Epoxide endocyclic

As a further application of the reaction, the conversion of an endocyclic double bond to an c.xo-methylene is possible[382]. The epoxidation of an cWo-alkene followed by diethylaluminum amide-mediated isomerization affords the allylic alcohol 583 with an exo double bond[383]. The hydroxy group is eliminated selectively by Pd-catalyzed hydrogenolysis after converting it into allylic formate, yielding the c.ro-methylene compound 584. The conversion of carvone (585) into l,3-disiloxy-4-methylenecyclohexane (586) is an example[382]. 

Trimethylsilyl alkyl and aryl sulfides react with allyl carbonates and vinyl epoxides to deliver the alkyl or aryl sulfide to the allyl unit. These species show typical regioselectivities by (i) adding to the less substituted end of alkyl substituted allyls (equation 290) (ii) adding to the allyl terminus more distant from remote oxygen functionality (equation 291) and (iii) showing substantial endocyclic addition to methylenecyclohexane-derived allyls (equation 292).223... 

These compounds have a similar steric arrangement of the oxolane ring as do ordinary derivatives of furanoses. Exocyclic oxirane-ring closure proceeds readily as compared to endocyclic epoxides because the required trans-orientation of the reacting groups is adopted without significant steric hindrance. 

In the metal-free epoxidation of enones and enoates, practically useful yields and enantioselectivity have been achieved by using catalysts based on chiral electrophilic ketones, peptides, and chiral phase-transfer agents. (E)-configured acyclic enones are comparatively easy substrates that can be converted to enantiomeri-cally highly enriched epoxides by all three methods. Currently, chiral ketones/ dioxiranes constitute the only catalyst system that enables asymmetric and metal-free epoxidation of (E)-enoates. There seems to be no metal-free method for efficient asymmetric epoxidation of achiral (Z)-enones. Exocyclic (E)-enones have been epoxidized with excellent ee using either phase-transfer catalysis or polyamino acids. In contrast, generation of enantiopure epoxides from normal endocyclic... 

Epoxidation of exocyclic enol lactones. Peracids, even under buffered conditions, are not useful for this epoxidation because of rearrangement and decomposition. Dimethyldioxirane effects epoxidation of y-methylene-y-butyrolactones (1) in 94-96% yield in 2-3.5 hours. It is also effective for epoxidation of endocyclic enol lactones such as 3. 

Prepared by addition of 4-pentenyl magnesium bromide to cyclopentanone, dehydration of the alcohol to l-(4-pentenyl)cyclopentene, epoxidation of the endocyclic double bond, and periodate epoxide cleavage.53... 

The coordination polymerisation of heterocyclic and heterounsaturated monomers consists in the nucleophilic attack of the metal initiating substituent (or the growing polymer chain) on the carbon atom of the coordinated monomer. Scheme 2.6 shows initiation and propagation steps in the coordination polymerisation of epoxides, as the most representative heterocyclic monomers with an endocyclic heteroatom, with catalysts containing an Mt-X active bond [68,114,115] ... 

Other heterocyclic monomers with an endocyclic heteroatom, such as higher cyclic ethers as well as cyclic sulfides like episulfides, undergo, similarly to epoxides, the coordination polymerisation in which the metal heteroatom a bond is regenerated in subsequent polymerisation steps [122,123]. 

Analogously to the mechanism of epoxide polymerisation, the mechanism of the polymerisation of -lactone and other heterocycles with both endocyclic and exocyclic heteroatoms involves multicentred transition states with the participation of at least two metal atoms. 

Epoxidation of endocyclic enol ether 37 with dimethyl dioxirane (DMDO) followed by one-pot reduction with DIBALH produced a 10 1 mixtrrre of alcohols, in which the major isomer possessed... 

Treatment of the 7a,8a-epoxy-5-oxo-5,6-secocholestane aldehyde (147) with mineral acid or Lewis acid gave the acetal (148) which with BF3-Et20 rearranged to the acetal (149). The olefin (154), which was formed in the BF3-catalysed fragmentation of the exocyclic epoxides (150) and (151) and, as previously reported, in the similar fragmentation of the endocyclic epoxide (152), was shown to arise from the oxetan (153) in all cases. 

In the epoxidation of /ra/w- -caryophyllcnc (12)158,16fi, conformations 12 A, B for both modes of attack are readily accessible. Interestingly, the less stable conformer of the alkene 12 A is preferentially attacked by the peracid. The reason for this is that torsional interactions of the partially formed C —O bonds differ considerably in the two diastereomeric transition states. This should be taken as a warning when prognosticating diastereomeric ratios of epoxidation of medium and large ring (i )-alkcnes from the relative population of alkene conformers. In this case, the correlation with relative stabilities of the diastereomeric epoxides seems to be generally better. The transition state force field reproduces the reported diastereoselectivities of epoxidation (80 20) of the endocyclic double bond perfectly. 

As shown in Section 4.5.1.3.2.2.1., epoxidation of the exocyclic conjugated double bond in terpenes usually leads to a mixture of diastereomeric epoxides. The situation changes when one moves to terpenes with endocyclic conjugated double bonds. Epoxidation of this class of compounds has been thoroughly studied35 40-42, and it has been shown that such epoxidation, under Weitz-Scheffer conditions, is stereospecific, resulting in the formation of a single epoxide 1-89 and 942. 

Epoxidation of Terpenes with an Endocyclic Bond General Procedure35 ... 

The unusual nucleophilic epoxidation of /i-hydroxyenones under Sharpless conditions (see Section 4.5.1.3.2.1.) is also applicable to compounds 1 with endocyclic double bonds. The. sj H-epoxides are produced with complete selectivity. The stereochemical outcome of the reaction under Weitz-Scheffer conditions significantly differs from that observed for acyclic compounds. While the acyclic enones afforded preferentially the moderate ratio, cyclic ones gave predominantly s>7i-epoxides32. 

---