Epoxidation ring expansion

This chapter begins by classifying the combinations of oxidation/reduction processes with subsequent cationic transformations, though to date the details of only two examples have been published. The first example comprises an asymmetric epoxidation/ring expansion domino process of aryl-substituted cyclopropyl-idenes (e. g., 7-1) to provide chiral cyclobutanones 7-3 via 7-2, which was first described by Fukumoto and coworkers (Scheme 7.1) [2]. 

Scheme 7.1. Domino asymmetric epoxidation/ring expansion reaction.

For another example of the strong dnality in the chemical behavior of distonic cation-radicals, see Moraes and Eberlin (1998). In the gaseons phase, m- and /7-dehydrobenzoyl cation-radicals ( CgH4C =0) react selectively either as free radicals or as acylium ions, depending on the choice of the reacting partner. Transacetalization with 2-methyl-l,3-dioxolane, ketalization with 2-methoxy-ethanol and epoxide ring expansion with epichlorohydrin demonstrate their acylium reactivity. 

Fig. 41 Mechanism of the epoxide ring expansion using [Co(CO)4] anion...

Epoxide ring expansion.1 The reaction of the monoepoxide (1) of 1,5-cy-clooctadiene with bromine in CH2C12 results in the two cyclic ethers 2 and 3 with high stereoselectivity. 

Iodine-promoted transannular epoxide ring expansion of /ram-1,2-epoxycyclododeca-l,5,9-triene or, better, related l,2-epoxycyclododeca-l,6,10-trien-3-ol 95 (Scheme 39) leads to specific bridged oxepane derivatives 96... 

The series of reactions leading to the 5-silyl-l-pentene - epoxidation, ring expansion, and Peterson elimination -are all stereospecific. Therefore, epoxides with different geometry can be transformed into the corresponding (E)- or (Z)-olefinic silanols <1994BCJ1694, 1991TL4545>. Subsequent Tamao oxidation affords stereodefined pentenols. 

Potassium Amides. The strong, extremely soluble, stable, and nonnucleophilic potassium amide base (42), potassium hexamethyldisilazane [40949-94-8] (KHMDS), KN [Si(CH2]2, pX = 28, has been developed and commercialized. KHMDS, ideal for regio/stereospecific deprotonation and enolization reactions for less acidic compounds, is available in both THF and toluene solutions. It has demonstrated benefits for reactions involving kinetic enolates (43), alkylation and acylation (44), Wittig reaction (45), epoxidation (46), Ireland-Claison rearrangement (47,48), isomerization (49,50), Darzen reaction (51), Dieckmann condensation (52), cyclization (53), chain and ring expansion (54,55), and elimination (56). 

Maltol. Otsuka Chemical Co. in Japan has operated several electroorganic processes on a small commercial scale. It has used plate and frame and aimular cells at currents in the range of 4500—6000 A (133). The process for the synthesis of maltol [118-71 -8], a food additive and flavor enhancer, starts from furfural [98-01-1] (see Food additives Flavors and spices). The electrochemical step is the oxidation of a-methylfurfural to give a cycHc acetal. The remaining reaction sequence is acid-catalyzed ring expansion, epoxidation with hydrogen peroxide, and then acid-catalyzed rearrangement to yield maltol, ie ... 

Ring expansions of suitable /3-lactams can also be achieved on treatment with base rearrangement of the Af-substituted azetidin-2-ones (133) occurs in the presence of LDA to give (134) (72JA9261). Aminolysis of the /3-lactam epoxide (135) and the aldehyde (137) affords (136) and (138) respectively (81JHC1239). 

Corey s ylide (1), as the methylene transfer reagent, has been utilized in ring expansion of epoxide 75 and arizidine 77 to provide the corresponding oxetane 76 and azetidine 78, respectively. 

The reactions of diazomethane with heterocycles containing a ketonic grouping in the ring do not differ, in principle, from those of alicyclic ketones (see footnotes 3 and 177) ring expansion and the formation of epoxides compete. In general, ring expansion is the more important reaction for example, tetrahydropyran-4-one (99) is converted to l-oxacycloheptan-4-one (100) (60%) and 4,4 -epoxy-4-methyltetrahydropyran (101) (23%). ... 

In a formal synthesis of fasicularin, the critical spirocyclic ketone intermediate 183 was obtained by use of the rearrangement reaction of the silyloxy epoxide 182, derived from the unsaturated alcohol 180. Alkene 180 was epoxidized with DMDO to produce epoxy alcohol 181 as a single diastereoisomer, which was transformed into the trimethyl silyl ether derivative 182. Treatment of 182 with HCU resulted in smooth ring-expansion to produce spiro compound 183, which was subsequently elaborated to the desired natural product (Scheme 8.46) [88]. 

Stereospecfic ring expansion can be done by taking advantage of the hydroxy-directed epoxidation and SnCl4-mediated rearrangement of 1-hydroxycycloalkyl epoxides.69... 

The same epoxide 335 was easily obtained in mild conditions (0°C, 5 min) by m-ehloroperbenzoic acid oxidation [13b]. Epoxidation of alkylidenecyclo-propanes by m-chloroperbenzoie acid has been greatly exploited as a route to the synthesis of cyclobutanones 638 via the well known ring expansion of oxaspiropentanes 637 (Scheme 98) [176,177,8]. 

Trost s approach to gibberellins applied the ring expansion of the epoxides (165)... 

Cyelobutanone has been prepared by (1) reaction of diazomethane with ketene,4 (2) treatment of methylenecyclobutane with performic acid, followed by cleavage of the resulting glycol with lead tetraacetate,s (3) ozonolysis of methylenecyclobutane, (4) epoxidation of methylene-cyclopropane followed by acid-catalyzed ring expansion,7 and (5) oxidative cleavage of cyclobutane trimethylene thioketal, which in turn is prepared from 2-(co-chloropropyl)-l,3-dithiane.8... 

Abstract Development in the field of transition metal-catalyzed carbonylation of epoxides is reviewed. The reaction is an efficient method to synthesize a wide range of / -hydroxy carbonyl compounds such as small synthetic synthons and polymeric materials. The reaction modes featured in this chapter are ring-expansion carbonylation, alternating copolymerization, formylation, alkoxycarbonylation, and aminocarbonylation. 

The ring-expansion carbonylation of epoxides is the most widely studied field in the epoxide carbonylation chemistry since the product lactones are highly attractive targets particularly, /1-lactones are useful compounds due to their versatility in organic synthesis [ 14,15] as well as their utilization as monomers to produce poly(3-hydroxyalkanoate)s, naturally occurring biodegradable polyesters [16-19]. 

The opened epoxide shown in Fig. 15 is also an intermediate in the catalytic ring expansion of epoxides to p-lactones (see Sect. 8.4). In this context, Drent and coworkers patented a catalytic system of dicobaltoctacarbonyl Co2(CO)g and... 

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