) and can be performed at low temperature (either 0 °C or —20 °C). Substrate scope encompasses both di- and tri-substimted allylic alcohols (24 and 26). However, substimtion at the 2 position, as in 28, leads to a drastic decrease in selectivity. The presence of additional oxygenated functionality (30) in the proximity of the alkene also lessens selectivity." The method is limited to the cyclopropanation of allylic alcohols. Other alkene-containing substrates, such as allylic ethers, homo-allylic alcohols and allylic carbamates, do not react with high selectivity. 

Appropriate quenching of a reductively formed lithium enolate with a carboxylic acid anhydride, chloride, methyl chloroformate or diethyl phosphorochloridate yields the corresponding enol esters, enol carbonates or enol phosphates. These derivatives may be transformed into specific alkenes via reductive cleavage of the vinyl oxygen function, as illustrated by the example in Scheme 8. 

An aldehyde with a-ether functionality was used in the synthesis of N-acetylneuraminic acid by Danishefsky (equation 37). A series of cis- and trans-alkenes with a-oxygen functionality were synthesized by Cinquinin and coworkers. Of the examples generated, the HWE reaction proceeded in the expected ( )-selective manner (164), while the trifluoroethyl phosphonate was used to form the (Z)-al-kenes (163) selectively (Scheme 26). Interestingly, 18-crown-6 was not used for (Z)-alkene formation. 

The reaction of olefins with lead tetraacetate has not been a useful method in organic synthesis, because reactions such as addition of an oxygen functional group to the double bond, substitution of hydrogen at the allylic position, and C-C bond cleavage can occur to give complex mixtures of products. With some specific alkenes, however, reaction with lead tetraacetate can afford synthetically important compounds cleanly. For instance, reaction of the diacid with 6 equiv. lead tetraacetate in acetonitrile gave the dilactone in excellent yield (Scheme 13.36) [59]. 

Fused ring C-glycosides. Ionization of glycosides in the presence of appropriate alkenes leads to C-C bond formation. When C-2 is protected by an acid- cleav-able group, formation of a tetrahydrofuran ring ensues. Since the alkene attacks from the axial direction, the C-2 oxygen function must be equatorial in order to form the new ring. 

C.i. Epoxides. Epoxides are easily generated from alkenes (sec. 3.4) and also from ketones and aldehydes 8.8.B). They are one of the most useful of all oxygenated functional groups. Reduction of epoxides leads to alcohols. Lithium aluminum hydride delivers hydride to the less sterically... 

Rhodium and ruthenium catalysts may alternatively be used and sometimes show useful selective properties. Rhodium allows hydrogenation of alkenes without concomitant hydrogenolysis of an oxygen function. For example, hydrogenation of the plant toxin, toxol 5 over rhodium-alumina gave the dihydro compound 6 (7.6) with platinum or palladium catalysts, however, extensive hydrogenolysis took place and a mixture of products was formed. 

---