Isomerization, of epoxides

Isomerization has been observed with many a,j3-unsaturated carboxylic acids such as w-cinnamic 10), angelic, maleic, and itaconic acids (94). The possibility of catalyzing the interconversion of, for example, 2-ethyl-butadiene and 3-methylpenta-l,3-diene has not apparently been explored. The cobalt cyanide hydride will also catalyze the isomerization of epoxides to ketones (even terminal epoxides give ketones, not aldehydes) as well as their reduction to alcohols. Since the yield of ketone increases with pH, it was suggested that reduction involved reaction with the hydride [Co" (CN)jH] and isomerization reaction with [Co (CN)j] 103). A related reaction is the decomposition of 2-bromoethanol to acetaldehyde... 

Sulphated zirconia catalysts can be acidic or superacidic depending on the method of treatment. A variety of acid-catalysed reactions, referred to earlier in this section, can be carried out with sulphated zirconia. Yadav and Nair (1999) have given a state-of-the art review on this subject. Examples of benzylation of benzene with benzyl chloride / benzyl alcohol, alkylation of o-xylene with. styrene, alkylation of diphenyl oxide with 1-dodecene, isomerization of epoxides to aldehydes, acylation of benzene / chlorobenzene with p-chloro benzoylchloride, etc. are covered in the review. 

A very common combination in Pd-catalyzed domino reactions is the insertion of CO as the last step (this was discussed previously). However, there is also the possibility that CO is inserted as the first step after oxidative addition. This process, as well as the amidocarbonylation, the animation, the arylation of ketones, the isomerization of epoxides, and the reaction with isonitriles will be discussed in this section. 

Activation of C—Zr bonds as electrophiles with Ag salts has also been applied to the reaction with epoxides (Scheme 1.33) [62—64]. In fact, the reaction has been shown to involve isomerization of epoxides to aldehydes and is therefore very closely related to that shown in Scheme 1.31. 

In conclusion, each of the three synthetic approaches to 256 has its advantages and disadvantages. Base-catalyzed isomerization of epoxides is certainly the simplest method however, it is not applicable in all cases. The use of a specific Base, not prone to side reactions, would be the best approach to the synthesis. 

Isomerization of epoxides to ally lie alcohols. The isomerization of (- )-ver-nolic acid (1) to ( + )-coriolic acid (2) is best effected with this magnesium amide in toluene at 0°.1... 

This isomerization is also catalyzed by more sterically hindered amines, and its rate increases with temperature and basicity of the amine used 80). In the absence of a-hydrogens, the isomerization of epoxide may lead to aldehydes 811... 

Antoon and Koenig671 reject this isomerization of epoxides in copolymerization with anhydrides on the basis of IR spectra. However, in the monomer feed, proton... 

Coordinatively labile ruthenium(II) porphyrins Ru(P)(THF)2 (P = TTP, TMP) catalyse the cis- trans isomerization of epoxides under mild conditions, probably by coordination of the epoxide and ring opening via a carbon radical [365]. The lifetime of the catalysts is restricted due to carbon monoxide abstraction from coordinated epoxide to yield inactive carbonylruthenium(II) complexes, e.g. RuCO(TMP)THF [366],... 

The isomerization of epoxides is discussed in Section II, D,l. The isomerizations of olefins and of alkyl- and acylcobalt carbonyls have been considered as side reactions to hydroformylation but studies dealing principally with these reactions will now receive attention. 

K. Ranganayakulu, U. P. Singh, T. P. Murray, and R. K. Brown, Base catalyzed isomerization of epoxides to bicyclic allylic alcohols, potential intermediates for the total synthesis of the DL-aldohexoses, Can. J. Chem., 52 (1974) 988-992 and preceding papers. 

Scheme 45 Isomerization of epoxides 134 prepared from 3-phospholene 130d to 2-phos-pholenes 135a-d...

It should be noted that addition of DBU (or HMPT) has often - particularly for the catalytic procedures - proven beneficial in terms of enantioselectivity. This effect has been attributed to the breaking-up of active but less enantioselective base aggregates [50, 56, 62-64]. Interestingly, when solid-phase-bound stoichiometric (achiral) bases were used instead of LDA, no addition of DBU was necessary [58, 59]. (An example of the stoichiometric use of a chiral solid-phase bound base is given elsewhere [52].) Both experimental and theoretical investigations [66-68] indicate that the base-induced isomerization of epoxides proceeds as a syn elimination, with the lithium ion of the base acting as a Lewis acid (Scheme 13.31). 

Acetone is the important product of the mimic s monooxygenase activity. It is synthesized at high temperature, 220 °C or higher. The yield of acetone noticeably increases with contact time and temperature, reaching 15 wt.%. In this case, two ways of acetone formation (catalytic isomerization of epoxide and direct synthesis from C3H6) are also probable. 

Isomerization of epoxides to ketones.3 This radical cation [as well as trityl hexachlorostibnate, (C6H5)3C+SbCl6 ] effects this reaction. 

J. K. Crandall, M. Apparu, Base-Promoted Isomerizations of Epoxides, Org. React. 1983, 29, 345—443. A. C. Cope, E. R. Trumbull, Olefins from Amines The Hofmann Elimination Reaction and Amine Oxide Pyrolysis, Org. React. 1960, 11, 317-493. 

Isomerization of epoxides to allylic alcoholsThis rearrangement has been effected with strong bases and various Lewis acids. Enantioselective rearrangement to optically active allylic alcohols can be effected with catalytic amounts of vitamin B, at 25°. Thus cyclopentene oxide rearranges to (R)-2-cyclopentene-l-ol in 65% ee. The rearrangement of the as-2-butene oxide to (R)-3-butene-2-ol in 26% ee is more typical. 

J. K. Crandall and M. Apparu, Base-promoted isomerizations of epoxides, Org. React 1983,29, 345-443. 

Recent results on the isomerization of epoxides to ketones accomplished via electrode-catalyzed, one electron-oxidizing reagents, and DCA-sensitized photoreaction under nitrogen atmosphere, seem to confirm this mechanistic pathway [142, 143]. On the other hand, the MCPBA oxidation of 42 affords the same reaction products 43, 44, through a plausible acid-catalyzed isomerization of the same intermediate 45 [144]. 

Acetals as Chiral Auxiliaries. There have been many applications of acetals of 2,4-pentanediol as chiral auxiliaries to control the diastereoselectivity of reactions on another functional group. Examples include cyclopropanation of alkenyl dioxanes, lithium amide-mediated isomerization of epoxides to allylic alcohols, and addition of dioxane-substituted Grignard reagents or organolithiums to aldehydes. 

Nozaki and Yamamoto reported a novel method for regiospecific isomerization of epoxides to allylic alcohols based on diethylaluminum 2,2,6,6-tetramethylpiperidide (DATMP) which can be prepared in situ from Et2AlCl and lithium 2,2,6,6-tetramethylpiperidide (LiTMP) (molar ratio 1 1) in benzene at 0 °C for 30 min [119], Reaction of DATMP (4 equiv.) with ( )-cyclododecene oxide in benzene at 0 °C for 3 h produced ( )-2-cyclododecen-l-ol in 90 % yield. Interestingly, the reaction of diepoxide 121, a useful synthetic intermediate in the preparation of Cig-Cecropia juvenile hormone, with DATMP furnished the triol 122 in 41 % yield (Sch. 83). 

In addition to the formation of silyl enol ethers, isomerization of epoxides to allylic alcohols is another highly typical transformation performed by combination of a silicon Lewis acid with a tertiary amine. Reaction with la was examined, and its scope and limitation reported, by Noyori [61]. Epoxide 44 can be successfully converted into the corresponding allyl silyl ether 45 (Sch. 34). 

The Beckmann rearrangement of ketoximes to the corresponding amides (31), the Fischer indole cyclization, isomerization of epoxides to the corresponding aldehydes, ketones, or alcohols, hydration and ammo-nolysis of epoxides, oxygen-sulfur interchange, formation of diaryl-ureas and -thioureas from condensation of aniline and carbonyl sulfide, and olefin carbonylation occur over zeolite catalysts (62). The oxo reaction over rhodium and cobalt containing zeolites recently has been claimed (22). 

Although cis-trans isomerization of epoxides is not formally associated with this section, it is clearly a potential problem in the conversion of an alkene to an epoxide. There are several catalysts for this process. 

Base-catalyzed isomerization of epoxides into allylic alcohols is a method of choice for the preparation of unsaturated sugars olefin 10 was prepared by treatment of oxirane 9 with a strong base. This process is similar to the Horwitz s)uithesis of d4T presented in Scheme 7. The Tipson-Cohen methodology is frequently applied for the synthesis of 2,3-unsaturated sugars as shown in Scheme 10 such reduction might be performed also under microwave irradiation [6]. 

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