Epoxides initiators

A novel approach to azabicyclic ring systems, based on an epoxide-initiated electrophilic cyclization of an alkyl azide, has been developed by Baskaran. A new stereo- and enantioselective synthesis of the 5-hydroxymethyl azabicyclic framework 91a, present in (+)- and (-)-indolizidines 167B and 209D, for example, was... 

Scheme 8.24 Epoxide-initiated electrophilic cyclization of azides.

Puskas, J.E., Pattern, W.E., Wetmore, P.M., and Krukonis, A. Multiarm-star polyisobutylene-polystyrene thermoplastic elastomers from a novel multifunctional initiator, Polym. Mater. Set Eng., 82,42 3, 1999. Brister, L.B., Puskas, J.E., and Tzaras, E. Star-branched PIB/poly(p-t-bu-Styrene) block copolymers from a novel epoxide initiator, Polym. Prepr., 40, 141-142, 1999. 

Puskas, J.E., Brister, L.B., Michef A J., I-anzenddrfer, M.G., Jamieson, D., and Pattern, W.G. Novel substituted epoxide initiators for the carbocationic pol3mierization of isobutylene, 7. Polym. Set, 38,444-451, 2000. Puskas, J.E. and Michel, A.J. New epoxy initiators for the controlled synthesis of functionalized polyisobutylenes, Makromol. Chem., Macromol. Symp., 161, 141-148, 2000. 

Michel, A., Brister, L.B., and Fhiskas, J.E. Novel epoxide initiators for carbocationic polymerizations, NATO ASI Ionic Polymerizations and Related Processes, NATO Science Series 359, Kluwer, Dordrecht, the Netherlands, 1999. 

Attempts by Fish and Johnson to effect a steroid synthesis using a standard epoxide-initiated pentacyclization of a polyene afforded complex mixtures [69]. Alternatively, the allyl alcohol 326 was synthesized and treated with TFA (Scheme 19.60). Protonation affords a symmetrical tetramethylallyl cation that undergoes cyclization to give pentacycle 327 in 31% yield. Simultaneous cleavage of the isopropylidene and vinylidene groups was carried out to furnish the diketone 328 in 88% yield, which was then converted to sophoradiol (329). 

Ring-Opening Polymerization of Epoxide Initiated with... 

Ring-Opening Polymerization of Epoxide Initiated with Aluminum Porphyrin in the Presence of an Organoaluminum Compound [60]... 

Visible Light Mediated Living and Immortal Polymerizations of Epoxides Initiated with Zinc Porphyrin [80]... 

Living and Immortal Ring-Opening Polymerizations of Epoxide Initiated with Manganese Porphyrin [91]... 

Squalene is an important biological precursor of many triterpenoids, one of which is cholesterol. The first step in the conversion of squalene to lanosterol is epoxidation of the 2,3-douhle bond of squalene. Acid-catalysed ring opening of the epoxide initiates a series of cyclizations, resulting in the formation of protesterol cation. Elimination of a C-9 proton leads to the 1,2-hydride and 1,2-methyl shifts, resulting in the formation of lanosterol, which in turn converted to cholesterol by enzymes in a series of 19 steps. 

Reddy, P. G., Baskaran, S. (2004) Epoxide-initiated cationic cyclization of azides a novel method for the stereoselective construction of 5-hydroxymethyl azabicyclic compounds and application in the stereo- and enan-tioselective total synthesis of (+)- and ( —) -indolizidine 167B and 209D. J Org Chem 69, 3093-3101. 

The fact that the epoxide yield decreases at higher temperatures, longer reaction time, higher catalyst concentration, and lower olefin concentration may be caused by two possible side reactions—decomposition of the hydroperoxide and addition of the alcohol to the epoxide. Initial kinetic studies of the decomposition of tert-butyl hydroperoxide in the presence of molybdenum hexacarbonyl showed second-order dependence on hydroperoxide and first-order dependence on catalyst concentration. These results indicate that the decomposition of hydroperoxide is caused by the reaction between the hydroperoxide-metal complex and another molecule of hydroperoxide. With higher temperature, higher... 

The results confirm the previously reported low reactivity of cyclohexene when the catalyst is TS-1 and indicate that Ti-beta is active for the oxidation of cyclohexene and other bulky olefins. However, for cyclohexene and the linear olefins, the major reaction products formed in the presence of Ti-beta are glycols and glycol ethers, whereas in the presence of TS-I, epoxides are predominantly formed. Also in this case, the epoxides initially formed in the presence of Ti-beta undergo secondary reactions catalyzed by the acidic centers associated with the aluminum in the material, as previously seen for allyl alcohol and for the epoxidation of 1-butene on aluminum-containing TS-1 (Bellussi et al., 1991a). A different product composition was observed for cyclododecene,... 

Scheme 1. Epoxide-initiated, enantiospecific polyene cyclizations of the substrates 3 and 5 to the synthetic precursors (4 and 6) of the marine natural products scaiarenedial and adociasulfate 1.

Inulin alkoxylates are reported to have surface-active, emulgating, and stabilizing properties. They may also be further altered via reaction with other epoxides. Initial synthesis methods utilized... 

Formulations of epoxy coatings can be varied over a wide range by combining different epoxy resins and cationic photoinitiators discussed in this paper. The reactivity of a formulation depends upon the structure of the epoxies and the photoinitiators which can be further influenced by additives such as fillers, pigm its, colorants, stabilizers etc. Complete crosslinking of epoxide/initiator systems can be achieved by an irradiation and a heat treatment step, both of a very short duration... 

Cp2TiCl-mediated 5-exo-trig cyclizations have been intensively exploited by Roy et al. for the total synthesis of ( )-dihydroprotolichesterinic and ( )-roccellaric acids, ( )-sesamin, ( )-dihydrosesamin, ( )-acuminatin, ( )-eudesmin, ( )-lariciresinol, ( )-pinoresinol, ( )-piperitol, ( )-acumi-natin methyl ether, (zh)-sanshodiol methyl ether, ( )-piperitol methyl ether, ( )-pinoresinol monomethyl ether, ( )-lariciresinol monomethyl ether, and ( )-lariciresinol dimethyl ether [ 100-103]. Moreover, this group has very recently reported the enantioselective synthesis of (-)-sesamin (Scheme 19), (-)-dihydrosesamin, (-)-acuminatin, and (-)-methyl piperitol by radical cyclization of chiral epoxides initiated by Cp2TiCl [104]. 

The first examples of high-yielding epoxide-initiated tri- and pentacarbocyclizations (Equation (4)) utilizing (2-propoxy)titanium trichloride ((Pr 0)TiCl3) have been reported <93TL7849>. 

This is followed by cationic polymerization of the epoxide initiated by the proton from HPFe-... 

Scheme 1.30. Ring-opening pol5mierization of epoxides initiated by BF3.

Polymerization of epoxides initiated by tertiary amines and the effect on the molecular weight distribution [92]. 

Formation of a low concentration of polyfunctional macromolecules dissolved in the monomer is a special feature of the early stage of the chain polymerization mechanism. Because of the usually high degree of polymerization of the formed chains, the gel-point conversion is relatively low. Exceptions are cases in which a very low concentration of a polyfunctional monomer (i.e. a divinyl molecule) is copolymerized with a high concentration of a difunctional monomer (i.e. a vinyl molecule), or cases where transfer or termination rates are comparable with propagation rates (i.e. homopolymerization of epoxides initiated by tertiary amines or Lewis acids). 

FIGURE 4 The dependence of the epoxidation initial rate on the contents of catalyst... 

Thus, the stated above results have shown that the methods of the fraetal analysis explain qualitatively and quantitatively the nonlinear character of the dependence of the epoxidation initial rate on the catalyst concentration, obtained by the authors [12]. Let s note that the continual models which use the representations of Euclidean geometry, do not allow to obtain the adequate explanation. 

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